US2839706A

US2839706A - Pulsed ion source

Info

Publication number: US2839706A
Application number: US598218A
Authority: US
Inventors: Carl E Anderson; Kenneth W Ehlers
Original assignee: Individual
Current assignee: Individual
Priority date: 1956-07-16
Filing date: 1956-07-16
Publication date: 1958-06-17
Anticipated expiration: 1975-06-17

Description

June 17, 1953 c; E. ANDERSON ETAL 3 0 PULSED ION SOURCE Filed Jilly 16, 1956 POWER SUPPLY PULSED POWER SUPPLY INVENTORS'. CARL E. ANDERSON KENNETH W. EHL ERS HIGH v VOLTAGE 02 POWER /HM4M SUPPLY ATTORNEY.

. PULSED ION SOURCE v The present invention relates to pulsed ion sources and more particularly to an ion source capable of producing betweenand normal to the pole faces 12. A vacuum very short high density pulses of ions without beam scattering from internal charge effects.

In the fields of nuclear physics research and application there have been developed various types of particle accelerators adapted to produce ion beams of high energies for target bombardment. Suchbombardment enables completion of many experiments for studying reactions of importance in the field and provides for target irradiation to a variety of ends. In order to reach higher and higher bombardment energies, it is necessary to employ heavier ions and to multiply charge same. Furthermore, the nature of materials that may be'ionized, is such that rapidity of ionization is important, as for example 4 in the case of substances having short half lives, and

maximum efliciency dictates the use of a pulsed beam of very short duration pulses.

The ion source of the present invention is particularly adapted to produce an intense pulsed beam of multiply charged ions and furthermore to overcome the tendency of such a high density beam to blow up" or expand in 'all directions from the coulomb scattering effect thereof.

As like electrical charges tend to repel each other, it

will be seen that a beam of positively charged ions will tend to expand in all directions by virtue of the mutually repulsive forces of the component ions. While this effect is not material in ion beams of low density, it becomes marked in very high density beams such as those herein contemplated and if not suppressed will destroy the beam identity. i

Inasmuch as high density ion beams are known in the art, beam' blow up has been previously experienced and certain corrective measures attempted. In a calutron, forexample, high beam densities are attained and therein a; relative high ambient gas pressure is maintained so that the beam causes material ionization in transit to free sufficient electrons for beam space charge neutralization. This solution is not here applicable, for gas bombardment with the ion beam causcsthe latter to diverge and weaken so that wherein a strong steady beam of a calutron may be so handled a pulsed beam as herein employed may not be subjected thereto without losing definition. There has been additionally put forward at least one other method of space charge neutralization of high density ion beams, including the operation of auxiliary arc discharges;

' however, same is subject to the difficulty of complexity that is overcome by the present invention.

It is an object of the present invention to provide an .It is a further object of the present invention to pro- "ice vide a pulsed ion source producing multiply charged ions in high density short duration pulses.

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

Figure 1 is a plan view in section of the'sourceyand Figure2 is a vertical view in section taken at 2-2 of Fig. 1 of the source and including associated electrical circuitry.

Considering now the structural details of a preferred embodiment of the present invention and referring to the drawing, there is shown a pair of parallel spaced pole pieces 11 adapted for energization from magnet windings, not shown, to'cstablish a strong magnetic field H therechamber 13 is formed between the pole faces 12 by means of a wall 14 extending between pole faces 12 in sealing relation thereto and extending about at least three sides thereof. A vacuum manifold 16 attaches to the chamber 13 on the-fourth side thereof and connects to vacuum pumps, not shown, whereby the chamber 13 is continuously evacuated to maintain a very low pressure therein.

Within the chamber 13 is disposed the ion generator 17 including an elongated anode block 18 having a cylindrical bore longitudinally therethrough and defining an ionization chamber 19 aligned with the lines of force of the magnetic field H. An ion exit slit 21 is formed longitudinally of the anode block communicating with the ionization chamber at the center thereof, and on the side facing the vacuum chamber 13 the exterior of the anode block is indented in the exit slit area, minimizing the depth of the exit slit. The anode block isencompassed on the top, bottom, and back by a C-shaped support member 22, and cold cathodes 23 of tantalum, for example, are mounted on the horizontal arms of the member 22 one at each end of the anode block in alignment with the ionization chamber therein. A pair of annular supporting insulators 24 are disposed one at each end of the anode block, retained for example by annular bosses on the anode block, and engaging the horizontal arms of the member 22 as in circular slotstherein.

