Losses in a pulsed electron beam during its formation and extraction from the diode chamber of an accelerator

The results of experimental studies of the current and charge balance in the diode unit of the ?ЭpU-500 high-current pulsed electron accelerator (an accelerating voltage of 350–500 keV, a half-height pulse duration of 60 ns, and a total kinetic electron energy of 250 J/pulse) during generation of an electron beam are presented. Planar diodes with multipointed cathodes having diameters of 43–60 mm and manufactured from graphite, copper, and carbon felt were studied. It is shown that the electron-beam divergence in the anode-cathode gap caused by a distortion in the electric field at the periphery of the cathode is the main source of parasitic losses in planar diodes. The half-angle of divergence is 68° at small anode-cathode gaps and decreases to 60° with an increase in the gap. When the diode impedance is matched to the generator’s output impedance (at a gap of 10–12 mm), the charge loss is within 12%.     

Increasing the reliability and service life of the cathode unit of an electron accelerator with extraction of a large-cross-section beam to the atmosphere

The design of an electron accelerator cathode unit with short filament elements in the form of a coil without a tightening mechanism is described. The advantage of the developed design over those of cathodes with long filaments with tightening mechanisms is shown. The nonuniformity of the beam-current density distribution behind the foil is <3%. The filament power required for cathodes in the form of coils is 20% lower than that for long-filament cathodes at the same beam-current density. Mechanical stresses leading to a breakdown of the cathode filaments are absent. The service life is determined mainly by the emission-caused wear of cathodes.     

A high-current subnanosecond electron accelerator with a gas-filled former

A subnanosecond electron accelerator prototype based on the ARSA small-size accelerator with a gas-filled former (nitrogen ~4 MPa) has been developed and studied. The operation principle of the former involves charging of a short storage line and its discharge to a stepped line with an accelerating tube that generates electrons. The recorded electron-beam current pulse length was t_0.5 = 0.3 ns, the current amplitude was at least 1.5 kA, and the maximum electron energy was ~850 keV.     

High-efficiency finishing process for metal mold by large-area electron beam irradiation

A new finishing process for metal molds by large-area electron beam (EB) irradiation is proposed in this study. In the large-area EB irradiation equipment used here, an EB with high-energy density is irradiated without focusing the beam, and so the EB with a maximum diameter of 60 mm can be used for melting or evaporating metal surface instantly. Experimental results show that the surface roughness decreases from 6 μmRz to less than 1 μmRz in just a few minutes under proper machining conditions. The corrosion resistance of metal mold surface also could be greatly improved by large-area EB irradiation. Furthermore, the surface roughness of tilting surface close to 90° could be well improved. Therefore, large-area EB irradiation method has a possibility to become a high-efficiency finishing process for metal molds.     

Crystal-assisted beam extraction and collimation at the U-70 circular accelerator

New crystal devices—an array of bent strips and a fan-type reflector based on thin straight plates—have been used to study extraction and collimation of the beam circulating in the accelerator at energies of 50 and 1.3 GeV. It is shown that these devices allow high-efficiency beam steering in a wide energy range. For 50-GeV protons, the efficiency of beam extraction and collimation is ～90%, which is the highest efficiency attained for this method to date. It has been observed that the use of different crystals causes the particle loss at the accelerator downstream of the absorber to decrease by a factor of 2–3 in comparison with the standard single-stage scheme of beam collimation by a steel absorber.     

A high-current picosecond electron accelerator with a low-impedance vacuum diode

A high-current picosecond (∼150 ps) electron accelerator with a beam energy of 50–100 keV is described. The use of a low-impedance vacuum diode at an amplitude of the arriving pulse of 300–400 kV made it possible to significantly increase the beam current (up to ∼15 kA) and the corresponding X-radiation intensity. One of the accelerator's applications in the X-ray therapy of malignant tumors. Some computational relations and results of measurements of the arriving and reflected voltage pulses near the diode are presented. The electron-accelerating voltage, beam current, vacuum-diode impedance, and other parameters are determined after the recovery procedure.     

A high-current nanosecond electron accelerator with a semiconductor opening switch

A compact nanosecond electron accelerator with an output energy of up to 4000 keV, a pulsed power of 100–180 MW, a beam current of 0.25–1.1 kA, and a pulse energy of 5–7 J is described. The accelerator operates with a pulse repetition rate of 200 Hz and ensures an average beam power of up to 1 kW. A nanosecond generator with a solid-state switching system, which is based on magnetic stages of pulse compression and a semiconductor opening switch, is used as a supplying device. The design and electric circuit of the accelerator are described, and test results are presented.     

