JP3564396B2 - Plasma gun and method of using the same - Google Patents

Abstract

A high pulse repetition frequency (PRF) plasma gun is provided, which gun inlets (74) a selected propellant gas into a column (16) formed between a center electrode (12) and a coaxial outer electrode (14), utilizes a solid state high repetition rate pulse driver (130) to provide a voltage across the electrodes (12, 14) and provides a plasma initiator (82) at the base of the column (16), which is normally operative when the driver is fully charged. For preferred embodiments, the initiator (82) includes a solid state simulated RF driver (130), the output is applied to the electrodes (91) affixed in an insulator (72) and producing a high voltage field at a surface of the insulator (72) which forms part of the base end of the column (16).

Description

【００４２】
中央チャンネル９２を通して供給されたガスがその単一電子状態に完全に変換されていない程度で、しかも、締め付け部において存在する温度で、大半のガスがこの状態に実質上イオン化されていない場合でさえ、放射線はまた、その元素の他のスペクトル波長で出力される。しかし、上述の式から明らかなように、これらの放射線は非常に低い強度を有し、その強度は単一電子状態の強度のほんの一部である。従って、例えば、原子番号５４のキセノンは小さな値の０．０４ｎｍの単一電子波長を有するが、また、略述するように、有用な１３ｎｍの波長でのエネルギを有する。しかし、１３ｎｍでのエネルギは、単一電子状態にとって最適な締め付け部での温度の単一電子波長でのエネルギの１／１０¹⁰であり、実質的に、レーザー締め付け温度がより低いとさらに低いオーダーの大きさとなる。その理由は、相対ラインの大きさを決定することに依存する黒体放射曲線の形のため、１３ｎｍにおいてエネルギの少量（≦１／４）以上を単独で強制的に発出させることが不可能であり、温度が大幅に変化するからである。
【００４３】
そのため、元素の最適な単一電子波長以外の波長で放射線を使用するためには、その元素のために放射されている一層高い強度の一層短い波長を濾波する必要がある。図３はこれを行う１つの方法を示し、ここでは、プラズマ銃９０から発出されている放射線９４は所望の標的の方へ反射される所望の波長を除いた放射線のすべての波長を吸収するように構成された当業界で既知の型式の鏡９６に供給される。所望の波長及びそれ以上の波長のための少なくともハイパスフィルタである他のフィルタも使用することができる。
【００４４】
従って、可能なら、ガスのための元素又は最大エネルギの単一電子状態における所望の波長で放射線を発生させる中央チャンネル９２へ供給される他の元素を使用するのが望ましい。しかし、単一電子状態における所望の波長で放射線を発出するいかなる元素もが存在しない場合、及び、表１から、約７．６ｎｍ以上の極めて少ない波長が最大エネルギ状態で元素のために実際利用できることが分かった場合は、所望の波長で放射線を発出する元素及び所望の波長での放射線を得るために利用されるフィルタとしての鏡９６の如き適当なフィルタを見つけ出さなければならない。この放射線は単一電子状態の波長での放射線よりも一層弱い強度なので、一層弱い強度で十分なエネルギを得るためには一層大型で実質上一層高価な装置すなわちプラズマ銃９０が一般に必要となる。一定の波長での放射線の強度はワット／（メートル）²／ヘルツの単位で与えられ、放射線の周波数又は波長、温度及び放射率の関数として変化する。放射率は１の最大値を有する関数であり、所望の出力周波数／波長で最大放射率を有するガスを選択するのが重要である。一定の波長λに対する最適な締め付け温度（Ｔ_OPT）はウィーンの変位則Ｔ_OPT＝０．２８９８ｃｍ×Ｋ゜／λ（ここに、Ｋ゜はケルビンにおけるプラズマの温度である）から決定できる。極めて少量のガスのみが各締め付け中に放射線を発生させるようにイオン化されるので、キセノンは１３ｎｍの放射線を得るために中央チャンネル９２を通って比較的遅い速度で流れることができる。しかし、先に述べたように、キセノンを使用した場合、１３ｎｍでの出力放射線は比較的弱い強度となり、この波長で有用な放射線を得るためには鏡９６のようなフィルタが必要となる。この理由のため、表１から実質上所望の波長で（即ち、１３．５ｎｍで）最大強度の波長を有することが分かるリチウムが、この波長での放射線にとって好ましい元素である。
【００４５】
図４は所望の放射線を生じさせるためにリチウム蒸気を利用する実施の形態のための中央電極１２を示す。この図を参照すると、中実のリチウムコア９８がステンレス鋼のような材料のチューブ１００内に保持され、チューブ１００の先端は中央電極１２に沿った先端近傍で、プラズマ締め付け中、約９００℃の温度になり、リチウムコア９８の端部から約１Ｔｏｒｒの圧力でリチウム蒸気を発生させる。このリチウム蒸気は、先端でアルゴン又は他のプラズマガスと置換するような流量で、中央電極１２の端部の開口１０２から流出し、この必要な流量は、図示の実施の形態に対しては、年間約１−１０グラムの範囲である。チューブ１００は適当な位置にリチウムコア９８の前端を保持するために適当な方法でゆっくり前進することができる。リチウムコア９８を使い切ったとき、これを交換することができる。少量のヘリウムガスが好ましくはチューブ１００のまわりに供給され、開口１０２から流出し、リチウム及びヘリウムのみが締め付け区域に存在するのを保証する。その理由は、少量ではあるがアルゴンは高エネルギで短い波長のラインを生じさせ、これが、濾波しなければ、所望の標的での１３ｎｍの放射線と干渉してしまうからである。
【００４６】
締め付け部にリチウム又は他の適当な材料を与える別の方法は、液体リチウム又は流体（即ち液体又は気体）状態のある他の適当な材料で飽和した焼結粉末耐火金属で、中央電極１２及び外側電極１４の少なくとも一方を形成することである。適当な結合剤と共にタングステンの如き粉末耐火材料をプレスし、次いで、出来上がった質量体を高温で焼結することにより、タングステン又はモリブデンの如き金属を所望の電極形状に製造できる。出来上がった多孔性の耐火金属母体を液体リチウム又は他の所望の材料で含漬して、改善された寿命及びリチウム／材料を放電部へ導入する別の手段を提供することができる。放射線発生材料を交換する必要なしに、プロセスの実質上無限の寿命を提供するように、所望なら、作動中に液体リチウムを電極の金属母体へ常に供給することができる。粉末耐火金属を選択する際の１つの制約は、金属が中で燃えている放射線発生材料内に溶けないことを保証することである。
【００４７】
１３ｎｍの放射線を得るためにキセノンを使用する場合は、キセノンはその波長でかなりの吸収性を有するため締め付け部のすぐ近傍にキセノンを閉じ込めなければならない。キセノンの場合のように、使用される放射線が中央チャンネル９２内の元素／ガスのための単一電子波長以外の波長である場合は、元素の少量をその単一電子状態へイオン化して、一層長い波長での多量の放射線及び一層短い波長での少量の放射線（ただし、一層強い強度の放射線）を提供するように、締め付け部での温度を制御することができる。
【００４８】
また、発出される放射線の円錐角は出来る限り小さい方が望ましい。締め付け部での放射ガスからの放射線の誘導放射が自然放射よりも一層大きい場合に、小さな円錐角が達成され、自然放射は一層分散的である。特に、ボルツマン定数ｋ×締め付け部での温度が放射線の周波数ｆ×プランク定数ｈよりも大きいと仮定すると、誘導放射Ａに対する自然放射Ｂの比率は（Ｂ／Ａ＝ｋＴ／ｈｆ）で与えられる。例えば、この比率が２０に等しい（即ち、プラズマ温度が問題の光子エネルギの２０倍である）場合、半円錐角は約２５゜となる。プラズマ温度が高いほど、円錐角は一層小さくなる。しかし、放射線の波長が短いほど、小さい円錐角を達成するのが一層困難になる。しかし、円錐角は締め付け部での所望の温度を達成するために電流及び他のパラメータを選択する際に考慮すべき因子の１つである。
【００４９】
図５は、電極長さの如き因子、及び、放射線発出元素／ガスが中央電極１２を通して導入されるか否かに応じて、スラスタ、放射線源又はプラズマ銃を利用する他の機能として使用できる本発明の別の実施の形態を示す。プラズマ銃は主ソリッドステートドライバ１１０により駆動されるものとして示され、好ましい実施の形態に対しては、この主ソリッドステートドライバ１１０は直流電圧源３２と、直流／直流コンバータ３４と、非線形磁気コンプレッサ（ＮＭＣ）３６とを含む。しかし、この実施の形態はプラズマ始動のために穴７４内のトリガ電極８２を利用するが、これは、トリガ電極又は他の電極が直流遮断コンデンサ１１４及び整合変圧器として機能する共振同軸ライン１１６を介してパルスＲＦ信号源１１２から駆動されるという点で、先の実施の形態とは異なる。好ましい実施の形態に対しては、ＲＦ信号は１０ＭＨＺないし１，０００ＭＨＺの周波数であり、主ソリッドステートドライバ１１０の付勢前に約１ないし１０マイクロ秒だけ付勢される。図５はまた交流フィルタコイル１２０を介して中央電極１２に接続された随意の直流バイアス源１１８を示す。直流バイアス源１１８は、駆動回路８６の如き整形及び制御回路を介して実質上給電される直流電圧源３２とすることができ、または、用途に応じて別の電圧源とすることができる。
【００５０】
図５において、コラム１６の両側に位置する２つのみのトリガ電極即ちスパークプラグ８２、９１を示すが、プラズマ銃は好ましくはコラム１６の周辺のまわりにおいて等間隔で離間した少なくとも４個のトリガ電極を有し、そして、６個又は８個（又は可能ならそれ以上）のトリガ電極を有することができる。４個のトリガ電極の場合、図示のトリガ電極に供給されたＲＦ信号は第１相となり、図示のものに対して９０゜の位置にあるトリガ電極に供給されたＲＦ信号は第１相に対して９０゜位相ずれした第２相となる。６個のトリガ電極を有するプラズマ銃に対しては、３相ＲＦ信号が使用され、各相はコラム１６の両側で一対のトリガ電極に供給される。８個のトリガ電極の場合、２相信号が好適には利用され、１つの相は１つおきの電極に供給され、第２の相はこれらトリガ電極間のトリガ電極に供給され、４相信号も使用できる。プラズマ始動のために直流信号ではなくＲＦ信号を使用する理由は、トリガ電極に供給されたＲＦ信号が一層均一及びほぼ完全に均一な容積的イオン化即ち始動をコラム１６内で生じさせるからである。好ましくはライン（単数又は複数）１２２上の制御信号に応答してＲＦ信号源１１２からのＲＦ信号と同時に供給される直流バイアス源１１８からの直流バイアスは更に、特に中央電極１２の近傍での均一なイオン化に寄与し、ＲＦ信号源１１２での必要電力を減少させる。直流バイアスは、図示のように中央電極１２へ供給することができ、または、例えば、ＲＦ信号が直流バイアスを変調するようにＲＦ信号と直列又は並列に中央電極１２へ供給することができる。
【００５１】
図６は、例えば互いに９０゜の角度で位置する２つのトリガ電極（スパークプラグ）８２、８２′へのＲＦ信号源１１２の接続を示す。プラズマ銃内には２つの付加的なトリガ電極が存在し、第２のトリガ電極８２は図示のトリガ電極８２に対して１８０゜の角度で位置し、トリガ電極８２のための図示の方法で接続され、第２のトリガ電極８２′は図示のトリガ電極８２′から１８０゜の角度で位置し、この電極と同じ方法で接続される。ＲＦ信号源１１２は四分の一導波路同軸ライン１２４、１２４′を介して同軸ライン１２６、１２６′の短くなった端部の近傍の地点に接続されるが、短くなった端部からそれぞれ距離Ｌ１、Ｌ２だけ離れている。同軸ライン１２６は四分の一波長長さであり、その短くなっていない端部でトリガ電極８２を有し、一方、同軸ライン１２６′は半波長長さであり、その短くなっていない端部でトリガ電極８２′を有する。四分の一波長長さの同軸ライン１２６及び半波長長さの同軸ライン１２６′では、トリガ電極８２、８２′でＲＦ信号のための所望の位相差が達成される。同軸ラインはまた大きな電圧逓昇を提供し、結合位置／距離Ｌ１、Ｌ２が正しく選択された場合は、破壊が達成されるまで、ＲＦ信号源を整合負荷として頼る。良質の同軸ラインを使用すると、１０−２０：１程度の電圧逓昇比率を容易に達成できる。破壊が達成されると、ラインは位置Ｌ１で短絡回路のようになる。位置Ｌ１からλ／４離れたＲＦ信号源１１２へ結合する入力において、見掛けのインピーダンスは開回路のようになる。更に、位置Ｌ２が正しく選択された場合、このラインは、破壊が開始された後に、整合負荷のようになる。同軸ライン１２６、１２６′を出来る限り短く保つのが望ましいが、所望の位相及びインピーダンス整合は（２Ｍ−１）λ／４、Ｍλ／２のそれぞれの長さでラインに対して実質上達成できる。それ故、ＲＦ信号源１１２は常に整合負荷を監視し、最初に一対のトリガ電極において電圧逓昇を生じさせ、次いで、プラズマが始動されたのち、第２の対のトリガ電極８２′において電圧逓降を提供するが、電流逓昇を提供する。次の表２は図示の実施の形態に対する図６のＲＦ信号源１１２のためのパラメータを与える。
【００５２】
【表２】

