US3143680A

US3143680A - Ion accelerators

Info

Publication number: US3143680A
Application number: US855176A
Authority: US
Inventors: Klein Siegfried; Pottier Jacques
Original assignee: Commissariat a lEnergie Atomique CEA
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA
Priority date: 1958-11-25
Filing date: 1959-11-24
Publication date: 1964-08-04
Anticipated expiration: 1981-08-04
Also published as: CH363105A; LU37902A1; NL245698A; FR1216446A; DE1168580B; GB927938A; BE584862A

Description

.zvi-mcl SR O l/ MTFQQQ XR 391439650 WW Wa Aug. 4, 1964 S. KLEIN E'I'AL ION ACCELERATORS Filed Nov. 24, 1959 United States Patent 3,143,680 ION ACCELERATORS Siegfried Klein, Paris, and Jacques Pottier, Orsay, France,

assignors to the Commissariat a lEnergie Atomiqne,

Paris, France, an organization of France Filed Nov. 24, 1959, Ser. No. 855,176 Claims priority, application France Nov. 25, 1958 14 Claims. (Cl. 313-63) The present invention relates to ion accelerators and more especially to linear accelerators of the electrostatic type which include an evacuated tube in which the ions produced by an ion source and introduced into the tube at one of the ends thereof are accelerated by an electrostatic field in the direction of the other end of the tube toward a target.

In the known linear electrostatic accelerators, the field is essentially produced by a very high direct potential dilference applied between the ends of the evacuated tube. However, in order to facilitate insulation and to achieve a constant electrostatic field, the difierence of potential is generally divided into several sections by making use of several metallic elements or electrodes provided inside the evacuated tube and brought to potentials which increase gradually in absolute value in the direction of the target, for instance by connecting them with successive points of a chain of resistors in series forming a kind of potentiometer between the two terminals of the very high potential source. Such electrostatic accelerators have the drawback of requiring a source of very high potential and therefore a source of very large dimensions such as the Van De Graatf generator.

On the other hand, there are known linear accelerators in which the acceleration of the ions is obtained by means of high frequency alternating electromagnetic waves applied to electrodes located inside the evacuated tube and the potential of which therefore undergoes alternating variations. Such accelerators do not require feed sources at voltages as high as the electrostatic accelerators above referred to but they can accelerate only a single kind of ions due to the fact that the geometrical characteristics of the accelerator and the electrical characteristics of the feed source are determined in accordance with the mass and the charge of the ions to be accelerated.

The purpose of the present invention is to provide an ion accelerator which, on the one hand, requires only a relatively low voltage source as in the case of the above mentioned electromagnetic wave accelerators so that it can be made of small dimensions and, on the other hand, permits of accelerating ditferent kinds of ions as in the case of the above mentioned electrostatic accelerators, owing to the fact that the velocity of the ions at different points of the accelerator has no influence upon the operation thereof.

With this object in view, the apparatus according to the present invention, intended to accelerate bunches of ions supplied at a reference potential by a high intensity pulsating ion source, includes an evacuated tubular chamber one end of which receives ions from said ion source, a series of electrodes partly permeable to the ions to be accelerated and disposed behind one another in said chamber, a direct high voltage source, a series of resistors of high resistance the number of said resistors being equal to that of said electrodes, conductor means for connecting one of the terminals of said high voltage source with each of said electrodes through one of said resistors respectively and conductor means for connecting the other terminal of said high voltage source with the ground corresponding to said reference potential. We thus obtain an ion accelerator, especially for heavy ions either positive or negative, which has the following properties:

(a) It permits of applying very high accelerations to the ions due to the fact that each of the acceleratingelectrodes, normally brought to a potential which is high in absolute value, has its potential suddenly reduced in absolute value just before it is struck by the ions, this potential becoming practically zero when the ions reach the electrode, which then produces between this electrode and the next one a difierence of potential equal to the high voltage that is applied, whereby it is possible to obtain by recurrence a total acceleration potential nearly equal (account having to be taken of the losses) to the product of said high voltage by the number of electrodes; and

(b) The apparatus is self regulating due to the fact that the time of transit or flight of the ions from one accelerating electrode to the next one (which time depends upon the mass and the charge of the ions) has no influence upon the operation of the device, contrary to what takes place in the case of an acceleration by means of high frequency currents.