Support for the ion generator 17 within the vacuum chamber 13 is provided by'a hollow stem 26 extending through the wall 14 via an insulator 27 and afiixed to the vertical portion of the support member 22. A suitable gasket or seal 28 may be provided about the stem 26 to insure a vacuum-tight connection with the wall insulator 27, and through the stem there extends a tube 29 into a transverse passage 31 in the anode block communicating with the ionization chamber 19. This tube 29 is adapted for connection to an external gas supply, not shown, for supplying gas or vapor to the ionization chamber for ionization thereat and the tube is mounted in the stem by an annular insulator 32 with sealing means 33 provided for insuring vacuum tight connection.

Means are provided for the extraction of ions from the ionization chamber 19 in the form of an accelerating electrode 34 having an elongated slit 36 therein and disposed in the anode block indentation with the slit 36 aligned with the ion exit slit 21 in the anode block. The

accelerating electrode 34 is maintained in adjustable close spaced relation to the anode block and out of contact therewith by mounting means including a support rod 37 secured to the vacuum chamber wall 14 and extending thercthrough to serve as-an electrical lead-as well as a support. The support rod is affixed to .the accelerating electrodeby any suitable means such as

fasteners

38 and 3. a seal 39 about the rod in the wall 14 insures a vacuumtight relationship thereat.

With regard to space charge neutralization of an ion beam produced by the generator, there are provided a pair of

metal plates

41 and 42, which may be flanged for structural rigidity and which are secured to the accelerating electrode and extend therefrom away from the anode block. The

plates

41 and 42 are oriented in parallel spaced relationship normal to the magnetic field H and are disposed with one above and one below the ion beam slit36 in the accelerating electrode so that an ion beam emerging therefrom will generally pass between the plates. It is the function of the

plates

41 and 42 to emit electrons upon bombardment and thus the plates are formed of a metal, such as aluminum, which readily emits under such conditions.

In the illustrated embodiment of the invention no accelerator or ion beam utilizing apparatus is depicted; however, the ion source is adapted for connection to such equipment and in the source shown such connection is made through a pipe 43 communicating with the vacuum chamber 13. It will be appreciated that it is not necessary to place a particle accelerator in direct line with the ion source for the magnetic field H is available to bend the ion beam and here the beam is bent through some 180 so as to leave the vacuum chamber in the opposite direction from which it it entered. In this case maximum use may be made of the

plates

41 and 42 by disposing them to project over the curved beam trajectory, as shown.

Electrical energization of the ion source is accomplished by power supplies disposed externally of the abovedescribed source structure and including a constant output direct current power supply 46 connected between the stem 26 and the gas tube 29. This power supply 46 applies a constant potential between the anode and cathodes to maintain a low order electron discharge from the cathodes. A pulsed power supply 47 is also connected between the anode and cathodes at the stem 26 and tube 29 and, like the first-mentioned power supply, has the negative terminal thereof connected to the cathodes via the stem 26 and the positive terminal connected to the anode through the tube 29. Thus, power supply 47 is adapted to deliver high voltage pulses of very short duration and may, for example, include a pulse line connected to a modulator that is, in turn, controlled by a high frequency trigger circuit and providing the output pulses. One further high voltage power supply 48 is provided to produce the ion accelerating potential and, in this case, the accelerating electrode is grounded at the support rod 37 thereof and the positive terminal of the high voltage supply 48 is connected to the anode 18 with the negative terminal of the high voltage supply grounded whereby the high voltage of this supply is impressed between anode and accelerating electrode to establish a strong ion extracting field. The high voltage supply 48 may also be pulsed so as to establish the ion extracting field only when required.