【００５３】
ＲＦ信号源１１２のみから又はＲＦ信号源１１２及び直流バイアス源１１８の双方からのＲＦ周波数及び電圧は、最大均一性を与えるように寸法及び作動圧力から決定される。一般に、ＲＦ周波数は臨界周波数以上となるように選択しなければならず、臨界周波数は、これよりも低い周波数では、ガス内の電子が各半サイクルにおいて全体の電極ギャップを横切って払拭されるような時間を有し、それ故消失するような周波数である。臨界周波数以上では、電子は電極間で前後に振動し、ガスのイオン化を容易にする。一定のプラズマ銃設計のための臨界周波数は、流動性を最初に計算することにより決定される。
【００５４】
【数１】

【００５５】
ここに、ν_cは衝突周波数、ω＝２πｆ（ここに、ｆは放射線の周波数）、ｑは電子チャージ、Ｅは電場、ｍは電子質量である。それ故、ガスを遷移させる時間は、
【００５６】
【数２】

【００５７】
【数３】

【００５８】
で与えられる。ここに、ｄは電極間の距離である。
スラスタの実施の形態に関しては、全体のプラズマ銃９０を近真空環境（ほぼ、ガス圧力≦１０Ｔｏｒｒ）内に維持する必要があり、これが更に必要な理由は、ＥＵＶ帯域内の放射線が容易に吸収され、近真空環境以外では有用な仕事を行うために使用できないからである。この実施の形態に対しては推進効率はさほど重要でないので、各弁操作即ち弁操作期間に対して単一の放射線バーストでよく、所望の期間だけ放射線を提供するために多数のパルス／バーストを選択することができる。
【００５９】
マグネトロン、クライストロン又はＲＦ増幅器の如き標準の高電圧のＲＦ信号源１１２は、前述のように、先の実施の形態に対してＲＦ信号源として利用することができるが、このような標準のＲＦ信号源は購買及び使用にとって高価であり、大型であり、利用する装置の熱管理負担を増やすかなりの熱を発生させる。それ故、このようなＲＦ信号源を、購買及び作動にとって大幅に安価であり、大幅に少ない熱を発生する一層小型のＲＦ信号源と交換できることが好ましい。図７Ａはこれらの要求を満たすソリッドステートのＲＦ信号源を示す。特に、ＲＦ信号源１３０の回路は標準のＲＦ信号源におけるコストの約１％のコストでＲＦ電力を生じさせ、大きなキャビネットではなく、例えば「６」又は「８」倍だけ小さい回路板の空間を占めることが判明した。
【００６０】
図７Ａを参照すると、ＲＦ信号源１３０は電圧源、例えば前述の直流電圧源３２から標準の様式で充電されるコンデンサ１３２を含む。例えばＳＣＲ、ＩＧＢＴ又はＭＯＳＦＥＴとすることのできるソリッドステートスイッチ１３４は、閉じたとき即ち通電したときに、コンデンサ１３２が前述の型式の多段非線形磁気パルス圧縮回路１３６の入力へ放電を行うのを許容する。多段非線形磁気パルス圧縮回路１３６は多段及び（又は）変圧器を含むことができ、このような形状の一例が示され、特殊化された出力セクション１３８で終端する。出力セクション１３８は接地部に対する飽和可能な共振分路を形成し、この出力セクション１３８の共振回路はコンデンサＣ_R及び飽和可能な誘導子Ｌ_Rを含む。コンデンサＣ_Rは多段非線形磁気パルス圧縮回路１３６の第ｎ段のコンデンサＣ_Nから共振的に充電される。コンデンサＣ_NはキャパシタンスがコンデンサＣ_Rよりも一層小さくなるように選定され、そのため、コンデンサＣ_RはコンデンサＣ_Nの充電中に反転する。代わりに、誘導子Ｌ_Rは、コンデンサＣ_NからコンデンサＣ_Rへのチャージの移送が完了する前に飽和するように選定することができる。これらの条件の一方又は双方が満たされると、誘導子Ｌ_Rが飽和しコンデンサＣ_Rがそのピークチャージに達する前に、コンデンサＣ_Nに対して逆電圧が生じる。これらの条件下で、誘導子Ｌ_Rの順次の飽和により、誘導子Ｌ_Rが図７Ｃに示すようにコンデンサＣ_Rを振動させる。本発明のプラズマ始動応用に対しては、ＲＦ信号源の３又は４回のみのサイクルが図７Ｃに示すように必要であるが、ＲＦ信号源１３０のパラメータは、応用に応じて、所望数のサイクルを提供するように選択することができる。出力セクション１３８の共振周波数ＦはコンデンサＣ_R、誘導子Ｌ_Rの値により決定され、これらの値のいずれか一方は回路のチューニングを許容するように調整可能にすることができる。抵抗性素子Ｒ_Oと容量性素子Ｃ_Oとからなる出力結合回路１４０が設けられ、これらの各々は適当に相互接続された多数の素子で形成することができる。出力結合回路１４０はコンデンサＣ_Rからのエネルギの一部を出力端子１４２に結合し、出力結合回路１４０のインピーダンスは各サイクルに対してコンデンサＣ_R内に貯蔵されたエネルギの一部のみ（例えば、サイクル当り２０％）を除去するように選定される。更に、図７Ａに示すＲＦ信号源１３０は本発明のプラズマ銃に使用するのに特に適するが、図７Ａに示すＲＦ信号源１３０の回路の性能特性を有するソリッドステートのＲＦ信号源は現在存在せず、それ故、このようなＲＦ信号源１３０はまた他の応用における使用を見出すことができる。それ故、このＲＦ信号源１３０もまた本発明の一部となる。
【００６１】
ＲＦ信号をプラズマ銃へ送給する際の２つの可能性のある問題は、コラム１６のベースで比較的大きな均一領域にわたって高電圧場が生じて、この領域で破壊が生じること、及び、コラム１６内で必要な真空に対する破壊を最小にした状態で、ＲＦ場をこの地点で得ることである。後者は宇宙応用では問題にならないが、放射線源としてのプラズマ銃の一層普通の応用では問題となる可能性がある。図８Ａは両方の目的を達成する１つの方法を示し、一方、図８Ｂは第２の目的のみを達成する方法を示す。
【００６２】
まず、図８Ａを参照すると、セラミック誘電体１５０がコラム１６のベースにおいて中心電極１２及び外側電極１４の間に設けられる。複数の電極１５２は、コラム１６の外側のセラミック誘電体１５０の表面に装着され、セラミック誘電体１５０によりコラム１６内のセラミック誘電体１５０の表面１５４から小距離だけ離間される。電極１５２と表面１５４との間のセラミック誘電体１５０の厚さは典型的には１／８インチ（約３．１８ｍｍ）以下とすることができ、セラミックの誘電体１５０が割れたり破壊したりしないことを保証しながら、出来る限り薄くなるように選択される。ＲＦ信号及び（又は）直流信号が電極１５２に供給されたとき、高電圧場が表面１５４上に現れ、所望のプラズマ破壊を開始させる。
【００６３】
図８Ｂの装置は、セラミック誘電体１５０′が中央電極１２の底部分にわたってカラーとして形成され、コラム１６内へ小距離延びているという点で、図８Ａのものとは異なる。電極１５２はセラミック誘電体１５０の外表面１５４′に装着され、ＲＦ信号及び（又は）直流信号を電極１５２に供給したときに、高電圧場が表面１５４′に形成される。全体のプラズマ銃が真空環境ではないような応用に対しては、電気リード線を真空のコラム１６内へもたらす必要がないという点で、図８Ａの形状が好ましく、図８Ｂの実施の形態に対しては、リード線１５６がコラム１６内へもたらされる。
【００６４】
また、始動後に初期の高電圧スパイクを中央電極１２、及び外側電極１４へ供給することにより、一層均一な破壊をプラズマ銃内で達成できることが判明した。図９Ａは図９Ｂに示す所望の波形を達成するための回路を示す。特に、この波形は初期のスパイク信号１６０を有し、持続信号１６２がこれに続く。初期のスパイク信号１６０は持続信号１６２の電圧の１０倍ほどの大きさとすることができるが、一層短い期間のもので、中央電極１２、外側電極１４へ供給されるエネルギの１／１０ほどの少ないエネルギを送給する。
【００６５】
図９Ａを参照すると、回路は第１の非線形磁気圧縮回路１６４（その最終段のみを図９Ａに示す）と、第２の非線形磁気圧縮回路１６６（その最終段のみをも図に示す）とを有する。第２の非線形磁気圧縮回路１６６は高電圧短期間のスパイク信号１６０を発生させ、一方、第１の非線形磁気圧縮回路１６４はスパイク信号１６０の端部で生じる一層長い期間の低電圧信号である持続信号１６２を発生させる。第１の非線形磁気圧縮回路１６４の最終段のためのリアクタ１６８は、第１の非線形磁気圧縮回路１６４からの信号の流れを許容するような方向に飽和されるが、第２の非線形磁気圧縮回路１６６から逆方向への信号の流れを遮断するように、バイアス巻線１７０を介して供給されるバイアス信号により、通常バイアスをかけられる。従って、この信号は第１の非線形磁気圧縮回路１６４、特にその最終段のコンデンサ１７２へ供給されず、電圧スパイクから第１の非線形磁気圧縮回路１６４を保護し、この信号のすべてが中央電極１２、外側電極１４へ供給されるのを保証する。スパイク信号１６０は、そこへ供給されるバイアスに部分的に打ち勝って飽和可能なリアクタ１６８のバイアスを反転させ始め、同時に、中央電極１２、外側電極１４において雪崩破壊を生じさせる。これが、破壊電圧を越える懸念なしに、主放電チェーンのための最適な電圧及び駆動インピーダンスレベルの選定を許容する。飽和可能なリアクタ１６８の反転バイアスは、リアクタ１６８が再飽和するまで第１の非線形磁気圧縮回路１６４からの持続信号１６２に対して遅延を提供し、２つの信号間に円滑な遷移を提供する。
【００６６】
例えば図３に示す型式のプラズマ銃についての１つの問題は、放射線の所望の周波数に応じて１００ｅＶないし１０００ｅＶの範囲にある所望の締め付け温度を達成するために、プラズマをマイクロ秒当り数センチメートルの速度に駆動するのに十分なテスラ程度の磁気圧縮場が必要となることである。これらの高速度により、プラズマは中央電極１２を下って駆動され、中央電極１２の端部から放出され、プラズマシースは中央電極１２の端部から離れた空間内へ移動し続ける。この結果、プラズマシースは最終的に締め付け部への電気的接続を失い、締め付けを終了させ、大きな電圧遷移を生じさせる。この電圧遷移は中央電極１２を激しく損傷させることのある高電圧再スパイクを生じさせることがある。プラズマシースとの電気接触の喪失はまた、プラズマ銃からの出力効率の実質的な減少を生じさせ、締め付けは、数マイクロ秒（例えば、２−４マイクロ秒）となることがある電気放電の実質上一層長い期間ではなく、ほんの約１００ナノ秒続く。
【００６７】
本発明の教示に従えば、プラズマ分離のこの問題はプラズマシースを中央電極１２の方へ戻るように再度導くために中央電極１２の出口端に隣接してブラストシールド即ち合焦装置１９４を設けることにより克服される。図１０Ａ−図１０Ｃは、それぞれ合焦空洞１９６Ａ、１９６Ｂ、１０６Ｃの形状が主として異なるこのようなシールド即ち合焦装置（以下、シールドとして総称する）１９４Ａ、１９４Ｂ、１９４Ｃの３つの可能な実施の形態を示す。特に、空洞１９６Ａはほぼ球状形状を有し、空洞は適当な装着コンポーネント（図示せず）により外側電極１４又はプラズマ銃の適当なハウジングコンポーネントに装着されて、空洞１９６Ａの壁が中央電極１２の先端からある距離だけ離れるようにする。このある距離とは、シールドと中央電極１２との間に接触を生じさせないのに十分なものであるが、プラズマ分離の前に中央電極１２へ戻るプラズマの放射線が生じるのに十分な短さのものである。これらの目的は、中央電極１２の半径をＲとした場合に、ほぼＲないし２Ｒの範囲内の間隔により達成される。しかし、このような距離はプラズマ銃１０の他のパラメータに応じてある程度変えることができる。空洞１９６Ｂは円錐形状を有し、空洞１９６Ｃは放物線形状を有する。中央電極１２の端部からの空洞の距離について上述したパラメータはすべての３つの空洞形状に適用される。
【００６８】
プラズマシースの分離を阻止し、プラズマシースをシールド１９４内に収容するのが望ましいが、シールド１９４がプラズマ銃１０からの所望の放射線の流出と抵触しないことが重要である。従って、各シールド１９４は、対応する空洞の頂部に形成され、中央電極１２の中心線と同軸の中心を備えた中央開口１９８Ａ、１９８Ｂ、１９８Ｃを有する。開口１９８は好ましくは円形であり、中央電極１２の先端で締め付け部から±１５゜の角度（ほぼ発出放射線の角度）で発出される放射線が邪魔されずに開口１９８を通るのに十分な直径を有する。各開口１９８の上方部分は外方へテーパしていて、プラズマシースのいかなる逃避をも実質上制限しながら、放射線の流出を容易にする。
【００６９】
シールド１９４の材料はほぼ１０００℃及びそれ以上の範囲の温度に耐えることのできる高温非導電性材料でなければならない。種々の高温セラミックは所望の特性を有し、図示の実施の形態に対しては、Ａｌ₂Ｏ₃（酸化アルミニウム）が利用される。種々のガラス、石英及びサファイアもシールド１９４の材料として役立つ所望の特性を有する。
【００７０】
上述の説明において、プラズマを再指向させるためのシールド１９４は特定の形状の放射線源と一緒に使用するものとして示したが、このシールド１９４は、プラズマ分離が可能性のある問題となるような任意の放射線源と一緒に使用するのに適し、それ故、本発明は図３の特定の放射線源形状により決して限定されない。同様に、陰極すなわち外側電極１４へ放射線を再指向するための３つの空洞形状を図１０Ａ−１０Ｃに示したが、この機能を遂行するのに適した他の空洞形状も利用できる。上述の特定の材料はまた単なる例示である。
【００７１】
更に、１３ｎｍでの放射線を発生させるためのパラメータを上述したが、放射線源としてのプラズマ銃９０の種々のパラメータを制御することにより、そして、特に、利用される元素／ガス、高電圧源からの最大電流、締め付け領域におけるプラズマ温度、コラム１６内のガス圧力、及び、ある場合は利用される放射線フィルタを注意深く選択することにより、ＥＵＶ帯域内における、又は、ある場合はこの帯域外における他の波長での放射線を得ることができる。
【００７２】
多くの種類のガスを上述のプラズマ銃のためのプラズマガスとして使用できるが、アルゴン及びキセノンの如き不活性ガスがしばしば好ましい。使用できる他のガスは窒素、ヒドラジン、ヘリウム、水素及びネオンを含む。上述のように、図３の実施の形態のようにプラズマ銃を放射線源として使用した場合、選択されたＥＵＶ又は他の波長を達成するために種々の元素／ガスを使用することもでき、ある場合は、プラズマ及び放射線ガスは同じガスである。例えば、ＶＵＶ帯域内で１２１．５ｎｍでの放射線を有効に得るために水素ガスを選択することができる。更に、種々の実施の形態を上述したが、これらの実施の形態は単なる例示であり、本発明を限定するものではないことは明らかである。例えば、図示のドライバは種々の応用にとって有利であるが、適当な電圧及び上昇時間を有し、高電圧切り換えを必要としない他の高ＰＲＦドライバも利用できる。同様に、好ましいソリッドステートのＲＦ信号源により駆動する電極トリガを用いた種々のプラズマ始動機構を説明したが、プラズマ破壊を開始させるための他の方法も適当な応用に利用できる。電極の形状及びプラズマ銃のために与えられた用途も例示である。従って、好ましい実施の形態について本発明を特に示し、説明したが、当業者なら、本発明の精神及び要旨内に留めたまま、詳細を構成する上述及び他の変更を行うことができ、本発明は特許請求の範囲によってのみ規定される。
【図面の簡単な説明】
【図１】本発明のプラズマ銃の第１の例示的なスラスタの実施の形態の半概略半切断側面図である。
【図２】本発明のプラズマ銃の別のスラスタの実施の形態の半概略半切断側面図である。
【図３】本発明のプラズマ銃の放射線源の実施の形態の半概略半切断側面図である。
【図４】本発明のプラズマ銃の１つの実施の形態についての図３の中央電極の拡大（実寸ではない）切断図である。
【図５】相対寸法に応じて、他の因子をスラスタ又は放射線源として使用でき、本発明の教示に従ったＲＦ信号で駆動するトリガ電極を有する、本発明のプラズマ銃の実施の形態の半概略側切断図である。
【図６】本発明のプラズマ銃におけるＲＦ信号源を得るための別の実施を概略的に示す図である。
【図７】図７Ａはトリガ電極を駆動するためのＲＦ信号源として使用するのに適したソリッドステートのＲＦ信号源の概略図であり、図７Ｂ及び図７Ｃは図７Ａの回路内におけるあるコンデンサを横切る電圧を示す線図である。
【図８】図８Ａ及び図８Ｂはプラズマ銃へ始動電圧を供給するのに適した２つの異なる始動器電極形状を示す、プラズマ銃の一部の切断部分側面図である。
【図９】図９Ａは別の実施の形態に従った本発明のプラズマ銃を駆動するために使用するのに適したパルスドライバ回路の概略線図であり、図９Ｂは図９Ａの回路からの出力信号を示す線図である。
【図１０】図１０Ａないし図１０Ｃは、本発明のそれぞれ球状、円錐状及び放物線状の実施の形態のための、中央電極の端部及びシールドを示す拡大側断面図である。
【符号の説明】
１０ プラズマ銃
１２ 中央電極
１４ 外側電極
１６ コラム
８２ トリガ電極
１３０ 回路
１３２ コンデンサ
１３４ ソリッドステートスイッチ
１３６ 多段非線形磁気パルス圧縮回路
１３８ 出力セクション
１４０ 出力結合回路 [0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to plasma guns and, in particular, selectable for use as space thrusters, ie including extreme ultraviolet (EUV), vacuum ultraviolet (VUV) and / or soft X-ray radiation in the high pulse repetition frequency band. Improved plasma gun suitable for producing radiation at various wavelengths. The present invention also includes a method utilizing such a plasma gun.
[0002]
[Prior art]
The improved plasma gun disclosed in U.S. Pat. No. 5,866,871 (patent) can only be implemented in previously unsatisfactory, poorly implemented, or relatively large and expensive facilities. It finds application in a variety of environments to perform functions that have not been possible. These functions include thrusters for satellites or other space stations that maintain and perform applications, and controlled generation of radiation at selected frequencies, generally in the extreme ultraviolet (EUV) band. The plasma gun disclosed for such an application is particularly advantageous in that it provides high reliability and pulse repetition frequency (PRF), and for space applications a PRF greater than nearly 100 Hz, preferably 5,000 Hz Especially for plasma guns having a PRF of at least 500 Hz, preferably 1,000 Hz for lithography or other applications requiring radiation generation.
[0003]
To achieve these objectives, the plasma gun of the above patent has two general embodiments, one of which is for space applications or other thrust applications, and the other or second The embodiments described above were for radiation generator applications. In both cases, the plasma gun had a central electrode and an outer electrode substantially coaxial with the central electrode, with a coaxial column formed between the electrodes. The selected gas was introduced into the column via an inlet mechanism, and a plasma starter was provided at the base end of the column. Finally, there is provided a solid state high repetition rate pulse driver operable to deliver a high voltage pulse across the electrode at the start of the pulse at the base of the column, with the plasma extending from the base end of the column, Get out of the department. For the thruster embodiment, the voltage of each pulse decreases over the duration of the pulse, and the pulse voltage and electrode length are such that the voltage across the electrodes reaches a substantially zero value as the plasma exits the column. Was selected. For this embodiment, the inlet mechanism preferably introduces gas radially from the center electrode at the base end of the column to improve uniformity of plasma velocity across the column, The plasma emerges from the column at a discharge rate generally ranging from approximately 10,000 to 100,000 meters per second, with the discharge rate varying somewhat depending on the application.
[0004]
For the radiation source embodiment of the present invention, the pulse voltage and electrode length are selected such that the current for each voltage pulse is substantially at its maximum as the plasma exits the column. . The outer electrode in this embodiment of the invention is preferably a cathode electrode and may be solid or may take the form of a plurality of substantially equally spaced rods arranged in a circle. . The inlet mechanism in this embodiment of the present invention provides a substantially uniform gas filling into the column, so that the plasma is first released from the center electrode, and the plasma is magnetically generated as it exits the column. Tightened to generate a very high temperature at the end of the center electrode. The selected gas / element sent to the clamp through the central electrode or otherwise as part of the gas is ionized by the high temperature at the clamp to provide radiation at the desired wavelength. The wavelength is achieved by careful selection of various parameters of the plasma gun, including the selected gas / element delivered to the clamp, the current from the pulse driver, the plasma temperature in the region of the clamp and the gas pressure in the column. . The above patent shows, for example, a combination of parameters for generating radiation at a wavelength of approximately 13 nm using, for example, lithium vapor as the gas sent to the clamp.