The ion accelerator according to the invention requires a source capable of supplying intensive ion bunches, for instance an ion source of the type described in the United States patent application Ser. No. 840,429, filed September 16, 1959, now US. Patent No. 3,015,745 to Siegfried Klein, issued January 2, 1962, due to the fact that a sufiicient intensity (of at least one ampere) is necessary to achieve within a sufiiciently short period of time the voltage drop Which brings the potential of the electrode struck by the bunch of ions down to ground potential and also due to the fact that the intensity of the ion beam is gradually reduced by its impact upon successive electrodes. This is why it is preferred to have an ion source of very high intensity averaging some tens of ampere.

Preferred embodiments of the present invention will be hereinafter described with reference to the accompany ing drawings, given merely by way of example and in which:

FIG. 1 show in diagrammatic sectional view a linear ion accelerator made according to the present invention and used in connection with an ion source made accord ing to the above mentioned prior patent application.

FIG. 2 is a front view of one of the accelerating elec trodes shown in the form of a grid on FIG. 1.

FIG. 3 shows a modified accelerating electrode.

According to the present invention, the accelerator, intended to accelerate ion bunches supplied by a pulsating ion source S of high intensity includes a tubular chamber 2 evacuated by means of a pump 3 and the end 1 of which receives bunches of ions from said ion source (said ions being possibly given an initial acceleration by means of an extracting electrode 4). The apparatus further includes a series of accelerating electrodes 5, 6, 7, 8, either in the form of grids g as shown in front view on FIG. 2 or of discs p provided with a central hole 0 as shown by FIG. 3, a movable diaphragm 9 (adapted to be replaced by the target to be bombarded by the high energy ions supplied by the accelerator) being disposed at the other end of tubular chamber 2. One of the terminals 10 of a high direct voltage source 11 (averaging several tens of kilovolts) is connected through a conductor 10, and a line 12 with each of the accelerating electrodes 5, 6, 7, 8, through a resistor 13, 14, 15, 16, respectively, of high ohmic value (of the order of magnitude of l megohm) with the provision of insulating elements 5,, 6 7,, 8 for the passage of the conductors through the wall of chamber 2, the other terminal 17 of source 11 being brought by means of conductor 17,, to the reference potential R owing to the connection of said conductor with a grounding part consisting either of the wall of chamber 2 when said wall is made of a conducting material, or preferably of a separate element when the wall of chamber 2 is made of an insulating material.

Source S may include, as described in the above mentioned prior patent application, a non-metallic flexible tube 18 one end of which is fitted on a metallic tube 19 of short length having a throttled part 20 and welded at 21 to the open tubular end of a bulb 22 made of aluminum and sodium bore-silicate glass such as designated by the trademark Pyrex. This bulb is also open at the other end 23 to form a tube which penetrates into a metallic ring 24 provided at the end 1 of the accelerator chamber 2, a flexible toroidal-shaped packing member 25 (for instance of rubber) being interposed between bulb 22 and chamber 2.

In order to ensure ionization in bulb 22, there is provided about said bulb a self-inductance coil 26 (of the order of some microhenrys) connected in series with a spark-gap 27 (which might be replaced by a discharge tube of the ignitron or thyratron type) across the terminals of a condenser 28 (of the order of 1 microfarad) capable of being charged, through a charge resistor 29, from a high voltage direct current source 30. Furthermore, the ends of coil 26 are connected respectively, one at 31 to metal tube 19 and the other at 32 to the wall of chamber 2, that is to say to a part at the reference potential R.

