EP0809271A2 - Canon à électrons - Google Patents

Canon à électrons Download PDF

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Publication number
EP0809271A2
EP0809271A2 EP97303476A EP97303476A EP0809271A2 EP 0809271 A2 EP0809271 A2 EP 0809271A2 EP 97303476 A EP97303476 A EP 97303476A EP 97303476 A EP97303476 A EP 97303476A EP 0809271 A2 EP0809271 A2 EP 0809271A2
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electrons
area
gun
cavity
electron
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German (de)
English (en)
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EP0809271A3 (fr
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Frederick Michael Mako
William Kalman Peter
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J3/00—Details of electron-optical or ion-optical arrangements or of ion traps common to two or more basic types of discharge tubes or lamps
    • H01J3/02—Electron guns
    • H01J3/023—Electron guns using electron multiplication

Definitions

  • the present invention is related to electron guns. More specifically, the present invention is related to an electron gun that uses an RF cavity subjected to an oscillating electric field.
  • High-current pulses are widely used in injector systems for electron accelerators, both for industrial linacs as well as high-energy accelerators for linear colliders.
  • Short-duration pulses are also used for microwave generation, in klystrons and related devices, for injectors to perform research on advanced methods of particle acceleration, and for injectors used as free-electron-laser (FEL) drivers.
  • FEL free-electron-laser
  • the difficulty in generating very high-current pulses of short duration can be illustrated by examination of a modern linac injector system.
  • a good example is the system designed and built for the Boeing 120 MeV, 1300 MHz linac, which in turn is used as an FEL driver [J.L. Adamski et al., IEEE Trans. Nucl. Sci. NS-32, 3397 (1985) ; T.F. Godlove and P. Sprangle, Particle Accelerators 34, 169 (1990)].
  • the Boeing system uses: (a) a gridded, 100 kV electron gun; (b) two low-power prebunchers, the first operating at 108 MHz and the second at 433 MHz; and (c) a high-power, tapered-velocity buncher which accelerates the beam bunches up to 2 MeV.
  • the design relies on extensive calculations with codes such as EGUN, SUPERFISH and PARMELA.
  • a carefully tapered, axial magnetic field is applied which starts from zero at the cathode and rises to about 500 Gauss.
  • Boeing obtains a peak current of up to about 400 A in pulses of 15 to 20 ps duration, with good emittance.
  • the bunching process yields a peak current which is two orders of magnitude larger than the electron gun current.
  • Space charge forces which cause the beam to expand both radially and axially, are balanced by using a strong electric field in the high-power buncher, and finally are balanced by forces due to the axial magnetic field.
  • the performance achieved by Boeing appears to be at or near the limit of this type of injector.
  • Micro-pulses are produced by resonantly amplifying a current of secondary electrons in an RF cavity operating in, for example, a TM 020 mode (Fig. 1) or a TM 010 mode (Fig. 2) [F. Mako and W. Peter, Part. Accel. Conf., IEEE Cat. 93CH3279-1 2702 (1993)].
  • Figure 1 shows a perspective view of the micropulse gun emitting electron-bunches in an annular geometry.
  • Figure 2 shows a side view of the micropulse gun emitting electron-bunches in a solid bunch geometry. Bunching occurs rapidly and is followed by saturation of the current density in typically ten to fifteen RF periods. "Bunching" occurs by phase selection of resonant particles.
  • the bunch that is formed is much shorter than the RF period which is due to the resonant nature of this process.
  • the micropulse gun produces a narrow bunch every RF period in the output direction. Bunch transmission is accomplished by use of a transparent grid. Localized secondary emission in the micropulse gun is dictated by material selection. Radial space charge expansion in the micropulse gun cavity can be reduced by using ⁇ either electric or magnetic focusing, or both. Radial electric focusing in the cavity is accomplished by a concave shaping of the cavity, as shown in Fig. 2.
  • the grid not only allows transmission of bunches but can also provide an emitting surface for electron multiplication. A path for the RF current can be maintained by using a grid of wires.
  • the double grid isolates an external accelerating field from "pulling out” non-resonant electrons which would form a dc baseline. Also, the two grids are electrically isolated to allow for dc biasing to create a barrier for low energy electrons. Axial and radial expansion of the bunch is minimized outside the micropulse gun cavity by using various combinations of rapid acceleration, electric and magnetic focusing.
  • This micro-pulse electron gun should provide a high peak power, multi-kiloampere, picosecond-long electron source which is suitable for many applications.
  • high energy picosecond electron injectors for linear colliders free electron lasers and high harmonic RF generators for linear colliders, or super-power nanosecond radar.
  • the present invention pertains to an electron gun.
  • the electron gun comprises an RF cavity having a first side with an emitting surface and a second side with a transmitting and emitting section.
  • the gun is also comprised of a mechanism for producing an oscillating force which encompasses the emitting surface and the section so electrons are directed between the emitting surface and the section to contact the emitting surface and generate additional electrons and to contact the section to generate additional electrons or escape the cavity through the section.
  • the section preferably isolates the cavity from external forces outside and adjacent the cavity.
  • the section preferably includes a transmitting and emitting screen.
  • the screen can be of an annular shape, or of a circular shape, or of a rhombohedron shape.
  • the mechanism preferably includes a mechanism for producing an oscillating electric field that provides the force and which has a radial component that prevents the electrons from straying out of the region between the screen and the emitting surface. Additionally, the gun includes a mechanism for producing a magnetic field to force the electrons between the screen and the emitting surface.
