High-current Electron Accelerator Research Articles

Operating features of a high-voltage spark gap switch with gas blow normal to the breakdown path in a repetitively pulsed mode

The results of studies of a high-voltage two-electrode spark gap switch (SGS) with forced gas blow at an operation voltage of up to 1.2 MV are presented. An SGS filled with nitrogen as the working gas at a pressure of up to 16 atm operated as the high-voltage switch of a high-current nanosecond electron accelerator. The gas flow was directed normally to the breakdown path. The SGS switched a 50-Ω forming line with an electrical length of 10 ns to a matched load. The voltage rise time across the electrodes before breakdown was ∼25µs. A stable repetitively pulsed mode is realized at operating voltages of 100–680 kV and pulse repetition rates of up to 270 Hz with a standard deviation of the pulse breakdown voltage of ≤1%. The physical mechanisms that determine unstable operation of the device during self-breakdown are analyzed.

Measuring the virtual cathode velocity

An electron beam with a virtual cathode (VC) transported in a two-section drift tube of a SINUS-7 high-current electron accelerator has been studied. The dependence of the VC velocity on the injected current has been experimentally determined for the first time. It is established that the VC motion in the drift tube is accompanied by microwave oscillations, which are caused by transient processes involved in the formation of a compressed electron beam.

Perveance of a planar diode with a multipoint cathode

The I–V characteristics of a planar diode with a multipoint explosive emission cathode are studied under the conditions when the impedance of the diode is matched to the output resistance of the nanosecond generator. The experiments are carried out on the TEU-500 (350–500 keV, 60 ns, 250 J per pulse) high-current electron accelerator. The perveances of the diodes with a spike (140 spikes) cathode, multipoint copper cathode, and cathode made of carbon felt are measured in the course of electron beam generation. Comparison is performed between the I–V characteristics of the diodes with multipoint and continuous cathodes made of the same material. It is found that the electron current of the diode is described well by Child’s law with regard to the shrinkage of the anode-cathode gap and an increase in the emissive surface due to the expanding plasma (formfactor). When the cathode is made of carbon felt or is multipoint, the space-charge-limited electron current mode sets in even 5–10 ns after the voltage is applied.

Efficiency of a planar diode with an explosive emission cathode under the conditions of delayed plasma formation

The energy and current balances in the diode unit of a high-current pulsed electron accelerator (350–500 keV, 60 ns, 250 J per pulse) are compared for an explosive emission cathode (made of graphite, copper, or carbon felt) and a multipoint (graphite or copper) cathode. The planar diode with the continuous cathode is shown to be more efficient in terms of conversion of the applied energy to electron energy (more than 90%) despite a delay in the plasma surface formation. With the impedance of the planar diode matched to the output resistance of the nanosecond generator, the efficiency of the diode does not depend on the time of plasma formation on the cathode. In the case of the graphite cathode, the plasma formation delay reduces the fraction of slow electrons in the forming electron beam and reduces electron losses in anode foil when the beam is extracted from the vacuum space of the diode chamber into the reactor.

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%.

Comparisons of Monte Carlo calculations with absorbed dose determinations in flat materials using high-current, energetic electron beams

International standards and guidelines for calibrating high-dose dosimetry systems to be used in industrial radiation processing recommend that dose-rate effects on dosimeters be evaluated under conditions of use. This is important when the irradiation relies on high-current electron accelerators, which usually provide very high dose-rates. However, most dosimeter calibration facilities use low-intensity gamma radiation or low-current electron accelerators, which deliver comparatively low dose-rates. Because of issues of thermal conductivity and response, portable calorimeters cannot be practically used with high-current accelerators, where product conveyor speeds under an electron beam can exceed several meters per second and the calorimeter is not suitable for use with product handling systems. As an alternative, Monte Carlo calculations can give theoretical estimates of the absorbed dose in materials with flat or complex configurations such that the results are independent of dose-rate. Monte Carlo results can then be compared to experimental dose determinations to see whether dose-rate effects in the dosimeters are significant. A Monte Carlo code has been used in this study to calculate the absorbed doses in alanine film dosimeters supported by flat sheets of plywood irradiated with electrons using incident energies extending from 1.0MeV to 10MeV with beam currents up to 30mA. The same process conditions have been used for dose determinations with high-current electron beams using low dose-rate gamma calibrated alanine film dosimeters. The close agreement between these calculations and the dosimeter determinations indicates that the response of this type of dosimeter system is independent of the dose-rate, and provides assurance that Monte Carlo calculations can yield results with sufficient accuracy for many industrial applications.

Experimental Setup for Studying Induced Backward Scattering of Microwaves by a Subnanosecond High-Current Electron Beam

This paper presents the results of an experiment in which the effect of generation of picosecond superradiation pulses was first observed during the induced backward scattering of a high-power pump microwave by a short high-density relativistic electron beam. These investigations became feasible due to an experimental setup built around two high-current electron accelerators of the Radan series that were set to operate synchronously to within ~300 ps. The pump wave was generated by a relativistic Ka-band backward-wave oscillator with a power of ~100 MW. To direct this wave into the scattering region, a quasioptical system was used. The scattered radiation detected in the experiment was a short (~200 ps) pulse with a spectrum-integrated power of up to 1 MW. Due to the relativistic Doppler shift, the scattered radiation spectrum covered frequencies up to 150 GHz

Matching a double forming line to explosive-emission diode

The regime of operation of an explosive-emission diode is affected by a matching transformer between this diode and a double forming line of a high-current electron accelerator. Preliminary forced demagnetization of the transformer core makes the shape of the voltage pulse applied to the cathode close to the optimum, corresponding to a decrease in the resistance of the anode-cathode gap related to expansion of the explosive-emission plasma. In addition, matching of the double forming line to the explosive-emission diode significantly decreases the amplitude of parasitic prepulses and increases (to 90–92%) the fraction of energy supplied to the diode during the main current pulse.

