Production Of Electron Beams Research Articles

Laser particle acceleration: beat-wave and wakefield experiments

In a plasma, some of the energy of a high-power laser beam can be transferred to a longitudinal plasma wave with a high phase velocity. This wave can in turn accelerate relativistic charged particles to very high energies. Several mechanisms have been proposed to generate these intense electric fields and some of them have already been tested experimentally. Using the beat wave method, electric fields of 1 - 10 have been produced and electrons have been accelerated with an energy gain from 1 MeV to more than 30 MeV. Some preliminary experiments have shown that electrons can be accelerated in plasma waves generated by the wakefield method. In the case of self-modulated wakefield, electric fields larger than 100 trap electrons and eject them from the plasma with an energy up to 100 MeV. The perspectives in the near future are the production of intense and short electron beams of a few MeV and the acceleration of electrons up to 1 GeV. To reach an energy of 1 TeV and get closer to the parameters required by the high-energy physicists, one will have to test some new methods to be able to guide the laser beam over large distances.

Small pulsed electron-ion sources for radiation technologies and surface modification of materials

A new type of small, combined pulsed electron-ion source for radiation technologies and surface modification of materials is reported. The source consists of a high voltage generator and particle emitter in the form of a vacuum diode. Explosive electron emission is used for production of electron beams and explosive ion emission for production of ion beams. The main parameters are the source output are: kinetic energy 200–700 keV, pulse length 0.3–1.0 μs, electron beam current 0.5–6 kA, ion beam current 0.1–200 A for ions of various conducting materials. The main applications of the device are presented.

Controlled production of warm electron beams

A warm beam is produced by passing a cold electron beam through a system of three closely spaced parallel grids. The beam and the grid structure are immersed in a magnetic field. The middle grid is biased at a large positive potential, and the resulting electric field near the grid wires scatters part of the electrons axial energy into transverse energy. Thus, although the beam remains monoenergetic, a controllable spread in both the transverse and axial energy distribution is obtained.

Electron beam damage Josephson junctions: The relation between beam damage profile and electrical properties

It has been found that in the production of electron beam damaged junctions in thin films of the high-Tc superconductor YBa2Cu3O7−δ, using a small condenser aperture results in a narrow damage width of 20nm, compared to over 100nm when a larger aperture is used. Junctions produced using the large aperture show a quadratic dependence of the current density on temperature, while the junctions produced using the smaller aperture have a more unusual cubic temperature dependence. A functional dependence of the Jc is given from which the sensitivity of the current density to the damage can be calculated.

Development of an electron-beam ion source for a relativistic heavy-ion collider preinjector

At Brookhaven National Laboratory (BNL), an electron-beam ion source (EBIS) is operational as a test bed for development of a high current EBIS for relativistic heavy-ion collider (RHIC). Previously, the goal of most EBIS research has been to produce bare or nearly bare nuclei. At BNL, the EBIS is required to produce only medium charge states of heavy ions, e.g., Au35+, since there is further stripping at higher energies. The BNL effort is directed at reaching intensities of interest to RHIC, approximately 3×109 particles/pulse, which will require an EBIS electron beam on the order of 10 A. Initial tests using a 1 mm LaB6 cathode have produced electron beam currents up to the design value of 110 mA. A 2 mm LaB6 cathode has been installed and in a first run has produced currents up to 350 mA. This source has so far produced charge states up to Ar16+ using neutral gas injection, and up to Tl50+ using external ion injection. Results of these studies and ion injection trials are presented.

Gyrotron experiments using cavities of different ohmic Q

Abstract The change in oscillating characteristics of a tunable (20-100GHz) gyrotron was studied with the change of cavity wall material, all other dimensions and operating parameters remaining constant. The gyrotron was based on a two-electrode electron gun consisting of a field emission, field-immersed cold cathode and a shaped anode cavity. The construction material of the cavity was changed from stainless-steel (σ=l-39×106Q−1m−1) to oxygen-free, high conductivity (OFHC) copper (σ= 5.92×107 Ω −1m−1with an associated 6-5-fold increase in the cavity ohmic quality factor for all modes. It was possible to determine, by measuring the relative millimetre wave output power and pulse duration, the optimum anode-cathode separation. This experimentally determined that the relativistic electron beam production was unaltered with the change in cavity construction material. The maser was operated with an anode-cathode separation of 15 cm and millimetre wave pulse duration of 150 ns at an intra-cavity B field of 10T...

