US2982917A - Method and apparatus for pulsing a charged particle beam - Google Patents

Description

y 1961 K. AALAND EI'AL 2,982,917

METHOD AND APPARATUS FOR PULSING A CHARGED PARTICLE BEAM Filed April 10, 1958 2 Sheets-Sheet 1 O c l O E a f I l 'ii T TIME @ INVENTORS.

KRIS AALAND 1 By ROBERT W KUENN/NG RAYMOND K. HARMON May 2, 1961 K. AALAND EI'AL METHOD AND APPARATUS FOR PULSING A CHARGED PARTICLE BEAM Filed April 10, 1958 2 Sheets-Sheet 2 wm sm o mwJDm o ATTORNEY.

6 M v M m MM T NDW w EN V A K J. WLW A D i ATN R0 gm RoA KRR B KWEEM E mofimwzmw M 92 O o EOE; o m Q w 555$ o ulzmmwtao 0 $225 9N mw ww Unite tates Patent i 2,982,917 Patented May 2, 1 961 METHOD AND APPARATUS FOR PULSING A CHARGED PARTICLE BEAM Kris Aaland and Robert W. Kuenning, Livermore, and Raymond K. Harmon, Hayward, .Calif., assigns to the United States of America as represented by the United States Atomic Energy Commission Filed Apr. 10, 1958, Ser. No. 727,758

'7 Claims. (Cl. 328--229) The present invention relates generally to the formation of pulsed beams of electrically charged particles, and more particularly to an improved method and apparatus for accomplishing same.

Pulsed beams of electrically charged particles areextensively employed for a variety of purposes in nuclear research, electronics, and various related fields. In'the operation of various particle accelerators, for example, it is often times necessary that a continuous charged particle beam be pulsed prior to introduction to an electric or magnetic accelerating field in order for the particles to be successfully accelerated. Moreover, in a wide variety of nuclear experiments,-e.g., neutron time-of- ,flight experiments, it is desirable that a particle beam impinging on a nuclear target to initiate certain nuclear events he pulsed with extreme precision such that the occurrence of the events can be accurately determined with respect to time.

While many of the present day charged particle accelerators are capable of producing precisely formed short duration pulses of charged particles in their normal operation, it. is often times desirable that a machine of the type that normally produces only a continuous particle beam be selectively adaptable to'precision pulsed operation for the sake of versatility. For example, at installations having only a continuous beam machine available, the provision of means for pulsing the beam 7 resourcefully facilitates the performance of time-of-flight experiments as well as a variety of other additional experiments which could not otherwise be conducted with the single'continuous beam machine. Moreover, it is often desirable that beam pulsing 'means beemployed with pulsedmachines inorder to pulse the pulsed beam at a repetitionrate diiferent than the normal operating repetition'ra'te, 'i.e., to chop the normal beam pulsesJ Conventional radio frequency sweeper systems, i.e.,

systems wherein a continuous beam is directed between a pair of deflection plates energized with radio frequency voltage and a fixed portion of the deflected b'eamis collected each radio frequency cycle" while the remainder is transmitted, are c'ommonlyemployed to pulse a continuous beam.- Such systems are variously inadequate, however, for purposes ofoptimum flexibility and performance. In such a system the pulse width is only conitrollable over an extremely narrow range and the reps-- tition rate is fixed. Moreover, due to the'sinusoidalconfiguration of the radio frequency energizing'v'oltagefthe shape of the beam pulses are not rectangular but Gaussian, and accordingly are terminated with relatively little precision. For the same reason, symmetrybetween consecutive pulses" is diflicult to obtain andmaintain.

