US2636999A

US2636999A - x x x x i

Info

Publication number: US2636999A
Authority: US
Publication date: 1953-04-28
Anticipated expiration: 1970-04-28

Description

3 ofaparticlsintheregionoithemagnetic isaplotofmagneticintensityncarthe tstruct l 6 is a plot of certain quantities derived .andusefulindesigningthemagnet Figs-land 2: l'lg.'1-isaplanviewandl"lg.8asectional elevation along plane VIII-VIII in rig. 'i of a magnet structure provided with an ancillary magnetic shielding arrangement; and

llgJisaplanviewandliigloanelevation insectionalongtheplaneX-Xinl'lg.9ofa magnetic structure provided with an ancillary m. l in detail. an electrified particlemaybeconsideredtobemovingaiong I from a region which is substantially electric and magnetic fields. and to by its movement directly in the cenbetwccnapairofironorother landt. Theconllle inpar- Thepoleplateslandlaremsinat diiIerent magnetic potentials by any inthe artand producein w them magnetic lines force l which are substantially perpendicular In accordance with well known magnetic principles. the density of the overmostofthesurtaceofthe The taneous direction of motion of the particle and constitutes a centripetal torce which is relatcdtothcradiusofcurvaturerofthecurve particle passingtransversetothe linesofforcetto Il where the intensity of the magneticiieldiachangingastheparticlemoves,

whmtheparticlehaareachedthemagneticlines ottorcelwhsrethemssnetic intensityiscon- 4 properly designing the shape of the plates t and 4 in the X-Y plane. their edges may be made to cross this circular path of movement at any desired point, thus determining the direction in which the particle will be moving at the time it re-enters the field-free space.

However. it is impossible as a practical matter to project electrined particles lying always exactly in the XQY plane above described. and in any actual electrical discharge device. some of the particles will follow paths such as I! which lie above the X-Y plane, while others follow paths Il which lie below the X-Y plane. Particles following either of the paths i2 and Il will cross the lines of magnetic force l to II at points where their direction is not normal to the X-Y plane, The directions of the linn of force l to Il where the particle following the path i2, forl example. intersects it can be resolved into two components. one along the Z axis normal to the X-Y plane and the other lying parallel to the X-Y plane.

If the electriiled particle continued in follow a path parallel to the X axis. the last-mentioned component of the magnetic force being along the X axis also would produce no deilecting force on it. However. the component of the field lying -along the Z axis causes the particle to follow a curve path having a component parallel to the Y axis. This component of velocity parallel to the Y axis. together with the abovementioned component of magnetic field parallel to the X axis. will produce a force accelerating the particle along the Z axis. Buch a movemmt oftheparticlesalongthezaxistendstodetocua the electron beam and creates considerable praetical diiiiculty in the use of magnets for the 911111058 here under discussion. The object d my invention is. in fact. to minimize the eileot of this deiocusing movement imparted to the particle along the Z axis.

It will be noted that once the particles tol lowing the paths I2 and il have passed into the region of the magnetic lines l. all lines of magnetic force which they encounter lie along the Z axis and they will correspondingly receive no further accelerations along the Z axis. They will, however, retain any velocity along the Z axis with which they left the tringing neld as long as they remain in the portion of the magnetic iield between plates 3 and l which is not subject to tringing." However. when the particles.

after following a circular arc between the platea 8 and 4. reach the edge of these plates at an'- other point than that at which they entered, *ahw again encounter the i'ringing neld. It can be shown that the eflect of the "fringing" field at the point of egress produces accelerations in the same direction along the Z axis al those encountered by the particle in the fringing" eld at entrance. Thus the effects of the fringing" at the entrance and the egress points ln producing velocity along the Z axis are cumulative.