Considering now the operation of the invention and referring to the drawing, the power supply 46 applies a steady voltage of nominal magnitude, say five kilovolts, between the cold cathodes 23' and the anode 18 in order to establish and maintain a low order discharge therebetween. This discharge is maintained so that very rapid arc buildup follows application of a pulsed voltage from the power supply 47 and the steady discharge prevents time variations in arc establishment by the pulsed voltage. During a pulse of voltage, electrons emitted from the cathodes, mainly by ion bombardment, move longitudinally through the ionization chamber 19 under lateral constraint from the magnetic field until they experience the repelling field of the opposite cathode and are repelled. There is thus produced elongated oscillatory electron paths between the cathodes whereby the probability of an ionizing collision with gas molecules entering through the tube 29 is maximized. Ions produced in the ionization chamber bombard the cathodes to increase electron emission therefrom which, in turn, increases ionization until an arc plasma is formed in the ionizing chamber.

The are struck within the ionization chamber is a so-' called high voltage are wherein high potentials are employed in the establishment thereof and electrons are accelerated from the cathodes with a large velocity and this is highly desirable in producing multiply charged ions. As the probability of ionization reaches a broad maximum at about five or more times the ionization potential of the charge state required, it is possible to accelerate electrons through a field a sufiicient strength to cause the electrons to fully strip gas molecules and produce fully charged ions or stripped nuclei. The high are voltage, as herein employed, produces multiple ionization to an extent depending upon the applied electron acceleration so that it is possible to achieve a large proportion of ions of a desired charge state by fixing the electron accelerating fields.

With the establishment of'a high voltage are of short time duration, the field of the accelerating electrode 3'4 produced by the power supply 48 reaches into the ionization chamber to attract ions from the outer arc edge whereby these ions accelerate through the anode exit slit 2'1 and thence through the accelerating electrode slit 36. In order to produce an ion beam of a single mass with a particular charge thereon the ion beam emerging from the anode is operated upon by the transverse magnetic field H, whereby each of the ions with like mass-tocharge ratios traverses an arc of like radius. Appropriate placement of the ion outlet pipe 43 in a wall 14 of the vacuum chamber 13 provides egress for ions of a chosen mass-to-cha'rge ratio which may then be operated upon in such as a particle accelerator.

Ion exit geometry at the ion generator provides an ion beam that is elongated parallel to the magnetic field and is very narrow perpendicular thereto. With a dense ion beam of multiply charged ions as herein produced the proximity of similarly charged ions in the beam causes mutual repulsion to such an extent that the beam tends to blow up or expand outwardly. The space charge forces in the beam cause such serious beam spreading that it is necessary to overcome same in order to produce an ion beam suitable for injection in many particle accelerators. To eliminate this spreading it is necessary to completely neutralize the beam by providing at least'the same density of electrons as the density of ions. With an ion beam of current density J and velocity v where and q is the ion charge and V the voltage, the ion density is equal to J v ions per cubic centimeter. If the electrons for beam neutralization are created only by ionization of the residual gas atoms in the vacuum chamber the rate of production of these electrons will be Jo'N, where a' is the ionization cross section and N is the number of residual gas atoms per cubic centimeter. Further, assuming no loss of electrons from the beam, the time required to completely neutralize the beam is vacuum chamber must be minimized to prevent scattering and charge exchange of the multiply charged ions. Maximum use may be made of the electrons that are created by gas ionization by reducing the rate of loss of electrons from the beam and this may be accomplished by increasing the electrostatic forcesacting on the beam. This is only a partial answer, however, for as an example, with the ionizing cross section cr= c1n. and employing nitrogen gas, the time T equals 40 microseconds at 7.6x lO millimeters of mercury, but T equals 400 microseconds at 7.6 l() millimeters of mercury. It will be seen from this thatwith a two millisecond beam pulse, the first twenty percent thereof would not be space charge neutralized at the lower pressure required. It is thus necessary to provide an auxiliary electron creating process. To this end there are provided by the present invention a pair of neutralizing

plates

41 and 42 disposed closely adjacent the ion beam above and below same and maintained at the accelerating electrode potential. consequence of these plates is to accelerate electrons away from same so that the electrons oscillate between the plates through the beam rather than completely escaping the beam. Further, these plates are formed of a material, such as aluminum, having a high ratio of secondary electrons to impinging ions. In operation, ions from the beam exterior move outward therefrom to strike theplates and thereby'produce electrons which are repelled from the plates into the beam for rapidly space charge neutralizing same. In this manner the high intensity beam is maintained substantially intact without blow up even in a high vacuum and in use neutralization times of less than 40 microseconds have been obtained with residual gas pressures as low as 7.6 10- millimeters of mercury.

