High Current Plasma Source

Abstract

A plasma source in which a plurality of sturdy filaments are distributed around the perimeter of a truncated cylindrical discharge chamber. A small area anode is situated axially at one end of the chamber with an accel-decel extraction grid system at the other end. A high current electron discharge pulse is established between the filaments and anode and a puff of gas is directed past the anode into the discharge chamber to be ionized to form an arc plasma from which a high current beam is extracted and directed by the extractor grids. The output contains a high proportion of energetic neutrals suitable for use in fueling a fusion reactor or other purpose.

Description

BACKGROUND OF THE INVENTION

This invention was conceived or made in the course of Contract No. W-7405-ENG-48 with the United States Atomic Energy Commission.

In nuclear fusion reactor practice certain modes of operation require the use of high current energetic neutral particle sources. For example, sources capable of delivering energetic particle current densities of the order of 1 ampere/cm.sup.2 at energies of up to 20 keV or more with a beam current which is relatively uniform over a large extraction area may be required. Moreover certain reactors such as the 2XII experiment require beam pulses lengths of the order of 0.03 to 0.1 seconds (c.f. F. H. Coensgen et al., "Plasma Containment" in the LRL 2XII Experiment, IAEA/CN/612 paper, Madison, Wis., June, 1971). Since ion extraction optics are quite sensitive to plasma density, little variation in beam intensity can be tolerated in order that the beam be usable in such reactors. Therefore, sources not subject to fluctuations or noise of the type and frequency encountered with most conventional sources are needed.

SUMMARY OF THE INVENTION

This invention relates, generally, to devices for producing beams of energetic particles and, more particularly, to a plasma source for producing a uniformly distributed high current beam of energetic particles.

A plasma source in accordance with the invention includes a vessel defining a discharge chamberhaving a truncated cylindrical configuration and an axial port opening in one endwall thereof. A plurality of refractory metal filament wires are arranged at spaced locations about the perimeter of the discharged chamber generally parallel to the cylindrical wall portion of said vessel. Preferably, such filaments are of a heavy generally hairpin construction self supported from electrical connectors or insulated seals secured to said ported endwall and arranged to pass a heavy pulse heating current through said filaments to provide copious electron emission therefrom. An anode body is disposed coaxially within said port with a relatively small surface area proximate the endwall surface level exposed within said chamber. Such anode is supported in insulated relation to said endwall and is arranged for application of a positive electrical potential so as to cause said copious emission of electrons from the filament creating an electrical discharge filling at least the portions of the discharge chamber enclosed by said filaments. A conduit means is arranged to direct gas, released in a relatively large volume by a puff valve from an exterior source, about the periphery of said anode to enter the electron discharge. The gas is ionized and heated by interaction with the electrons forming a dense plasma body filling the discharge chamber. Extractor electrode means arranged outwardly from the second end of the discharge chamber are energized with a relatively high voltage net potential thereupon extracting a high current of positive ions from the discharge plasma. The ion current has a uniform distribution across the extraction area and emerges, e.g., in an extraction channel, as an ion beam with a very low angle of divergence. Moreover, the ion beam which isproduced contains a high proportion of energetic neutral particles due to the high gas density existing in the extractor channel and thereby being highly suitable for injection into the magnetic containment field of a controlled fusion reactor to provide a high density temperature reaction plasma therein. Accessory equipment including pulsed filament, anode and extractor power supply means is also provided for operating the plasma source.

Accordingly, it is an object of the invention to provide a high beam current ion or plasma source in which the beam current is relatively uniform over a large area.

Another object of the invention is to provide a high current energetic particle source adapted for use in establishing a plasma of high density and elevated temperature in a controlled fusion reactor.

Still another object of the invention is to provide a plasma source in which the beam has minimal modulation noise components.

Other objects and advantageous features of the invention will be apparent in the following description and accompanying drawing of which the single FIGURE is a longitudinal sectional view of the ion or plasma source of the invention.