Finally, in order to facilitate extraction of the ions from bulb 22, we may provide an extraction electrode 4 connected, through a conduit 33 passing through the wall of chamber '2 by means of an insulating member 4 to a terminal 34 of source 11 so that its potential ranges between that of the wall of chamber 2 and that of the accelerating electrodes 5, 6, 7, 8, being preferably closer to that of the wall of chamber 2.

The operation of the device above described to produce and to accelerate positive ions (the polarities of sources 30 and 11 being as indicated by FIG. 1) is as follows.

Concerning first the ion source, there is produced through tubes 18 and 19 and bulb 22 a small but continuous stream of the gas or vapor to be ionized. The high voltage of source 30 charges condenser 28 through resistor 29 until the voltage across the terminals of spark-gap 27 is higher than the breakdown potential corresponding to the distance between the electrodes thereof. Condenser 28 then discharges into coil 26 within a very short time in the form of a current pulse of high intensity, thus producing in bulb 22 an intensive electromagnetic field which ionizes and heats the gas flowing through bulb 22. This ionized gas expands and therefore tends to escape very quickly through the outlet 23, the throttled portion 20 of tube 19 preventing the gas from flowing back toward flexible tube 18. A column of plasma (strongly ionized gas) is therefore formed between tube 19 and ring 24. As this plasma is equivalent to an electric conductor branched across the terminals of the high voltage source between 24 and 31 (both ends of coil 26 being connected to tube 29 and 31 and to the wall of chamber 2 at 32), this plasma is subjected, according to the law of Lenz, to an electromagnetic force which facilitates the outflow of plasma through nozzle 23 while imparting thereto an important acceleration in the direction of arrow F.

The extraction electrode 4, which is at a negative potential with respect to the ions and electrons of the plasma thus produced, further increases the force with which the positive ions are ejected from bulb 22, while rejecting the electrons toward the inside of the bulb.

As for the accelerator proper, which constitutes the subject matter of this invention, the positive ions which are substantially at the reference potential R are accelerated, after they have been extracted from bulb 22, toward electrode the potential of which is strongly negative. When they get close to electrode 5, there is produced a phenomenon analogous to a variation of capacity, the bunch of positive ions and the negative electrode 5 at very high voltage constituting two elements behaving like the plates of a capacitor. Considering the circuit formed by the high voltage source 11, resistor 13 and the capacitor-like structure thus defined, any variation of the value of the capacity of this capacitor-like structure involves a variation of the characteristics of said circuit so that, when the ions come close to electrode 5, the charges flow through resistor 13, which connects electrode 5 to source 11. Consequently, the potential of electrode 5 decreases in absolute value.

When the bunch of positive ions strikes electrode 5, it causes a complete discharge of said electrode. Electrode 5 remains discharged until the whole bunch of positive ions has passed therethrough. Therefore, at this time, there is produced between this electrode 5 and the next electrode 6, a potential diflerence equal to the high voltage of source 11. This diflference of potential imparts to the bunch of ions a new acceleration. Considering a series of successive accelerating electrodes such as 5, 6, 7, 8, it will be understood that the total acceleration imparted to the positive ions is substantially equal to the product of the voltage applied to each electrode, as all electrodes successively traversed by one bunch of positive ions remain discharged until the head of said bunch has reached electrode 8 by the number of electrodes.

Furthermore, the creation of a potential difference between the electrode struck by the bunch of ions and the next electrode being wholly independent of any external synchronizing means, since it is produced just when the ions strike the electrode, the device according to the present invention is self-regulating.

Of course, when it is desired to produce and to accelerate negative ions, the polarities of the voltage sources 11 and 30 are reversed but the operation remains unchanged.

In the case of linear accelerators of the type exemplified in the drawings, it may be advantageous in some cases to dispose the successive accelerating electrodes 5, 6, 7, 8, at increasing distances from one another in the direction of diaphragm 9 so that the time of transit, or time of flight, of the ions between two accelerating electrodes is substantially constant, account being taken of the increase of velocity of the ions due to their successive accelerations by electrodes 5, 6, 7, 8.

