US3509490A - Inductive excitation system for plasma - Google Patents

Description

April 28, 1970 c. B. ZAROWIN 0 5 5 INDUCTIVE EXCITATION SYSTEM-FOR PLASMA Filed April 26, 1967 18 RE WAVE RATOR P FIG.2 v w TIME (MILLISEC) v +50- 1 16.?) vous L 1 1 1 0 0.2 0.4 0.0 0.0 1.

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(MILLISEC) ATTORNEY United States Patent M 3,509,490 INDUCTIVE EXCITATION SYSTEM FOR PLASMA Charles B. Zarowin, University Heights, N.Y., assignor to International Business Machines Corporation, Armonlt, N.Y., a corporation of New York Filed Apr. 26, 1967, Ser. No. 633,964 Int. Cl. H01s 3/00 US. Cl. 33194.5 7 Claims ABSTRACT OF THE DISCLOSURE An inductively excited ion gas laser is described including a plasma tube containing a gas. Inductive excitation means including a cylindrical core envelopes a portion of the plasma tube. A primary winding is Wound on the core and is connected to a source of low frequency square wave signal. The primary winding, the core, and the plasma tube essentially form a transformer. The square wave signal whose frequency is below the upper cut-off frequency of the transformer passbaud is applied to the primary winding to excite the gas in the plasma tubes.

BACKGROUND OF THE INVENTION Field of the invention The present invention relates to the excitation of gases into the plasma state, and more particularly to a plasma tube containing a gas which is inductively excited by means of a square wave signal. The plasma tube is a 9 related element of an ion gas laser.

SUMMARY OF THE INVENTION The present invention may be summarized as the inductive excitation of a gas by means of an applied square wave signal. The square wave signal permits the use of low frequencies while reducing modulation of the excitation to a desired level.

An object of the present invention is to provide a system for exciting a gas into the plasma state.

Another object of the present invention is to provide an inductive plasma excitation system employing a low frequency excitation signal without modulation.

A further object of the present invention is to provide a low frequency inductive plasma excitation system employing a square wave driving signal.

Another object of the present invention is to provide an ionic gas laser employing low frequency inductive excitation.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 is a schematic drawing of an embodiment of an ion gas laser employing inductive excitation by a low frequency square wave signal according to the principles of the present invention.

FIG. 2 is an illustration of a waveform representative of a square wave driving signal used in the system of FIG. 1.

FIG. 3 is an illustration of a waveform representative of the signal on the secondary of a transformer employed in the system of FIG. 1.

FIG. 4 is an illustration of a waveform representative of the power which is used in the explanation of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Excitation of a gas into the plasma state as required in an ionic gas may be accomplished in several ways,

with inductive excitation by transformer action being a preferred method because no electrodes are required and there is no limit on the power that can be transferred to the plasma in contrast to capacitive excitation.

Heretofore inductive excitation has been employed only using high frequency (i.e., above the upper cut-off frequency of the transformer passband) since low frequency excitation of the plasma causes modulation, and such modulation prevents continuous wave (C.W.) operation of the laser. High frequency excitation, however, produces a leakage inductance which must be overcome by the inclusion of a capacitor in the excitation circuit to produce resonance. This in turn results in undesirable high voltage conditions.

The present invention relates to means for inductively exciting the plasma of an ion gas laser by a low frequency square wave signal. It is significant in that when low frequency excitation is used modulation of the excitation is reduced to a desired degree and acceptable C.W. operation is obtained. Thus, a C.W. ion gas laser which is inductively excited by a low frequency signal is obtained.

The square wave signal is applied to the primary of a transformer whose secondary is the plasma forming the active medium of the ion gas laser. The resultant excitation appears to the plasma as direct current, yet provides the necessary alternating current to produce the voltage required to maintain the plasma.

