US3011087A - Device and method for producing electron beams - Google Patents

Description

Nov. 28, 1961 M. R. JEPPSON ETAL 3,011,087

DEVICE AND METHOD FOR PRODUCING ELECTRON BEAMS Filed Feb. 8, 1955 EIIE 2 JNVl-JVTORS. Mar/71s A? J? pson By Richard F 052 LOLQLMZD. A

United States Patent 3,011,087 DEVICE AND METHOD FDR PRODUCING ELECTRON BEAMS Morris R. Jeppson, Orinda, and Richard F. Post, Walnut Creek, Calif., assignors to Applied Radiation Corporation, Walnut Creek, Califl, a corporation of California Filed Feb. 8, 1955, Ser. No. 486,831 9 Claims. (Cl. 315-5.42)

The present invention relates to an improved and simplified method and means for producing high energy electron beams and particularly adapted to accelerate relatively high current electron beams to a substantial potential.

The present invention contemplates the production of a high current electron beam which is then velocity modulated so as to comprise alternate high and low electron density portions. This incompletely bunched beam is then employed by taking energy from the high density portions thereof and adding it incrementally to the low density portions for accelerating same whereby the low density portions attain potentials many times that of the original beam potential. In so far as means for accomplishing the above, only conventional apparatus is required and in fact the equipment normally employed in electron beam acceleration may be used although in a different manner and relationship than is normal. Also, at least a part of the equipment formerly employed to produce and accelerate electron beams is not here required so that the resultant device is simplified over conventional apparatus with a consequent reduction in cost and maintenance.

It is an object of the present invention to provide an improved method and means for producing high energy electron beams.

It is another object of the present invention to provide method and means for accelerating one portion of an incompletely bunched electron beam with other portions thereof for producing a high energy bunched electron beam.

' Various other objects and advantages of the invention will become apparent to those skilled in the art from the following description taken together with the accompanying drawing wherein; FIGURE 1 is a graphic represenation of typical beam current waves and cavity voltage waves as related in the present invention, and

FIGURE 2 is a schematic representation of apparatus of the invention.

Considering now the invention in some detail and particularly with regard to the method of acceleration thereof, there is first produced as by thermal emission a large quantity of free electrons which are formed into a continuous or pulsed beam by suitable electric fields attracting the electrons in one direction. This beam of electrons is velocity modulated by a time-varying electric field through which the beam passes and which periodically varies in intensity so that successive portions of the beam receive greater and lesser acceleration therefrom. The velocity modulated beam is then allowed to travel for some distance through a field free space. The electrons of the spaced portions of the beam which received the greater acceleration from the time varying field travel faster than adjacent electrons of the beam, and thus as the beam drifts the faster electrons catch up with slower preceding electrons and bunches of electrons are formed. The beam intensity then becomes periodically varying with spaced high current portions separated by low current portions, and in FIGURE 1 of the drawing there is illustrated by curve A the time-current relationship of such a beam.

In accordance with the present invention the bunched beam described above is operated upon to the end of employing energy in the high current bunched portion of the beam to accelerate the low current portion of the beam to high potentials. This is accomplished by directing the beam through a plurality of adjacent resonant cavities whose dimensions are related in a well known manner to the bunch frequency of the beam so that a resonant interchange of energy is produced in each cavity by the beam passage therethrough. In an idealized situation the cavity oscillations will lag the beam pulsations by ninety degrees and the curve B of FIGURE 1 represents a cavity voltage-time relationship in such relation to the beam of curve A. Of course cavity losses and various other factors tend to decreases the phase difference between the beam current variations and the cavity voltage wave; however, with careful construction and with optional cavity tuning means the phase difference need differ from ninety degrees by only a few degrees. Considering the relationships shown in FIGURE 1, it will be seen that during the positive half cycle of cavity voltage the beam gives up energy to the cavity to sustain oscillations therein and that during the negative half cycle of cavity voltage the beam receives energy from the cavity. Although the beam is accelerated during the entire negative half cycle of cavity voltage only a negligible acceleration occurs near the start thereof to affect the remainder of the high current portion of the beam. Maximum beam accelereation occurs at the maximum cavity voltage so that during the time in which this voltage is a maximum a certain part of the low current portion of the beam is accelerated substantially to the beam voltage or maximum cavity voltage. This portion of each cycle of the beam is additionally accelerated in each cavity an amount substantially equal to the beam voltage. Considering a beam potential of V and n cavities the accelerated portions of the beam thus attain a resultant potential of nV, disregarding losses.

It will, be appreciated that there are certain limiting factors upon the maximum beam acceleration by adding successive cavities, i.'e., the number of cavities n is limited. Energy'is removed from the beam bunches to sustain cavity oscillations and a net debunching efiect is produced which. tends to broaden and flatten the beam bunches and additionally if the distance travelled by the beam through. the cavities is sufiicient the inherent velocity variations in the beam bunches will cause further flattening of the beam bunches. On the other hand a net focusing effect on a portion of the beam is produced by the acceleration thereof which tends to increase the current of the accelerated portions of the beam. It is advantageous to stack the resonant calvities as close together as possible, so that substantially no flattening of the beam bunches results in the passage of the beam through the cavities, i.e., insufficient time is allowed for the velocity variations to produce debunching.

