US2941077A - Method of enlarging and shaping charged particle beams - Google Patents

Description

1 l l I June ,1960 R. c. MARKER 2,941,077

METHOD OF ENLARGING AND SHAPING CHARGED PARTICLE BEAMS Filed July 7, 1958 2 Sheets-Sheet 1 F/ELD STRENGTH 2 INVENTOR. ROY C. MARKER BEAM DIAMETER ATTORNEY June 14, 1960 R. c. MARKER 2,941,077

METHOD OF ENLARGING AND SHAPING CHARGED PARTICLE BEAMS Filed July '7, 1958 2 Sheets-Sheet 2 /2 W VAR/ABLE ac.

H POWER ,3? 29 I /0 SUPPLY -n l I 37 f f U INVENTOR. /3 l R0Y c. MARKER LL) m BY A T TORNE Y METHOD or ENLARGING AND SHAPING CHARGED PARTICLE BEAMS The present invention relates to the enlarging and shaping of charged particle beams, and more particularly, tothe spreading and shaping of such beams with steady state spatially heterogeneous fields.

In the irradiation of materials by intense beams of charged particles, i.e., electrons, ions, or the like, issuing, for example, from a high energy particle accelerator, it is generally advantageous to appropriately disperse the particle beam in order to irradiate large areas of material. Similarly, in order to transmit the charged particle beam through thin windows in the vacuum envelope of the accelerator at sufiiciently low power densities to avoid overheating ofthe window material, the beam is conventionally ditsributed over as large a Window area .as possible. Moreover, the intensity of a charged particle beam is often preferentially distributed for purposes of producing desired effects in the material being irradiated and, by porper overlapping of the irradiated areas, to obtain uniformity of dose in the irradiated material.

Heretofore, the desired distribution or dispersion of charged particle beam intensity to accomplish the foregoing, as well as other effects, has been conventionally obtained by scanning the beam through a predetermined 'angle withtime varying magnetic or electric fields. The beam center is cyclically displaced by the time varying field to opposite sides of the direction of beam propagation. A moving spot of charged particles thus continuously traces a line on the material undergoing irradiation whereby the beam intensity is distributed .along such line with respect to time, The material may accordingly be appropriately moved relative to the line traced by the scanned beam to thereby uniformly disperse the beam over the entire material or other wise distribute the beam as desired. In order to assure 100% material coverage by the beam as well as to produce various preferred effects in the material, it is necessary to appropriately coordinate or synchronize the scanning with the rate of material flow. This entails the use of complicated synchronizing apparatus resulting in disadvantages from the standpoint of complexity and difiiculty of adjustment together with attendant high cost. Conventional time varying beam scanning methods are further disadvantageous in that such methods are subject to a relatively high degree of unreliability and frequency of breakdown by virtue of the electrical components and energization necessitated therein.

The present invention overcomes the foregoing disadvantages and difiiculties associated with conventional scanning methods of charged particle beam distribution by providing a method of distributing a particle beam without the entailment of time varying magnetic or electric fields, thus resulting in simplicity of construction and control. More particularly, in accordance with the present invention, a charged particle beam is spread and shaped by a steady state field of force having spatial heterogeneity, i.e., a fringe field or field produced for example at the leakage air gap between magnet sole faces or at the edges of spaced electrodes. The resulting beam States ate pattern produced upon the material to be irradiated may be advantageously made uniform over the entire area of the material or may be concentrated in specific regions to take advantage of consistant non-uniformities in the particular material being irradiated by appropriate adjustment of the spatial heterogeneity of the field. The method of the invention moreover contemplates the generation of the spatially heterogeneous fields by means of permanent magnets thus resulting in extreme simplicity, ruggedness and reliability in the 'beam spreading apparatus.

It is therefore a primary object of the present invention to provide a method and apparatus for the steadystate spreading of a charged particle beam in order to irradiate large areas of material and/or to transmit the beam through thin exit windows in the beam producing apparatus at sutficiently low power densities to avoid overheating of the window material.

Another object of the invention is the provision of spatially heterogeneous fields for the enlarging of beams of charged particles.

Still another object of the present invention is to provide a method and apparatus for enlarging and shaping a charged particle beam which is readily adapted to empirical adjustment in the distribution of the beam to produce exactly the dosage pattern desired.