[0005]
It is important to the present invention that the pre-ionization of the gas by the starter provides an absolutely uniform pre-ionization of the gas in order for any of the above applications to work effectively. In the above patents, this was accomplished by forming equally spaced holes around the column and introducing or guiding gas through the holes.TriggerElectrodes are provided, which are preferably mounted in holes or preferably mounted on the base of the column outside or close to the column, theseTriggerThe electrodes were energized to start the plasma. The trigger electrodes are preferably equally spaced around the base end of the column and are energized at substantially the same time to provide a uniform start of the plasma at the base end,TriggerA DC signal was used to energize the electrodes. This mechanism provides a more uniform plasma start than is possible with any conventional configuration, and is suitable for most applications, but an even more uniform plasma start is desirable, especially when using a plasma gun as the radiation source. Such applications exist. This more uniform plasma startTriggerThis can be provided by using an RF signal to energize the electrodes. However, currently available RF such as magnetrons, klystrons or RF amplifierssignalThe operation is relatively expensive, costing almost $ 1 per watt of peak power, and is relatively large, requiring cabinet sized enclosures to produce, for example, 20 kilovolts at 8 megawatts. Therefore, RF is generated using small solid-state circuits that generate power at lower cost and that, in addition to lower cost and substantially smaller dimensions, also provide a significantly lower heat removal burden on the device.signalIn such a way thatTriggerIt was desirable to generate an RF signal that was used to energize the electrodes. The above modelRFSignal sourceAre particularly useful for the plasma gun applications of the present invention, but are not currently available in the art.Na rFsignalSources are also useful for other applications.
[0006]
Also used for plasma startTriggerElectrodeCenterElectrodes andOuter electrodewhileofIt is desirable to provide the high voltage field over as large an area as possible at the base of the column, and to produce the required high voltage field at the base of the column without having to bring wires into the vacuum environment of the column.TriggerIt is desirable to be able to energize the electrodes. Maintaining a vacuum around such wires increases the cost of the plasma gun.
[0007]
Another problem with plasma guns is providing the necessary gas / material to the clamp where the material is to be ionized to produce the desired radiation. Therefore, an improved technique for retaining such material and releasing it into the column to the clamp is desirable.
[0008]
In addition, while plasma guns of the type described above can act as a radiation source and provide useful radiation at the desired wavelength, high-speed plasmas driven down the column and out of the central electrode do so. WhatradiationsourceAsCan cause problems that severely limit the usefulness of In particular, temperatures in the range of 100 eV (ie, about 11,000 ° C.) to 1000 eV at the clamp are sufficient to drive the plasma to speeds of a few centimeters per microsecond, depending on the desired frequency of radiation. Requires a magnetic compression field. Central at such speedelectrodeDown the plasma moving out of the edge forming the clampelectrodeThe plasma sheath will eventually lose its electrical connection to the clamp, with the tendency to continue to move into the space away from the end of the clamp. This terminates prematurely after periods as short as 100 nanoseconds, and causes large voltage transitions in the range of thousands of volts, causing re-hits that can severely damage the electrodes.
[0009]
Since the discharge can last for a few microseconds, if the early loss of the electrical connection between the plasma sheath and the electrode could be eliminated, the life of the clamp would be greatly extended and the possible damage re-established You can eliminate the blow. As a result, the plasmagunThe output efficiency forPlasma gunElectrode life can be greatly extended, for example, can be expensive in lithography applicationsPlasma gunReduce downtime and maintenance. Therefore, very good performance can be obtained at low cost.
[0010]
Finally, it is desirable to achieve the breakdown as uniformly as possible, and in particular, a technique that uses improved drive signals to enhance such uniformity of the breakdown.
[0011]
It therefore provides a more uniform plasma start at a lower cost than is possible with conventional devices, facilitates the introduction of the material to be ionized at the clamp into the column, premature termination of the clamp and ( Or) stop re-strike,CenterelectrodeAnd outer electrodeThere is a need for an improved plasma gun and method of use that provides more uniform breakdown when a high voltage is applied across the plasma gun.
[0012]
[Means for Solving the Problems]
According to the foregoing, the present application provides a center electrode and an outer electrode substantially coaxial with the center electrode,Between the center electrode and the outer electrodeForming a coaxial column between them,ColumnAn outer electrode having a closed base end and an open outlet end, and an inlet mechanism for introducing a selected gas into the column;,When starting the plasma at the base of the ramCentral electrode and outerSolid state high repetition rate pulse driver operable to deliver high voltage pulses across electrodesAndPlasma expands from the base end of the column and exits at the outlet end of the columnSolid state high repetition rate pulse driverWhen,A plurality of trigger electrodes at the base end of the column, and a radio frequency (RF) signal source connected to the plurality of trigger electrodes for selectively providing an RF signal to the plurality of trigger electrodes; A non-linear magnetic pulse compression circuit having a number N of signal sources equal to or greater than one and optionally a capacitor connected to an input of a first stage of the non-linear magnetic pulse compression circuit; Operable solid state switch and capacitor (C _R ) And a saturable inductor (L _R ), Wherein the final stage of the non-linear magnetic pulse compression circuit has a capacitance (C). _N ) And a capacitor (C _R ) And inductor (L _R ) Is a capacitor (C _R ) Is fully charged before the capacitor (C _N ) Is selected such that a reverse voltage at the inductor (L _R ) Continues to saturate and the capacitor (C _R ), And a capacitor (C) for driving the plurality of trigger electrodes. _R And an output coupling circuit (140) for coupling the energy fromProvide a plasma gun. RFsignalThe source can operate at a frequency in the range of, for example, 10 MHZ to 1,000 MHZ, alone or at DCsignalIt can be used in combination with a source.
[0013]
For the preferred embodiment, the solid state switch is an SCR, IGBT or MOSFET.Capacitor (C _N ) Is a capacitor (C _R ) Smaller thanCan orThe inductor (L _R ) Is a capacitor (C _N ) To the capacitor (C _R )It can be selected to saturate before the transfer of charge to is completed. outputsectionIs preferably a saturable resonant shunt for ground,outputThe coupling circuit is preferablyCapacitor (C _R )Of energy stored inpartButCapacitor (C _R )During each vibration cycle ofMultiple trigger electrodesHas an impedance that is coupled to For the preferred embodiment,Inductor (L _R ) And a capacitor (C _R )Is the outputsectionAre selected so that only three to four oscillation cycles of RF mentioned abovesignalThe source can also be utilized independently of the high PRF plasma gun application.
[0018]
The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings or otherwise discussed herein.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
First, referring to FIG.Plasma gun as10(Hereinafter referred to as "thruster 10")Is a central electrode 12, which in this embodiment is a positive or anode electrode, and a concentric cathode, ground or return electrode.Is the outer electrode14 and two electrodes12, 14Has a substantially cylindrical shape formed betweencolumn16.column16 has its base end defined by an insulator 18;18The central electrode 12 is mounted inside. The outer electrode 14 is mounted on a conductive housing member 20 connected to a ground via a conductive housing member 22. The center electrode 12 is mounted at its base end in an insulator 24,24Is mounted in the insulator 26. Cylindrical outsideofA housing 28 surrounds the outer electrode 14,Centerelectrode12, outer electrode 14Area beyond the front or exit end of theIs the expanding endIt becomes 30 and expands.CenterElectrode 12,Outer electrode14 can be formed, for example, from triatungsten, titanium or stainless steel.
[0020]
The positive voltage is applied to a dc-dc (DC / DC) inverter 34, a non-linear magnetic compressor 36, and a center electrode.12Can be supplied from the DC voltage source 32 to the center electrode 12 via a terminal 38 connected to the center electrode 12. The DC / DC inverter 34 has a storage capacitor 42, which can be a single large capacitor or a series of capacitors, a control transistor 44, a pair of diodes 46, 48, and an energy recovery inductor 50. Control transistor 44 is preferably an insulated gate bipolar transistor.DC / DCInverter 34 is DCVoltageIt is utilized in a manner known in the art to transfer power from source 32 to nonlinear magnetic compressor 36. As explained later,DC / DCInverter 34 may also provideCenterElectrode 12,Outer electrodeIt functions to recover the wasted energy generated from 14 and improve pulse generation efficiency.
[0021]
The non-linear magnetic compressor 36 is shown as having two stages, the first including a storage capacitor 52, a silicon controlled rectifier 54, and an inductor or saturable inductor 56. Non-linear magnetic compressor36The second stage includes a storage capacitor 58 and a saturable inductor 60. If necessary, additional compression stages can be provided to obtain shorter and more rapidly rising pulses and higher voltages. A method for achieving nonlinear magnetic compression in this type of circuit is disclosed in U.S. Pat. No. 5,142,166. Basically,Non-linear magnetic compressor36CircuitUse a saturable core as an inductor in a resonant circuit. The core of each stage saturates before a significant portion of the energy stored in the capacitors of the previous stage is transferred. Non-linear saturation phenomena increase the resonant frequency of the circuit by the square root of the decrease in transmittance when the core saturates. Energy is more and more rapidly coupled from one stage to the next.Non-linear magnetic compressorIt should be noted that 36 is effective in transferring power in both directions. The reason is that this circuit not only acts to upshift the frequency in the forward direction, but also downshifts the frequency when the voltage pulse is reflected, cascading for backup of the chain. Because. The energy resulting from an improper load / electrode isstorageIt can be cascaded to back up the chain so that it appears as a reverse voltage stored in capacitor 42 and is added to the next pulse. In particular, when the reflected charge is re-switched to the initial energy storage capacitor 42, current begins to flow into the energy recovery inductor 50.storageWith capacitor 42Energy recovery inductor (coil)The combination with 50 forms a resonance circuit. Half point [here, t = π / (L₅₀C₄₂)^1/2]rear,storageThe polarity of the voltage on the capacitor 42 is reversed and this energy isDCThe energy required to recharge this capacitor from voltage source 32 is reduced.