After passing the beam through a plurality of cavities, as above noted, the emergent beam has substantially the same current-time relationship as shown in curve A of FIGURE 1; however, the beam voltage distribution has been materially altered so that a part of the low current portion of each cycle of the beam has been accelerated to a high voltage. Should even further acceleration be required the low voltage portion of the beam may be materially reduced as by suitable electrostatic fields to pro duce a beam having as the high current portions that part of the original accelerated substantially to the nV potential and then a further series of resonant cavities may be employed in the same manner as above wherein the beam operates uponitself to further accelerate a portion of each cycle thereof. A suificient drift space thanis produced by conventionall methods. the steps of the present invention may be accomplished should be provided between separate stages of acceleration, such as above-noted, to allow the velocity modulation to produce current variations in the beam.

The above described method of producing a high energy electron beam will be seen to comprise the controlled interchange of energy from one portion of each cycle of-the beam to another lower current portion thereof, and although parts of the beam are herein referred to as low current portions this term is employed only as relative to other beam portions and the resultant ac celerat'edbeam portions are of a much higher current Further,

with: a minimum of equipment, each portion of which is in itself conventional. Thus referring to FIGURE 2 of the drawing there is shown at 1 1 a velocity modulat-- ing. electron tube which may be of conventional design; An electron source 12 adapted for energiz'ation by, suitable external power supply means, not shown, produces free electrons which are attracted by a positive potential applied to an anode 13 spaced therefrom and insulated from the emitter 12' by an insulator 14'. A- potential is applied from an external source (not shown) between the cathode I2 and anode 13' with the cathode'll being maintainedat a: negative potential with respect to the anode so that electrons are attracted toward the anode 13 and pass throughan opening therein.- Disposed on the opposite side of the anode 13 from the cathode 12 is aresonant cavity 16 which is excited by radio-frequency energy applied thereto by a coupling loop 15 from an external source (not shown), and there is thus established in the cavity 16 a time varying electric field throughwhich electrons from the cathode pass aftertraversing. the anode opening. As the electronbeampasses through this time-vanlin'g field, it receives a per-i odically varying acceleration, whereby the beam is velocity modulated in a conventionalmanner. The velocity modulated electron beam passes through an elon- V gated chamber 17 which is substantially field free and in:

which: the beam drifts with the faster electrons of each cycle catching up with: preceding slower electrons to produce periodic electron-density variations in the beam. This beam then has a current-time relationship similar 1 to-that shown in FIGURE-1.

' Acceleration of electrons in the above described bunched beam isaccomplished in aplurality of adjacent communicating resonant cavities 18. These cavities 18' are dispcsed directly adjacent one another and may comprise a single: housing with transverse walls, as shown; together defining an' accelerator 19. The accelerator 19 has" an axial openingtherethrough in alignment and in communication with the tube 11 and closed at the far end whereby the interior may be maintained at a subat'mospheric pressure; A- target or window 21 in the end wall of the accelerator 19'provides-an exit for the electron beam and if desired this end wall or other struc ture adjacent thereto may be insulated and maintained ata: suflicient negative potential to repel all or most of the electron beam that is not accelerated in the accelera tor 19.

Operation of the above described apparatus follows from thepreviously described method of the invention; Electrons emitted from the emitter 12 the tube 11. areattracted toward the anode 13 by a positive potential applied thereto so that the electrons form into beam; This beam of electrons in passing, through the cavity 16 is subjected to a periodically varying electric field produced by a time varying potential applied to the radiofre'que'ncy coupling loop 15. Certain spaced portionsof the beamthus receive more acceleration than other portions whereby the beamis velocity modulated and in drifting through the chamber 17 this velocity are designed to resonate at the frequency of the beam density variations so that the beam bunches give up energy to the cavities to establish and sustain oscillations therein. The voltage variations within the cavity being somewhat more than ninety degrees out of phase with the current variations of the beam thereby return energy to the relatively low current beam portions by accelerating same particularly in the beam cycle portions corresponding to the cavity voltage maxima. The beam is thus actually further velocity modulated in the accelerator 19, however, as the combined length of the cavities 18 is short this velocity modulation substantially does not affect the beam current distribution. The resultant beam includes in each cycle a high potential portion with the potential thereof being about fZV and despite the fact that relatively low current ortions of the beam are accelerated these portions represent a substantial current, as for example of the order of ten milliamperes so asto compare favorably with known electron accelerators.