Still another object of this invention is to provide a method of spreading a charged particle beam in two dimensions simultaneously.

A further object of the invention is the provision of a simple, reliable beam spreading system which assures coverage of material continuously passed under the beam.

The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the following specification taken in conjunction with the accompanying drawings, of which:

Figure 1 is a schematic representation of magnet means for generating a spatially heterogeneous 'field in accordance with the present invention for spreading a charged particle beam passed therethrough;

Figure 2 is a side elevation sectional view of the magnet means and field taken along the line 22 of Figure 1 and showing the spreading action of the field upon the particle beam;

Figure 3 is a graphical illustration of the-field strength profile of the field of Figure 2 across the diameter of the beam; I

Figure 4 is a schematic representation of magnet means for generating a spatially heterogeneous field transverse to the center line of a charged particle beam for spreading the beam equally about the center line;

Figure 5 is a side elevation view of the magnet means and field of Figure 4 taken along the line 5-5 thereof and showing the symmetrical spreading action of the field on the beam;

Figure 6 is a side elevation view of an embodiment of beam spreading apparatus in accordance with the presembodiment 3 tiorifield for increasing the dimensi particle beam in only one direction -or plane.

Proceeding now with a description of the beam enlarging and shaping method of .the present invention,.it is contemplated that there will .first be provided a high energy chargedparticle beam as indicat'ed'genera'lly 'at 10 of Figures 1 and 2. The beam 10-may be' composed of electrons or ions. Such charged particles are preferably issued from an exit window 11,'or"equivalent' me'ans, of a suitable accelerated charged particlesource 12 such as a linear accelerator, cyclotron, Van de Graafl generator, resonant transformer, or the like. The beammay be continuousor'pulsed depending upon the mode of operation 'of, the particular source 12 employed. 7

The beam 10 emerging from window 11 maybe ad'- vantageously directed upon a material 13 to be irradiated in accordance with conventional practice. In this connection, material 13' is often supported upon a moving conveyor (not shown) transversely "disposed relative to the axis of beam 10 and appropriately spaced from window 11. The beam '10 .accordingly travels in substantially a straight line or path towards the material 13 to be irradiated. I-n the absence of appropriate beam modification, the-beam 10 would impinge upon but a small area of the material '13 substantially'equal to-the cross sectionaliarea-of the beam. With the material-moving transverse to the bearnfthe irradiated areawould accordingly appear as a narrow line having a width substantially oils f a charged diametrically opposed therefrom.

equal to the diameter of beam 10. It is-to be' appreciated that under the foregoing circumstancesfthe' beam energy is delivered to but a small percentageof the material surface resulting in intense irradiation of a correspondingly small proportion of the material mass and substantially less orno irradiation of the remaining larger proportion of s'aid mass. f l In accordance with thesalient features of the present invention, the beam 10 is modified bya spreading action prior to impingingthe material 13 so-as to adapt the beam cross sectional area to the size of the material and thereby distribute the beamintensity or energy-Fover all parts of 'the material as desired. More particularly, the beam 10 issuing from window 11 is directed'thr'ough a suitable steady-state spatially heterogeneous 4 field [14 "which is established intermediate the accelerated particle source 12 .and material 13 to be irradiated. The field 14 may be electric or,-' m'ore preferably,-magnetic and the field heterogeneity is in adirection top'roduce a non-uniform distributionof chargedparticle forces transverseto the beam axisi Hence, an' electricfi'eld'in accordance with the present invention is of a type which-exists .at the edges-or-corners of direct currentjenergizedelectrodes disposed in axially spaced-apart relationship along the axis of beam 10 and spaced radially outward therefrom.

erated as by means of C-shaped magnet 16km Figures 1 and 2) which is disposed radially outward from beam 10 with its pole faces17, 18 in atransverse plane relative to the axis of beam'propagatiori. The plane of magnet 16 is positioned intermediate exit window ll and material 13 and themagnetic field 14 generated insuch plane is spatially heterogeneous. The lines of magnetic. flux (denoted by dashed lines in the figure) generated by the magnet extend along arcuatepaths'between the pole'facefs 17, 18. Thespacing between adjacent flux lines progresj sively increases transversely away from the magnet '16 towards beam 10. Since the strength of a magneticfi'eld at a point in space is dependent upon the number of flux lines per unit of area proximate such point, the strength of field 14 transverse to beam ltlprogressively decreases with respect to distance away from magnet 16. Accordingly, the strength of magnetic field r14 varies across the diameter of beam 10 as shown generally by' he diam tric field strength pro-file of Figure 3. As shown'in thefigure,

mate magnet 16 to a relatively low strength at the edge It isto be appreciated that the field strength gradient (i.e., heterogeneity) may be accentuated through the employment of non-parallel or non-planar pole faces for pole faces 17, '18.

