US2668260A - Ion source - Google Patents

Description

ION SOURCE Filed Feb. 7, 1951 BY Clarence A Barnezz ATTOPN'Y Feb. 2, 1954 S& N EM 6 V m5 9 fi A -5 m a e a 4/ 3 7 a a -34 5 M. 4 e A 2 2r M C a CM a ||||L Al 4 O O 0 O 2 3 w u m 5 Patented Feb. 2, 1954 ION SOURCE Clarence F. Barnett and Carroll B. Mills, Oak Ridge, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commission Application February 7, 1951, Serial No. 209,766

6 Claims.

The present invention relates to electric discharge devices, and more especially to an improved source of ions for a particle accelerator such as a cyclotron.

Ion sources wherein the ionizing electrons are obtained from a cold cathode discharge perpendicular to a magnetic field are described by Bleakney, Patent No. 2,221,467, and Penning, Patent No. 2,146,025. Backus, in Patent No. 2,499,289, shows a cylindrical anode with two cathodes disposed perpendicular to the axi thereof and a short distance outside opposite ends of the cylinder. In sources of this general type, a very high potential, sumcient to cause an electrical glow-type discharge between the cathodes and anode, is impressed upon the electrodes. The electrons in the discharge describe helical paths along the magnetic lines of force through the anode cylinder, and oscillate between the two cathodes, so that their paths are substantially lengthened, and more collisions will occur in the source.

While such sources provide generally suitable ion generators, they do not entirely satisfy the requirements of a high current accelerator. For one thing, they must be cooled in some manner, usually by flowing water adjacent the electrode surfaces. The necessary tubing is itself difficult to thermally insulate, makes construction of the source unduly complicated, and is troublesome to install, remove, and maintain in operation.

In addition, for such a device, it is essental that the source furnish a relatively large and Wellfocussed particle output. Moreover, it is desirable that the ratio of the desired ion, which may be, for example, protons (311+), to the molecular ions (H2+ and H3 delivered by the source be high, for increased source eiiiciency, to avoid excessive power dissipation, and to reduce interference with proper operation due to space charge effects. Additionally, it is essential that the source provide a high ratio of desired ion output to feed gas input, for economy of operation and also in order to maintain adequate vacuum conditions in the accelerator. For a high-current cyclotron, it is also desirable that the physical cross-section of the source be small, so that the source will not obstruct the initial path of the low-energy ions during their first orbital revolution, and that it be ruggedly constructed for long, continuous operation at high intensities.

With aknowledge of the shortcomings of the ion sources of the prior art, and of the particular requirements of a source suitable for a relatively high current particle accelerator such as a cyclotron, we have as a primary object of our invention provision of an ion source characterized by its large, relatively intense, ion output.

Another object of our invention is to provide an ion source which may operate at very high temperatures, yet does not require water cooling or other systems of forced cooling.

Yet another object of our invention is to provide an ion source characterized by its relatively high overall efiiciency.

A further object of our invention is to provide a low-voltage, high current ion source utilizing positive ion bombardment of thermionic cathodes in a magnetic field to produce a low voltage, high density arc,

The above objects are attained in the present invention by providing a carbon anode envelope of relatively small cross-section and a pair of thermionic cathodes disposed therevvithin and at opposite ends thereof. The cathodes are of such material that they are readily heated, so that when they are bombarded by ions of sufficient energy and intensity, they become incandescent and supply thermally emitted electrons to the plasma. The effective resistance of the arc discharge is decreased, the required potential difference across the source is very greatly reduced, and the character of the arc accordingly changes to that of a low-voltage, high density type, very intense and stable, and singularly suitable for production of ions.

A preferred embodiment lustrated in detail and of our ion source is ilseveral pertinent operating characteristics thereof are shown in the appended drawings, in which Figure 1 is a sectional view of one form of our ion source particularly adapted to furnish protons for a cyclotron;

Figure 2 is a sectional View taken along the line- 2--2 of Figure 1; and

Figure 3 shows the relationship existing be tween cathode current and ion output of the device of Figure 1.

