US3131300A

US3131300A - Apparatus for reducing energy variations of a van de graaff ion beam

Info

Publication number: US3131300A
Application number: US238307A
Authority: US
Inventors: Thomas R Jeter; John D Baldeschwieler
Original assignee: Individual
Current assignee: Individual
Priority date: 1962-11-16
Filing date: 1962-11-16
Publication date: 1964-04-28
Anticipated expiration: 1981-04-28

Description

April 28, 1954 T. R. JETER ETAL 3,131,300

APPARATUS FOR REDUCING ENERGY VARIATIONS OF A VAN DE GRAAFF ION BEAM Filed Nov. 16, 1962 Can/ro/kz 5/6 9 '12 I f v0 H1998 sup 04v 2K:

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United States Patent 3,131,369 APPARATUS FQR REBUCENG ENERGY V- TTONS 0F A VAN DE GFF ION BEAM Thomas R. Jeter, Bel Air, Md, and John D. Baldeschwieler, Cambridge, Mass, assimors to the United tates of America as represented by the Secretary of the Army Filed Nov. 16, 1962, Ser. No. 238,307 2 Claims. (Cl. 250-495) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates generally to an electrostatic generator apparatus and more particularly to a Van de Graatf positive ion or electron accelerator and the method and means for reducing energy variations of the ion beam thereof.

Electrostatic generators, and especially Van de Graafi generators, have been long used for the production of high voltages in many applications. Briefly, apparatus of this type comprise a high voltage terminal and a grounded tank which constitute two plates of a capacitor. A column supports this high voltage terminal, provides space for a belt and maintains essentially a uniform potential gradient from the terminal to ground. An upcharge unit sprays positive charges on the moving belt by an arrangement of corona points. The motor driven belt transports the charges from ground to the high voltage terminal and here the positive charges are collected from the belt within the terminal and deposited on the terminal shell thereby charging the terminal as a capacitor to the desired high voltage. Located adjacent to this terminal is a source of ions and means which cause the emission of the ions into an accelerator tube. This accelerator tube contains the proper fields, and by means of such electrostatic fields the positive ions are focused into a beam, and accelerated to an energy determined by the potential of the high voltage terminal. The tube is also used to carry the ions to an exit portal, the latter providing an outlet to which apparatus or a suitable target can be attached for the required utilization of the beam.

Recently, accelerators of this type have become extremely important in the field of nuclear physics. In such work accuracy is of prime importance and energy fluctuations of the ion beam have to be kept to a minimum.

Since the resonance of A1 (p, 'y)Si is commonly known in work pertaining to gamma-ray yield, it is ordinarily used as the target upon which the ion beam operates. This Al (p, 'y)Si resonance at 0.992mev. is used to measure the ion beam energy spread. Measurements made in this manner indicate that further improvements in the beam energy distribution are possible with optimum voltage and phasing of the signal applied to the target.

The energy distribution of ions produced in a Van de Graaflf accelerator depends on the homogeneity of the ion source, the straggling of ions in the accelerating tube and vacuum system, and fluctuations in the high voltage terminal. In many Van de Graafi facilities a signal taken from the slit system following an analyzing magnet is used to control the voltage of the high voltage terminal. The energy distribution of the beam then depends on how effectively the slit system cancels voltage fluctuations of the high voltage terminal.

In one instance it has been suggested that the electro static mass analysis of the H fraction of a Van de Graaff ion beam be used to give an error signal. This high voltage error signal could then be applied directly to cancel energy fluctuations in a proton beam. By applying a correction voltage from the H beam to the target it is possible to reduce only certain variations in the ion beam energy. The energy distribution of the beam due to 3,131,300 Patented Apr. 28, 1964 inhomogeneity of the ion source, or straggling of ions in the system cannot be changed by applying only a voltage to the target.

The variation of the signal from the slit system and the variation of the terminal voltage are proportional to the time dependent variation in the energy of the beam. In this invention, the signals from the slit system and the terminal have been amplified and applied to the target in proper phase to reduce the time dependent variations in beam energy in a manner which will be later described. The energy variation of the beam has been estimated from measured half-widths of the Al (p,'y)Si resonance at 0.992-mev.