The inductive excitation system of the present invention will be described relative to an ion gas laser since it is particularly useful with such device. An ion gas laser differs from an atomic laser in that the gas atoms are in the ionic state. The ion laser requires a much higher current density than the atomic laser to produce laser action. All other factors being equal, the ion laser requires in the order of one hundred times more current than the atomic laser. The high current requirement of the ion gas laser presents a problem of excitation. Excitation by capacitive coupling is undesirable because the plasma resistance in series with the capacitor results in high voltage conditions and there is a practical limit to the voltage which can appear across the capacitor. Direct current excitation by cathodes is also undesirable because there is a limit to the amount of current that cathodes can produce and to obtain high currents the cathodes must be impractically large.

Inductive excitation is desirable for ion gas lasers because there is no limit on the amount of current that such excitation can produce. Thus, the present invention, which is a low frequency inductive excitation system, is useful with ion gas lasers.

Referring to FIG. 1, a gas ion laser is shown including a plasma tube 10 and an inductance coil 12. Plasma tube 10 is topologically toroidal, being composed of a closed single surface having a continuous center region. For ease of explanation. plasma tube 10 is shown as aconventional doughnut-shaped toroid, one leg of which may include Brewster windows 14 and 16 at each end to enhance transmission of one plane or polorization. The structure of FIG. 1 is a laser amplifier, if a laser oscillator is desired, a pair of mirrors may be added opposite the Brewster windows 14 and .16.

Inductance coil 12 includes a core 18 and a winding 20 surrounding core 18. Core 18 is connected to plasma tube 10 such that it passes through the loop of plasma tube 10. Winding 20 is connected to a source of square wave signal 22. Core 18 is preferably a ferrite or other mag netic material, however an air core may be employed.

Plasma tube 10 is filled with a suitable gas such as argon or an argon-xenon mixture at a suitable pressure. Any of the gases heretofore used in gas ion lasers may be employed in the present invention. The combination of the plasma tube 10 and the inductance coil 12 is essentially a transformer, with winding 20 being the primary winding and the closed gas loop being a one turn secondary winding.

Winding 20 is connected to a signal source which in the present invention is a source of square wave signal 22. The function of the signal source is to produce a changing magnetic field by means of the inductance coil 12 which in turn produces a changing voltage in the gas in the plasma tube 10 to excite the gas into the plasma state. As previously mentioned, a source of alternating current signal has been used for inductive coupling in order to produce the changing voltage in the gas. The alternating signal had to be above a given frequency, since alternating signals below such given frequency resulted in modulation of the excitation of the plasma. Modulation of the excitation of the plasma is undesirable since it results in intermittent operation (i.e., the laser will not operate as a continuous wave device). The use of alternating frequencies above such given frequency, however, produces a high leakage flux, the compensation for which results in high voltage conditions.

The aforesaid given frequency is defined herein as an alternating current frequency above the upper cut-off frequency of the passband of the inductive excitation means, and the term low frequency as used hereinafter and in the claims is defined as a frequency below the upper cutoff frequency of the passband of the excitation means and may be any frequency lower than the alternating current frequency at which modulation of the excitation of the plasma is produced. For most gases used in the plasma tube, the high excitation frequencies are in the range from 1 megahertz to 10 megahertz. Argon, for example, would employ a high frequency of approximately megahertz. Thus, the low frequency of the present example would generally be below the 1 megahertz frequency. The term low frequency, that is, the frequency below the alternating current frequency at which modulation of the excitation of the plasma is produced can be determined for any gas by methods known to one skilled in the art. In FIG. 1 square wave signal generating means 22 provides a low frequency signal, that is, the square wave signal has a frequency at which an alternating current signal would produce modulation of the plasma excitation in the plasma tube 10.

Referring to FIG. 2, an illustration of the waveform of the signal applied to coil 20 from square wave generator 22 is shown. For purposes of explanation only the signal has a maximum voltage of 150 volts, a frequency of 2.5 kilohertz, and a current of 100 amperes. Considering a three turn primary winding 20, the secondary voltage has a peak value of 50 volts, a frequency of 2.5 kilohertz, and a current of 300 amperes. An illustration of the waveform of the secondary winding (plasma tube) voltage is shown in FIG. 3.