DETAILED DESCRIPTION OF THE INVENTION

As illustrated in the single FIGURE of the drawing, the ion source of the invention comprises a generally cylindrical vessel 11 defining a truncated cylindrical arc discharge chamber 12. Vessel 11 includes a cylindrical tubular wall portion 13 with an endwall portion 14 centrally apertured to receive anode and gas inlet fixtures and secured to one end, i.e., the anode end thereof. More specifically, wall portion 13 may be cut-away to provide an enlarged shoulder portion 16 to receive endwall 14 with flange portion 17 thereof bearing upon insulating gasket seal 18 which in turn bears upon the end of wall portion 13. An O-ring sealing member may also be similarly disposed between flange 17 and said end of wall 13.

A plurality of hairpin type electron-emitter filaments 21 of tungsten or other refractory metal are arranged in spaced relation somewhat inwardly of and parallel to vessel wall 13, i.e., circumferentially in the outer perimeter region of chamber 12. It has been found efficacious that no portion of the means for supporting and supplying electrical current to such filaments project within chamber 12. A convenient arrangement for supporting filaments 21 is obtained by providing counterbores 22 in endwall portion 14 with apertures 23 leading therefrom into chamber 12 at spaced locations appropriate to accommodate the respective ends of filaments 21. Cylindrical copper slug connectors, bored and crimped at one end to receive and secure the ends of filaments 21 are disposed in spaced relation within such counterbores 22. Relatively shorter slug connectors 24 (one only shown) are secured to one respective end of filaments 21 and the second ends of the slugs 24 are secured to an intermediate annular copper plate conductor 27 separated from endwall portion 14 by gasket 28 and O-ring seal members 29. The other respective ends of filaments 21 similarly secured and supported at one end of relatively longer slug connectors 31 (one only shown) which connectors 31 project in spaced relation through bores 32 matching with alternateones of counterbores 22 and are sedured at the other end to an outermost annular plate conductor 33. Plate conductor 33 may be insulated from plate conductor 27 with an insulating gasket 34 disposed therebetween. With the foregoing arrangement the filaments are connected in parallel across plate conductors 27 and 33 which provide a low resistance high current electrical connection thereto. Machine screws 37 which pass through matching bores (not shown) in plate conductors 27, 33 and flange 17 of endwall portion 14 to engage threaded holes in wall portion 13 may be used to secure such members when assembled as described. Moreover, an insulating sleeve (not shown) about the shanks of screws 37 and an insulating washer 38 beneath the head of the screws may be used to preserve the insulated relation therebetween or, alternatively, non-conducting plastic screws may be used.

Gas inlet and anode structure may be supported by means of an annular plate 39 disposed in the central aperture of endwall portion 14 as by engagement of outwardly projecting flange portion 41, thereof, with inwardly projecting lip portion 42 of endwall portion 14. An insulating gasket 43 or plastic (Nylon) screws (not shown) may be used to secure such members in insulated sealed relation. A tubular conduit section 44 may be secured as by brazing to the periphery of the central aperture in plate 39 and extending outwardly therefrom. A cup-shaped hollow anode 46, e.g., of copper is disposed in coaxial relation in conduit section 44 and may be supported by an insulating seal means 47 situated between the outer ends of the conduit and anode. While a cup-shaped anode is shown, and is preferred for maximum output, it may be noted that a water cooled-cylindrical or circular plate anode mounted similarly may be used and often yields a more uniformly distributed output. The inward end surface of such an anode may terminate about flush with the inner surfaces of the endwall portions. An insulating sleeve such as a silica sleeve 48 is disposed in loose fitting relation in the space between the conduit section 44 and the anode 48 to minimize arcing therebetween. With button and disc type anodes a conduit 49a may communicate with the space between the conduit and anode for directing a puff of an appropriate gas to flow therethrough and emerge about the periphery of the anode. However, with a hollow anode 46, it is preferred that a conduit 49b be used instead to direct the gas through the central hollow portion thereof. The puffer stream of gas may be supplied from a gas source reservoir 51 through a needle or similar valve 52 as controlled by a solenoid valve 53. Gases such as hydrogen, deuterium, deuterium-tritium mixtures may be used as required in fusion reactor practice.

