US2836759A - Linear accelerator - Google Patents

Description

-May 27, 1958 s. A. coLGATE LINEAR ACCELERATOR Filed July 22, 1955 INVENTOR.

STIHL/NG A. COLGATE TTORNE Y United States Patent LINEAR ACCELERATOR Stirling A. Colgate, Livermore, Calif., assignor to the United States of America as represented by the United States Atomic Energy Commission Application ruiy 22, i955, serial No. 527,892

3 claims. (ci. sis- 5.41)

The present invention relates to a linear accelerator for charged particles and, more particularly, to a spiral beam linear accelerator.

With the increased utilization of accelerators for charged particles as tools in the eld of nuclear physics for research purposes and in the held of production of radioactive isotopes, such accelerators must be capable of handling beams of charged particles having larger current intensities than has heretofore been achieved and with a higher degree of eiciency. The linear type of particle accelerator is inherently capable of handling higher values of current than cyclotrons, betatrons, or synchrotrons because the latter depend upon revolving charged particles an enormous number of times While increasing the energy thereof and thereby increase the possibility of losing particles with the resultant decrease in the value of the output current. Another disadvantage arises from the ditliculty in removing the accelerated beam out of the accelerator apparatus. With a linear accelerator of the Sloan-Lawrence type the charged particles are increased in energy by a series of favorable electric fields disposed along a linear path and a given particle energy can be readily achieved in a much shorter length of path.

In the past, it has been found that a foremost problem has prevented the achievement of a stable focussed beam with a linear accelerator in that beam instability has arisen from the defocussing elfect of the accelerating electric elds and from the mutual repulsion of the charged particles within the beam (sometimes referenced as space charge defocussing). The problem of focussing has been met in one type of linear accelerator (Alvarez, U. S. Patent No. 2,545,595, March 20, 1951) by the use of focussing foils; however, such foils tend to decrease the efciency of the accelerator.

The above-mentioned diiiculties and disadvantages of previously-known linear accelerators are overcome with the present invention by providing a radial electric ield transverse to the accelerating electric lields and angularly introducing the beam of particles into the elds. The result of the foregoing is to achieve a beam which spirals about the axis of the acceleration path. The combination of the electric elds and angular motion of the beam cooperate to provide stability and focussing.

lt is therefore an object of the present invention to provide a new and improved linear accelerator.

Another object of the invention is to provide a new method and apparatus for focussing a beam of charged particles during acceleration.

A further object of the invention is to provide a linear accelerator having a spiralling beam.

Still another object of the invention is to provide a linear accelerator having means to neutralize the eects of space charge.

Another object of the invention is to provide a linear accelerator wherein charged particles are angularly introduced into a series of accelerating fields substantially parallel to the acceleration path and radial iields transverse to such path.

2,836,759 Patented May 27, 1958 Other objects and advantages of the invention will be apparent in the following description and claims considered together with the accompanying drawing which is a schematic cross-section of the invention.

Referring to the drawing in detail, there is provided an elongated vacuum-tight tank 11 having conventional evacuating means (not shown) communicating therewith. Disposed at one end of the tank 11 is a conventional ion source 12 which provides a beam of charged particles 13 having a high value of current. The ion source 12 may readily be mounted within the tank 11 or, as illustrated, disposed externally to inject the beam 13 through a communicating tube 14. A plurality of hollow and openended drift tubes 16 is disposed coaxially within the tank 11. Such drift tubes 16 are of graded lengths with the shorter disposed adjacent to the ion source end of the tank 11 and are spaced-apart along the axis to provide gaps of increasing length with the shorter gap occurring at the ion source end of the tank.

A rst conductor 17 is disposed parallel to the ams of the tank 11, in insulated relation with respect to the tank and is connected electrically to alternate drift tubes 16 commencing with the drift tube closest to the ion source end of the tank (i. e., the odd numbered drift tubes). A second conductor 18 is similarly disposed parallel to the axis of the tank 11, in insulated relation with respect to the tank, and is electrically connected to alternate drift tubes 16 commencing with the second closest drift tube to the ion source end of the tank. The mountings (not shown) of the two conductors 17, 1S, as well as the two conductors and the electrical connections, may readily support the drift tubes lo within the tank 11.