In order to obtain higher beam currents on a target and better beam collimation in Van de Graatf accelerator work it is necessary to apply a correction voltage to the target and simultaneously adjust fluctuations at the high voltage terminal of the Van de Graatf. The method contemplated by the present invention comprises essentially the use of a slit system ahead of the analyzing magnet to give an error signal, i.e., if the terminal voltage is low, the ions will leave the magnet at an angle and strike one side of the slit system, and conversely, if the voltage is too high, the ions will strike the other side of the slit system. This error is then translated to the high voltage terminal of the Van de Graafi generator to adjust the current. If nothing further is done however, the energy of ions that have already left the accelerator will not be adjusted and will spray over the target. In order to adjust this portion of the ion beam, the present invention contemplates picking up the adjustment at the high voltage terminal by means of a capacitor or the like and then amplifying and adjusting the voltage at the target in order to add or subtract a voltage increment proportional to the error signal as appropriate to the ion beam.

It is, therefore, a general object of this invention to provide for energy control devices to give a significant improvement in the energy distribution of a proton beam.

A further object of this invention is to provide improved energy control of ion beams in order to enable more accuracy in studies of shapes and widths of various resonance absorptions, and in measurement of gamma emission from specific excited nuclear states.

Another object of this invention is to provide a simple and accurate technique of reducing energy variations of a Van de Graaif ion beam.

More specifically an object of this invention is to provide method and means for essentially cancelling out unwanted energy fluctuations of an ion beam, when such beam is used to bombard a target under consideration.

In accordance with one aspect of this invention, signals from the slit control system and the terminal of a Van de Graaff accelerator are amplified and applied to the target in proper phase to reduce time dependent variations in beam energy.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to organization and method of operation, as well as additional objects and advantages thereof, Will best be understood from the following description when read in connection with the accompanying drawing, in which there is shown a simplified illustration of the ion beam acceleratorvand the corrective means therefor suitably embodying the invention.

To enable investigation of the improved energy resolution and beam intensity of an ion beam it is necessary to consider the effects of high-resolution neutron crosssection work. The observed width of a resonance depends upon the Breit-Wigner shape of the resonance itself, the target Doppler elfect, and the energy distribution of the beam. Usually the half-width (the full width of the Since the thick target results correspond to the integrated resonance curve, the interquartile range, 2s, is just the half-Width of the integral curve. For the Breit-Wigner distribution, the half-width and the interquartile ranges are equal. For other distributions this is not necessarily true.

Generally, it can be assumed that:

( he= +z i T =observed interquartile range for a given resonance, I=Breit-Wigner half-width for resonance, and

T =interquartile ranges for energy distribution arising from other effects.

The Breit-Wigner half-width for the A1 (p, *y)Si resonance at 0.992 mev. is well known. For this work it is therefore assumed that I=l ev.

The I, values are considered to be made up generally of three effects. The first of these is the Doppler broadening of a resonance which arises from the thermal motion of the target nuclei. This eifect can be considered in the equation:

2E kT m (3) M where A=standard deviation of Doppler distribution, E =proton energy K=Boltzmann constant (8.61X 10 ev./ BL), T =effective temperature of target atoms, m=mass of proton, and

M=mass of target nuclei.

For the Al "(p, Si reaction, A=48 ev. The Doppler spread is given by a Gaussian distribution function where the interquartile range is:

where I =interquartile range of Doppler distribution, and A=standard deviation of Doppler distribution.

where R=full Width at /e of maximum for Gaussian distribution. Thus:

(6) I =l.36o' =l44 ev.

The most important portion for purposes of this invention is the value of I,,, which can be estimated from the geometry of the magnet and slit systems. The maximum energy diiference between ions that pass through the magnet and slit system in most magnetic analyzing systems can be given approximately by:

AE maximum energy difference between ions, E =mean proton energ s=slit spacing,

r distance from center of magnet to slits, and 6=deviation of analyzed beam (usually 25).