Although the present invention has been described in connection with a single preferred embodiment thereof, it will be appreciated by those skilled in the art that numerous modifications and variations are possible within the spirit and scope of theinvention and thus it is not intended to limit the invention except by the terms of the following claims.

What is claimed is:

1. An ion source comprising an apertured anode having a pair of cathodes disposed one at each end of the anode aperture, means establishing a magnetic field through said anode aperture between said cathodes, means supplying a gas to said anode aperture, power supply means energizing said anode and cathodes whereby an arc discharge is established in said anode aperture, ion accelerating means communicating with said anode aperture for withdrawing an ion beam therefrom, and neutralizing plates disposed immediately above and below said ion beam along said magnetic field for producing electrons that space charge neutralize said ion beam.

2. An ion source comprising means establishing a high voltage are discharge, ion accelerating means adjacent said discharge and expelling ions from said are discharge in dense beam, and a pair of neutralizing plates disposed one immediately above said beam and one immediately below said beam and maintained at a highly negative potential whereby ions strike same to produce electrons that oscillate therebetween for space charge neutralizing said beam.

3. An ion source comprising means establishing a high voltage arc discharge, an apertured accelerating electrode disposed adjacent said are discharge and maintained at One a negative potential relative thereto for accelerating ions in a beam from said are discharge through said accelerating electrode aperture, and a pair of plates formed of a material having a high ratio of secondary electrons to incident ions electrically connected to said accelerating electrode and disposed close to said ion beam along the path thereof for producing beam space charge neutralizing electrons from ion bombardment.

4. An ion source comprising means establishing a magnetic field, means establishing an oscillating electron discharge Within said field, means supplying a gas to said discharge for establishing therefrom an ionizing are discharge, an apertured accelerating electrode disposed adjacent said discharge and maintained at a highly negative potential relative thereto for accelerating ions therefrom in a beam, and a pair of neutralizing plates disposed on opposite sides of said ionbeam along said magnetic field in close proximity with said beam and electrically connected to said accelerating electrode for producing electrons from ion bombardment and oscillating electrons therebetween to space charge neutralize the ion beam.

5. An ion source comprising an anode block having an elongated opening therethrough, means establishing a magnetic field having lines of force longitudinally through said anode opening, a pair of cold cathodes disposed in alignment along said field with one at each end of said anode aperture, means admitting gas into said anode opening, high voltage pulsed power supply means establishing said cathodes at a relatively negative potential With respect to said anode for producing a pulsed high voltage arc discharge through said anode opening, an accelerating electrode communicating with the interior of said anode opening over a substantial length thereof and having an elongated ion exit slit therein, high voltage power supply means maintaining said accelerating electrode at a highly negative potential relative to said anode whereby ions are withdrawn from said pulsed are discharge in a beam traveling transverse to said magnetic field, and a pair of neutralizing plates disposed immediately adjacent said ion beam and electrically connected to said accelerating electrode for supplying electrons to space charge neutralize said beam.

6. An ion source as claimed in claim 5 further defined by said neutralizing plates being formed of a material that produces a high ratio of electrons to incident ions and said plates being displacedalong said magnetic field with one directly above said beam and the other directly below same for intercepting outer ions of said beam to produce secondary electrons that oscillate through said beam between said plates for space charge neutralizing said beam.

7. An ion source as defined in claim 5 further defined by said high voltage power supplyproducing a pulsed output in synchronism with said anode-cathode pulsations.

References Cited in the file of this patent UNITED STATES PATENTS Washburn Nov. 27, 1956