[0022]
The drive circuit shown in FIG. 1 can also be matched to very low impedance loads and can produce complex pulse shapes if needed. thisDriveThe circuit is also designed to operate at very high PRF,k VCan be adjusted to provide voltages in excess of
[0023]
Propellant gas is fed from line 64 through valve 66 under control of a signal on line 68 to manifold 70 which feeds a number of inlet ports 72 in housing 28, as shown in FIG. It is shown. For example, four to eight equally spaced apart about the periphery of housing 28 near the base of the housing.entranceA port 72 can be provided.entrancePort 72 isOutsideGas is supplied into the holes 74 formed in the electrode 14, and these holes are supplied.74Near the center electrode 12columnPropelling radially and inward toward 16 basesgasIt is angled to lead. Propellant gas alsocolumn16 can be supplied from the rear.
[0024]
The thruster 10 is designed to operate in space or some other low pressure close to a vacuum environment, especially at pressures that would cause failure on the low pressure side of the Paschen curve. The pressure curve for which this is the case will vary somewhat depending on the gas used and other parameters of the thruster, but this pressure is typically in the range of 0.01 to 10 Torr, and is On the other hand, it is almost 1 Torr. For a pressure in this range, an increase in the pressure in a region will decrease the breakdown potential in that region, thereby increasing the likelihood of breakdown in such a region. Thus, in theory, the introduction of the propellant gas at the base of the column 16 and therefore merely increasing the pressure at this point can cause the desired breakdown / plasma start at this point. However, as a practical matter, controlling the gas pressure enough to cause predictable failure, and16Not in the selected section of the column16It is difficult to make the pressure sufficiently uniform around the periphery of the column 16 to cause a uniform breakdown within.
[0025]
To ensure that the plasma start occurs uniformly at the base of the column 16 and that such breakdown occurs at the desired time, at least two things can be done. To understand how such a breakdown enhancement is achieved, the plasma gun of the present invention typically operates at pressures between 0.01 Torr and 10 Torr, and in particular, the Paschen curve It should be understood that it operates at a pressure such that a breakdown occurs on the low pressure side of For the preferred embodiment, the pressure in column 16 is approximately 1 Torr. In such low pressure discharges, there are two important criteria that determine gas destruction or startup:
1. The electric field in the gas must exceed the breakdown field for the gas, which depends on the gas used and the gas pressure. Breakdown field is cathode known as Paschen criterionIe outer electrode14 is the electron source.plasmaIn the low pressure region where the gun operates, for a device size, the breakdown field decreases with increasing pressure (occurring on the low pressure side of the Paschen curve). Thus, breakdown occurs in the column 16 at the point where the gas pressure is at a maximum.
[0026]
2. Second, an electron source must be provided. If the average electric field exceeds the breakdown field, nothing happens until the negative surface starts emitting electrons. In order to extract electrons from the surface, one of two conditions must occur. The first condition is that a cathode drop or a potential difference exceeding the cathode potential must be generated near the surface. Cathode drop / cathode potential is a function of gas pressure and is a function of surface composition and geometry. The higher the local gas pressure, the lower the required voltage. Like a holeLooking inwardThe geometry provides a significantly improved level of surface area to volume and also reduces cathode drop. This effect, in which the hole acts preferentially as an electron source with respect to the adjacent surface, is called the hollow cathode effect. In the second condition, the electron source can be formed by a surface flash trigger source. These conditions can be met individually, or both can be used. However, to prevent spurious starting, the voltage across the electrodes should be less than the sum of the gas breakdown potential and the cathode fall potential.
[0027]
Accordingly, in FIG.Is the outer electrode14 formed in these holes74Through the gas to the base of column 1674Is a column16Terminate near the base of. For the preferred embodiment, multiple such holes74Are equally spaced around the periphery of the column 16. These holes74Penetrate through these holes74The gas combined with the hollow cathode effect resulting from the presence of74Pressure in the region of the16At this point in the plasma start occurs. Although this method of plasma initiation is sufficient for plasma initiation in certain applications, for most applications of the plasma gun of the present invention, especially for high PRF applications, providing a trigger electrode in the manner described in the next embodiment, Preferably, both conditions are met to ensure both uniformity and timeliness of plasma initiation.
[0028]
If a thruster 10 is to be utilized, the valve 66 is first opened and thePromotionGas passes through manifold 70column16 to allow it to flow into the hole 74 leading to it. Because the valve 66 operates relatively slowly compared to other components of the device, the valve 66 has a sufficient amount to deploy the desired thrust over multiple plasma starts.PromotionGascolumnIt remains open long enough to flow into 16. For example, the cycle time of a solenoid valve that can be used as valve 66 is 1 millisecond or more. Because plasma combustion can occur in a few microseconds, and down the electrode length of 5-10 cm used in the thruster of the preferred embodiment, the gas is typically about 1/4000 second. Because only one pulse for each valve cycle is present, only about one-tenth of the propellant gas will be utilized because it can flow. Therefore, in order to achieve a high propulsion efficiency, a plurality of (for example, at least 10)burstAlternatively, a pulse is generated. Each individual of the pulseburstMedium, peak power is on the order of hundreds of kilowatts to produce the required power. The peak PRF is determined by two criteria. The impulse time must be long enough for the plasma from the previous pulse to leave the thruster exit or recombine. Besides, the impulse time is cold propulsiongasMust be shorter than the time required to operate the electrode length. The latter criterion is determined by the gas used. For argon, for a typical length of 5 cm column 16, propulsiongasIs a thrusterTen ofThe time period spread over the electrode surface is only 0.1 ms, while for heavier gases such as xenon, the time increases to approximately 0.2 ms. Thus, a high thruster pulse repetition rate (ie, about 5,000 pps or more) allows a plasma gun to achieve high propulsion efficiencies approaching 90%. Pulse of fluid during one valve operationburstThe length can vary from a few pulses to a few millions, during which time some fuel is consumed and therefore shorterburstLower propulsion efficiencies for length are achieved. So, if possible,burstThe cycle is the propulsion provided during the minimum time cycle of valve 66gasShould be at least long enough to allow full use.
[0029]
PromotiongasBefore the gate reaches the end of the column 16, the gate transistor 44 is enabled or opened and the capacitor 58 is fully charged to provide a high voltage (400-800 volts for the preferred embodiment) across the electrodes. Provided, this voltage, alone or in combination with the activation of the trigger electrode in a manner described below, causes a plasma start at the base of the column 16. as a result,Central electrode 12 and outer electrode 14Through the plasma sheathCenterelectrode12 and the outer electrode 14Current flows radially between them, generating a magnetic field. The resulting magnetic pressure pushes the plasma sheath axially,Centerelectrode12 and outer electrode 14Provides a J × B Lorentz force that accelerates the plasma mass as it moves along. This results in very fast plasma velocities, with the electrode length and initial charge first increasing with time and then decreasing to zeroCenterelectrode12 and outer electrode 14The rms current acrossstorageThe voltage that is reduced upon discharge of the capacitor 58 is both selected to be exactly zero upon discharge of the plasma from the tip of the electrode. When the plasma reaches the end of the coaxial structure, substantially all of the gas has been entrained or drawn into the plasma and is emitted from the end of the electrode. This gives rise to the maximum gas mass and thus the maximum momentum / thrust for each pulse. If the length of the structure is selected so that the capacitor is completely discharged when the plasma leaves the electrode, the current and voltage will be zero and the ionized slag of the gas will leave the thruster at high speed. By operating the thruster in this manner and optimizing the ejection speed utilized for a given thruster application, ejection speeds in the range of, for example, 10,000 to 100,000 meters per second can be achieved. Thruster10The expansion end 30 of theDoBy facilitating controlled expansion of the gas, it is possible to convert some of the residual thermal energy to thrust via isentropic thermodynamic expansion, but this effect has been found to be quite negligible. , TaperedDevelopmentEnd 30 is not typically used. In fact, not generally needed in spaceCenterExcept for the protection of the electrodes 12, the weight of the thruster 10 can be reduced by completely eliminating the housing 28.controlDisable transistor 44 or otherwiseDC voltageSource 32Non-linear magnetic compressorBy disconnecting from circuit 36, the pulseofThe rupture can be terminated.
[0030]
