Plasma generator

Aug. 18, 1959

W. H. BOSTICK ET AL

2,900,548

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PLASMA GENERATOR

Filed June 6, 1956

D.C. °---VWW

POWER

SUPPLY

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TRIGGER

PULSE

GENERATOR

OCCLUDED GAS ELECTRODES

14^ // ·

OCCLUDED GAS ELECTRODES

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INVENTORS.

WINSTON H. BOSTICK _BV VIRGIL G. MCINTOSH oi

ATTORNEY.

United States Patent Office

2,900,548

Patented Aug. 18, 1959

2,900,548

PLASMA GENERATOR

Winston H. Bostick, Danville, and Virgil G. McIntosh, Livermore, Calif., assignors to the United States of America as represented by the United States Atomic Energy Commisison

Application June 6,1956, Serial No. 589,831

Claims. (Cl. 313—131)

The present invention relates to method and means for producing a plasma and in particular to the production of a shaped plasma.

At the outset, it is noted that the term plasma, as herein employed, is taken to define a space charge neutralized ionized gas. Within a plasma there are disposed free positively charged ions and a potential balancing number of free electrons with the physical plasma features resembling a conventional arc.

In an ion source art it is common to generate a plasma from which ions may be extracted and, likewise, in other arts plasmas are employed. It has been found in ion sources employing plasmas that the efficiency as well as the total possible output and various other factors are in. large part dependent upon the plasma itself and, in this respect, the plasma density and density gradients as well as plasma geometry are of importance. With regard to the properties with which it is desired to provide a plasma, many ion source criteria are applicable. Thus for pulsed operation it is advantageous to produce a plasma that is capable of accurate timing as to initiation and period, as well as having a readily reproducible size and configuration. As regards the means for plasma generation, it is desired for same to have a low power input for a given output, minimum size and weight, simplicity of construction, and extended period of reliable operation without such as repair or replenishment.

The present invention provides an improved plasma generator having the above-noted qualities and in general comprises a pair of occluded gas electrodes between which an arc is struck for producing a plasma and so oriented as to establish a magnetic field advantageously shaping the resultant plasma.

It is an object of the present invention to provide a plasma generator producing a magnetically shaped plasma of advantageous configuration.

It is another object of the present invention to provide a plasma generator of high output and small size.

It is a further object of the present invention to provide a plasma generator producing a shaping magnetic field with the plasma generating current.

. It is yet another object of the present invention to provide an improved compact occluded-gas plasma generator.

Numerous other advantages and possible objects of the present invention will become apparent to those skilled in the art from the following description taken together with the attached drawing, wherein:

Figure 1 is a longitudinal sectional view of a preferred embodiment of the invention together with an associated circuit for energization thereof;

Figure 2 is a transverse sectional view taken at 2—2 of Fig. 1; and

Figure 3 is a schematic illustration of the establishment of a shaping magnetic field by the plasma generating current.

The invention generally comprises a pair of metal electrodes loaded with a gas to be ionized, as by occlusion therein, and spaced by insulation. These electrodes are adapted for pulsed energization to produce an arc discharge therebetween whereby the gas therein is released therefrom and ionized to form a plasma. The electrodes 5 are disposed out of axial alignment so that the arc current path therebetween is inclined with respect to the current path through the electrodes for establishing a magnetic field that expands the plasma and shapes same.

Considering now a preferred embodiment of the inven10 tion in some detail and referring to Figs. 1 and 2 of the drawings, there will be seen to be provided a solid insulating plate or disc 11, formed for example of ceramic, and having a pair of metal electrodes 12 and 13 extending transversely therethrough. These electrodes 12 and 13 15 terminate flush with the outer face 14 of the insulator 11 and extend rearward beyond the insulator for connection to a pair of spaced conductors 16 and 17 as by connectors 18 and 19, respectively. The electrodes 12 and 13 preferably fit snugly through the insulator and are loaded 20 with a gas to be ionized. This gas loading is best accomplished with certain metals having an affinity for gas and thus the electrodes may be formed of titanium or another of the transition metals with gas loading being accomplished in a conventional manner.