Aslwillnowproceedtoshomitispossibleto substantially reduce to pero the velocity along thezaxisofthcparticleleavingthefringing" neld, by the 'expedient of forming the edges o( the pole pieces I and l as planes normal to the XYp1ane,butwhiehmakeanangleawith thexaxisalongwhichtheparticleismovingin held-free space before entering the magnetic fleidduetoplateslandl. Rieferringindetail tnFlg.3,theedgcofthepolepieces8andlmay thsparticlefollowaacircularare. By'lmaybcrepresentedbythelinesmakingan 7 ponent, but at poinis above and below this plane have horizontal ts which are normal to Fig. 4). it will be noted that at the point A. the velocity of the may be represented by the vector NP (118.3) paralleltothexaxiaandthisvelocity may be resolved into two components nm whichisnormaltothelineSBandmpparallel tothelineSB. Sincethevelocitynmisparaliel tothetsmhofthemagnetici'ieidin Flg.2.itwillcausenodenectionofthemoving particle, but the velocity component mp which isparalleltothelines.andhencetotheedge of the magnet plates I and 4, lies In the X-Y plane perpendicular to the magnetic component mhandwillcauseaforceequaitotheproduct mh times mp to produce accelerations of the movingparticlealongthezaxis. Itwillbenoted that the magnetic component mh for points above the X-Y plane is opposite in direction from that for points below the X-Y plane, while the direction of mp is the same; hence particles moving in paths above the X-Y plane are accelerated in one direction and particles moving below the X-Y plane are accelerated in the opposite direction along the Z axis.

Whilethevelooitynpinllig.3wasreferredto as the velocity of the particle to the left oi' point Ainrlg.2.itwillbenotedthatsimilarresolu tions of the velocity at other points on the AEB portion of the curve could be made in directions similar to nm and mp, but that, in general, the component mp would decrease as the particle moved from A and soon reach a point at which it would become sero. Calling the latter point E, the component mp would be of the reverse direction for points beyond E. This means that at the point E the acceleration of the particle along the Z axis has fallen to pero, although its velocity along the Z axis is not nero. Thus the particle is being accelerated in the opposite direction alongtheZaxiswhileintheEBportionofthe path,tothatwhileintheAEportion. Porthe correct value of angle a. the retardation along the last-mentioned path EB will counter-balance the Z axis acceleration imparted along the path AE and the particle will enter the uniform field inside the pole pieces s, 4 with aero velocity along the Z axis. In short, the particles which left the held-free space moving in the X-Y plane will enter the space between the plates 3, 4 moving along a curved path in a plane parallel to the X-Y plane, although displaced from the latter.

It will be evident from the foregoing that this elimination of velocity along the Z axis by the time the particle fully enters the space between pole-plates 3 and 4 is effected by giving the edgesoftheplatess,4aparticularanglea relative to the direction of movement of the particle in held-free space. where a may be determined from the kuation 1 as previously described.

As has previously been stated. the particle movesbeyondthepointalongacirculararc determined by the density of the magnetic field between the plates 3, 4 and will continue to follow this curve until it arrives close to another edge o! the plates s, 4 at which it leaves the dense portion ci the magnetic neld therebetween. The fringing field adiseent this other edge of the plates3.4willbesimilartothatalreadyde 8 ticlewillaocordingiyfollowacurvesimilarto AEB after leaving the space between the plates. If it is desired that the particle upon again reaching field-free space shall move in a directiondirectlyparalleltothepath I inl"lg.2,a mirror image of the path IAEB may be applied atapointKhavingthesameXcoordinateas point B in Fig. 2. The point K is, accordingly. a point of exit on the other edge of the plates 8. 4 corresponding to the point of entry B.

When it is desired that the path of the particle on re-entering held-free space should have some angle, diering by angle U from 180 degrees relative to the original held-free path I, it will be obvious that the point K should be displaced along the curve KB by an angle U, the radius of curvature of the path through the "fringing" neld in the regions of emergence being retained unaltered.

A moments consideration will show that the velocity of the particle emerging from the space directly between the plates 3, 4 has a. component parallel to the magnet edge which is similarly directed along that edge to the homologous component on entrance; i. e. the vectors mp in the diagrams corresponding to Fig. 3 are the same for homologous points in the fringing ilelds at both entrance and exit paths of the Particle. Likewise, the magnetic component mh is the same as that shown in Fig. 3 for homologous points in both "fringing tields; hence the forces and accelerations along the Z axis are exactly the same. point by point along the exit path, to those existing along the entering path AEB. Hence the particle, while it experiences a displacement along the Z axis in passing through the "fringing" field. will emerge from the fringing ileld with zero velocity along the Z axis. Its path in field-free space will, accordingly, be parallel, to, and so coplanar with. the path I which which it entered the magnet ileld.