  • the present invention pertains to a method for producing electrons.
  • the method comprises the steps of moving at least a first electron in a first direction. Next there is the step of striking a first area with the first electron. Then there is the step of producing additional electrons at the first area due to the first electron. Next there is the step of moving electrons from the first area to a second area and transmitting electrons through the second area and creating more electrons due to electrons from the first area striking the second area. These newly created electrons from the second area then strike the first area, creating even more electrons in a recursive, repeating manner between the first and second areas.
  • Figure 1 is a perspective view of the micropulse gun for a hollow beam using the TM 020 mode. The inner conductor is not shown.
  • Figure 2 is a schematic of micropulse gun for solid beam using the TM 010 mode. This side view of solid beam micro-pulse gun cavity showing double grid and emitting and transmitting surfaces. Beam pulses and concave shaping of the micropulse gun cavity are shown. Figure is not to scale. A coaxial feed or side coupling or coupling loops can be used for RF input (not shown).
  • Figure 7 Emittance growth due to double-grid extraction with an injection beam energy of 114 kV.
  • the wire thickness is set 0.1 mm.
  • Figure 9 Expansion of micro-pulse from space charge during acceleration, neglecting energy spread.
  • the acceleration field is 50 MV/m and the axial space charge electric field is 1.33 MV/m (or about 35 nC/cm 3 ).
  • the electron gun 10 comprises an RF cavity 12 having a first side 14 with an emitting surface 16 and a second side 18 with a transmitting and emitting section 20.
  • the gun 10 is also comprised of a mechanism 22 for producing an oscillating force which encompasses the emitting surface 16 and the section 20 so electrons 11 are directed between the emitting surface 16 and the section 20 to contact the emitting surface 16 and generate additional electrons 11 and to contact the section 20 to generate additional electrons 11 or escape the cavity 12 through the section 20.
  • the section 20 preferably isolates the cavity 12 from external forces outside and adjacent the cavity 12.
  • the section 20 preferably includes a transmitting and emitting screen 24.
  • the screen 24 can be of an annular shape, or of a circular shape, or of a rhombohedron shape.
  • the mechanism 22 preferably includes a mechanism 26 for producing an oscillating electric field that provides the force and which has a radial component that prevents the electrons 11 from straying out of the region between the screen 24 and the emitting surface 16. Additionally, the gun 10 includes a mechanism 28 for producing a magnetic field to force the electrons 11 between the screen 24 and the emitting surface 16.
  • the present invention pertains to a method for producing electrons 11.
  • the method comprises the steps of moving at least a first electron 11 in a first direction. Next there is the step of striking a first area with the first electron 11. Then there is the step of producing additional electrons 11 at the first area due to the first electron 11. Next there is the step of moving electrons from the first area to a second area and transmitting electrons through the second area and creating more electrons 11 due to electrons from the first area striking the second area. This process is repeated until the device is shut off by removing the RF power source.
  • FIG. 1 A schematic of one embodiment of the proposed device is given in Figures 1 and 2. Although the design shown is not necessarily optimum it provides a basis for describing the invention. Shown in figures 1 and 2 is a side view of a cylindrically symmetric device. RF power may be fed into the cavity through several means including: a low impedance coaxial transmission line connected to the back perimeter of the cavity, side coupling using a tapered waveguide or coupling loops. The appropriate mode is then set up in this case a TM 010 for a circular cavity (as in Fig. 2) or a TM 020 for the circular cavity as in Figure 1. An annular bunch is generated by secondary emission at the second peak of the electric field in the cavity operating in a TM 020 mode (Fig. 1).
  • the first peak may be eliminated by placing an inner conducting cylinder at the first zero of the TM 020 mode.
  • a solid bunch is created on the axis by secondary emission.
  • the right wall of the cavity in Figure 2(also see detail) is constructed with a transmitting double screen or grid which allows for the transmission of electron bunches.
  • the crossed wires of the grid maintain a path for the RF current.
  • the double grid isolates an external electric field from pulling out non-resonant electrons.
  • the grids are electrically insulated to allow for biasing to create a barrier for unwanted electrons.
  • Fig. 2 is shown an RF cavity operating in for example a TM 010 mode. Assume that at the grid-ded wall of the cavity there is a single electron at rest on axis, which transits the cavity in about one-half the RF period and is in proper phase with the field. This electron is accelerated across the cavity and strikes the surface S. A number ⁇ 1 of secondary electrons are emitted off this electrode, where ⁇ 1 is the secondary electron yield of surface S.
  • the seed current density J seed can be created by several sources including: thermionic emission, radioactivity, field emission, cosmic rays, a spark or ultraviolet radiation.
  • the secondary emission yield ⁇ is defined to be the average number of secondary electrons emitted for each incident primary electron and is a function of the primary electron energy ⁇ .
  • ⁇ for all materials increases at low electron energies, reaches a maximum ⁇ max at energy ⁇ max , and monotonically decreases at high energies.
  • Table I gives some commonly used materials with high and low values of ⁇ [D.E.Gray(coord. Ed.), Amer. Inst. of Physics Handbook, 3rd Edition, McGraw-Hill; E. L. Garwin, F. K. King, R. E. Kirby and O. Aita, J. Appl. Phys. 61, 1145 (1987); A. R. Nyaiesh, et al., J. Vac. Sci.
  • GaP is not sensitive to oxygen but is sensitive to water. With very thin coatings on the surface of GaP, it can be made to allow secondaries to leave and at the same time prevent contamination. Also, GaP can be doped to eliminate charge build-up. Thus GaP could be an excellent candidate at high energy (up to 100's of keV). MgO is a good candidate for lower particle energy ( ⁇ 60 keV) and would have to be applied in a thin layer in order to minimize charge build-up.