The output window of a large-area accelerator with an increased electron-beam current density

The results of theoretical and experimental investigations of a large-area accelerator with a new type of output window are presented. With this window, it is possible to increase the current density of the extracted electron beam, reduce the operating temperature of the foil, and extend its service life. A two-level support structure with cooling of each level is used for this purpose. The heat load of the foil mounted on the second level of the support structure that carries the main mechanical load is reduced by partial interception of the thermal power due to direct loss of the electron beam, which is released on the first level experiencing no mechanical load. The current loss at the structural elements of the output window is estimated, and the hydraulic characteristics and foil temperature in the output device are calculated. A higher current density of the beam extracted over the foil is attained in comparison with the conventional design of the support structure.     

Demonstration of a real-time interferometer as a bunch-length monitor in a high-current electron beam accelerator

A real-time interferometer (RTI) has been developed to monitor the bunch length of an electron beam in an accelerator. The RTI employs spatial autocorrelation, reflective optics, and a fast response pyro-detector array to obtain a real-time autocorrelation trace of the coherent radiation from an electron beam thus providing the possibility of online bunch-length diagnostics. A complete RTI system has been commissioned at the A0 photoinjector facility to measure sub-mm bunches at 13 MeV. Bunch length variation (FWHM) between 0.8 ps (~0.24 mm) and 1.5 ps (~0.45 mm) has been measured and compared with a Martin-Puplett interferometer and a streak camera. The comparisons show that RTI is a viable, complementary bunch length diagnostic for sub-mm electron bunches.     

A study of collimation and extraction of the U-70 accelerator beam using an axially oriented crystal

Extraction and collimation of the 50-GeV proton beam with a bent silicon crystal at the U-70 accelerator of the Institute for High Energy Physics (Protvino, Russia) was investigated. Until recently, proton beam extraction (and collimation) from accelerators has been effected using crystals with the (111) or (110) plane orientation, when the beam propagates far from the crystal axes. In the described experiment, the silicon crystal was oriented so that the proton beam was incident on it near the 〈110〉 axis. Under these conditions, a part of the beam was deflected by the crystal owing to the dynamic chaos phenomenon. The maximum beam extraction efficiency was as high as ~80%.     

A source of a high-power relativistic electron beam with high brightness

A ribbon diode of a U-2 accelerator (800 kV, ∼30 kA) intended for generating a high-intensity electron beam for heating plasma in a GOL-3 multimirror magnetic trap (Budker Institute of Nuclear Physics, Siberian Branch, Russian Academy of Sciences) is described. The parameters of the beam characterized by a high brightness (∼7 kA/(cm² sr)) in a magnetic field of ∼5 T resulting from a numerical simulation of the beam formation process are presented. The results of simulation of the beam transport and transformation of the profile of its cross section during movement of electrons in a curvilinear guiding magnetic field are presented. The calculated cross section is compared to the beam imprint on a target.     

ILU-14 industrial electron linear accelerator with a modular structure

A new high-power (up to 100 kW) industrial electron linear accelerator ILU-14 for energies of 7.5–10.0 MeV has been developed by the Budker Institute of Nuclear Physics. The operating frequency of the accelerator is 176 MHz, and the total efficiency is 26%. Owing to the modular structure of the accelerator, the electron energy and the beam power can be varied within certain limits by changing the modular arrangement. A 5-MeV prototype of this accelerator has been produced and successfully tested. Its design parameters verified in the experiments are as follows: the beam current averaged over the RF period is 600 mA, the beam pulse power is 2.5 MW, and the electron efficiency of the accelerating structure is 68%. By applying an additional RF voltage to the electron gun cathode-grid gap, a 96% transmittance of the beam current has been attained at a minor beam energy spread. The prototype of the ILU-14 accelerator can be used as an accelerator with a beam power of 50 kW.     