As an example, a slit setting of inch provide a value of about 2.5 kev. Thus, AE-2.5s'/50, where s is the slit spacing in inches x10 This value of AB is the maximum energy spread to be expected with a beam that has no distribution in space. For beams with a finite distribution in space, the energy spread will be less than AE.

For the purpose of calculating the magnitudes of various effects contributing to the beam energy spread, the distribution of the beam in space and in energy must be known. However, certain assumptions can be made and the various distributions can be approximated. With such approximations the value of I can be estimated with a fair degree of accuracy.

As mentioned previously, the time dependent variations in proton beam energy can be reduced by using a correction voltage applied to the target. Thus the control system to be described reduces I Referring now to the drawing, there is shown an airtight enclosing tank 1 generally filled with compressed nitrogen and carbon dioxide, which is used to prevent arc-over. Inclosed in tank 1 is a rapidly moving insulating belt 2, and a hemispherical high voltage terminal 3. Belt 2 runs in the direction indicated between two pulleys 4 and 5; pulley 4 being a drive pulley mounted on the tank base, and terminal pulley 5 being mounted in high voltage terminal 3. The belt is driven at a constant speed by a motor 6 suitably attached to pulley 4. A positive charge is sprayed onto belt 2 by a corona discharge system generally indicated by reference numeral 7. The system normally consists of a comb-like head held adjacent to the belt by proper support means. This continuous discharge is generated from a small DC. power supply incorporated into the discharge system 7 mounted inside tank 1. Insulating belt 2 mechanically carries the charge into high voltage terminal 3, where it is automatically transferred by means of a collector electrode 8 onto the high voltage shell. Most of the charges are used to maintain the terminal voltage at a high DC. potential while some of the charges leak away by various paths. By varying the flow of the electric charge to the terminal 3, the voltage can be correspondingly varied. In practice, excess charge flow is utilized for the accelerator tube load.

Ions are provided by an ion source 9 which generally consists of a gas, usually hydrogen, which is introduced into source 9 by means of a remotely controlled valve (not shown). Normally radio frequency power from an RF. oscillator supplies the energy to ionize the ion gas. A positive potential is applied to a probe or ejection electrode of the source; causing positive ions to be withdrawn from the plasma into accelerator tube 10. The positive ions are focused and accelerated by means of the electric field along tube 10, said tube being normally evacuated and of a glass-and-metal construction. The positive ion beam, designated by dashed line 11, is accelerated to extremely high velocity by the potential difference between terminal 3 and the base of tank 1, which is connected to ground as indicated at 12. Because the potential is DC. in nature, the particles in the beam are homogeneous in energy at any instant. This energy corresponds to the voltage of the high voltage terminal 3. A practical range is from 1.0 to 3.0 million electron volts. By regulating the probe potential and the gas flow to the ion source, the ion beam current can be adjusted from any remote control station. The flow of positive ions through accelerator tube 10 is therefore generally controlled by controlling the rate of spraying charges onto the belt, controlling the ion emissipn into the tube, and

maintaining the high voltage terminal at a constant value by controlling the leakage current.

For precise measurement and control of ion beam energy, the ion beam is deflected by a magnetic field provided by an electromagnet 13. The strength of the field is held constant by an electronically regulated DC. power supply (not shown), which is connected to the electromagnet 13.

An insulated slit system consisting of plates 14 and 15 is provided beyond the exit portal of the electromagnet 13 to permit the ions of a particular mass-energy product to proceed to a nuclear target 16. The relative amount of the ion beam hitting the plates 14- or 15 is a measure of the energy spread or variation of the beam. The current thus induced in the slit system is fed to an amplifier, 19, by conductors 1'7 and 18. This correction voltage is then supplied to a controhable high voltage supply 21 by means of a conductor 20. The controllable high voltage supply 21 in turn energizes corona electrode 22 by means of conductor 23 and in this manner the current of the slit system is utilized as a correcting signal to high voltage terminal 3. The current between the corona points of electrode 2.2 and high Voltage terminal 3 thus provides a measure of control of the high voltage terminal voltage which reduces fluctuations in the ion beam energy at its source.