To provide suitable acceleration of charged particles injected into the opening of the rst drift tube 16, a high frequency alternating voltage is impressed upon the rst and second conductors 17, 18 from a radio-frequency power supply 21. Since such accelerating voltage has a high frequency, the feeder system comprises two stein conductors 22, 23 electrically connected at a point of zero voltage by a shorting bar 24. Output terminals 26, 27 of the radio-frequency power supply 21 are respectively coupled to proper impedance points on the stems 22, 23 by capacitors 2S, 29 so that the combination of the stems, rst and second conductors 17, 18 and drift tubes 16 provide a resonant system at the frequency of the radio-frequency power supply. With the ion source 12', operating to inject a beam of charged particles through the drift tubes 16 along the axis of the tank 11 and with a target 31 disposed to receive the accelerated beam at the end of the tank opposite the position of the ion source, the structure thus far described and the operation thereof is that of a conventional linear accelerator.

To overcome the aforementioned diiculties and disadvantages inherent in conventional linear accelerators, structural modifications and additions are required which will be set forth hereinafter. An elongated electrode 32, having a substantially small cross-section in comparison to that of the drift tubes 16, is extended through the drift tubes along the axis of the tank 11. To impress a suitable voltage upon such elongated electrode 32, a conventional high voltage direct current power supply 33 is provided with one terminal 34 connected to ground and another terminal 36 connected to the electrode. A second conventional direct current power supply 37 is connected with a terminal 38 tied to the shorting bar 24 of the stem conductors 22, 23 'and another terminal 39 connected to ground. Such second power supply 37 provides a source of bias voltage to the drift tubes 16 and, together with the high voltage power supply 33, establishes a uniform radial electric eld Within each of the drift tubes 16. With 2': Diameterqf elongatedele'ctrojdez:'%lindi. Y A, Y 3. Erequ'en'c'y of radio-frequency power supply 21':l 12.6

the elongated' electrode 372"installed; as stated above;

the ion source 12 is disposedjin such manner as to inject sa i041 beam. 1 3 'enguafy .lfath v radial ,demic Y eld f the first drift tube 16; that iSf, the ion'asourcehisdispoised olf-center with VrespectV td the center 'ofthe endswall is angular-ly to the'elongated electrode 32 and the positiye terminal of such power supply is grounded. y A low value of bias voltage is providedy to the drift tubes i6 by the ,connection of the' power supply 57 to the short'ving bar 2'4 of thest'emY Y. conductors 22,23. It willfbe readily apparent then that 'ai uniform radial electriciieldisest'ablished between the elongated electrode 32 and the drift tubes V16. `It is well known` that chargedparticles moving along a path transverse to an electric field are'a'ct'ed upon' by a force which causesI saine to transverse a curved path'. Thus by inject-k ing particles frorn the ion source 12' as abearn alongf a pathV which has' a component'paralleltothe electric held and also a component transverse to' the field, the' beam is forced to follow a spiral-path because-of they attractive force established toward the elongated electrode 32. The particles of such beam are also a'ctediupo'n' by `a centrifugal force' which isa repulsive .force away Yfrom the elongatedV electrode 32. When the attractive force and the repulsive forceacting upon the particles are balanced there exists a stable potential well whichconiiues the charged particles to a substantially constant spiral path for the length ofthe accelerator. j It can be readily proven mathematicallythat the motion of the particles parallel to the elongated electrode 32 is independent of the established potential Well so that accelerating electric elds disposed along the spiral path do' not disturb the potential Well. V.. f .f

Thus, inV operation theion source i2 introduces' charged particles in 'a beam 13 with an initial velocityV at an angle to the direction of theelectric held between the elongated Y electrode 32 andtheA drifttubesl. An angular momenturn is thenimparted to the particles of the beam bythe radialelectriciield and the beanifollows a constant spiral nearest the ion rsource 12, the radio-frequency power supply 21 impresses an increasing electric iield between theY two/drift 'tubes to increase the energy of the yparticles during traversal of the gap. Because of the strong forces acting upon the particles to maintain the beam in the potential well the substantially lesser defocussing forces at the gap produced by the accelerating electric field are not effective to disrupt the spiral path. Thus phase stability, aswell as particle focussing, are maintained during the vacceleration action. f While the Voltage of the radio-frequency lpower supply 21 is reversing polarity the particles drift through the second drift tube 16 and reachl the following gap. whenV an accelerating `electric eld is presented. The foregoing action ,continues4 successively While the bearn 13 spiralsin the potential well between the'` elongated electrode 32 and the drift tubes 16 until the linal gap has been crossed; and the target 31 reached. Since the bearn 13 has a spiral conguration about the elongated electrode 32, the presence of the elongated electrode at the center of the bearn introduces a chargefof opposite polarity to that of the particlesl and tl'nereby v neutralizes the effects of spaceV charge defocussing.