Referring now to Figures 1 and 2, our source comprises simply a rectangular carbon block anode 3, flared at one end into a bulbous body 2, which is flanged to fit within a circular recess in one face of a mounting stem l. A ring 24.

threaded on its periphery, engages a threaded re-- cess in the stem to hold flange 25 rigidly against the stem. A cylindrical chamber 1 is milled through the block 3 to form the arc chamber, and carbon cap it! is press fitted over one open end to close the chamber. it cut in one side, hereinafter called the front, of block.3 to allow emergence A slit or aperture 4 of ions generated in the source. Cathodes and 6, which may be simply plates of a thermally-emissive metal, are mounted near opposite ends of the slit 1, towards the front of the chamber I, on stems 9, M, which attach to cathode support member l2. Thi support is in turn held rigidly in position by electrical insulators l5, 16 carried on an insulating sleeve, not shown, over bolt 11, which engages a correspondingly threaded hole in stem I. A second cathode support I8 is secured to the first support 12 by means of nut l9, carried on the threaded end of stem 9, which also engages a correspondingly threaded holein support! 2. The other end of support 18 is "rig-idly joined to tube ll, cathode voltage conductor and gas inlet means. The tube is held in position inside the stem -I by means of annular insulator 2|, which is in turn press fitted into an aperture at the inlet end o'r mounting stem l. Plate 22 forms the back of the deed-chamber 23,'=and maybe dastened-tostem I by screws -or other suitable iastenings, not shown, to make the chamber substantially gas tight. Quartz tube 8 is provided as an-electrical insulator for the stem 9, so that the stem, at cathode potential, may extend through the anode block 3 to reach thecpposite end of-chamher -'I.

In a preferred form of mansions of block -3 may be and =4" long, while the chamber 5 may be in dia'meter. A carbon block is employed inconstruction and the chamber l is milled within it. This particular construction eliminatesthe need for water cooling of the anode, which would otherwise-be required because 'of the high temperature at which the source normally operates. The plate 22 may be copper, while the insulators 15, it maybe steatite. A suitable electrical supply for energizing the source provides volts D. Cxat 10 amperes, with the positive terminal-of 'thesupply at ground potential. A series resistor is suitably incorporated in the cathode circuit. As will be apparent to those skilled in the :art, this will provide a limitation on current flow when the resistance of the discharge decreases. The slit or aperture '4 may be rectangular, 2%" long and s%" wide, and may be located substantially midway between the ends of the chamber 1. The cathode plates 5, Bare preferably --of tantalum, and "do not "require water cooling. On the contrary, they emit more electrons as their temperature increases, providing a more intense arc. Thus -a troublesome disadvantag of prior art "sources has been utilized in our invention togreatly improve theefficiency of source operation. The stems 9, H,-are also preferably tantalum. The plates are preferably .050 thick and .200" in diameter. They may be disposed equidistant from their Tespective ends of the-slit 4, and should be very close to the front of the chamber I, the front edges of the two cathodes being aligned with-the inneredg-es-orlips of the aperture lin the embodimentshown.

Referring now to Figure 3,curve A "shows the output current of H+ ions (protons) delivered as a :function of amperes-of cathode current input, while curve B shows the output current of the molecular ions H2 and 1-13 as a'function'of cathode current'input. It is obvious-from the curves that -as the 'input current increases, .proton output increases rapidly, whilemolecular ionoutput decreases. This extremely desirable characteristic of our-source-along with the maintenance the invention, the 'di- A" wide, /8" deep,

which serves the combined functions of a 1 of maximum total ion output has been achieved by placing the primary electron stream from the cathodes very close to the lips of the exit aperture, thereby exposing the part of the are at the highest temperature to the force field of the accelerating system of the particular particle accelerator with which the source is utilized.

In operation, the axis of the anode tube of the illustrated embodiment of our source is exactly aligned in the direction of a magnetic field of about 7000 oersteds, indicated by the arrows H in Fig. 1, and a D. C. potential difference of about 1000 volts is applied between the anode and the end of tube Ll, which serves as a conductor to the cathodes. -A glow-type discharge is formed. in the 'chamber between the cathodes, and that discharge is maintained by secondary electron emission from the cathodes 5, 6. Hydrogen gas isthen introduced through tube ii and flows into the arc chamber '1. Electrons in the glOW discharge collide with hydrogen molecules to :form many positive ions, which are accelerated :along magnetic lines of force toward the cathodes. By ion bombardment of the cathodes more electrons are released to ma'intainthe glow discharge. We have'employed as cathodes not simply conductive metals, like the sources of the;pri0r art-but thermally-emissive metals, such .as tantalum or Wolfram. When the cathodes in our source are bombarded, they do not overheat, rupture, or otherwise cease operation. Rather they become incandescent, and supply the arc with both seccndary electrons and copiousquantities of thermally emitted electrons. By this means, the effective resistance of the arc discharge is greatly decreased, so that the potential required thereacross decreases from 1000 to 200 volts. The 1,0 30 volt power supply 25 is connected :in series with a dropping resistor 23. As the arc .current increases, the voltage drop across ith'eire'sistor increases, lowering the potential across the arc. The source will continue tooperate, .even without water cooling. And of greatest significance, the'electron-density of the arc in ournovel source is thereby caused to be many orders c'f magnitude greater than that possible with the coldcathode sources of the prior art. Yet we are able to continue to utilize the primary advantage of those -sourcesthe oscillating volume of trapped electrons between the cathodes which increases the number of ionizing collisions per electronwithout sufiering the severe currentlimitations characteristic thereof.