A low voltage D. C. power supply means 54, providing rectified and filtered current, may be connected, as by means of conductors 56, 57 to plate onductors 27, 33 to supply energizing current to filaments 21. A pulse power supply 56 with the negative terminal connected to the negative terminal of the filament supply as well as to conductor 57 and with the positive terminal connected through conductor 58 to a terminal post 59 attached to anode 46, may be used to establish the electron arc discharge in chamber 12. High amperage at a few hundred volts, e.g., 300 volts usually suffices for such purpose. To minimize the tendency of the gas to pass through the chamber with too short a transit time, a deflector plate or hat 61 may be positioned outwardly from anode 46 as by means of a rod 62 attached thereto.

At the open end of the discharge chamber 12 an annular plate member 63 is provided as a support or mounting means. Member 63 may be mounted by means of a flange 64 on wall portion 13 through which screws 66 pass but which are insulated therefrom by a washer 67 and sleeve 68. The screws 66 engage threaded portions of plate member 63 while an insulating gasket seal 69 is disposed between flange 64, the end of wall portion 13 and a recessed portion of member 63. The central opening therein defines an ion extraction port.

As employed for injecting energetic particles into a fusion reactor the foregoing arrangement is provided with an extractor arranged outwardly of the open end of the discharge chamber. To accommodate a conventional accel-decel extractor arrangement, an extension of the vessel 11 may be provided as a cylindrical tubular section 71 having at one end a flange 72 attached in sealed insulated relation to flange as by means of non-conductor machine screws 73. A flange 74 at the opposite end may be used to attach the plasma source to an injector port of a controlled fusion reactor, for example, the 2XII, Baseball II or any other appropriate reactor. When so connected the source is evacuated by the vacuum pumping equipment of the fusion reactor. Insulators 74 mounted on the inner walls of tubular 71 may serve to support the grid elements of the accel-decel system which grids may comprise plates provided with matching slots or apertures usually circular. More specifically, a first accel-grid 76 is disposed transversely across the extraction channel which extends through section 71 facing the plasma in discharge chamber 12 while a decel grid 77 is disposed thereafter in spaced-parallel, matching-orifice relation. A third grid 78 is disposed in similar relation to grid 77 at the outlet of section 71. A particularly effective accel-decel grid geometry is disclosed in Report UCRL-72880 referenced in Nuclear Science Abstracts 25 38451, Aug. 31, 1971. Electrical potentials for energizing the extractor may be supplied from a pulsed D.C. power supply 71. Power supply 71 delivers a potential, V.sub.1, through a conductor 81, through a sealed feedthrough insulator (not shown) to grid 76. Similarly, a second potential, V.sub.2, is delivered through conductor 82, to grid 77 and a third potential, V.sub.3, to grid 78. Potential, V.sub.1, is generally a high positive potential, e.g., 22 kilovolts and, V.sub.2, a negative potential, e.g., -2 kilovolts (may be varied, e.g., 1 to 4 kilovolts) to repel electrons from the extracted beam. Potential, V.sub.3, may then be 20 kilovolts which yields a beam of 20 kilovolt particles.

In typical operating cycle a timing sequence may comprise turning filaments power on at -4 seconds, i.e., for a few seconds adequate to reach emission temperature; pulsing gas at -20 milliseconds; applying arc power at t=o; and turning arc power off at t=+15 milliseconds (or other period as desired).

With such a pulsing technique overheating of the components is minimized despite the large input energies which are used. Cooling water coils (not shown) may be attached as by brazing to various of the external surfaces of the various vessel components and exposed end of the anode to cool the source more rapidly. Insulating varnish may be applied to interior vessel surfaces or insulation such as mica used to minimize undesired arcing. Also, a solenoid, circumjacent vessel section 13 and capable of generating a low intensity magnetic field, e.g., up to 20 Gauss in chamber 12 may assist in normalizing arc current across extraction port.

In the ion or plasma source using the described geometry including a small anode a relatively high electron current ionising efficiency is obtained so that relatively large gas flows can be ionized, e.g., to provide ion currents of at least about 1A/cm.sup.2 at the extraction port. For example, a 20A beam may correspond to a gas flow rate of about 300 cm.sup.3 /min (STP) or 3,600 .mu.1/sec. The ion current has a uniformity within about 15 percent to as low as 5 percent of the mean as measured by means of a traversing probe (not shown) and with a tungsten screen disposed across the extraction port to simulate the extractor.