Inasmuch as a relatively large potential is to be impressed between the electrodes 12 and 13 in order to establish an arc therebetween, it is necessary to provide adequate insulation between the electrode conductors and to this end a sheet insulator 21 may be disposed there30 between. This sheet insulation 21 preferably extends past the far ends of the conductors or at least past a point Where same are bent outwardly away from each other, as shown. The unit may be neatly packaged for ease of handling and maximization of insulation by the provision 35 of an enveloping insulation in the form of a cylinder 22 affixed to the back side of the insulating disc 11 about the conductors 16 and 17 and insulating sheet 21. This cylinder 22 may be formed from a liquid material poured about the aforementioned elements and solidified in place 40 so as to. seal all of the elements into a solid block.

. Energization of the generator may be accomplished with a conventional pulsed circuit such as that shown in Fig. 1. The circuit includes a direct current power supply 31 with a resistor 32 coupling the positive terminal thereof to the anode of a gaseous discharge tube 33. The negative power supply terminal is connected to one of the generator conductors 16 as by a ground line, while the other generator conductor 17 is connected to the cathode of the tube 33. A capacitor 34 is connected across the tube and generator from the tube anode to conductor 16, and for controlling operation of the discharge tube 33 there is provided a trigger pulse generator 36 connected to the tube control electrode. It will be appreciated that with the power supply 31 energized, the capacitor 34 will _eg be charged thereby and, that upon receipt by the tube 33 of a trigger pulse from the pulse generator 36, the tube conducts to connect the capacitor across the plasma generator conductors 16 and 17.

With regard to operation of the plasma generator, the ₆θ application of a high voltage pulse across the conductors 16 and 17, as above noted, produces a large potential difference between the electrodes 12 and 13 at the insulator face 14. This potential is of sufficient magnitude to establish a discharge between the electrodes 12 and 13 _6g across the face 14 of the insulator 11. Note at this point that the generator is adapted for disposition in an evacuated chamber so that the discharge occurs between the electrodes in a high vacuum. Establishment of a discharge between the electrodes releases gas occluded or _7Q otherwise stored in the electrodes and as this gas leaves the electrodes it is ionized to produce an arc discharge. The ionized gas includes electrons and positive ions which

2,800,548 form a space-charge neutralized discharge or plasma 37. As the discharge occurs in a high vacuum substantially all of the plasma is formed of the gas released from the electrodes so that a “pure” plasma is produced without Contamination with ions of ambient gas. As substantially 'all gas released is ionized a very dense plasma is produced with only a minute quantity of gas, and further no increase in ambient pressure results from unionized gas.

The plasma produced is shaped as shown in Fig. 1 by the action of self-generated magnetic fields and in this respect note Fig. 3. A discharge current flowing, for example, down electrode 13 and up electrode 12 produces magnetic lines of force about these conductors as indicated. This current also flows across the electrode gap through the plasma as a so-called arc current, so that similar lines of force are produced about the discharge, as shown. It will be noted that as the current flow bends in the discharge area from a downward direction in electrode 13 to an upward direction in electrode 12, the lines of force are closer together above the discharge than below. The magnetic field strength above the discharge is thus stronger than below the discharge and elementary magnetic field theory shows that there is thus produced a net force F downward on the discharge. This force F urges the discharge downward, i.e., expands the plasma axially of the generator into a configuration such as that shown in Fig. 1. There is thus produced an advantageous plasma configuration which, for example, is desirable for ion extraction.

In order to produce an unbalanced self-induced magnetic field for deforming the plasma it is necessary that the plasma has other than a straight axis, i.e., that the discharge between electrodes lies on other than a straight line. The amount and disposition of the discharge curvature determine the ultimate plasma configuration and ³⁵variations therein are possible. In the illustrated embodiment there is shown a portion of an insulator interposed between the electrodes to prevent the discharge from passing directly therebetween so that the original discharge is curved. With the original discharge curved in space, ⁴⁰it will induce an unbalanced magnetic field to deform the resultant plasma as set forth above. It will be appreciated that various geometries are possible to provide a desired plasma configuration.

The plasma generator of the present invention is ⁴⁵adapted for pulsed operation with each pulse extending for a period determined by the charge on the capacitor 34 and the pulses being initiated by the triggering of the discharge tube 33 from the pulse generator 36. As sub- „ stantially all of the gas released from the electrodes 12 ⁶⁰and 13 is ionized during each discharge, a very strong plasma is obtained with a minimum gas expenditure so that sufficient gas supply lies in the electrodes for long term pulsed operation. Additionally the generator is quite compact and may be made very small particularly in view of the plasma densities produced.

While the invention has been disclosed with respect to a single preferred embodiment, it will be apparent to those skilled in the art that numerous Variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.