FIG. 2 shows another embodiment which differs in some respects from that shown in FIG.Plasma gunShows a thruster 10 '. First, a single storage capacitor 80 is used in place of the non-linear magnetic compressor 36, which in practical applications is typically a series of capacitors achieving a capacitance of approximately 100 microfarads. Second, the cathodeIs the outer electrode14 tapers slightly towards its outlet end. Third, a spark plug-like trigger electrode 82 is shown positioned within each hole 74 with a corresponding drive circuit 86 for the trigger electrode, and an internal gas manifold 72 'formed by the housing member 77. A gas inlet hole (not shown) is provided for delivering propellant gas to hole 74.housingProvided within member 77, gas outlet holes 84 are shown formed in insulator 24 and central electrode 12. With respect to the embodiment of FIG. 1, typicallyOuter electrodeThere are, for example, four to eight equally spaced holes 74 around the circumference of the perimeter 14, a trigger electrode 82 is located in each hole 74, and the gas outlet (s) 84 are preferably in each hole 74. And guides the gas there. For reasons described below,columnMost of the gas at the inlet to 16 is from a suitable source, which can be the same source as for manifold 72 'and hole 74.gasThe gas flowing through outlet 84 into the chamber near central electrode 12 and flowing through hole 74 is primarily82Facilitates starting.
[0031]
In some applications, instead of the non-linear magnetic compressor circuit 36, to store the voltage and provide a high voltage drive pulse,storageAlthough a capacitor 80 can be utilized, such an arrangement is typically used in applications requiring a lower PRF and / or lower voltage. The reason is,Nonlinear magnetismThis is because the compressor 36 is suitable for providing both shorter and higher voltage pulses.Non-linear magnetic compressor36 alsostorageVoltage across the capacitor 58 and a non-linear coilInductor asA pulse is provided at a time determined by the saturation of 60, which basically charges until a breakdown occurs at the base of the column 16 to allow discharge of the capacitor.storageThis is a more easily predictable time than can be achieved with the capacitor 80.
[0032]
The trigger electrode 82DC voltageActivated by a separate drive circuit 86 that receives voltage from source 32, but in other respects:DC / DCInverter 34 andNonlinear magnetismCompressor 36 orstorageIt is independent of the capacitor 80. The drive circuit 86 has two non-linear compression stages, and a trigger electrode82Can be activated in response to an input signal to the SCR 87 to initiate activation of the SCR 87. The signal to the SCR 87 is, for example,storageRespond to the detection of the voltage or charge across the capacitor 80 and the start of energization when this voltage reaches a predetermined value, orstorageA timer that is started when charging of the capacitor 80 begins,ButActivation occurs when sufficient time has elapsed to reach the desired value.Nonlinear magnetismCompressor 36 can be timed so that energization occurs when inductor 60 saturates. column16Controlled starting at the base of the hole 74Looking inwardDepending on the geometry, andcolumnThis is enhanced by the fact that 16 is narrower at its base end and further increases the pressure in this area, and thus, for the reasons mentioned above, guarantees the onset of rupture in this area.
[0033]
Each trigger electrode 82 fits into an opening 89 of the housing 77,Triggerelectrode82Opening to secure in place892 is a spark plug-like structure having a screw section threaded into it.TriggerThe front end of the electrode 82 is open89Has a smaller diameter than the diameter of the82Can flow through the hole 74 around. For example, hole74Can be 0.44 inches (about 11.18 mm) in diameter, while the trigger electrode82Is 0.40 inches at its lowest point. Trigger electrode82Trigger element 91 extends near the end of the hole 74 adjacent to the column 16, but preferably against plasma forces developing in the column 16.Triggerelectrode82Do not extend into the column 16 to protect it.Triggerelectrode82The end of the hole, for example,74Can be separated from the end of the hole 74 by a distance approximately equal to the diameter of the hole 74 (7/16 inches).
[0034]
Trigger electrode 82 and, Center electrode and outer electrodeBoth 12 and 14,CommonDCIt is energized from the voltage source 32,Central electrode 12 and outer electrode 14Are independent and generate different voltages and powers while operating substantially simultaneously. For example,CenterelectrodeAnd outer electrodes 12, 14Typically operates at 400-800 volts, but the trigger electrode82Can have a voltage of 5 Kv across it. However, this voltage exists for only a very short period of time, for example 100 ns (nanoseconds), so that the energy is very small, for example 1/20 Joules.
[0035]
Another potential problem with the thrusters of the type shown in FIGS. 1 and 2 is that the Lorentz force across the column 16 is not uniform and is greatest near the center electrode 12 and from there the cathodeIsThe decrease is somewhat uniform toward the outer electrode 14. As a result, the gas plasma is angledToWithWake upForwardFlowOut and gas is first applied to the central electrode12Out of and after the outer electrode14Extend toward. As such, the outer electrode 14 can be made shorter to facilitate uniform outgassing of the gas across the thruster, but this has not been done for the preferred embodiment. This outer electrode14Of the housing 28Expanding endIt is provided for the same reasons as the taper in 30 and is optional for the same reasons as described in connection with this taper.
[0036]
The problem of non-uniform velocities in column 16 is also due to the fact that most of the gas is12From and / or into column 16 so that the outer electrode14Central electrode than gas mass at12The larger gas mass at, is addressed in FIG. Central electrode12If this is done carefully so that the greater mass near the offset of the greater acceleration force there, a more uniform velocity can be achieved radially across the column 16 so that the gas / plasma is It flows out uniformly from the edge (ie, with the front side perpendicular to the electrode). This fix is for the shorter outer electrode14Is one reason that is virtually unnecessary.
[0037]
Except for the differences described above, the thruster of FIG.10 'Is the thruster in Figure 110Works in the same way as. Further, while the figure shows a single thruster, in space or other applications, multiple thrusters, such as twelve thrusters, may be utilized, with each thruster operating at less than one joule per pulse. Have a weight of 1 kg or less. All thrusters are powered by a central power source, have a central controller and are propelled from a common sourcegasReceive. In the latter case, the operating life of the thruster-based spacecraft is not affected by the fuel supply for the most frequently used thrusters, as in the case of the same solid fuel thrusters, and the totalgasIt is particularly advantageous for the thruster of the invention in that it is only affected by the thruster.
[0038]
FIG. 3 illustrates another embodiment of a plasma gun in accordance with the teachings of the present invention.plasmagun90Are more suitable for use as a radiation source than as a thruster. This embodiment of the invention uses a driver similar to that shown in FIG. 1 with a DC / DC inverter 34 and a non-linear magnetic compressor 36, andOuter electrode 14And a manifold 72 'for supplying gas around the trigger electrode 82 through a hole 74 in the housing. However, for this embodiment, no propellant gas is input from center electrode 12.Outsideelectrode14Is also not tapered and has approximately the same length as the center electrode 12. For this embodiment of the invention,CenterElectrode 12,Outside electricity veryThe length of 14 is also much shorter than for the thruster embodiment, so that the gas / plasma reaches the end of the electrode / column when the discharge current is at a maximum. Typically, the capacitor now approaches the 1/2 voltage point. Further, for radiation source applications, outer electrode 14 may be solid or perforated. The best result is that the outer electrode typically consists of a collection of rods evenly spaced to form a circle14Has been found to be achieved by: According to the shape described above, the plasma is12When ejected from the end of the magnetic field, the magnetic field generates a force that drives the plasma into the clamp and significantly increases its temperature. The higher the current and hence the magnetic field, the higher the final plasma temperature. Also, to achieve a more uniform velocity across the column 16ToMothTheNo profiling effort is required, and static and uniform gas filling is typically used. Thus, there is no need to introduce gas at the base end of the column 16, which is more preferred. Unprofiled gas is outsideelectrodeCentral speed faster than speed at 14electrodeAt 12. The capacitance, gas concentration and electrode length at the driver are such that when the current is near its maximum, the plasma surface is12Adjusted to ensure release from the end of the
[0039]
Plasma is centerElectrode 12The plasma surface is pushed inward when emitted from the end of the plasma. The plasma forms an umbrella or fountain shape. CenterElectrode 12The magnetic field of the current flowing through the plasma column immediately adjacent to the tip of the fin provides an inward pressure, which forces the plasma column inward until the gas pressure reaches equilibrium with the inward magnetic pressure. Tighten to
[0040]
Using this technique, temperatures in the clamp that are more than 100 times higher than the surface of the sun can be achieved. Radiation at the desired wavelength is obtained from the plasma gun by introducing a generally gaseous element having a spectral line at that wavelength at the clamp. This is a plasma that functions as an elementgasOr by an element introduced into the clamp in some other way, but for the preferred embodiment, the element isCenterIt is introduced through a central channel 92 formed in the electrode 12. The central electrode 12 is preferably cooled at its base end by a flow of cooling water, gas or other material in contact therewith over a portion of the housing. This is the cathodeThat is, the outer electrode 14Provides a large temperature gradient with the tip of the plasma, which can be at a temperature of approximately 1200 ° C. when the clamping of the plasma occurs. In particular, at high temperatures, the radiation intensity is inversely proportional to the fourth power of the wavelength (ie, intensity ≒ 1 / λ).^Four= (F / c)^FourWhere λ is the desired radiation wavelength, f is the desired radiation frequency, and c is the speed of light. Therefore,CenterFor certain gases / elements sent to the clamp or otherwise fed to the clamp through channel 92, the maximum intensity is the shortest wavelength signal emitted from the element during decay from the 2P → 1S state. This signal is obtained for elemental atoms in a single electron state (ie, atoms that have risen to a high energy state such that most of the atoms have been removed from the molecule). For atoms in the single electron state, the wavelength λ is λ = 121.5 nm / N^Two(Where N is evaporatingCentral channel92, which is the atomic number of the element within). Using this equation, the wavelengths with the highest energies for the first six elements of the periodic table are shown in Table 1 below.
[0041]
[Table 1]