. The charged particles of beam 10 upon passing through magnetic field 14 are subjected to forces which act at right ,angles to both the direction. ofthefield lines and to thedirection of particle motion (i'.e;,;;the.direction of beam propagation). Such forces accordinglyjdeflect the beam: particle's transversely away from the axis of beam 10 along-curved paths as depicte'dfby .the phantom lines of Figure 2. Moreover, the magnitude of the force'acting upon eachcharged particle is proportionalfto the field strength at the particular point ofthe field 14 through which the particle passes." Thus particles in difierent portions of the beam are subjected to forces ofdifi'erent magnitudes by virtue ofspatially heterogeneous distribution of magnetic field strength across the beam. 'The curved paths traversed by the individual charged particles of bear'n' 10 upon passing through magneticfield14 accordingly have different radii of eurvature-proportional to the-heterogeneous field strength distribution across the.

beam diameter. Particles passing through magnetic field 14 in the'regions of relatively strong field --near"magnet 16 are deflected radiallyoutward'from the beam axis, by

substantial amounts; 'Particles' passing through the field in the regions of relatively weakfieldfdisposed distally withi'reference to magnet 16 are'c'o'rrespondin'gly deflected bysm'al-l amountsand the particles. at the distal edge of the beam with reference to magnet 16 V are; substantially undellected. The effect of spatially heterogeneous magnetic field 14, is thus to spread the, beam overalarger cross sectional area at material 13 by elongatin'g' the transverse-dimension of the bearn to one side offthe axis thereof (see Figure 2). 7

Inasmuch as field 14 is steady-state, beam 111 is continuously spread by substantially any desired amount by appropriately adjusting i'the fieldstrength. The field strength "may be advantageously adjusted; for" example,

kqgspreaa j the beam to the width .of' material 1Qbei ng V V irradiated] The 'materiabmay consequently be'continuouslyl'passed under the enlarged be'a rn'withpomplem assurance of 100% beam coverage; Moreover, ,where a' magneticfield'i'semployedas spatially heterogeneous field 14'1injthe manner previously described; it is t'ojbenote'd that either "a direct current electromagnet or a'perrnanent' magnet maybe employed. as jrnagriet"16. I A permanent magnet may thus be advantageously used [to generate field 14, and thereby spread 10with' a maximum of reliability; there'being no' electrical cornponents subje'ct fto breakdown employed infthe beam 7 V g v spreading apparatusJ'f j Similarly a magneticspatially heterogeneousfield is gen- It-will be appreciated that insorn gfnsta'ncesfit is desirable to modify-the pattern of beam spreadingto configurations other than the. one sided spreading effected by thebeam spreading ,heterogeneous'field 14; of Figures 1- 3 and hereinbefore described. Substantialvariation in the beam "spreading pattern may be effected by altering thespatial heterogeneity of the spreading field.. In this; connection, a wide variety of geometric configurations may be employed in the design of pole tips 17,? 18 of magnet 16't0 obtain substantially any desired spatiallyhet erogeneous distribution of field flux. In addition, aplu- 'ra'lity of magnets suitably disposed relative to the faxisiof beam propagation may be utilized to generate a-[beam the field strength progressively decreases across the beam spreading field having a desired spatial heterogeneity. For example, it is often desirable to spread a" charged particle beam equally about the center line thereof and to'accomplish this end a beam spreading field 1911s, illus- 'trated in Figure 4 may be advantageously employed. More particularly, field 19 is preferablyalspatially heterogeneous magnetic field in a planetransverse to the'c'enter line of beam 10 and progressiyely increases equally in intensity in diametrically opposed directions from the center of the beam. In addition, the flux lines of field 19 at diametrically opposed positions of the field relative to the center line of beam are in opposing directions. Accordingly, field 19 isbest generated by means of a pair of C-shaped magnets 21, 22 transversely disposed on opposite sides of the center line of beam 10 intermediate the charged particle source exit window 11 and material 13 to be irradiated (see Figure 5). Magnets 21, 22, moreover, are disposed with the pole faces thereof in polar opposition (i.e., with the north pole face of magnet 21 opposite the south pole face of magnet 22 and vice versa). Heterogeneously distributed flux lines thus extend between the opposing pole faces respectively of magnets 21, 22 as well as between the opposing pole faces of each individual magnet to thereby establish field 19 of the character hereinbefore described. Charged particle beam 10 upon passing through field 19 is spread in substantially the same manner as that previously described in detail in relation to field 14. In the present instance of field 19, however, by virtue of the heterogeneous field strength distribution on all sides of beam 10 and the opposing directions of the field on opposite sides thereof, the beam is spread subsantially equally about the center line thereof as depicted in Figure 5. It will be appreciated that although magnets 21, 22 are illustrated in Figures 4 and 5 and hereinbefore described as being in the same transverse plane relative to beam 10, such magnets 21, 22 may alternatively be respectively disposed in separate planes spaced along the beam axis thereby producing substantially the same results.