  • Another very robust emitter material that is currently under intensive study is diamond film [M.W. Geiss, et al., IEEE Electron Device, Letters, 12, 8 (1991)]. Single crystal alumina (sapphire) or polycrystalline alumina are also excellent robust emitters.
  • the entire MPG cavity (except for the specified secondary emission sites) needs to be built with a low secondary emission coefficient.
  • Cavity surface coatings can reduce secondary emission and also isolate electrical whiskers from the cavity and serve as a trap for slow electrons [W. Peter, Journal of Applied Physics 56, 1546 (1984)].
  • CaF 2 and TiN [E. L. Garwin, F. K. King, R. E. Kirby and O. Aita, J. Appl. Phys. 61, 1145 (1987); A. R. Nyaiesh, et al., J. Vac. Sci. Tech. A, 4, 2356 (1986)] are excellent candidates for cavity coatings.
  • cavities built from 304 stainless steel or titanium work well for the low secondary emission areas.
  • the ⁇ 3 scaling law is an important characterization of the micropulse gun.
  • Kilpatrick's criterion W.D. Kilpatrick, Rev. Sci. Inst. 28, 824 (1957)] is based phenomenologically on electrical breakdown due to secondary electron emission from ion bombardment. However, with advances in cleaning, conditioning and better vacuum techniques (that do not introduce contaminants), Kilpatrick's criterion overestimates the likelihood of breakdown by a factor of two or three for cw [R.A. Jameson, High-Brightness Accelerators, Plenum Press, 497 (1988)] and five to six for short pulses [S.O. Schriber, Proc 1986 Linear Accelerator Conference, June 2-6 (1986)] .
  • the micropulse gun has been fully characterized using an FMT developed proprietary 2 1/2-D relativistic electromagnetic PIC code FMTSEC (that includes secondary emission).
  • Input parameters for the micropulse gun are: RF voltage, frequency, cavity gap spacing, and magnetic focusing field.
  • Output parameters are: current density, particle energy, transverse emittance and pulse width.
  • the saturated current density is defined to be the peak current density after 10 to 15 RF cycles, i.e. where the amplitude becomes constant.
  • the curve obeys a power law J s ⁇ ⁇ 3 .
  • Excellent agreement is shown between theory and simulation for the ⁇ 3 scaling law. Note that V o ⁇ ⁇ 2 must be maintained for resonance at fixed ⁇ o .
  • ⁇ o eV 0 / m ⁇ 2 d 2
  • the saturated current density is the peak current density, after 10 to 15 cycles, from the current density vs time traces.
  • the current density plots also show the "tuning range" for the micropulse gun. A very tolerant tuning range is a key result. Even if the electric field changed by 30% from, say, beam loading, resonance would still occur but at a lower current density.
  • Figure 6 shows that the micro-pulse width can be adjusted using the drive voltage. Depending on gap spacing and ⁇ 0 the pulse width can be adjusted from 1.5% to 10% of the rf period. For the case: ⁇ 0 ⁇ 0.373, the bunch length is 7 ps at a frequency of 2.85 GHz.
  • the Q L loaded by the beam is 816.
  • a coated copper cavity could be used to keep the cavity secondary emission low and the unloaded Q U high.
  • FMT has performed 3-D PIC code (SOS) simulations to determine bunch emittance growth from the micropulse gun grid. Emittance was evaluated before and after the grids. Emittance can be substantially reduced by using a dense grid of wires, but at the expense of reducing transmission. Results have shown a lower emittance than anticipated. This occurs because the transverse electric field at the grid wires is small during bunch extraction. The inherent mechanism for bunch formation captures the bunch at a phase angle near zero. Bunch arrival at the grid ⁇ / ⁇ later occurs when the electric field is again near zero. This is a big advantage for the micropulse gun as compared to a Pierce gun with a grid which exposes electrons to the maximum field. Input parameters are supplied from the results of FMTSEC.
  • SOS 3-D PIC code
  • ⁇ p 114 keV
  • J s 1150 A/cm 2
  • ⁇ 7 ps
  • Figure 7 shows the beam emittance and transmission versus the grid wire density for a wire thickness of 0.1 mm and density of 28 wires/cm the results give a transmission of -53% and a total (all sources) normalized transverse beam emittance of about 2.3 mm-mrad. This final emittance is nearly the same as the emittance before the double grid.
  • the grids will heat up primarily due to electron beam impact.
  • a molybdenum grid with a thin coating of secondary emitting material A thin layer of secondary emitter is used in order to reduce charge build-up, thus most of the charge is deposited in the molybdenum.
  • the material will be made electrically conducting by doping, thus eliminating charge build-up.
  • ⁇ d and f r are the macro-pulse duration and repetition rate, respectively
  • ⁇ is the FWHM of the micro-pulse current
  • f is the cavity drive frequency
  • micropulse gun cavity is designed for a TM 010 mode and is fed from an L-band (1.2-1.3 GHz) magnetron which delivers about 50 kW to the beam load.
  • L-band 1.2-1.3 GHz
  • the magnetron is operated at 300 Hz repetition rate.
  • Each microwave pulse lasts for 5 ⁇ s and contains about 6500 electron bunches (1 for each RF period).
  • a very accurate timing system is used to trigger the scope so that, if the micropulse gun pulses are reproducible, it is possible to measure the current as a function of time.
  • the collected charge generates a signal that propagates through a custom made 50 ohm, 50 GHz coaxial feedthrough. We were successful in performing a direct measurement of the bunches and were able to prove the feasibility of the micropulse gun concept.
  • Figure 8 shows a measurement of the bunches on a 500 ps/div time scale.
  • the bunches appear with the periodicity of the RF field (-800 ps), in excellent agreement with simulation. More detailed measurements show that the actual bunch length is about 50 ps (FWHM) which is about 6.5 % of the RF period at a current density of about 22 A/cm 2 . This is about 1.1 nC or 7 x 10 9 electrons per RF period.
  • E sc is the space charge field in the moving frame of the micro-pulse.
  • the inductive electric field reduction of the space charge electric field is taken into account in Eq. (5) by the additional ⁇ 2 .

