US2901628A

US2901628A - Ion source

Info

Publication number: US2901628A
Application number: US479255A
Authority: US
Inventors: William A S Lamb
Original assignee: Individual
Current assignee: Individual
Priority date: 1954-12-31
Filing date: 1954-12-31
Publication date: 1959-08-25
Anticipated expiration: 1976-08-25

Description

W. A. S. LAMB Aug. 25, 1959 ION SOUR'CE AMPLIFIER Filed Dec. 31, 1954 INVENTOR.

WILLIAM A. S. LAMB ATTORNEY.

ION SOURCE William A. S. Lamb, Berkeley, Calif., assignor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Application December 31, 1954, Serial No. 479,255

4 Claims. (Cl. 250-495) The present invention relates to an improved ion source wherein the ion extracting fields do not interfere with the fields employed in the mechanism of ion production and a simplified and readily controllable pulsed ion extraction is provided.

One of the greatest difficulties in the production of an ion beam for use in apparatus, such as particle accelerators for example, is the extraction of ions from the ion generating means. One difficulty lies in the extraction of a sufiicient number of ions from the generator, even though an adequate number are generated. Additional difficulties lie in the formation of an ion beam that can maintain a very high intensity without blowing up and in controlling ion extraction to produce an ion beam having desired current variations.

The ion source of the present invention overcomes the above-noted difiiculties. The source configuration and ion extraction pattern is such that a hollow ion beam is produced wherein space charge effects normally limiting beam intensity are materially reduced. Ion extraction is accomplished by the establishment of fields which do not counteract or disturb the ion generating fields so that ion generation is substantially independent of extraction and suffers no interference therefrom. Also, the ion extraction means operates upon a transformer principle with high frequency feedback control, whereby ready and accurate extraction control provides desired ion beam current variations.

It is an object of the present invention to provide an improved ion source having a high frequency controlled magnetic field ion extraction means providing phased ion extraction.

It is another object of the present invention to provide an ion generator having a radially extending ion sheath with means establishing an ion extracting magnetic field acting thereon and controlled by high frequency feedback signals from ion beam utilization apparatus associated with the source.

It is a further object of the present invention to provide, in conjunction with an ion source and an accelerator, an ion extracting mechanism including magnetic field producing means for ion extraction and controlled by accelerator voltages.

Numerous other advantages and possible objects of the invention will become apparent to those skilled in the art from the following description of a single preferred embodiment of the invention taken together with the accompanying drawing wherein:

Figure l is a central sectional view of the ion source in schematic representation with relative potentials of elements indicated by connection to batteries; and

Figure 2 is a partial sectional view taken at 22 of Figure 1.

Considering now the details of the illustrated embodiment of the present invention and referring to the drawings, there will be seen to be provided magnet structure including a toroidal pole piece 11 and a central cylindrical pole piece 12 with same being coaxially disposed to ice define a toroidal space therebetween. These pole pieces comprise the ends of magnet cores adapted for energization as by electrical windings to establish a magnetic field therebetween with the central pole piece consisting, for example, a south magnetic pole and the surrounding pole piece 11 a north magnetic pole, so that a radial magnetic field is established through the toroidal space separating the pole pieces. Within this space there is disposed an annular conductor 13 about the pole piece 12 and adapted to emit electrons upon electrical energization. Extending radially outward from the vicinity of the central pole piece 12 are electrode means 14 which preferably in cludes an annular back wall 16 and a parallel annular front wall 17 which may be mounted in any suitable manner in insulated relation to the emitting conductor 13 and between the pole pieces. This electrode means comprises an anode 14 in that it is adapted to attract electrons.

With the aforesaid disposition of elements, electrons emitted from the emitting conductor or filament 13 are attracted by the anode structure 14, but are constrained from reaching same by the radial magnetic field between the pole pieces 11 and 12 in a well-known manner and the emitted electrons thus travel radially outward toward the pole piece 11. The discharge is intensified by the provision of a pair of reflectors comprising an outer annular metal strip reflector 18 disposed directly inward of the outer pole piece 11 and a second annular metal strip reflector 19 disposed between the inner pole piece 12 and the filament 13. These reflectors 18 and 19 extend longitudinally across the pole pieces about the inner and outer circumferences thereof respectively and are adapted to be maintained at a relatively negative potential whereby electrons are repelled therefrom. The anode 14 and reflecting electrodes 18 and 19 define a toroidal chamber 20 within which the discharge occurs. The electron discharge becomes oscillatory in that electrons are reflected back and forth between the two reflectors 18 and 19 within the chamber 20 and an elongated electron path is thereby produced.