In addition to enlarging a charged particle beam in order to distribute same over a relatively large area, it is often desirable in the irradiation of material to further modify the beam distribution to suit the particular material being irradiated. In accordance with the present invention, subsequent to spreading of a charged particle beam by means of a steady-state, spatially heterogeneous field, the enlarged beam may be subjected to small localized fields acting normal to the plane of beam spreading to empirically shape correspondingly small portions ofthe beam pattern as desired. The foregoing may be advantageously accomplished by the charged particle beam adjusting apparatus illustrated in Figures 6 and 7. As shown therein, there is provided a suitable open-ended housing 23 which is adapted for end attachment to charged particle source 12 in coaxial communication with exit window 11. Within housing 23 are secured a pair of transversely spaced opposed polarity beam spreading magnets 24, 26 or equivalent means to generate a spatially heterogeneous magnetic field in accordance with the present invention transverse to the axis of housing 23 and therefore to a charged particle beam issuing from window 11. More particularly, magnets 24, 26 are preferably mounted in the end region of housing 23 proxirnate' window 11 in axially space-apart relation. It is to be appreciated, however, that magnets 24, 26 may be alternatively mounted in the same transverse plane in the manner illustrated in Figures 4 and 5 and hereinbefore described, or various other means for generating a spatially heterogeneous field may also be employed.

To generate the localized empirical shaping fields of previous mention a plurality of small trimming magnets 27 are disposed within the distal end of housing 23 with reference to window 11 in axially spaced relation from magnets 24, 26. Pairs of the trimming magnets 27 are transversely spaced in polar opposition on opposite sides of the axis of housing 23 with the pole faces of such trimming magnets parallel to the pole faces of spreading magnets 24, 26. The pairs of trimming magnets 27 moreover are preferably arcuat ely arrayed across the axis of housing 23 and are secured to gap adjusting plungers 28 extending through the walls of housing 23. The small localized fields generated between respective pairs of the trimming magnets 27 are thus normal to the plane of a spread beam emanating from the spreading field estabdesired to produce an enlarged beam of correspondingly of the individual trimming fields may be varied by altering the magnetizing force of the magnets 27 by any one of several conventional means well'known in the art. In either case, the strengths of the individual trimming fields may be adjusted to further deflect corresponding localized portions of the spread beam and thereby empirically modify the beam distribution to suit the particular material 13 being 'irradiated asdisposed beneath housing 23.