Considerable advantage istherefore to be gainedby the provision of aihighly flexible beamfpulsing method and means for producing, fromia 'continuousfparticle beam, rectangular pulses' which are always consecutive 1y of the same shape. Other advantages will be realized where the beam pulsing method and means include varia-f is variable over an extremely widerange.

many of the limitations and advantages of prior art beam.

pulsing systems. -More particularly in the beam pulsing method of the invention a continuous beam of electrically charged particles (i.e., ions or electrons) which may be produced for example by a particle accelerator are directed between a pair of spaced deflection plates.v Provision is made for the deflection of the beam to one side of the beam;traverse axis between the plates by establishing a unidirectional electrostatic field therebetween. The deflected beam may then be collected as-by disposing a collector electrode transversely on one side of the axis so as to intercept the deflected beam. The electrostatic field, however, is repetitiously neutralized during precise time intervals by successively applying a train of substantially flat front voltage pulses to one of the deflection plates with the pulses being of equal magnitude and opposite direction with respect to the electrostatic field, and then after a time delay, to the other deflection plate. The beam is not deflected during the neutral interval of the electrostaticfield and is not collected but directed axially past the collector electrode to emerge as a pulsed beam. Moreover, since the same flat pulse front is employed to initiate as well as terminate each neutral electrostatic field interval between the deflection plates, theresulting beam pulses are consistently rectangular and of precise time durations. vThepreciscly' formed pulsed beam may be further shaped as by passing same through a buncher cavity. and then employed as desired.

The invention also provides apparatus in which the foregoing method may be conducted, such apparatus comprising in general means for providing the indicated unidirectional electrostatic' field between the spaced deflection plates, means for collecting the deflectedcharged particle beam, means for generating the flat front voltage pulses, and means for-applying said pulses directly to one deflection plate and after a time delay to the other deflection plate. Accessory apparatus for utilizing the pulsed beam is also provided. v 1

Accordingly, it is an object of the present invention to provide a method and apparatus for pulsing a continuous beam of electrically charged particles.

Another object of the invention is the provision of a charged particle beam pulsing method and apparatus for producingprecisely formed beam pulses having durationswhich may be varied over an extremely wide range.

It is still another object of the invention to produce charged particle beam pulses at a repetition rate which An important object of the invention is to provide a pulsed beam of. charged particles having equal symmetry between consecutive pulses.

.Yet another object of the invention is to'providean optimurnly'focused pulsed beamfof charged particles A further object of the invention is the provision-of a highly.flexible system for producing substantially recclaims consideredtogether withfthe accompanying drawing, of which; 7 I

, Fi gurej l is a graphicalfillustrajtion of voltage and the manner in which they are applied to? 'beam'de- 'flection 'plates in the/present. invention; and

Figure 2 is'fajschematic block'diagram' of preferre ap ra e e en v ti t t t 9 the men n embodied as a beam pulsing system for a charged particle accelerator.

Considering now the method of the present invention in some detail with refer ence to the accompanying 'd'ra'wing, it is contemplated that there Will'first be p'rovided in the initial steps of the method aeontinuous beam of electrically charged particles," i.e., electrons or ions,- which may be produced for example in various continuous beam charged particle accelerators. The' continu- "ous beam. is deflected laterally of the normal axial direction of beam motion preferably by subjecting the beam to a unidirectional electrostatic field established at right angles to the beam axis. The above-noted electrostatic field may be provided as by disposing deflection plates on oppositesides of thebeam in equal spaced relationship therefrom, iQe., the continuous beam is directed axially between the spaced deflection plates. -The term axially is takenherein as referring to the beam traverse front of each pulse b be as vertical as possible, viz., t, should be extremely small.

. Each pulse b is additionally applied, to the electrostatic field in aiding relationship to the direction thereof after an adjustable time delay t less than the duration of the flat crest (i.e.,' the positive or negative peak) of thelpulse. Theforegoing mayfbe accomplished as by applying each pulse b produced by the previously mentioned pulse, generating means in. negative relationship to the electrically unbiased deflection plate through a suitable variable time delay line to introduce the time delay i The resulting voltage distribution at such electrically unbiased deflection plate is thus as illustrated by curve d ofFigure 1. The voltageabruptly changes during rise time t of the Verticaltrontpulse b from zero potential to the negative amplitude (negative crest) of such pulse which is equal to the magnitude of bias a. Moreover, inasmuch as pulse b is applied to the unbiased plate after the time delay I the voltage as such unbiased plate. with respect tothe applicationof pulseb to the biased plate, abruptiyrchangcs after such] time delayt toth-ereaft'er rcach the negative value of bias a Subsequent to the deflection of the continuous charged particle beam, the deflected beam maybe intercepted and collected. The foregoing is preferably accom plished by disposing a collector electrode laterally of the originalbeam direction and positioned to receive the deflected beam after passage between the deflection plates;