By way of example, we will indicate the following magnitudes for the elements of the device shown by FIG. 1:

(I) Pulsating Ion Source Flow rate of gas G: 1 cm. per minute at atmospheric pressure, the gas being hydrogen or deuterium to obtain protons or deuterons respectively,

Volume of bulb 22: cm.

Source 30: 10,000 volts,

Condenser 28: 0.2 microfarad,

Coil 26: 10 microhenrys,

Frequency of the discharges of condenser 28: 50 discharges per second,

Duration of a discharge impulse: about 1 microsecond,

Energy of every impulse: about 10 joules,

Instantaneous power of a discharge: about 10 megawatts,

Ionic intensity at the output of source S: 10 amperes,

Difference of potential between 17 and 34: 20,000 volts.

(11) Linear Accelerator Proper Length of the accelerator: 1 meter,

Pressure in chamber 2: 10 mm. of mercury,

Number of accelerating electrodes: 10,

Nature of the accelerating electrodes: gride g constituted by nickel wires 1 occupying about 10% of the area of every grid,

Source 11: 100,000 volts,

Resistors 13 to 16: 1 megohm,

Peak current flowing through resistors 13 to 16: about 100 ma.,

Loss of ionic intensity for the whole of the ten accelerating electrodes: 1 ampere,

Acceleration obtained: it corresponds to an electrostatic potential of about 1 million volts in a conventional electrostatic accelerator of the Van de Graatf type.

We thus obtain an ion accelerator which, making use of a source of relatively moderate value (100,000 volts), permits of obtaining ions of very high energy (corresponding to an acceleration normally achieved by a difference of potential of about 1 million volts), this accelerator having a self-regulating operation and being therefore capable of accelerating without any modification of structure ions of different nature and charge, whether these ions are positive or negative.

In a general manner, while we have, in the above description, disclosed what we deem to be practical and efiicient embodiments of our invention, it should be Well understood that we do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the accompanying claims.

For instance, the invention might be applied in the case of an accelerator of circular type including a series of electrodes such as 5, 6, 7, 8, placed in an evacuated chamber in the form of a ring of rectangular cross-section of relatively low axial height (as in the case of cyclotrons) or of toroidal shape (as in the case of synchrotrons); means being provided to form a magnetic field parallel to the axis of revolution of the ring or of the tore so as to curve the path of travel of the ions respectively in spiral or circular shape. Of course, when it is desired to keep the ions on circular trajectories, it is necessary gradually to increase the intensity of the magnetic field as in the case of synchrotrons.

In such circular accelerators, the ions would travel a great number of times through the evacuated chamber between the ion source and the target, every electrode 5, 6, 7, 8, acting several times as accelerating electrodes in the manner above described.

What we claim is:

1. An ion accelerator, for accelerating bunches of ions supplied at a reference potential by a high intensity pulsating ion source, this accelerator comprising, in combination, an evacuated tubular chamber having one portion thereof connected with the outlet of said ion source so as to receive ions therefrom, a series of electrodes partly permeable to ions, said electrodes being disposed behind one another in said chamber transversely to the axis thereof, a high direct voltage source having a first terminal at said reference potential and a second terminal, a series of resistors of substantially identical high resistance and of negligible inductance having each one end connected to one of said electrodes, respectively, and conductor means for connecting said second terminal of said source to all the other ends of said resistors.

2. An ion accelerator according to claim 1 in which the voltage of said source averages some tens of kilovolts and each of said resistors has a resistance of an order of magnitude of 1 megohm.

3. An ion accelerator according to claim 1 further comprising an extraction electrode located between said portion of the tubular chamber and that of said electrodes which is nearest thereto, and a conductor for bringing said extraction electrode to a potential ranging between those of said first and said second terminal of said high voltage source and closer to the potential of said first terminal than to the potential of said second terminal.

4. An ion accelerator according to claim 3 in which said extraction electrode is connected to an intermediate tap of said high voltage source.