[0042]
CenterThe gas supplied through the channel 92 isOne trainTo the extent that it has not been completely converted to the child state, and at the temperature present at the clamp, even if most gases are not substantially ionized to this state, radiation will alsoThatelementofIt is output at another spectral wavelength. However, as is clear from the above equation, these radiations have a very low intensity, and the intensity isofOnly a small part of the strength. Thus, for example, xenon at atomic number 54 has a small value of a single electron wavelength of 0.04 nm, but also has useful energy at a wavelength of 13 nm, as outlined. However, the energy at 13nm is,Optimum temperature at the clamp for single electronic stateof1/10 of energy at single electron wavelength^TenIn effect, lower laser clamping temperatures are of lower order magnitude. The reason is that because of the shape of the blackbody radiation curve that depends on determining the relative line size, it is not possible to force more than a small amount (≦ １ ／) of energy alone at 13 nm. Yes, because the temperature changes significantly.
[0043]
Therefore, the elementofIn order to use radiation at wavelengths other than the optimal single electron wavelength, it is necessary to filter out the higher intensity shorter wavelengths being emitted for that element. FIG. 3 shows one way to do this, in which the radiation 94 emitted from the plasma gun 90 absorbs all wavelengths of the radiation except for the desired wavelength that is reflected towards the desired target. Is supplied to a mirror 96 of a type known in the art configured as described above. Other filters that are at least high-pass filters for desired wavelengths and beyond can also be used.
[0044]
Thus, if possible, generate radiation at the desired wavelength in the elemental or maximum energy single electron state for the gasCenterIt is desirable to use other elements provided to the channel 92. However, in the absence of any element that emits radiation at the desired wavelength in the single-electron state, and from Table 1, that very few wavelengths above about 7.6 nm are actually available for the element in the maximum energy state. If known, the element emitting radiation at the desired wavelength and the filter used to obtain the radiation at the desired wavelengthAsA suitable filter, such as mirror 96, must be found. Since this radiation is less intense than radiation at the wavelength of the single electron state, larger and substantially more expensive devices are needed to obtain sufficient energy at lower intensity.Ie plasma gun90 is generally required. The intensity of radiation at a certain wavelength is watts / (meter)^Two/ Hertz and varies as a function of radiation frequency or wavelength, temperature and emissivity. Emissivity is a function with a maximum of 1, and it is important to select the gas that has the highest emissivity at the desired output frequency / wavelength. Optimal tightening temperature (T_OPT) Is the Vienna displacement law T_OPT= 0.2898 cm × K ゜ / λ (where K ゜ is the temperature of the plasma in Kelvin). Since only a very small amount of gas is ionized to generate radiation during each clamp, xenon has to be used to obtain 13 nm radiation.CenterChannel92Through it at a relatively slow speed. However, as mentioned above, when xenon is used, the output radiation at 13 nm has a relatively weak intensity, and to obtain useful radiation at this wavelength,mirrorA filter such as 96 is required. For this reason, lithium, which from Table 1 is found to have a wavelength of maximum intensity at substantially the desired wavelength (ie, at 13.5 nm), is the preferred element for radiation at this wavelength.
[0045]
FIG. 4 shows a central electrode 12 for an embodiment utilizing lithium vapor to produce the desired radiation. Referring to this figure, a solid lithium core 98 is held in a tube 100 of a material such as stainless steel, with the tip of the tube 100 near the tip along the center electrode 12 and at about 900 ° C. during plasma clamping. Temperature goes to the lithium core98At a pressure of about 1 Torr. This lithium vapor is at a flow rate such that it replaces argon or other plasma gas at the tip,CenterOf the end of the electrode 12OpeningExiting from 102, this required flow rate is in the range of about 1-10 grams per year for the illustrated embodiment. Tube 100 can be advanced slowly in any suitable manner to hold the front end of lithium core 98 in place.lithiumWhen the core 98 is used up, it can be replaced. A small amount of helium gas is preferably supplied around the tube 100 and flows out of the opening 102 to ensure that only lithium and helium are present in the clamping area. The reason for this is that, although small, argon produces high energy, short wavelength lines that, if not filtered, would interfere with the 13 nm radiation at the desired target.
[0046]
Another method of providing lithium or other suitable material to the clamp is a sintered powdered refractory metal saturated with liquid lithium or other suitable material in a fluid (ie, liquid or gaseous) state, with a central electrode 12 and an outer material. electrode14At least one of them. By pressing a powdered refractory material, such as tungsten, with a suitable binder, and then sintering the resulting mass at an elevated temperature, a metal, such as tungsten or molybdenum, can be made into the desired electrode shape. The resulting porous refractory metal matrix can be impregnated with liquid lithium or other desired material to provide improved life and another means of introducing lithium / material to the discharge. If desired, liquid lithium can be constantly supplied to the metal matrix of the electrode during operation so as to provide a virtually unlimited life of the process without having to replace the radiation generating material. One limitation in selecting a powdered refractory metal is to ensure that the metal does not dissolve in the radiation producing material burning therein.
[0047]
If xenon is used to obtain 13 nm radiation, xenon must be confined in the immediate vicinity of the clamp because xenon has considerable absorption at that wavelength. As with xenon, the radiation used isCentral channelIf at a wavelength other than the single electron wavelength for the element / gas in 92, a small amount of the element is ionized into its single electron state, resulting in a large amount of radiation at longer wavelengths and a small amount at shorter wavelengths. The temperature at the clamp can be controlled to provide more radiation (but higher intensity radiation).
[0048]
It is desirable that the cone angle of the emitted radiation is as small as possible. If the stimulated emission of radiation from the radiant gas at the clamp is greater than the spontaneous emission, a small cone angle is achieved and the spontaneous emission is more dispersive. In particular, assuming the Boltzmann constant k × the temperature at the tightening part is greater than the radiation frequency f × Planck constant h, the ratio of the natural radiation B to the stimulated radiation A is given by (B / A = kT / hf). For example, if this ratio is equal to 20 (ie, the plasma temperature is 20 times the photon energy of interest), the half cone angle will be about 25 °. The higher the plasma temperature, the smaller the cone angle. However, the shorter the wavelength of the radiation, the more difficult it is to achieve a small cone angle. However, the cone angle is one of the factors to consider when selecting current and other parameters to achieve the desired temperature at the clamp.
[0049]
FIG. 5 shows a book that can be used as a thruster, a radiation source or other feature utilizing a plasma gun, depending on factors such as electrode length and whether the radiation emitting element / gas is introduced through the central electrode 12. 7 shows another embodiment of the invention. The plasma gun is shown as being driven by the main solid state driver 110, and for the preferred embodiment,Main solid stateDriver 110DCA voltage source 32, a DC / DC converter 34,Non-linear magnetic compressor (NMC)36. However, this embodiment utilizes the trigger electrode 82 in the hole 74 for plasma initiation,Trigger electrodeAlternatively, the pulse RF is applied via a DC blocking capacitor 114 and a resonant coaxial line 116 functioning as a matching transformer.signalIt differs from the previous embodiment in that it is driven from the source 112. For the preferred embodiment, the RF signal is at a frequency between 10 MHz and 1,000 MHZ,Solid stateIt is energized for about 1 to 10 microseconds before energizing driver 110. FIG. 5 also shows an optional DC bias source 118 connected to the center electrode 12 via an AC filter coil 120.DC biasSource 118, DrivePowered substantially through a shaping and control circuit such as circuit 86DCVoltage source32Can beComeOr different depending on the applicationVoltageCan be a source.
[0050]
In FIG.columnAlthough only two trigger electrodes or spark plugs 82, 91 are shown on either side of the 16, the plasma gun is preferablycolumnAt least four evenly spaced around the perimeter of the sixteenTriggerWith electrodes and 6 or 8 (or more if possible)TriggerIt can have electrodes. FourTriggerFor electrodes,TriggerThe RF signal supplied to the electrodes is in the first phase, 90 ° relative to the one shown.Trigger at positionThe RF signal supplied to the electrode has a second phase shifted by 90 ° from the first phase. For a plasma gun with six trigger electrodes, a three-phase RF signal is used, with each phasecolumnA pair of 16 on both sidesTriggerSupplied to the electrodes. EightTriggerIn the case of electrodes, a two-phase signal is preferably used, one phase being oneYouAnd the second phase isTriggerBetween the electrodesTriggerThe four-phase signal supplied to the electrodes can also be used. For plasma startToDC signalnotRFsignalThe reason for usingTriggerRF supplied to the electrodesignalProvides more uniform and almost completely uniform volumetric ionization or startupcolumnThis is because it is generated within 16. Preferably line (s)1In response to a control signal onRF signalSupplied simultaneously with the RF signal from source 112DC biasThe DC bias from source 118 may also be12Contributes to uniform ionization in the vicinity ofsignalThe power requirement at source 112 is reduced. DC bias is applied to the center electrode as shown12Or, for example, in series or parallel with the RF signal such that the RF signal modulates a DC bias.Centerelectrode12Can be supplied to
[0051]
FIG. 6, for example, shows two 90 °Triggerelectrode(Spark plug)RF to 82, 82 'signalsource112Shows the connection. In the plasma gun there are two additionalTriggerElectricPoleExists, the secondTriggerThe electrode 82 is shownTriggerIt is located at an angle of 180 ° to the electrode 82 and is connected in the manner shown for the trigger electrode 82, the second trigger electrode 82 'TriggerIt is located at an angle of 180 ° from electrode 82 'and is connected in the same way as this electrode.RF signalSource 112 is connected via quarter-waveguide coaxial lines 124, 124 'to a point near the shortened ends of coaxial lines 126, 126', respectively, from the shortened ends at distances L1, L2 away. The coaxial line 126 is one quarter wavelength long, and at its non-shortened end.TriggerWith the electrode 82, the coaxial line 126 'is half-wavelength long and at its non-shortened endTriggerIt has an electrode 82 '. Quarter wavelength longCoaxialLine 126 and half-wave lengthCoaxialAt line 126 ',TriggerThe desired phase difference for the RF signal is achieved at the electrodes 82, 82 '. The coaxial line also provides a large voltage ramp and, if the coupling locations / distances L1, L2 are correctly selected, until the breakdown is achievedRF signalRelies on source as matching load. With a good quality coaxial line, a voltage step-up ratio on the order of 10-20: 1 can be easily achieved. Once the breakdown is achieved, the line will look like a short circuit at location L1.positionFrom L1λ / 4DistantRF signalsource112At the input that couples to, the apparent impedance becomes like an open circuit. Furthermore, if location L2 is selected correctly, this line will look like a matched load after the break has begun.CoaxialWhile it is desirable to keep the lines 126, 126 'as short as possible, the desired phase and impedance matching can be substantially achieved for the lines at respective lengths of (2M-1) λ / 4, Mλ / 2. Therefore, RFsignalsource112Always monitors the matched load and first pairTrigger electrodeCauses a voltage step-up, and then, after the plasma has been started, the second pair ofTrigger electrodeAt 82 ', a voltage step down is provided, but a current step down is provided. Table 2 below shows the RF of FIG. 6 for the illustrated embodiment.signalsource112Give the parameters for
[0052]
[Table 2]