It will be appreciated that in some instances it is desirable to enlarge a charged particle beam beyond the normal beam spreading capabilities of a spatially heterogeneous field of given fixed field strength in accordance with the present invention. In this connection a beam having a relatively small diameter prior toentering the, heterogeneous beam spreading field is enlarged therein by a correspondingly small amount. Conversely, a beam having a relatively large diameter prior to entering the beam spreading field is enlarged therein by a correspondingly large amount. A small diameter beam may thus be enlarged beyond the normal amount attained in passing through a spatially heterogeneous spreadingfield of fixed strength by enlarging the beam diameter-prior to entry into field. Moreover, the beam diameter may be varied prior to entering the beam spreading field to produce corresponding variations in the resultant beam enlargement over a wide range. To accomplish the fore going beam diameter enlargement or variation, the beam 10 may be passed through an axially symmetric magnetic solenoidal field 29 of variable strength as illustrated in Figure 8. More particularly, the solenoidal field'29 is established coaxially intermediate the exit window 11 of charged particle source 12 and the beam spreading field, e.g., beam spreading field 19 as generated by magnets 21, 22 of Figure 5. Solenoidal field 2.9 is preferably generated by a solenoid 31 disposed coaxially between exit window 11 and the plane of magnets 21, 22 and energized by means of a variable direct current power supply 32. The .beam 10 issuing from window 11 thus passes through field 29 established within solenoid 31. prior to entering beam spreading field 19. Solenoidal field 29 acts as a focusing lens and causes the beam to diverge beyond the focal point thereof in the region of beam spreading field 29. The amount of divergence is dependent upon the strength of the field 29 which may be adjusted as desiredflby. variation of the output of power supply 32. Thediameter of the beamentering beam spreading field 19 may thus be readily varied as variable diameter at the irradiated material 13.

Inasmuch as the solenoidal field 29 as generated by the conventional solenoid 31 issubstantially circumferentially uniform, the magnification of the beam diameter or cross section thereby effected is similarly uniform. Although a uniform magnification of the beam diameter prior to introduction to a spatially heterogeneous spread-' ing field is generally suitable for the purposes of the present invention, it is sometimes advantageous to magnify the beam dimensions in one direction or plane only. To accomplish this end the uniform solenoidal magnifying field 29 may be replaced by an astigmatic solenoidal field having a preferential intensification of field strength in one diametric plane. Such a field may be generated by an astigmatic solenoid lens 33 as shown in Figure 9. Lens 33 preferably includes a conventional solenoidal winding 34 which is encased by a core 36 of high magnetic permeability and of a suitable configuration. More particularly, core 36 is substantially open at the inner periphery thereof and includes diametrically opposed solid projecting portions 37, 38 circumferentially slotted at the mid-points thereof as shown generally at 39 and 7 41 respectively. The magnetic field generated by lens 33 accordingly includes regions of relatively higher strengths proximate the slot ted; solid portions 37, 38 and thereby magnifys the @dimensions of charged particle beam introduced. theretoin substantially but one plane extending throughsuch solid portions 37, 38. The astigmatically' magnified beam emer ging from the field generated by solenoid lens33 ma y thereafter be introduced toa spatiallyheterogeneous beam spreading field in the manner of thvpresent invention hereinbe fore described or employed per se to irradiatelarge areas of material, Y v. a l

I hthou ghfihe 'present invention has been hereinbefr'ore described as to specific steps in the method thereof and withjreg'ardto {but .sevieral preferred ftypes of apparatus for p'racticing the same; many variations and modifications are. possible withoutjde'parting', from; the scope of the invention. f In this connection,'the beam enlarging and shapingmethod andappara'tus of ithepresent invention have been illustratedand hereinbefore described primarily inrelation to exte'rnal fbeams emanating from. charged particle sources for the purpose-19 distributing the beam over large areasof rnaterial to be irradiated. v Such method and apparatus may be;a s wellemployed however, to

. internal beamswithin charged particle sources prior- .to

passing through the exit windows thereof-vior-purposes of distributing the. beam energy equally over the'window materialv and thereby avoid overheating of sameis Thus it is not intended to limit the invention except by the terms of the appended claims.

- What is claimed is;

eter charged particle beam comprising a solenoidal winding disposed coaxiall y inthe path of said beam, a core of magnetically permeable material encasing said winding and substantiallyopen at the inner peripheryythereof witl diam'et'rically opposed solid projecting portions ,situ ated thereat, said projecting portions circumferentially slotted at a central transverse planetherethrough, and

variable direct current power supply means connected in energizing relation to said solenoidal winding.

f 2 ,-A-pparatu s astdefinediby claim I, further defined by a pair of C-shaped magnets disposed on opposite sides of the pathbf said "beam in a transverse, plane spaced from said solenoidal windingin the direction of beam motion,;' the pole faces;of saidl'pair of magnets respectively disposed in polar opposition.

2,866,902 .Nygard Dec. 30, 1958 1 1.- Apparatus for transversely enlarging a sm all diam:

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