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EP97303476A 1996-05-22 1997-05-21 Canon à électrons Withdrawn EP0809271A3 (fr)

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CA (2) CA2205989A1 (fr)
IL (1) IL120874A0 (fr)
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Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111341631A (zh) * 2020-04-07 2020-06-26 电子科技大学 一种利用二次电子倍增的电磁波发生器

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US9065244B1 (en) 2014-05-01 2015-06-23 The Boeing Company Compact modular free electron laser (FEL) architecture

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US2381320A (en) * 1940-11-28 1945-08-07 Westinghouse Electric Corp Electromagnetic apparatus
US2591322A (en) * 1946-08-30 1952-04-01 Csf Generator of ultra-short electromagnetic waves

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
MAKO F.M & PETER W: "A high-current micro-pulse electron gun" PROC: PARTICLE ACCELERATOR CONFERENCE, 17 - 20 May 1993, pages 2702-2704, XP002128262 ISBN: 0-7803-1203-1 *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN111341631A (zh) * 2020-04-07 2020-06-26 电子科技大学 一种利用二次电子倍增的电磁波发生器

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EP0900446A1 (fr) 1999-03-10
IL120874A0 (en) 1997-09-30
EP0809271A3 (fr) 2000-03-15
JPH1055763A (ja) 1998-02-24
CA2254137A1 (fr) 1997-11-27
WO1997044804A1 (fr) 1997-11-27
CA2205989A1 (fr) 1997-11-22

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