In accordance with the foregoing, it has been found that a bearnof protons is accelerated to provide a stable well-focussed output beamV with higher4 energy and with substantially no vloss of particles under the following con-V ditions:

l. Diameter of'drift tubesld: 6 inches.

niegacycles per second;

4: Ratio of gap' between' drift tubes 1'6 to' length between Y center-lines of successive gaps: 0.2.

Y 5. Constant voltage gain per gap: l kilovolts. Y

6. Voltage between elongated electrode 32 and drift tubes 16: 50 kilovolts. 7. Injection voltage: 25 kiloYOl-,tgj 8. Twelve drift tubes 16 givinga nal energy of l2()v kilovolts. n

n will". be readily' apparent-fram the have example that, by a mere alteration .ofptlievalues'setfortd a n iu'ch greater iinal energy is attainable.r Also, because Yof theV necessary increased lengths ofthe final drift tubesin an isvsimplviied because of the greatstability andwfocussing of the beainat theoutputrof the .first section and, permits the use of shorter lengths of drift tubes atrthefhigh energy end ofthe accelerator than `isotherwisepossible.

f Whilevthe salient features ofthe present invention ,haveV been described in detailwith respect to vltSingle e`rrihodi= nient, it willV be apparent thatnumerous' modiiications Vmay be inadewithiu the spirit and'scop'e of the, invention audit isftherefore not Vdesired to liniitttli'e `invention to the exactj detailsv shown anddescribed: eXcept insofar as they inlay be definedin theA following claims. y Y

said electrode, whereby said'part'iclesare accelerated in YVs/'hatis lairnedis:; v Y 1. VIn a linear accelerator, the combination! comprising a vacuum-tight tank,V aV pluralityrof l,drift tubes mounted in coaxialspacedfapart, relation along a linear path in said tank, means connected to said drift tubes t'o'provide` .a

plurality of radio frequency. accelerating electric fields along said linear path, anV elongated electrode extendedY through said `drift' tubes, direct currentpo'p/erfsupply posed at one end of said'tanlcfor inectingparticles having lan angular momentum into said radial. electric fieldY and' said particles having a charge attracting same ktoward a substantially constant spiral about` saidelect'r'ode.

2. In a linearV accelerator, Ythecorrlbination conrprising layacuunfi-tight tank, Ia plurality of drift tubesl mounted l irrcoaxial spaced-,apartrelationj along alinear path,v Va V source of radio-frequency accelerating voltage connected between successive pairs of said diiftl tubes tosprvidea plurality of accelerating electric Vfields along said linear path, an elongated electrode mounted coaXially with t respect to said drift tubes along Vsaid linear path, direct disposed to inject positivelyy charged particles into .said

radial electriceld withan angular momentum, whereby particles are accelerated in a substantially constant spiral about said electrode. Y

3. In a proton linear acceleraton-the yccnnbination coni? v prising a vacuum tight tank, Va plurality of driftt'ubes t, coaxially mounted in spaced-.apart ,relation along a linear path, a source of 4radio-frequency accelerating voltage connected betweenV successive pairs of said drift tubes to provide a plurality ofV accelerating I'electric fields along said linear path, an elongatedy electrode mounted coaxially with'r'e'spect tev said drift tubes' along said linear' path, direct current power supply'meafns hav-v a negative and a positive terminal said,v negative terminal connected to said electrode and saidV positive terminal connected to said drift tubes, and a proton ion source mounted angularly at one end of said tank to provide a beam of protons directed into the region between said electrode and drift tubes with an angular momentum, whereby said protons are accelerated in a substantially constant spiral about said electrode.

References Cited in the le of this patent UNTTED STATES PATENTS Alvarez Mar. 20, 1951 FOREIGN PATENTS Great Britain Oct. 6, 1954

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