Ionization is accomplished by the introduction of gas or vapor into the oscillatory electron discharge with the gas molecules being ionized by collision with electrons in the discharge. The gas or vapor to be ionized may be admitted to the discharge space by means of a slot or slots 21 through the back plate 16 of the anode 14 and about same with an annular pipe 22 communicating with the slot for carrying the gas or vapor to be ionized. An ion egress aperture 23 is provided in the form of a slot about the front plate 17 of the anode 14 with this slot, or slots, extending circumferentially thereabout near the radial center thereof.

It is to be appreciated that the embodiment of the invention herein shown and described is merely illustrative and that numerous structural modifications and variations are possible. With regard to the mounting of elements of the ion generator within the space between the magnet pole pieces for example, a hollow insulating toroid may be formed of glass or the like with suitable exterior mounting and carrying the anode, filament and reflecting electrodes in suitable spaced relation therein, or these elements may be separately mounted upon insulators which are in turn secured to exterior mounting structure. Likewise as to the energization of the above-described members, there is to be provided energizing windings for the magnet with a suitable power supply connected thereto for supplying magnet winding current and one or more power supplies are provided for establishing and maintaining the relative potentiais of the ion generator elements and the filament current. There is shown as representative of these power supplies a first battery 24 having a grounded negative terminal and connections from thepositive terminal to the front and back walls of the anode 14. A second battery 25 has a grounded positive terminal and connections from the negative terminal thereof to the filament 13and reflecting electrodes 18 and 19.- A'-filament current supply is shown as a-third battery 26 connected across leads 27" secured to ends of the filament 13which are spaced apart to provide a'heating current path so that current'therethrough' renders thefilament electron emissive.

Considering now the extraction of ions from the abovedescribed ion generator, there will be seen to be provided a pair of'concentric toroidal magnet cores 31' and 32 di'sposedcoaxially with the ion generator and lying in a plane parallel to and spaced from the plane ofthe front wall 17 of'the anode 14. The annular space between the two

toroidal magnet cores

31 and 32, which may be termed an ion accelerating space or slit 33, is aligned with the opening 23 in the front wall 17 'of the anode structure 14 so thata straight line of ion egress is provided about a median circumference of the ionizing discharge. The extracting -magnets include, in addition to the

cores

31 and 32, energizing means including a loop 34 of one or more turns of electrical conductor about the cores, as shown. This energizing loop is adapted to conduct high frequency electrical currents to produce with the cores 31 and 32' a transformer action; Electrical energization of the loop 34 is' derived from-an ion accelerator 36 with which the ion source is adapted to cooperate and this accelerator may comprise a plurality of resonant cavities in which case the ion source is adapted to inject high frequency bursts of ions into the cavity resonator of the particle accelerator for acceleration ofthe ions therein, there being shown part of a wall 37 of this accelerator 36. A pickup loop 38 is disposed within the first cavity of the accelerator 36 for receiving electrical energization in synchronism with field fluctuations in the cavity and a lead connected to the pickup loop extends through the cavity wall37 in insulated relation thereto and is connected to theinput terminal of a phase control circuit 39. The output of this phase control circuit 39.is connected to the magnet loop 34 through an amplifier 41. The pickup loop 38 receives energy from the cavity of the accelerator in phase with the cavity energization in a known manner and applies this energy in amplified form to the magnet loop 34 forestablishing a magnetic field about the

cores

31 and 32. The phase control circuit 39 operates to provide small'phase corrections as required so that the magnetic field extracting ions from the ion generator and injecting same into the accelerator shall have the proper phase relation to inject the bursts of ions at the desired accelerating field intensity within the particle accelerator 36.