Novel cathode for field-emission applications

A CsI salt-based cathode which is capable of producing a modest perveance, 10 s of A/cm2 electron beam for several microseconds pulse lengths, and has little susceptibility to diode closure has been experimentally characterized. This explosive field-emission CsI-coated carbon fiber cathode has operated in modest 10−5 Torr vacuums at voltages up to 160 kV, and can easily be configured to provide space-charge-limited solid or annular electron beams in arbitrarily large diameter configurations. The CsI cathode has demonstrated negligible closure for 2 μs pulses, and has operated for 200 shots with no degradation in cathode performance. Data on the operating performance of this salt cathode, including effective gap time history and streak photographs demonstrating uniformity of the current density, are presented. A comparison of CsI cathode performance with a velvet explosive field emitting cathode used in electron-beam production is also presented.

Optimum field size and choice of isodose lines in electron beam treatment

Purpose : A method is provided for the optimum filed size and the choice of isodose line for the dose prescription in electron beam therapy. Methods and Materials : Electron beam dose uniformity was defined in terms of target coverage factor (TCF) which is an index of dose coverage of a given treatment volume. The TCF was studied with respect to the field size, the beam energy, and the isodose level for prescription from the measured data for various accelerators. The effects of the TCF on air gap between electron applicator/cone and the surface was investigated. Electron beams from scattering foil and scaned beam units were analyzed for the target coverage. Results : A mathematical method is provided to optimize a filed size for target coverage by a given isodose line in terms of TCF which is strongly dependent on the type of accelerator and the design of the collimator. For a given type of collimating system, the TCF does not depend on the type of electron beam production (scattering foil or swept scanned beam). Selection of isodose line for dose prescription is very critical for the value of the TCF and the dose coverage. The TCF is inversely proportional to the isodose value selected for the treatment and nearly linear with field size and beam energy. Air gap applicator and the surface reduces the dose uniformity. Tertiary collimator moderately improves the lateral coverage for high energy beams. Conclusions : To adequately cover the target volume in electron beam treatment, lateral and depth coverage should be considered. The coverage at depth is strongly dependent on the choice of isodose line or beam normalization. If the dose prescription is at d max (i.e., the 100% isodose line is selected), the choice of beam energy is not critical for depth coverage since d max is nearly independent of energy for smaller fields. The 100% isodose line should not be chosen for treatment because of the significant constriction of this isodose line and inadequate coverage at depth. For a higher TCF, a minimum air gap between the cone to the surface of the patient is desired. If such is not possible, then a tertiary collimator at the skin is required. Whenever, a tertiary collimator is used, it is advised to icrease the collimator field size by a factor of 1.4.

First detection of correlated electron beams and plasma jets in radio and soft X-ray data

From a common analysis of solar radio spectral and imaging data of a fast drift burst of type U(N) together with Yohkoh soft X-ray images it is shown that the radio emission is compatible with electron beams injected and reflected in extended loops. The electron beam production coincides with the injection of hot matter, visible as a jetlike soft X-ray feature in the underlying loop system.

Electron-beam generation during the hollow cathode phase of pseudospark discharges

From calculations using a two-dimensional, hybrid fluid-particle model, the mechanism for the production of the high-current electron beam observed during the hollow cathode phase of pseudospark discharges is identified. The predicted beam properties are in good qualitative agreement with experiments; the onset of the beam current is abrupt and the current in the beam is some 10%–30% of the discharge current. Two distinct components appear in the energy spectrum of the beam. A sizable fraction of the current is carried by electrons with an energy corresponding to the voltage applied to the discharge. A distinct low-energy beam is also predicted with a spread in energy of several 100s of eV.

High brightness, long pulse, electron beam production with SC photo-injectors

In this paper we discuss the design of a possible superconducting photoinjector able to produce long pulses of high charge electron bunches, as required for the TESLA project. An extrapolation to high brightness cw electron beams for inertial fusion and X-UV FEL experiments is also given, together with an analysis of the R&D needed for the actual injector development. The requirements for the photocathode which has to deliver high current density in a SC environment are discussed on the basis of the preliminary results of the Wuppertal experiment, performed in the context of an international collaboration.