The foregoing describes a way in which the position of the edges of the plates 3 and 4 ma! be determined in order to minimize defocusing 0l the electron beam as a result of accelerations of the particles along the Z axis in the fringing nux. I may point out that where anyone desires to avoid the plotting and integrations described above. it will be possible to make up a number of specimen magnets having edges lying at various values of angle a in Fig. 2. These may then be tried successively and the degree of defocusing plotted against the various values of the angle o. In this way, a curve may be plotted from which an optimum angle a may be determined by inspection.

An arrangement which may be applied either alternatively, or as a supplement, to that described in the foregoing, for improving the magnetic field distribution of the plates l, 4 and reducing the fringing" iield distant from the edges of the plates 8 and 4 is shown in Figs. 7 and 8. It comprises providing a pair of magnet poles like those of Figs. l and 2 with a series of plates 3i, Il. Il of magnetic material such as iron. which are perpendicular to the mid-plane between the D010 plate 3, 4 and parallel to the edges of poles. These plates have openings or windows to permit passage of the beam. forming closed loops around the beam. so that the lower halves 34, 35 and Il are at the same magnetic potential as the corresponding upper halves Il, 32, 33. While such magnetic plates may increase the component o! magnetic force along the X axis (i. e. the comscribed in. with Fig. 8.and the pal- 'Is lwnntmhinl'is) directlyinfrontofthepoll ll whereBisthemagneticiiuLBrandBzarer and z tsof said magnetic neld at any pointxontheparticlespatnandlistheradius ofthecirculararcfollowedbysaidparticlewhen well within said magnet gap.

2. An arrangement for altering the direction of movement of an electrified particle comprising a pair of pole pieces having a magnetic ileld between them which is normal to the path of movement of said particle in ileld-free space before entering said magnetic eld, and electrodes for producing an electric eld in the fringing magnetic ileld of said pole pieces where said particles enter the space between said pole pieces, said electric ileld having a magnitude substantially equal to the product of themagnitude of the magnetic held and the velocity of the particle along the path of the particle, said electric field having adirectionsoastoproduceaforceonsaidparticle opposite to that produced by said fringing magnetic field.

3. An arrangement for altering the direction of movement of an electrified particle comprising a pair of pole pieces of z material having a magnetic eld between them which is normal to the path of movement of said particle in held-free space before entering said magnetic field, and a set of plates of magnetic material for each pole piece positioned adiacent the edges of said pole pieces in the region where said particle passes between them forming closed magnetic paths around said path, said plates being normal to the mid plane between said plates.

4. Means for operating upon an electrified particle moving without acceleration along the X axis of a set of rectangular coordinates. means for producing a magnetic ileld along the Z axis of said coordinates and which ig substantially uniform over an area in the X-Y plane having boundaries, and which is in the path of movement of said particle, said boundaries having a rectilinear portion which makes withthexaxisatapointwheresaidpath intersects said area an angle of eos-1 12 whereisthemagneticflux wellwithinsaid area. B: and Bz are the .1: and z components of said magnetic field at any point being traversed by the particle and R is the radius of the circular arc followed by said particle when well within mid area.

5. Means for operating upon an electriiled particle moving without acceleration along the X axis of a set of rectangular coordinates, means for producing a magnetic ileld along the Z axis of said coordinates and which is substantially uniform over an area in the X-Y plane having predetermined boundaries, and which is in the path of movement of said particle, said boundaries having a rectilinear portion which makes with the X axis at a point where said path intersectssaidareaanangleof90degrees.apairof electrically charged sheets of conducting material symmetrically positioned relative to said X axis and perpendicular to the X-Y plane. and intersecting said X-Y plane in curves which duplicate the curves formed by the intersection of the X-Z plane with magnetic equipotential surfaces of said magnetic held.

GEORGE W. HEWI'I'I.

elereneescitedintheilleofthispatent UNTI'EDBTATIBPATENTS OTHER REFERENCES Deection and Impedance of Electron Beams atHighPrequenciesinthePresenceofaMagneus meid, by n naine. nos Review, pp. 43s

through 4M: vol. V. No. 4, April 1941.