The secondary voltage causes the electrons in the gas to move in one direction for positive portions of the secondary voltage cycle and in the opposite direction for negative portions of the voltage cycle. When the electrons move in either direction, they collide with the ions and give up to the ions the energy supplied by the magnetic field. Energy is a scaler quantity, thus there is no polarity involved in whether the ions collide with electrons moving in one direction or the other. An illustration of the waveform of the energy supplied to the ions is shown in FIG. 4. From FIG. 4 it can be seen that the excitation appears to the plasma substantially as direct current so that modulation of the excitation is reduced to a desired degree such that the laser operates in a continuous wave mode.

On the other hand the excitation voltage is shown in FIG. 2, being a square wave, provides the necessary polarity changes to produce the magnetic field energy which maintains the gas in the plasma state.

The present invention is particularly useful in the in- .4 ductive excitation of wide bore lasers. Wide bore lasers are those having plasma tubes with relatively large diameters or cross-sectional areas. Wide bore lasers have several advantages over narrow bore lasers: they can support larger mode diameters and thus result in smaller beam divergencies; the larger volume per unit length allows greater output power per unit length for a more compact device; and, the wider bore results in an active medium of higher numerical aperture which is desired for intracavity processing. In phase plasma current I is expressed as follows:

w is a function of excitation frequency E is plasma voltage L is plasma inductance R is plasma resistance k is coupling coefficient.

For a fixed plasma inductance L and coupling coefiicient k (which are functions of geometry and core permeability only), and voltage E the plasma current I is inversely proportional to the frequency of excitation, so that for the high currents, low frequencies as provided in the present invention are desirable.

What has been described is a system for the inductive excitation of gas enclosed in a plasma tube, such as in an ion gas laser. The plasma tube may vary in size and shape, the only requirement being that it be a topologically toroidal closed loop. The inductive excitation results from an applied square wave signal, preferably at a low frequency as defined. The gases used in the plasma tube are well known in the art and many include agon, krypton, xenon, etc. and chlorine, bromine, etc. The pressures at which the gases are maintained and other working parameters of the system should be within the knowledge of those skilled in the art.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A system for exciting a gas comprising:

a closed loop plasma tube containing a gas,

excitation means having a passband with an upper cutoff frequency, said excitation means coupled to said gas filled plasma tube,

and a square wave signal generating means connected to said excitation means for producing a square wave signal having a frequency lower than said upper cutoff frequency for operating said excitation means for exciting gas into a plasma state.

2. A system according to claim 1 wherein said excitation means includes a cylindrical core, the center axis of said core lying substantially along a section of said plasma tube thereby allowing said core to fully envelope said tube section therein,

and a primary winding wound on said core, Said primary winding being connected to said square wave signal generating means.

3. A system according to claim 1 wherein said closed loop plasma tube is topologically toroidal consisting of a closed single surface having a continuous center region containing a gas.

4. A system according to claim 2 wherein said square wave signal frequency is a frequency lower than one megacycle.

5. An ion gas laser comprising a closed loop plasma tube containing a gas a cylindrical core, the center axis of said core lying substantially along a section of said plasma tube I 5 6 thereby allowing said core to fully envelope said square wave signal frequency is a frequency lower than section therein, one megacycle. a primary winding wound on said core to form with References Cited said core and said plasma tube a transformer means Induction Excitation of Visibh Laser havmg an uppe? cut'ofi frequency Transitions in Ionized Gases, J. P. Goldsborough et al., a a square wave slgnal generating means for producing 5 App Phy Ltrs Mar 15 1966 vol 8 N0 6 pp a square wave signal having a frequency below said 139 upper cutoff frequency and connected to said primary winding for producing a changing magnetic field RONALD WIBERT Primary Examiner around said core for exciting said gas in said plasma tube into the plasma state to produce continuous K- GODWINJRwASSIStam Exammer wave operation of said ion gas laser. a 6. An ion gas laser according to claim 5 wherein said US closed loop plasma tube is topologically toroidal. 315 111 7. An ion gas laser according to claim 5 wherein said

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