Electron emission from ferroelectrics - a review

The strong pulsed emission of electrons from the surface of ferroelectric (FE) materials was discovered at CERN in 1987. Since then many aspects and properties of the method of generation and propagation of electron beams from FE have been studied experimentally. The method is based on macroscopic charge separation and self-emission of electrons under the influence of their own space-charge fields. Hence, this type of emission is not limited by the Langmuir-Child law as are conventional emission methods. Charge separation and electron emission can be achieved by rapid switching of the spontaneous, ferroelectric polarization. Polarization switching may be induced by application of electrical-field or mechanical-pressure pulses, as well as by thermal heating or laser illumination of the ferroelectric emitter. At higher emission intensities plasma formation assists the FE emission and leads to a strong growth of emitted current amplitude, which is no longer limited by the FE material and the surface properties. The most attractive features of FE emission are robustness and ease of manipulation of the emitter cathodes which can be transported through atmospheric air and used without any problems in vacuum, low-pressure gas or plasma environments. Large-area arrangements of multiple emitters, switched in interleaved mode, can produce electron beams of any shape, current amplitude or time structure. The successful application of FE emission in accelerator technology has been demonstrated experimentally in several cases, e.g. for triggering high-power gas switches, for photocathodes in electron guns, and for electron-beam generators intended to generate, neutralize and enhance ion beams in ion sources and ion linacs. Other applications can be envisaged in microwave power generators and in the fields of electronics and vacuum microelectronics.

On the design of systems for production of intense electron beams in the space-charge-limited emission regime

Numerical modeling of electron-optical systems in the space-charge-limited regime ({rho}-regime) includes finding the current distribution on the emitter in addition to calculation of the electron trajectories. The conventional scheme for solution of this problem is based on an iteration procedure in which the current is determined in each iteration by the three-halves law using values of the potential in the vicinity of the emitter. It is known that this scheme is in general incorrect, and convergence of the process is achieved only by the use of special relaxation procedures. Moreover, this method is applicable in principle only to systems with single-component, purely electrostatic, single-stream charged-particle fluxes, i.e., the three-halves law is satisfied only in such systems. A more-universal approach to study of the {rho}-regime consists of direct satisfaction of the natural physical condition (for a zero initial particle velocity) E{vert_bar}{sub {Gamma}{sub e}} = 0 (1) on the electric-field component that is normal to the emitter surface. A method has been proposed that implements (1) and is based on solution of the system of nonlinear equations f(E{sub i}) = {var_phi}{sub i}(I{sub 1}, I{sub 2}, ..., I{sub N}) = 0, i = 1, 2, ..., N (2) for the currents I{sub k}more » in the current pipes by Newton`s method. Here E{sub i} is the electric-field component that is normal to the emitter surface at the i-th starting point, N is the number of current pipes, and f(E) is a specially selected function that vanishes when E = 0. The form of the function f(E) greatly affects the convergence of the process. The function f(E) = E{sup 2} has been recommended for trajectory analysis. An important aspect of the procedure is the use of a diagonal approximation of a Jacobian matrix with elements U{sub ik} = {delta}f(E{sub i})/{delta}I{sub k} and calculation of that matrix only in the first two iterations.« less

High current density small divergence electron beam production by laser-induced various-order multiphoton photoeffect

The relative roles of (1) surface and volume type photoelectric effects, (2) slow and fast laser-induced thermionic electron emission, and (3) optical tunnel emission are discussed from the point of view of the production possibilities of efficient high current density, small divergency electron beams. Angular distribution of photoelectrons emitted from gold target surface by high-intensity pulsed laser radiation at wavelengths of 1060, 343, and 248 nm for the s and p polarization of the grazing incident radiation are investigated experimentally. For the longer wavelengths, where the order of the multiphoton photoelectric emission increases, the ratio of the yields of the surface photoeffect and of the volume photoeffect also increases. The comparative advantages of the surface multiphoton photoeffect of metals as a promising method for the realization of small emittance, high brightness, high current density electron sources are pointed out.

Relaxations in electron beams and adiabatic acceleration.