It may be noted that due to the high gas density employed in the system a very substantial rate of charge exchange occurs. This effect also occurs in the channels which communicate between the plasma source and interior of the reactor chamber. As a consequence the energetic particle beam which emerges may comprise at least about 50 percent energetic neutrals, e.g., D, which with an effective extraction potential of 20 Kv has an energy of 20 Kev. Such particles are of sufficient energy to produce a substantial fusion rate when trapped to produce a plasma of adequate density. The efficiency of charge exchange neutralization can therefor be increased by providing a channel of adequate length or augmented by using a conventional gaseous charge exchange cell at the output of the source.

With the 2XII experimental device mentioned above the present source would be mounted to direct the neutral particle beam transversely through the containment zone just after the normal plasma has been trapped to augment the plasma density therein (c.f. Application Ser. No. 53,056(70) of F. H. Coensgen et al., filed July 8, 1970).

EXAMPLE

Arc discharge chamber diameter 9 inches Arch discharge chamber length 2 inches Filament perimeter diameter 81/4 inches Filament length in chamber 13/4 inches Filaments 0.020 inch thickness tungsten wire hairpin 26 only Filament current 25A each (approx 1800 amp.) Arc/current (35-60 volts) Approx 600-1200 amp. Pulse length 5-100 millisec. Anode diameter O.D. circular plate 5 inch. Accel potential (V.sub.1) +22 Kv Decel potential (V.sub.2) (-1 to -4Kv) -2 Kv (Net accel potential) (V.sub.3) +20 Kv Ion Current 0.5-1am/cm.sup.2

While there has been described in the foregoing what may be considered to be preferred embodiments of the invention, modifications may be made therein without departing from the teachings of the invention and it is intended to cover all such as fall within the scope of the appended claims.

Claims

What we claim is:

1. In a high current plasma or ion source, the combination comprising:

vessel means defining a generally cylindrical arc discharge chamber and having a centrally apertured wall at one end together with a mounting plate means at the other end defining a port for extraction of ions;

electron emitter means including a plurality of refractory metal filaments extending longitudinally in the outer perimeter area of said chamber;

anode body means disposed in spaced insulated relation within the aperture of said wall at one end of said vessel means;

gas source reservoir means including a conduit arranged to direct a quantity of a selected gas about a perimeter of said anode body into said discharge chamber; and

first power supply means for energizing said metal filaments to cause emission of electrons therefrom and second power supply means for applying a positive potential to said anode relative to said filaments. thereby creating an arc discharge through the gas directed into said chamber so as to create a plasma therein.

2. A high current plasma or ion source as defined in claim 1 wherein a vessel extension is attached to said mounting plate means and ion extractor means are provided, said ion extractor means comprising at least one apertured grid member disposed transversely proximate said ion extractor port.

3. A high current plasma or ion source as defined in claim 2 wherein said ion extractor means comprises accel-decel grid electrodes disposed in said vessel extension and power supply means for energizing said grids so that a beam of energetic particles is extracted from said plasma and is directed through the channel defined by said vessel extension.

4. A source as defined in claim 3 wherein said filaments comprise hairpin shaped wire filaments and the ends of said filaments are supported in insulated sealed relation at said one vessel endwall, said filaments being connected in a parallel circuit.

5. A source as defined in claim 4 wherein said anode body comprises a circular plate disposed in coaxial spaced relation within the aperture of aid one vessel endwall.

6. A source as defined in claim 4 wherein said anode body comprises a hollow cylinder disposed coaxially within the aperture of said one vessel endwall

7. A source as defined in claim 4 wherein said vessel extension is provided with mounting means at the ion beams outlet end for attaching said source to an injector port of a controlled thermonuclear fusion reactor.

8. A source as defined in claim 7 wherein said gas source reservoir means supplies a gas selected from the group consisting of hydrogen, deuterium and tritium and mixtures thereof.