The deflected continuous beam is accordingly terminated at 'thecollector electrode and, in the absence of further collector electrode.

i :In the method of the present invention a pulsed beam is formed by periodically neutralizing the aforementioned electrostatic beam deflecting field for precise increments of time. During such increments of neutralized field,=the particle beam is not deflected and accordingly operation upon the beam, is not transmitted beyondsuch' not intercepted at the collector electrode, but instead is transmitted axially 'past the electrode. Consequently as the beam is periodicallyswept from the collector elec- The voltage pulses are 'bestprovided at an adjustable repetition rate as byxineans of variable .rate 'pulse generating means in orderthatithe beam pulse frepetition rate may be correspbndinglyvaried. Each pulse bis applied tozthe electrostatic field in opposition to the directionthereof asby applying-the pulses negative relationship to the positively biased one of thefdeflection plates. The resulting voltage distribution at the biased deflection plate is consequently as depictedby curvec of Figure: 1; 'As shown thereinithe voltage abruptly.

changes Efrem the positive bias maintained thereon (curve a) to zero uponthe occurrence of. applied pulse b More particularly, the electrostaticfield'due to fbias fiz'is neutnalizcd,by pulse b in a1tirneequal,toithei ise time t, 'of'the. 'frbnt of such negative goiiig pulse}. AQ-

l adi 'r h he w s sa i ie e u r m tev aa ss be r s i .e r sqsaisa as precisely formed as possible, it is necessary that the after rise time t,. a

The resultant:voltagedistribution existing between the deflection electrodes as obtained by a summation of the individual plate voltage'distributions c and d, respectively, accordingly comprises a series of negative pulses e deviating essentially from positive bias a to zero voltage as illustrated by curve f of. Figure 1. It is to be noted that each pulse e is symmetrical in that the rise and fall times of 'thexpulse are-identical. This is dueto/the fact that thBvfl'OIllT of the same applied pulse b isv employed to drive the ,voltagewbetween. the deflection plates from zero to the magnitude of positive bias a a was, previously employed to drive the voltage from positive bias 0 to zero. Itwill be further noted that the negative crest Width z,, to each pulse 0, Where r =t t,. determines the "time. during -which the volta'gebetween the plates is zerd, viz., the time during which the electrostatic deficction fieldis neutralized and the beam is undeflected toform a beam pulse. The width of the; beam pulses may, accordingly be varied by varyingwthe delay time t between applications of each orient the pulses b to first one and then the other of the deflection plates.

Preferred apparatus for conducting. (the foregoing method of the present invention is illustrated in Figure 2 .of the drawing wherein such, apparatus is embodied as a system for pulsing the beam of a continuous beam erably comprises a 'p-air of flat deflection plates 13, 14.

equallyspaced on opposite sides of an axial pathbetween the" accelerator" beam, extraction structure and1 target. While plateslS, '14 maybe diametrically opposed, it is .moreiprefenable that the plates 13, 14 be disposed in axial-- 1y staggered relationship grounded, plates 16, 1?,

respectively spaced. in; diametri'c' opposition therefrom. The latter -arrangement minimizes undesirable capacitative effects that existwith 'the former, arrangement of the deflecting plates in diamet ric opposition. 1.

In-order to bias deflectionpl ates 13,14 and establish a unidirectional electrostaticyfield therebetween, a suit- .able 'D.C. voltage-supply 18 is connected to oneplate 13. and .the other plate l4 is unbiased It will.,,be appreciatcd that plate 13 rnay be biased either positively or negatively, I. however, .in the interest of consistency with the examples previously.descr ibed inrelation to the methol of the invention, plate is positively biased by. connection to the positive terminal of'voltage supply 18. The

thus deflects the beam fromaccelerator v1 1Vlaterallyiiof the axial path between the accelerator and target 12.,

Beam collecting means are preferably disposed in receiving relationship to the deflected beam andsuch means is best provided as a collector electrode '19 disposed between plates 13,v14 and target 12. Electrode 19 is transversely oriented and closely spaced laterally of the axial path between accelerator 11 and target 12 in order to intercept the deflected beam and dissipate the energy of able pulse generating means 21 for applying substantially vertical front-flat crest pulses to deflection plate 13 and, after a time delay, to plate 14. More particularly, pulse generating means 21 preferably includes a square wave generator 22, e.g., a free running multivibrator, the repetition rate of which may be varied over a wide range.