Moreover, our source utilizes forces within the arcit-self to'maintain an-intense, stable self-regulated discharge or arc. Since the arc plasmais positive with respect to both the anode and thetwo cathodes, positive'ions therein may drain to any .of the three electrodes. If a large quantity of those .positiveions Jdrains from the arc to an electrode, the potential of the are immediately falls, decreasing the gradient tending to accelerate secondary :electronsinto the arc until equilibrium is'r'eestablished.

The source illustrated has produced currents up'to 339inilliamgweres, of which 82% is' rotons. Greater source efficiency is obtained at lower outputs by reducing the feed rate of hydrogen gas. For example, at an output of only 137 milliamperes, 84% thereof being protons'the efficiency may be-410%. The output and eificiency will increase with an increase in accelerating potential employed to remove ions from the source, 'will increase as the FCElJhOde current input increases, and will increase 1 withianincrease in l the applied magnetic field. In a typical utilization of the source of the dimensions given above, a magnectic field of 7000 oersteds may be applied, cathode current of amperes at 200 volts may be drawn, hydrogen may be introduced at cc./min., and an accelerating potential of 10 kilovolts may be applied between an external accelerating electrode and the present source.

While the preferred embodiment of our improved ion source described has taken the form of a proton source for a cyclotron, it will be apparent to those skilled in the art that the novel features thereof may be adapted to produce alpha particles, deuterons, other ions for different types of particle accelerators and other scientific tools requiring a supply of ions for analysis or acceleration. Accordingly, our invention is not to be construed as limited to the preferred embodiment shown, but only by the scope of the appended claims.

We claim:

1. An ion source for utilization in a magnetic field comprising a longitudinally apertured anode defining a cylindrical chamber disposed in and parallel to said magnetic field, a pair of cathodes disposed within said chamber, near opposite ends thereof, and characterized by the property of thermally emitting electrons when heated by ion bombardment within the chamber, a source of electrical energy connected between said anode and said cathode, and means for introducing a gas to be ionized into the chamber formed by said anode and cathodes.

2. An ion source for utilization in a magnetic field, comprising a longitudinally apertured tubular anode member definin a chamber disposed within and parallel to said magnetic field, said chamber being closed at one end and internally communicating at the other end with a second chamber, means for admitting gas to be ionized into said second chamber, a pair of thermionic cathode members disposed in confronting relation at opposite ends of said first chamber and substantially equidistant from the center of the aperture in said anode and in close proximity to that wall of said anode containing said aperture, and a source of electrical energy connected between said anode and said cathodes, said source being of sufficient magnitude to initiate a glow discharge therebetween in the absence of said gas, but in the presence of said magnetic field.

3. An ion source for utilization in a magnetic field comprising a longitudinally apertured anpair of cathodes disposed within said chamber near opposite ends thereof and characterized by the property of thermally emitting electrons when heated, a source of electrical energy connected between said anode and said cathode, means for introducing a gas to be ionized into the chamber formed by said anode and cathodes, electrically conducting support stems for said cathode members, the stem supporting the oathode nearest the end of said chamber distal from the point of said gas entry extending 1ongitudinally within the wall of said anode from a mounting means near said point of entry, and means for electrically and thermally insulating said stem from said anode.

4. In an ion source of the character described, a conductive tubular anode envelope, and a pair of thermally-emissive cathodes disposed in confronting relation therein, means for striking an are between said anode and said cathodes, and means for establishing a magnetic field parallel to a line joining said cathodes.

5. In an ion sourceof the character described, a tubular anode envelope, a pair of thermionic cathodes mounted in confronting relation therein and defining a chamber therebetween, means for introducing gas to said chamber said envelope being provided with a longitudinal exit aperture and a pair of inwardly-extending lips, said lips beginning adjacent said cathodes and terminating in said aperture, the mid-point of said aperture being substantially equidistant from said cathodes.

6. In an ion source of the character described, a tubular anode envelope, first and second thermionic cathodes mounted in confronting relation therein near one wall thereof, a tubular electrie cal insulator disposed within said envelope, near ode member defining a cylindrical chamber disposed in and parallel to said magnetic field, a

the opposite wall, a support for said first cathode disposed Within said insulator, means for admitting a gas to be ionized into said envelope, said envelope being provided with a recessed portion adjacent to and between said cathodes and a longitudinal slit in the central portion of said recessed portion.

CLARENCE F. BARNETT.

CARROLL B. MILLS.

References Cited in the file of this patent UNITED STATES PATENTS

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