[0053]
RFsignalsource112Only or RFsignalsource112And the RF frequency and voltage from both the DC bias source 118 are determined from the dimensions and operating pressure to provide maximum uniformity. In general, the RF frequency must be chosen to be above the critical frequency, below which the electrons in the gas are swept across the entire electrode gap in each half cycle. Frequency that has a long time and therefore vanishes. Above the critical frequency, the electrons oscillate back and forth between the electrodes, facilitating ionization of the gas. The critical frequency for a given plasma gun design is determined by first calculating the fluidity.
[0054]
(Equation 1)

[0055]
Where ν_cIs the collision frequency, ω = 2πf (where f is the frequency of the radiation), q is the electron charge, E is the electric field, and m is the electron mass. Therefore, the time to transition the gas is
[0056]
(Equation 2)

[0057]
(Equation 3)

[0058]
Given by Here, d is the distance between the electrodes.
For the thruster embodiment,Plasma gunIt is necessary to maintain 90 in a near-vacuum environment (approximately gas pressure ≦ 10 Torr), which is further necessary because radiation in the EUV band is easily absorbed and performs useful work outside of the near-vacuum environment Because it cannot be used for Since propulsion efficiency is not as important for this embodiment, a single radiation for each valve operation or valve operation periodburstTo provide radiation for a desired period of time.burstCan be selected.
[0059]
Standard high voltage such as magnetron, klystron or RF amplifierofRFsignalSource 112 is, as described above, an RF source for the previous embodiment.signalCan be used as a source, but such standard RFsignalSources are expensive to purchase and use, bulky, and generate significant heat that adds to the thermal management burden of the equipment utilized. Therefore, like thisRF signalThe source is a smaller RF that is significantly less expensive to purchase and operate, and generates significantly less heat.signalPreferably, it can be exchanged for a source. Figure 7A shows a sled that meets these requirements.TDostateofRFSignal sourceIs shown. In particular,Circuit of RF signal source 130Is the standard RFsignalIt has been found that it generates RF power at a cost of about 1% of the cost at the source and occupies less board space, for example by "6" or "8" times, rather than a large cabinet.
[0060]
Referring to FIG. 7A,RF signal source130 is a voltage source, for example,DCA capacitor 132 is charged from the voltage source 32 in a standard manner. A solid state switch 134, which may be, for example, an SCR, IGBT, or MOSFET, allows the capacitor 132 to discharge to the input of a multi-stage nonlinear magnetic pulse compression circuit 136 of the type described above when closed or energized. .Multi-stage nonlinear magnetic pulse compressionCircuit 136 may include multiple stages and / or transformers, one example of such a configuration is shown, terminating in specialized output section 138. Output section 138 forms a saturable resonant shunt to ground,outputsection138Is a capacitor C_RAnd saturable inductor L_Rincluding. Capacitor C_RIsMultistageNon-linear magnetic pulseCompression circuit 136Of the n-th stage capacitor C_NIs charged resonantly.CapacitorsC_NIs the capacitanceCapacitorsC_RSelected to be even smaller thanCapacitorsC_RIsCapacitorsC_NInvert while charging. instead of,InductorL_RIsCapacitorsC_NFromCapacitorsC_RCan be selected to saturate before the transfer of charge to is completed. If one or both of these conditions are met,InductorL_RIs saturatedCapacitorsC_RBefore it reaches its peak chargeCapacitorsC_N, A reverse voltage is generated. Under these conditions,InductorL_RDue to the sequential saturation ofInductorL_RAs shown in FIG. 7CCapacitorsC_RVibrates. For the plasma start application of the present invention,RF signalOnly three or four cycles of the source are required as shown in FIG.RF signal source 130Can be selected to provide the desired number of cycles, depending on the application. Output section138Is the resonance frequency F ofCapacitorsC_R,InductorL_RAnd any one of these values can be made adjustable to allow tuning of the circuit. Resistive element R_OAnd capacitive element C_OAn output coupling circuit 140 is provided, each of which can be formed of a number of suitably interconnected elements. The output coupling circuit 140 includes a capacitor C_RA portion of the energy fromoutputCoupling circuit140The impedance of each cycleCapacitorsC_RIt is selected to remove only a portion of the energy stored within (eg, 20% per cycle). Further, as shown in FIG.RF signal source 130Is particularly suitable for use in the plasma gun of the present invention, but is shown in FIG. 7A.Circuit of RF signal source 130Solid state with high performance characteristicsRF signal sourceDoes not currently exist, and thereforeRF signal source 130Can also find use in other applications. Therefore, thisRF signal source 130It also forms part of the present invention.
[0061]
RF signalAre two possible problems in delivering the plasma to the plasma gun: a high voltage field is created over a relatively large uniform area at the base of the column 16, causing breakdown in this area;columnFor the vacuum required in 16DestructionThe goal is to obtain an RF field at this point while minimizing. The latter is not a problem in space applications, but can be a problem in more common applications of plasma guns as radiation sources. FIG. 8A shows one way to achieve both goals, while FIG. 8B shows a way to achieve only the second purpose.
[0062]
Referring first to FIG. 8A, a ceramic dielectric 150 is provided at the base of column 16 between center electrode 12 and outer electrode 14. The plurality of electrodes 152,column16 outsideceramicdielectricbody150 mounted on ceramic dielectric150By column 16ceramicDielectric150From the surface 154 by a small distance. Between the electrode 152 and the surface 154ceramicDielectric150Can typically be less than 1/8 inch (approximately 3.18 mm) thick, and can be a ceramic dielectric.150Are selected to be as thin as possible, while ensuring that they do not crack or break. RFsignalAnd / or when a DC signal is applied to electrode 152, a high voltage field appears on surface 154 and initiates the desired plasma breakdown.
[0063]
The device of FIG. 8B differs from that of FIG. 8A in that the ceramic dielectric 150 ′ is formed as a collar over the bottom portion of the center electrode 12 and extends a small distance into the column 16. The electrode 152ceramicDielectric150On the outer surface 154 'of thesignalAnd / or electrode DC signal152A high voltage field is formed at surface 154 '. For applications where the entire plasma gun is not in a vacuum environment, the electrical leads must be evacuated.ofThe shape of FIG. 8A is preferred in that it need not be brought into the column 16, and for the embodiment of FIG.16Brought inside.
[0064]
Also, the initial high voltage spikes after startingCentral electrode12, And outer electrodeIt has been found that more uniform destruction can be achieved in the plasma gun by feeding to. FIG. 9A shows a circuit for achieving the desired waveform shown in FIG. 9B. In particular, this waveformofspikesignal160, followed by a persistence signal 162. initialofspikeSignal 160Can be as large as ten times the voltage of the persistence signal 162, but for a shorter period of time,CenterElectrode 12,Outer electrodeThe energy is supplied as little as 1/10 of the energy supplied to 14.
[0065]
Referring to FIG. 9A, the circuit includes a first nonlinear magnetic compression circuit 164 (only the last stage is shown in FIG. 9A) and a second nonlinear magnetic compression circuit 166 (only the last stage is also shown in the figure). Have.Second nonlinear magnetic compressionCircuit 166 generates a high voltage short duration spike signal 160, whileFirst nonlinear magnetic compressionCircuit 164 is a spikesignalA sustain signal 162, which is a longer term low voltage signal occurring at the end of 160, is generated.First nonlinear magnetic compressionThe reactor 168 for the last stage of the circuit 164First nonlinear magnetic compressionSaturated in a direction that allows the flow of signal from circuit 164,Second nonlinear magnetic compressionA bias signal provided via the bias winding 170 to interrupt the reverse signal flow from the circuit 166, typicallyBiased. Therefore, this signalFirst nonlinear magnetic compressionCircuit 164, especially to its final stage capacitor 172, from voltage spikes.First nonlinear magnetic compressionProtects the circuit 164 so that all of this signalCenterElectrode 12,Outer electrode14 is guaranteed. spikesignal160 begins to reverse the bias of the saturable reactor 168, partially overcoming the bias supplied thereto,CenterElectrode 12, Outer electrode 14Causes avalanche destruction. This allows the selection of optimal voltage and drive impedance levels for the main discharge chain without fear of exceeding the breakdown voltage. The reverse bias of the saturable reactor 168 is168Until it resaturatesFirst nonlinear magnetic compressionThe persistence signal from circuit 164162And provides a smooth transition between the two signals.
[0066]
For example, a plasma of the type shown in FIG.gunOne problem with is that enough Tesla to drive the plasma to a speed of several centimeters per microsecond to achieve the desired clamping temperature in the range of 100 eV to 1000 eV depending on the desired frequency of radiation. That is, a certain degree of magnetic compression field is required. Due to these high velocities, the plasma is centeredelectrodeDriven down 12, centerElectrode 12From the end of the plasma sheathElectrode 12Continue to move into the space away from the end of the. As a result, the plasma sheath eventually loses its electrical connection to the clamp, terminating the clamp and causing a large voltage transition. This voltage transition isCenterelectrode12Can cause high voltage respikes that can severely damage the device. Loss of electrical contact with the plasma sheath alsoPlasma gunCausing a substantial reduction in power efficiency from the squeeze, and the clamping lasts only about 100 nanoseconds, rather than a substantially longer duration of the electrical discharge, which can be several microseconds (eg, 2-4 microseconds). .
[0067]