-Squa're wave generator 22 functions as the rate generator for-the beam pulsing system and may be synchronized in thevconventional manner with an externally generated injection signal. The output of square wave generator 22 is coupled to a differentiator 23 which may b60011- ventionally provided, for example, as a series capacitor and shunt resistor. Ditferentiator 23 is eifective in pro-- ducing a series of alternately positive and negative spikes which respectively correspond to the leading and trail- 1 ing edges of the square wave pulse from square wave generator 22.

The. output of differentiate: 23 is coupled to a shaper and inverter 24 which-is advantageously selected as one having parallel outputs 26, 27. For optimum performance, shaper and inverter 24 is arranged to produce substantially vertical front and flat crest output Pulses, e.g., unsymmetrical trapezoidal pulses having fast rise'times of the order of seconds, in rcsponse'to the-positive I .is-normally deflected and intercepted by collector elec- ,trode '19, is abruptly swept from such electrode to the axial pathbetween the accelerator; and target 12. Consequently, precisely formed beam'plulses occurring at the repetition rateofsquare wave generator 22 are directedtoward target 12. It isto be appreciated, moreover, that an electrostatic lens eifect is produced by the field configuration established between deflection plates 13, 14 and ground plates 16, 17. During 'the'neutral time interval the individual fields between plates 13 and 16, and 14 and 17, respectively, which in'combinationjestablish the resultant'field o flzero strength, tend to focus the beam upon target '12. Conversely, the beam is defocused during the remainder of the time when deflected by the field for collection by collectorelectrode 19. Such defocusing of the beam at the collector facilitates dissipation ofthe beam energy thereat over a relativelywide surface-area resulting inminimized burning of the collector electrode and an attendant decrease in the frequency with which same must be "replaced. The pulsed beam of the present invention may accordingly be said to be optimumly focused in as much as the portions of the beam useable at the target 12 are concentrated was pulses of increased current density ,while the unused portions of the beama're dispersed as pulses'of decreased current density for collection atthe collector electrode 19resulting in optimum dissipation of directed at target 12 be further bunched or shortened in spikes applied thereto from ditferentiator 23, the negative spikes being eliminated. The pulses are additionally inverted in the shaper and inverter whereby the output at each of parallel outputs 26, 27 comprises a series of negative substantially vertical front and flat negative crest unsymmetrical trapezoidal pulses occurringatthe repetition rate of square wave generator 22; It is to be appreciat'ed that many. circuits are well known in the electronics art for accomplishing the purposes of shaper and inverter 24, and therefore no specific circuit is described nor illustrated herein.

' 1 One output 26 of shaper and inverter, 24 is directly connected to a variable bias pulse amplifier28, the output of which is coupled to positively biasedrdeflect'ion plate 13. "The other output 27 of the shaper and inverter is connected through suitable pulse delay means, 'e '.g., a

variable time delay line 29, to a similar pulse amplifier 31. 'The output of the latter amplifier 3 1 is in turn coue pled to the unbiased deflection plate 14. Identical substantially vertical front and flat negative crest pulses-are thus. applied first to deflection plate 13' and then,-after a time delay introduced by delay line 29, to deflection vplate 14. Moreover, the bias of amplifiers 28, 31 may be'varied to adjust the amplitude of the output pulses therefrom I equal to the magnitude ofthe bias supplied by voltage sup- .spectivelyfor lprecise increments of time deterr'nined,