In order to produce electron beams with the minimum possible energy spread we have investigated the possibility of replacing the usual thermocathodes with a photoemissive source such as GaAs. A comparison carried out between these two sources indicated the latter as the best device to obtain a very-low-energy spread. Beam relaxation after emission also leads to an increase in the energy spread. Therefore an experimental study on the nature of relaxations occurring in electron beams, yielded both by a thermocathode and by a photocathode, has been performed. More specifically, we investigated the possibility of reducing the transverse-longitudinal and the longitudinal-longitudinal relaxations. With this aim, the features of adiabatic acceleration, which damp the pure longitudinal relaxation, have been examined. The experience gained during the measurement cycle demonstrated that an adiabatic structure, accelerating electrons emitted by a GaAs photocathode, leads to the best performances.

Broad beam electron sources with plasma cathodes

Plasma cathodes are used in large-cross-section electron beam sources. The cathode plasma is created by a low-pressure arc discharge with a hollow anode. The physical processes in the discharge anode region associated with electron emission from the plasma and the formation of a stable plasma boundary as well as the methods used for reaching a more uniform electron current density distribution are discussed. The electron source designs allowing one to produce electron beams with currents of 10−10 3 A, durations of 10 −5–10 −3s, current densities of 0.1−1.0 A/cm 2, and cross sections of ∼10 4cm 2 are described.

Magnetic microlens focusing of electron beams

Recently, field emission sources in combination with either magnetic or electrostatic microlenses have been predicted to produce electron beams with high brightness and low aberrations. The key advantage to such systems is the short focal length and thus small coefficients of spherical and chromatic aberration. In addition, by using either small apertures or extremely sharp emission sources, the emission angle can be made small and thus the effect of the aberrations reduced further. We present here our initial results on building a magnetic microlens system. The unique feature of this work is the use of permanent magnets as the sole focusing elements in the lens, resulting in an extremely simple and inexpensive system.It has been shown that the aberrations in a focused electron beam can be reduced by selectively scaling the column dimensions. The focused spot sized is limited by the spherical aberrations,and the chromatic aberrations

Outer heliospheric radio emissions: 1. Constraints on emission processes and the source region

The Voyager 1 and 2 spacecraft observed low‐frequency radio emissions near 2 and 3 kHz during the interval 1983–1987 while at heliocentric distances from 15 to 27 AU and 11 to 20 AU, respectively. We consider the detailed theoretical and observational requirements for this radiation to be produced near multiples of the plasma frequency ƒp by nonlinear, weak turbulence, wave‐wave processes involving electrostatic Langmuir waves. Constraints on the emission processes and source characteristics are discussed. The minimum brightness temperature of the radiation, 3×1014K, requires the Langmuir waves to be generated by a plasma instability, most plausibly by an electron beam instability. This requires electron acceleration and beam production in the near vicinity of the source. Minimum Langmuir wave electric fields in the source region, based solely on the kinematics of the radiation processes, lie in the range 1–100 μV m−1 for nominal source and electron beam parameters. Path lengths for fundamental and harmonic emission processes are calculated: the observed levels of radiation can be produced in path lengths (and source dimensions) smaller than 1 AU provided the participating Langmuir waves have effective temperatures TL ∼ 1017K. These Langmuir wave levels are plausible, based on observations in planetary foreshocks and theoretical calculations. Previous source models for the radiation are discussed. Suggestions that the radiation is generated on the downstream side of the inner heliospheric shock or in the vicinity of the heliopause are shown to face severe theoretical and observational problems, based on present knowledge of the outer heliosphere. However, the upstream (sunward) side of the inner heliospheric shock is predicted theoretically to have a foreshock region containing electron beams and associated Langmuir waves. Theoretically this source region can plausibly produce radiation at multiples of ƒp with the observed brightness temperatures.

Production of high-energy electron beams in moderate and high-pressure gases

The feasibility of producing high-current, high-energy electron beams was studied for the application of nanosecond voltage pulses with amplitudes to 300 kV in accelerating gaps. The pressures of the gap gases (helium and nitrogen) measured from 10−1 torr to atmospheric. Experiments showed that the placement of two barriers in the accelerating gap significantly increased the electron beam current due to an increase in the burn time of the volume discharge under conditions involving significant over-voltage in the electrical field. Electron beams with energies of up to 250 keV and currents up to 260 A were obtained.

Outline of a 15–20 GeV CEBAF like machine

Transverse beam dynamics problems in a recirculating linac and synchrotron radiation effects in beam transport lines are briefly summarized. A broad outline of a CEBAF like accelerator capable of producing electron beams with energies of up to 15–20 GeV is given.