Considering now in rsum the overall operation of the ion source of the present invention, energization of the filament and generator electrodes withthe establishmentof. a radial magnetic field between'the pole pieces Hand 12 produces an electron discharge from the filament 13radially outward. This discharge .is reflected by the electrode 18 radially inward and, thence, reversed in direction by the inner electrode 19 so as. to oscillate radially under the influence of the'magnetic field limitingtransverse electron travel. Introduction of a gas or vaporinto the discharge area from the tube 22 produces ionization by bombardment, whereupon the ions so formed are constrained to the general area of the discharge within the chamber 20. by the magnetic field between the pole pieces 11 and 12 to form a generally planar toroidal ion sheath. Pulsed energization of the first cavity of the particle accelerator 36, with which the ion. source is adapted to cooperate, energizes. the pickup loop-38 which in turnenergizes the magnet loop 34. Intermittent current flow through the magnet loop 34-sets up a magnetic field in the

toroidal cores

31 and 32. This, field alternately builds up and collapses with magnetic lines of force extending through the ion exit slot 33--between the cores 31 and 32'andinto the chamber 20 of the ion source. This magnetic field applies a force upon the ions and electrons in the chamber 20 causing them to precess about the annular discharge space, but does not otherwise deflect them except to provide an attractive force thereon normal to the plane of the discharge toward the-.ion exit slit 33 by virtue of the transformer action of charged particles cutting magnetic linesiof force..- Ofcourse, thegcuttingtresultsfrom thesuccessive-riseand fall of' the-magnet-ic field'and, although no conductor lies between the cores 319 and 32, the free ions are acted upon much as if they lie alonga solid conductor between; the. cores. Consequently, ions are attracted from the toroidal ion sheath through the circular slit or egress aperture 23 and, thence, through the magnet core separation or slit 33 whereby there emerges a hollow cylindrical ion beam. A feature of-this emergent ion beam is the absence of limiting space charge eflects for the beam is concentrated in a thinshellso that insulficient beam. cross section resultsfor large scale interaction of ion charges. The ion beam in hollow cylindrical form contains a larger number of ions per unit length, i.e., beam current, than is possible with conventional beams. Further, with the improved ion extraction structure, no interruption or interference with the mechanism of ion generation results during recurrent periods of ion extraction and a ready phase identity or relationship is established between ion injection and accelerating forces.

It is noted that the ion source of the present invention including both ion generator and ion extractor may be readily employed with a variety of apparatus utilizingion beams, and it is notnecessary that resonant cavities be employed in the particle. accelerator for the magnet loop may be energized in conventional manner from any source of ion accelerating potential employed in the accelerator.

What is claimed is:

1. An, improved ion source-comprising means establishing an annular ionizing discharge, means providing gaseous material to said discharge whereby ions thereof are produced, a ring of material having a high magnetic permeability disposed adjacent said discharge coaxially therewith in a plane parallel thereto, and a loop about said ring for energization with high frequency electrical energy for attracting ions from said generator to form a hollow cylindrical ion beam.

2. An ion source comprising means establishing a radially oscillating toroidal electric discharge, means feeding vaporized source material into said discharge for ionizing same, a pair of coaxial toroidal. magnet cores having a high permeability disposed in a plane adjacent said discharge with the core separation aligned with at least a part of said discharge, and a conductor looped about said cores andconnected for energizationbyapparatus employing ions fromisaid source whereby said cores establish an ion-extracting field removing'ions from the discharge areain the form of a hollow cylindrical ion beam.

3. An ion source comprising a toroidalfilament, an anode attracting electrons radially outward from said filament to establish a discharge, means introducing a material to be ionized'into said discharge for producing a toroidal sheath of ions, a pair of toroidal magnet cores disposed coaxially with said filament and displaced axially therefrom, a conductor looped about said cores, and means energizing said loop with high frequency energy forv establishing a magnetic field intersecting said ion sheath whereby ions generated in said discharge are extracted therefrom.

4. An ion source for a cavity accelerator comprising means establishing a toroidal electric discharge, means feeding a vapor to be ionized into said discharge whereby ions. are formed therein by bombardment, a pair of co- 6 circuit and wound about said magnet cores for energizing same to establish a magnetic field thereabout whereby ions are attracted from said discharge in a hollow cylindrical ion beam.

No references cited.