A long pulse width and high extraction rate arc plasma electron beam source

Based on the principle of vacuum arc discharge under magnetic field, a novel plasma cathode electron- beam source was designed. This device can be used to regulate electron-beam current so as to improve the extrication efficiency of electron beam through regulating the exciting current and thus controlling the density of the plasma electron beam source. Experiment results showed that the arc current change with the magnetic field, to be specific, the stronger the magnetic field was, the smaller the arc current will be, then the density of plasma that penetrated the anode hole to serve as electron beam will be higher. From this experiment, it can be seen that under the condition of 10^?3 Pa air pressure, 100 V arc voltage, 30 A exciting current, we can obtain the electron beam of 40 ms pulse width, and 828 mA current in the extraction rate of 6.1%.     

Experimental study of ion beam optics in a two-stage accelerator

Hydrogen ion beam optics in a two-stage linear acceleration system is studied by examining the beam divergence as a function of the voltage and gap distribution, the beam perveance, the background gas pressure, the aspect ratio, and the total accelerating energy (60-110 keV). The system consists of four electrodes with single, cylindrical, straight-bore apertures acting as an extraction-accel-decel column. An optimum relation between the field ratio and the extraction perveance is obtained from measurements for the minimum beam divergence condition. The HWHM divergence angle is <0.3 degrees under optimum conditions. Qualitative agreement between the measurements and a previous theoretical study is noticed. A potential application of the results to high energy neutral beam injectors for fusion research is also discussed.     

Design and tuning of a 40-MeV electron linear accelerator

The 40-MeV electron linear accelerator RELUS-6 with a 100-mA pulse current has been designed. The standing wave accelerator with a biperiodic accelerating structure is composed of four sections and is powered from two 6-MW pulse klystrons. The operating frequency is 2856 MHz. The total length of the accelerating structure is 2.73 m. Calculations have been performed with the aim of selecting the lengths of the first three cells in the first accelerating section and the values of the accelerating field in them, so that the width of the energy spectrum at the end of the accelerator is 2%. The geometry of the accelerating structure and the power input cells has been designed. Four accelerating structures have been manufactured based on the simulation results. The structures have been tuned to the operating frequency, the predetermined accelerating field distribution, and a fixed coupling between the feed waveguide and the accelerating section.     

Simulations of slow positron production using a low-energy electron accelerator

Monte Carlo simulations of slow positron production via energetic electron interaction with a solid target have been performed. The aim of the simulations was to determine the expected slow positron beam intensity from a low-energy, high-current electron accelerator. By simulating (a) the fast positron production from a tantalum electron-positron converter and (b) the positron depth deposition profile in a tungsten moderator, the slow positron production probability per incident electron was estimated. Normalizing the calculated result to the measured slow positron yield at the present AIST linear accelerator, the expected slow positron yield as a function of energy was determined. For an electron beam energy of 5 MeV (10 MeV) and current 240 μA (30 μA), production of a slow positron beam of intensity 5 × 10(6) s(-1) is predicted. The simulation also calculates the average energy deposited in the converter per electron, allowing an estimate of the beam heating at a given electron energy and current. For low-energy, high-current operation the maximum obtainable positron beam intensity will be limited by this beam heating.     

A collector assembly for measuring a subnanosecond-duration electron beam current

The design and test results of the collector assembly intended for recording electron beams of sub-nanosecond duration are presented. The collector assembly features a passband of up to 12 GHz and is capable of transmitting slightly-distorted voltage pulses with a half-height duration longer than ～50 ps. The collector assembly was used to detect current pulses of a subnanosecond-duration electron beam generated in a gas-filled diode.     

A facility for metal surface treatment with an electron beam

A design for a facility for the surface treatment of metal samples is described, and the results from investigating the source of a high-current low-energy electron beam are presented. The electron beam, which has a current as high as 300 A, a pulse duration of 30 µs, and a pulse repetition rate of up to 10 Hz, is formed in a plasma-cathode gas-filled diode at an accelerating voltage of 20 kV. The space-charge compensated electron beam is transported a distance of 20 cm in a longitudinal magnetic field to the region of its interaction with a solid body. At a current density as high as 100 A/cm², the power density produced by the beam is sufficient for the metal surface to be melted in the duration of one or several pulses. Samples can be replaced in the facility without breaking the vacuum.Translated from Pribory i Tekhnika Eksperimenta, No. 1, 2005, pp. 135–140.Original Russian Text Copyright © 2004 by Koval, Shchanin, Devyatkov, Tolkachev, Vintizenko.