According to the teachings of the present invention, this problem of plasma12This can be overcome by providing a blast shield or focusing device 194 adjacent to the exit end of the center electrode 12 to redirect it back toward. FIG. 10A-FigureFIG. 10C shows three possible embodiments of such shields or focusing devices (hereinafter collectively referred to as shields) 194A, 194B, 194C that differ primarily in the shape of the focusing cavities 196A, 196B, 106C, respectively. In particular, the cavity 196A has a substantially spherical shape, and the cavity may be formed with the outer electrode 14 or the outer electrode 14 by a suitable mounting component (not shown).Plasma gunIn a suitable housing component such that the wall of cavity 196A is at a distance from the tip of central electrode 12. This distance is the distance between the shield and the center electrode.12That is sufficient to prevent contact between12Is short enough to produce the plasma radiation returning to These objects are achieved by a spacing approximately in the range of R to 2R, where R is the radius of the center electrode 12. But such a distancePlasma gunIt can vary to some extent depending on the other 10 parameters. The cavity 196B has a conical shape, and the cavity 196C has a parabolic shape. The parameters described above for the distance of the cavity from the end of the center electrode 12 apply to all three cavity shapes.
[0068]
Prevents plasma sheath separation,plasmaIt is desirable to house the sheath in the shield 194, but the shield 194Plasma gunIt is important not to conflict with the outflow of the desired radiation from 10. Thus, each shield 194 is formed at the top of the corresponding cavity and has a central electrode12Has a central opening 198A, 198B, 198C with a center coaxial with the center line of Opening 198 is preferably circular and has a central electrode12Radiation emitted at an angle of ± 15 ° (almost the angle of emitted radiation) from the tightening part at the tip of198Have a diameter sufficient to pass through. The upper portion of each opening 198 tapers outwardly to facilitate radiation outflow while substantially limiting any escape of the plasma sheath.
[0069]
The material of the shield 194 must be a high temperature non-conductive material capable of withstanding temperatures in the range of approximately 1000 ° C. and higher. Various high temperature ceramics have desired properties, and for the illustrated embodiment, Al_TwoO_Three(Aluminum oxide) is used. Various glasses, quartz and sapphire also have desirable properties that serve as materials for the shield 194.
[0070]
In the above description, the plasmaToReorientationTo makeAlthough shield 194 has been shown for use with a particular shaped radiation source, this shield194Is suitable for use with any radiation source where plasma separation is a potential problem, and therefore the invention is in no way limited by the particular radiation source configuration of FIG. Similarly, the cathode orOuter electrode 14Although three cavity shapes for redirecting radiation to are illustrated in FIGS. 10A-10C, other cavity shapes suitable for performing this function may be utilized. The specific materials described above are also merely exemplary.
[0071]
In addition, the parameters for generating radiation at 13 nm have been described above,Plasma gun asBy controlling the various parameters of the 90, and among others, the elements / gases used, the maximum current from the high voltage source, the plasma temperature in the clamping area, the column16By carefully selecting the gas pressure within and, in some cases, the radiation filter utilized, radiation at other wavelengths within the EUV band, or in some cases, outside this band can be obtained.
[0072]
Many types of gases can be used as the plasma gas for the plasma gun described above, but inert gases such as argon and xenon are often preferred. Other gases that can be used include nitrogen, hydrazine, helium, hydrogen and neon. As described above, when using a plasma gun as the radiation source, as in the embodiment of FIG. 3, various elements / gases may be used to achieve selected EUV or other wavelengths. In the case, the plasma and the radiation gas are the same gas. For example, hydrogen gas can be selected to effectively obtain radiation at 121.5 nm in the VUV band. Furthermore, while various embodiments have been described above, it is clear that these embodiments are merely illustrative and do not limit the invention. For example, the driver shown is advantageous for various applications, but has other voltages that have adequate voltage and rise times and do not require high voltage switching.PRF drivers are also available. Similarly,Electrodes driven by preferred solid-state RF signal sourcesVarious plasma starting mechanisms using triggers have been described.,Other methods for initiating plasma destruction can be utilized for appropriate applications. The shape of the electrodes and the applications given for the plasma gun are also examples. Thus, while the present invention has been particularly shown and described with reference to preferred embodiments, those skilled in the art will be able to make the above and other changes which constitute the details while remaining within the spirit and spirit of the invention. Is defined only by the claims.
[Brief description of the drawings]
FIG. 1 of the present invention.Plasma gunFIG. 4 is a semi-schematic half-cut side view of a first exemplary thruster embodiment.
FIG. 2 of the present invention.Plasma gunFIG. 7 is a semi-schematic half-cut side view of another thruster embodiment.
FIG. 3 of the present invention.Plasma gunFIG. 4 is a semi-schematic half-cut side view of an embodiment of a radiation source.
FIG. 4 of the present invention.Plasma gunFIG. 4 is an enlarged (not to scale) cutaway view of the center electrode of FIG. 3 for one embodiment.
FIG. 5 shows that other factors can be used as thrusters or radiation sources, depending on the relative dimensions, and that RF in accordance with the teachings of the present invention.Trigger electrode driven by signalHaving,Of the present inventionPlasma gunIt is a semi-schematic side cutaway view of an embodiment.
FIG. 6 shows RF in the plasma gun of the present invention.Signal sourceFig. 3 schematically shows another implementation for obtaining
FIG. 7A isTrigger electrodeTo driveofRFsignalSolid state suitable for use as a sourceofRFsignalFIG. 7B is a schematic diagram of a source, and FIGS. 7B and 7C are diagrams illustrating voltages across certain capacitors in the circuit of FIG. 7A.
FIGS. 8A and 8B are cut-away partial side views of a portion of a plasma gun showing two different starter electrode configurations suitable for providing a firing voltage to the plasma gun.
FIG. 9A is a schematic diagram of a pulse driver circuit suitable for use in driving the plasma gun of the present invention according to another embodiment, and FIG. 9B is a schematic diagram of a pulse driver circuit from the circuit of FIG. 9A. FIG. 4 is a diagram illustrating an output signal.
FIGS. 10A to 10C are enlarged side cross-sectional views illustrating the end and shield of a central electrode for spherical, conical and parabolic embodiments, respectively, of the present invention.
[Explanation of symbols]
10 Plasma gun
12 Central electrode
14 Outer electrode
16 columns
82 trigger electrode
130 circuits
132 capacitor
134 solid state switch
136 Multi-stage nonlinear magnetic pulse compression circuit
138 output section
140 output coupling circuit

Claims (7)

In high PRF plasma guns (10, 10 ', 90)
Central electrode (12);
An outer electrode (14) substantially coaxial to the center electrode, wherein a coaxial column (16) is formed between the center electrode and the outer electrode , wherein the column is closed and the base is open; An outer electrode having an outlet end;
An inlet mechanism (70, 72, 74) for introducing the selected gas into the column ;
A solid state high repetition rate pulse driver (32, 34, 36) operable to deliver a high voltage pulse across the center electrode and the outer electrode during plasma initiation at the base of the column, wherein the plasma is A solid state high repetition rate pulse driver extending from the base end of the column and adapted to exit from the outlet end of the column;
A plurality of trigger electrodes (82) at the base end of the column (16);
A radio frequency (RF) signal source (130) connected to the plurality of trigger electrodes (82) for selectively providing an RF signal to the plurality of trigger electrodes (82);
Has,
The RF signal source (130) comprises:
A non-linear magnetic pulse compression circuit (136) having N stages that is an integer equal to or greater than 1;
A solid state switch (134) selectively operable to connect a capacitor (132) to an input of a first stage of the non-linear magnetic pulse compression circuit;
An output section (138) having a resonance circuit with a resonance frequency (F) including a capacitor (C _R ) and a saturable inductor (L _R ), wherein the last stage of the nonlinear magnetic pulse compression circuit has a capacitance ( C _N ) and at least one of the capacitor (C _R ) and the inductor (L _R ) develops a reverse voltage across the capacitor (C _N ) before the capacitor (C _R ) is fully charged. An output section selected so that the inductor (L _R ) is subsequently saturated, causing oscillation of the capacitor (C _R ) at frequency (F) ;
An output coupling circuit (140) for coupling energy from a capacitor (C _R ) to drive the plurality of trigger electrodes (82) ;
A plasma gun comprising: The plasma gun according to claim 1 , wherein the solid state switch (134) is one of an SCR, an IGBT and a MOSFET. The plasma gun of claim 1 in which the capacitance of the capacitor (C _N) is equal to or smaller than the capacitor (C _R). 2. The inductor according to claim 1 , wherein the inductor (L _R ) is selected to saturate before the transfer of charge from the capacitor (C _N ) to the capacitor (C _R ) is completed. Plasma gun. The plasma gun according to claim 1 , wherein the output section (138) is a saturable resonant shunt for ground. The output coupling circuit (140) that has an impedance such as a portion of the energy stored in the capacitor (C _R) is coupled to the plurality of trigger electrodes during each oscillation cycle of the capacitor (C _R) The plasma gun according to claim 1 , characterized in that: 2. The plasma gun according to claim 1 , wherein the inductor (L _R ) and the capacitor (C _R ) are selected such that only three to four oscillation cycles of the output section (138) occur.

Images