5. An ion accelerator according to claim 1 further characterized in that said partly permeable electrodes are grids.

6. An ion accelerator according to claim 1 in which said partly permeable electrodes are plates provided with a central hole.

7. An ion accelerator according to claim 1 in which said evacuated tubular chamber is rectilinear, said portion of said chamber being located at one of the ends thereof.

8. An ion accelerator according to claim 7 in which the distance between two successive electrodes increases in the direction of the other end of said tubular chamber.

9. An ion accelerator according to claim 1, in which said tubular chamber is made of an electricity conducting material and said chamber is connected to ground and to said first terminal.

10. An ion accelerator, for accelerating succesive copious bunches of ions supplied at a reference potential at the outlet of a pulsating ion source, comprising in combination: an evacuated tubular rectilinear chamber constituted by a cylinder having a length of about one meter and being located around a geometrical axis, said chamber having one end thereof connected to the outlet of said source so as to receive therefrom successive copious bunches of ions; a potential source of about hundred thousand volts having a first terminal maintained at said reference potential and a second terminal; a series of substantially identical electrodes partly permeable to said ions, the number of said electrodes being of the order of ten and said electrodes being disposed behind one another in said chamber transversely to said geometrical axis of said cylinder; a series of resistors, the number of resistors being equal to the number of electrodes and each of said resistors having a resistance of about one megohm and a negligible inductance, each of said resistors including a first end connected to one of said electrodes respectively and a second end; and conductor means of negligible impedance connecting said second end of each of said resistors to the second terminal of said potential source.

11. An ion accelerator according to claim 10, in which each of said electrodes is a grid.

12. An ion accelerator according to claim 10, in which each of said electrodes is a plate pierced by a central hole, the holes of said plate being disposed along said geometrical axis of said cylinder.

13. An ion accelerator according to claim 10, in which said cylinder is made out of an electricity-conducting material and is connected to said first terminal of said potential source and to ground.

14. An ion accelerator according to claim 10, further comprising an extraction electrode located around said geometrical axis between said end of said chamber and that one of said series of partly permeable electrodes which is nearest thereto, and a conductor connecting said extraction electrode to an intermediate tape of said potential source located between said first terminal maintained at said reference potential and said second terminal thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,261,569 Schutze Nov. 4, 1941 2,611,878 Coleman Sept. 23, 1952 2,688,088 Berry et al Aug. 31, 1954 2,770,755 Good Nov. 13, 1956 2,845,571 Kazan July 29, 1958 2,880,356 Charles et a1. Mar. 31, 1959

Claims

1. AN ION ACCELERATOR, FOR ACCELERATING BUNCHES OF IONS SUPPLIED AT A REFERENCE POTENTIAL BY A HIGH INTENSITY PULSATING ION SOURCE, THIS ACCELERATOR COMPRISING, IN COMBINATION, AN EVACUATED TUBULAR CHAMBER HAVING ONE PORTION THEREOF CONNECTED WITH THE OUTLET OF SAID ION SOURCE SO AS TO RECEIVE IONS THEREFORM, A SERIES OF ELECTRODES PARTLY PERMEABLE TO IONS, SAID ELECTRODES BEING DISPOSED BEHIND ONE ANOTHER IN SAID CHAMBER TRANSVERSELY TO THE AXIS THEREOF, A HIGH DIRECT VOLTAGE SOURCE HAVING A FIRST TERMINAL AT SAID REFERENCE POTENTIAL AND A SECOND TERMINAL, A SERIES OF RESISTORS OF SUBSTANTIALLY IDENTICAL HIGH RESISTANCE AND OF NEGLIGIBLE INDUCTANCE HAVING EACH ONE END CONNECTED TO ONE OF SAID ELECTRODES, RESPECTIVELY, AND CONDUCTOR MEANS FOR CONNECTING SAID SECOND TERMINAL OF SAID SOURCE TO ALL THE OTHER ENDS OF SAID RESISTORS.