the delay tin1e of delay line 29.and occurrin'g' at there e'tiQ t me rarest uarewa 'e n at r 2 D' i e em I tragltime'incrementtthe beam from accelerator 11, which emerging fromthe'buncher. cavity 32 in driftingithrough v time durationprior to impingement on the target. To accomplish the foregoing a buncher cavity 32 maybe disposed between collector electrode 19 and target 12 in I axial alignment with the path between the target and accelerator 1 1; Buncher cavity 32 is designed to resonate at a frequency corresponding to the repetition rate of the beam pulses or a multiple thereof and is energized by a radio frequency source (not shown) in the conventional manner. The radio frequency signal is applied at the fre-' quency of the" pulsed beam or at a multiple thereof to velocity modulate the-beam pulses. In this connection it is necessary that the beam pulses be in proper phase with the radio frequency modulating signal, and to accomplish this'purp osesquare wave generator122 is'synchronized to the modulating signal as by connecting a conductor 33 between buncher cavity 32 and the; injection input of the square wave generator; The velocity modulated pulses the field free space between thebuncher and target 12 then form shorter bunches in impingement with 'the' target. As regards the adjustment of various'of the characteristicsof the, pulsed beam produced by the-above described apparatus'of. the present invention, it is to be notedthat the repetition r'ate of the beam pulses may be varied over a'considerable range by correspondingly varying the repetition rate of square ,wave generator'. 22. In this connection' conventional square wave generators are readily 1 available which may be employedas generator 22-to correspondingly produce beam pulses at readily attain able repetition rates in a range of the order of from 1 utilizing a modulating, frequencyw atbuncher cavity V which is 'a multiple of the generator 22.

rs tiq i 9 q a W tt- The duration ofthe pulses'is also continuously assent? adjustable over a wide range by variation otthe delay time i of delay line129 inasmuch as the pulse negative crest durationi t is substantially'given byfthelexpressionfr are commonplace in the art which may be employed as delay line 29 to produce timedelays in a range of the order of 10- to 2 -10 -seconds whereby an adjustable range of beam pulse durations of the order of 10,to 10 seconds is easily attainable inpractice.

While the present invention has been hereinbefore described in terms of specific steps in the method and with respect to a single embodiment, it will ,be apparent that numerous modifications and variations are possible within the spiritand scope of the invention and thus it is not intended to limit theinvention except by the terms of the following claims. a 1

What is claimed is: t 1 V 1. Apparatus for pulsing a continuous beam of electrically charged particles comprising deflection plate means equally spaced on opposite sides of the axis of beam traverse, bias means connected to'said deflection plate means for establishing a unidirectional electrostatic field at right angles .to said, axisv to deflect said beam laterally thereof,pulse generatingfmeans for; generating unidirectional vertical front and flat crest, electrostatic field neutralizing pulses, and means connecting said pulse generating means to said deflection plate meansfor successively applying said pulses ,to said deflectionplat'e means in opposed relationship to the direction, of said electrostatic field and after a time delay in aiding relationship to the direction of said field whereby a pulsedtbeam is produced along said axis. 1 3

3 2. Apparatus asdefined by claim 1 further defined by said deflection plate means comprising a pair of deflection plates equally spaced on opposite sides ofsaid axial path of motion and disposed in axiallystaggered relationship, and a pair of grounded plates equallytspaeed on opposite sides of said axial path of. motion in diametric opposition to said deflection plates. l. l l :3. Apparatus as defined by claim 1 further defined by said-means connecting saidpulse generating'meansrto said deflection plate means comprising a conductor and a time ,delaylline cornmonlyfcoupledfto said pulse Igeneratingmeansand respectively coupled to said deflection platetmeansn 4. Beam pulsingapparatus,cgmprising a pairfof staggered deflection platesequally spaced, on opposite sides of anaxial path of,motion ota-continuous .beam of electrically charged particle's, apair of grounded plates equally spaced onnopposite sidesilof said axial ,path of 5 motion in diametric opposition to said deflection plates, a D.CL,voltage supply connected-to onelofjfsaiddflee; tion plates forestablishingialielectrostatic field to deflect said beamtlater'ally oflsaid axial path, 'ofnaotion, beam collecting means spaced from said e and disposed in rceivipgl relationshipwwitli jthe, laterally deflected beam, :pulse generatingrneans havingafpair of parallel outputs for producing repetitiou sly occurring uni: directional vertical front and flat crestelectrostatic field neutralizing pulses, ,th'eifirst of said lparallel. outputs a directly coupled to oneof -said deflection plates to apply said pulses in opposition to the direction ofsaid electro static field, and a time delay line connected; between said, second parallel eutput and said second deflection plate; to