What is claimed is:

1. A method of producing a high current electron beam comprising the steps of establishing a high current electron discharge, directing said discharge into a beam, operating upon said beam with a periodically varying electrostatic field for velocity modulating same, passing said modulated beam for a substantial distance through a substantially field-free space whereby said beam becomes intensity modulated with regular periodic current variations of al-' ternate high and low current portions, establishing a plurality of separate resonant electromagnetic oscillations with said beam by giving up energy from the high current beam portions to support the oscillating electromagnetic field, and passing said beam through said oscillating. fields with low current beam portions in phase with the maxi mum field voltage whereby said low current beam por tions are accelerated by the electromagnetic fields.

2. A method of producing a high energy electron beam comprising the steps of producing an electron beam hav-- ing periodic current variations of consecutive high and low current densities, energizing a plurality of consecutive substantially electrically decoupled resonant cavities with said beam by passing same therethrou'gh for establishing resonant electromagnetic field oscillations within said cavities, and accelerating low current density portions of said beam by maximum cavity voltages whereby said low current beam portions attain a high energy.

3. A method of electron beam acceleration comprising the steps of velocity modulating an electron beam at a fixed frequency, passing the modulated beam through a field-free space whereby same becomes intensity modulated with spaced high density portions connected by low density portions, establishing with said beam a plurality of individual adjacent electromagnetic field oscillations by resonant energy exchange from the high density beam portions with said fields having voltage maximums in phase with low'de'nsity beam portions for accelerating same modulation causes the beam to become bunched with whereby said low density beam portions receive from each of said holds an acceleration substantially equal to the original beam voltage and thereby attain a high energy level.

4. A method of producing a high energy electron beam' comprising the steps of velocity modulating an electron beam to provide spaced high current portions thereof separated by lowcurrent beam portions, energi'z ing" a succession of four to ten substantially electrically decoupledr'esonant cavities solely with said velocity modulated electron beam whereby said high current beam por tionsresonantly establish electromagnetic oscillations within said cavities which in turn accelerates said low current beam portions to a potential substantially equal to the number of cavities times the original beam voltage.

5. An improved electron beam accelerator comprising. means establishing an electron discharge, means directing said discharge into a beam, means velocity modulating said beam, means defining an elongated field-free chamber disposed in said beam path wherein said beam becomes bunched with spaced current maxima, and a plurality or" closely spaced substantially electrically decoupled resonant cavities with communicating openings therethrough aligned with said beam path and energized solely by said beam whereby said beam passing therethrough excites same with the high current portions thereof to establish individual cavity oscillations accelerating low current beam portions between said current maxima.

6. An electron accelerator comprising a velocity modulated electron tube producing an electron beam having periodic current pulses of a fixed frequency separated by relatively low current portions, said tube having an opening the end thereof in the beam path for beam exit, a plurality of like resonant cavities resonating at the frequency of said beam disposed in alignment adjacent said open tube end and communicating therewith, said cavities having an opening through each in the beam path With decoupling means therebetween whereby said beam traverses said cavities for resonately exciting each cavity individually and a window in the end of the cavity farthest from said tube for beam passage therethrough and sealing said cavities.

7. An electron beam accelerator as defined in claim 6 further characterized by a single envelope connected to said tube about the open end thereof and having equally spaced transverse walls defining said resonant cavities, each cavity having an opening transverse to the beam path which in the beam path direction is smaller than the spacing between the cavity walls.

8. An electron accelerator comprising an electron emitter, electron accelerating means directing said electrons into a beam with an energy V, means establishing a fixed frequency varying electrostatic field in the path of said beam for velocity modulating same, means defining an elongated field-free chamber in said beam path wherein said beam becomes bunched from said velocity variations therein, means defining n cavities resonating at the frequency of said varying electrostatic field and disposed adjacent one another in said beam path with a common passage therethrough for beam traverse, said last mentioned means including means for substantially separating the electrostatic fields of each of said cavities one from another, whereby said cavities are individually resonantly energized by said beam to accelerate beam portions between beam bunches to a final potential of substantially nV.

9. An electron beam accelerator comprising an electron source producing a beam of electrons, velocity modulating means disposed in the path of said beam for producing successive high and low current portions thereof, a ten or less plurality of resonant cavities disposed contiguously in alignment in the path of said beam with aligned openings therethrough for passage of said beam, each cavity having dimensions causing same to be resonantly energized by said high current beam portions, contiguous ones of said cavities forming as between themselves a short tube at least partially defining said aligned openings and limiting coupling of the resonant energizations thereof, each cavity being thereby individually energized by said high current beam portions and in turn proportionately accelerating said low current beam portions passing through said aligned openings.

References Cited in the file of this patent UNITED STATES PATENTS 2,367,295 Llewellyn Jan. 16, 1945 2,406,379 Hansen et al Aug. 27, 1946 2,637,001 Pierce Apr. 28, 1953 2,653,270 Kompfner Sept. 22, 1953 2,813,996 Chodorow Nov. 19, 1957

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