As pointed out previously however, such a feed back correction system will not reduce the fluctuations of ion beam energy at any point before or beyond the slit system. It is therefore necessary to provide for a means and method to reduce the time-dependent variations of the ion beam further than such as can be accomplished by trying to get the high voltage terminal voltage constant. Since, a corrective signal from the slit system itself to the target w ll not provide the proper time relationship required, a corrective signal to the target must be derived from another source. Capacitive sensing of the high voltage terminal potential fiuctuations, and then applying such sensed signals, after amplification, to the target provides the necessary reduction of I,,.

In the drawing, a pickup plate 24, which acts as one plate of a capacitor, the other being the shell of high voltage terminal 3, senses the changes in electric field due to the corrective changes supplied by electrode 22, or any other fluctuations in the terminal potential. The corrective signal thus obtained by the insulated pickup plate 24 is then transmitted via conductor 25 to an amplifier 26. An Epsco DA-IOZ low level diflerential D.C. amplifier or the like can be properly used to amplify the signal thus received. After amplification the signal is passed along conductor 27 to a step-up transformer indicated generally at 28. A 50:1 transformer, for instance, along with am plifler 26 can produce a signal from 1000 to 4000 volts, peak to peak, for application to target 16. Target 16, which is insulated from the remainder of the accelerator tube, is thus provided with a voltage which reduces the efiects of energy fluctuations of the ion beam.

In addition to the disclosed advantages of the invention, it should be stated that the corrective signal from plate 24 has other useful purposes. As the energy of the beam fluctuates, for instance, the beam traces a line on the target. By applying the signal from the plate, via the amplifier to a pair of deflection plates located ahead of the target, the beam can be held in the center of the tube. Thus it is possible to obtain higher beam currents on the target or better beam collimations.

Although a specific embodiment of this invention has been illustrated and described, it will be understood that this is but illustrative and that various modifications may be made therein without departing from the scope and spirit of our invention.

We claim:

1. Electrostatic generator apparatus comprising a high voltage terminal; means for transporting electric charge to said terminal; an airtight conductive tank enclosing said terminal and transporting means; a corona leak electrode disposed within said tank; a controllable voltage supply connected to said electrode; a source of ions adjacent to said high voltage terminal to provide an origin for an ion beam; accelerating means for focusing and accelerating said ions to thereby form an ion beam of high energy; control means including a magnet, the field of which is used to deflect the ion beam on to a target; a slit system located between said control means and target to detect variations of said ion beam, said slit system being connected to said controllable voltage supply whereby the current produced in said slit system due to said variations is connected to said controllable voltage supply, said controllable voltage supply coupled to said corona leak electrode whereby potential variations of the high voltage terminal are obtained; capacitance means disposed within said tank for deriving a signal responsive to the said potential variations of the high voltage terminal caused by said controllable voltage supply; and means for feeding said signal to said target to cancel out any energy fluctuations.

2. A system for reducing energy fluctuations of the ion beam of a Van de Graalf ion beam accelerator having a charge collecting electrode, means for transporting electrical charge to said electrode, a high voltage terminal, and a corona electrode for controlling the charge upon and voltage of said high voltage terminal, said charge collecting electrode electrically connected to said high voltage terminal whereby said electrical charge is supplied to said high voltage terminal, said system comprising a controllable high voltage supply for supplying voltage to said corona electrode, means for sensing energy fluctuations of said ion beam and varying the voltage of said controllable high voltage supply to correct for said ion beam fluctuations, the varying voltage of said controllable high voltage supply applied to said high voltage terminal by means of said corona electrode whereby variations in the potential of the high voltage terminal are obtained, capacitance means responsive to said variations in the high voltage terminal potential for deriving a signal responsive to said energy fluctuations, and amplifying means responsive to said capacitance means to cancel said energy fluctuations at a target which is bombarded by said ion beam.

No references cited.