apply said pulses in aiding relationship to thefdirection", 70

of said electrostatic. field whereby beamjpuls'es liaving durations substantially .et ual tov the time ,oi time idelayline, emerge beyond said beanrcollecting on said axialpath erases-n; In a system forrproducing -a pulsed chargeflparticle beam at a target, the combination comprisinga'cliarged path o'f motion e L axially, disposed with respect to said; axial path between said collector electrode and said targ'et for velocity modu letingna charged "particle beam, and conductorjnieans supply connected to said deflection plate means for establishinga unidirectional electrostatic fieldtrans'verse to' said axial path, beam collecting means transversely disposed between said deflection plate means and said target and spaced from said axial path, pulse generating means for generating unidirectional vertical front and flat crest voltage pulses, means connecting said pulse generating means to said deflection plate; means in opposite sense to the direction of said ee'lctrostatic field, time delay means connecting said pulse. generating means to said deflection plate meansin aiding relationship tothe direction oisaid electrostatic field whereby beam pulses having durations substantially equal to the time delay of said timedelay means are produced along said axial path and a buncher cavity disposedin alignment with said axial path and between said beam collecting means and said, target for velocity modulatingthe. beam pulses. I

6. Apparatus for pulsing the beam of a continuous beam charged particle accelerator comprising a pair of deflection plates disposed in axially staggeredrelationship on opposite sides of a path in axial alignmentwith said accelerator, a pair of ground potential platesrespectively disposed in diametric opposition to said deflection plates on opposite sides of said path therefrom, a positive D.C. voltage source connected to the first, one of said deflection plates forapplying a D.C. bias voltage thereto, a collector electrode transversely disposed in lateral spaced relationship from said path on the distal side of said deflection plates with respect to said accelerator, variable repetitionrate negative pulse generating means for produeing vertical front and fiat negative cr est voltage pulses having amplitudes equal toflthemagnitude of said DlC. bias voltage, said pulse generating means having a pair of parallel outputs, means directly coupling one of said outputs to saidfirst deflection plate,and a variable time delay line coupling said secondoutput to said second deflection plate whereby the beam, of said accelerator is preciselyipulsed at the repetition ratef of said pulse generating means and for durations substantiallyequal to the delay time of said time delay line. e

7. In a charged particle accelerator for bombarding an axially aligned target with substantially,rectangular beam pulses of adjustable time duration and at a: variable repetition rate, the combination comprising first and second flat deflection platesdisposed in axially staggered relationship and equally spaced on opposite sides oi an axial path between said accelerator andrsaid target, apair of ,ground potential plates respectively disposed in ldiametric opposition to said deflection plates n. oppositesides of said ,axialfpath, a; positive D.C. voltagegsource c onnected tdsaid first defleeti on' plates fer applying a D.C. bias voltage, thereto, acollector electrede. transversely disposed in laterallyspaced relationship from said path between said deflection plates -and said, target, a variable repetition rate square wave generator, a differ} entiator connected tothe output of said generator for producing a series of alternately positive and negative spikes; a shaper and inverter connected to the output of said differentiator and having first and second outputs in parallel, said shaper and inverter producing atsaid first line connected to -said second output, 'asecorid variable bias pulse' amplifier connected between said, In eny line and saidsecond deflection plate,a,bunchercay1tyco connected betweensaid'buncher cavity andsaid square wave generator for injecting synchronizing signals from said cavity to said square wave generator.

References Cited in the file of this patent UNITED STATES PATENTS Aekermann July 1, 193 Opsahl Oct. 24, 1933 Schlesinger Dec. 31, 1940 10 Larson Nov. 24, 1947 Labin et a1 Apr. 6, 1948 Opsahl July 18, 1950 Van Overbeek Nov. 4, 1952 Robinson Sept. 22, 1953 Ross Dec. 27, 1955 Jurgens Aug. 20, 1957 Ross Oct. 22, 1957

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