The RADAN series of compact pulsed power Generators and their applications

This paper presents results of development of a compact pulsed power high-voltage generators and high-current electron accelerators of the RADAN series. The basic high-voltage units of RADAN instruments are built around coaxial pulsed forming lines and efficient charging device represented by a Tesla transformer. The fields of applications in science and in practice are rather wide and include formation of nanosecond and subnanosecond voltage and ultrawideband RF pulses, high-power microwave generation, X-ray radiography, radiation physics, chemistry, and biology. The designed technique provided achievements of outstanding specific parameters of dense e-beams, microwaves, and ultrawideband pulses.

A High-Current Pulsed Accelerator with a Matching Transformer

An externally biased matching transformer was used to increase the efficiency of transfer of the energy stored in the double forming line (DFL) of a high-current electron accelerator to the electron beam. The transformer was placed between the DFL and the diode module. Saturation of the transformer core prior to line charging allowed us to significantly reduce the amplitude of the prepulse and its destructive effect on the anode foil without using a pulse-sharpening gap. An electron beam with a pulse energy of 200 J was produced; the kinetic energy of the electrons was 500 keV and the pulse duration was 60 ns.

Subnanosecond 1-GW pulsed 38-GHz radiation source

The regime of excitation of subnanosecond high-power microwave pulses has been studied in a Cherenkov device with an extended periodic slow-wave structure, using an electron beam from a compact pulsed high-current electron accelerator (290 keV, 2.3 kA, 1 ns). Conditions are established for which the power conversion coefficient can reach up to 1.5 at an output pulse power of 1.2 GW and a pulse duration of 200 ps.

High-current Pulsed Accelerator with Matching Transformer: Construction and Exploitation Characteristics

To increase energy-to-electron-beam transfer effectiveness accumulated in the double forming line of high-current electron accelerator, the matching transformer with outside magnetization placed between forming line and diode module was used. Transformer core saturation to line charging phase allows to decrease greatly pre-pulse amplitude and its influence on anode foil durability without switch sharpening application. Accelerator provides electron beam generation with pulse energy up to 200 J at electron kinetic energy 500 keV and pulse length 60 ns. Nanosecond voltage generator and explosive emission diode effectiveness were investigated.

Analysis of the heat load of thin metal foil during electron beam scanning

The thermal loading of thin metal foils used as extraction windows in high current electron accelerators is one of the most important processes which should be analysed during the design of such devices. Heat loads at metal surface are also of special interest in many other plasma applications. The distribution of heat loads at the surface of thin metal foil is evaluated from the energy absorption in the foil and forced cooling with air flow. Time dependence of temperature distribution at the foil surface is calculated from the two-dimensional heat conduction equation. The simplified heat transient analysis is also given to determine the influence of scanning frequencies, electron beam size and shape ,shape of current impulses in scanning coils, energy loss in foil, the beam current and air cooling coefficient on time evolution of temperature for an arbitrary fixed point at the foil in first period of electron beam scanning. In most d.c. high current electron accelerators the concentrated electron beam generated at the cathode is accelerated along multi-gap structure and after that is dispersed over the surface of thin metal window (usually titanium (1)) by two sets of perpendicular scanning coils to obtain as uniform distribution of thermal load as possible (2). Thin metal foil (usually 50 µm thick) separates vacuum region of the accelerator from the atmosphere and is supported by a rigid frame usually cooled by water flow . The energy of the electron beam is absorbed in the foil and causes serious heating of the material, so thin foil is cooled from atmosphere side by transverse forced air stream delivered from manifolds located alongside the foil (3). The amount of the energy absorbed in window material from electron beam with given energy and current is fundamental information for design of reliable extraction system. The modification of the energy spectrum and angular distribution of primary electron beam is the result of electron interaction with the matter. In case when scattering foil is 'thin' i.e. small compared to the incident electron range, the multiple Coulomb scattering is observed together with other processes of less importance. The Monte Carlo SHOWME code was used to calculate the absorption, transmission and reflection of 800 keV electrons falling on 50 µm flat titanium foil perpendicular to its surface. The following results were obtained from the analysis (4): TE =0 .95 ± 0.01 (the energy transmission coefficient) RE =0 .005 ± 0.001 (the energy reflection coefficient) φA =0 .042 ± 0.004 (the energy absorption coefficient)

A vircator with electron beam premodulation based on high-current repetitively pulsed accelerator

The paper describes theoretical, numerical, and experimental studies of a decimeter wavelength vircator with electron beam premodulation. Possible mechanisms for the excitation of electromagnetic oscillations in virtual cathode systems are analyzed (modulation of current, reflex klystron effect, and inertial bunching of reflected particles). It is demonstrated that the microwave efficiency in a double-gap virtual-cathode system may be substantially higher than that in a single-gap one. Based on one- and three-dimensional numerical simulations, an experimental mockup of double-gap vircator without external magnetic field has been developed. In experiments employing the SINUS-7 high-current repetitively-pulsed electron accelerator, single-mode microwave generation in the S-band was obtained with /spl sim/1 GW peak power and /spl sim/25 ns pulse width at about 5% efficiency. The generator showed frequency stability from pulse to pulse and throughout each pulse, which proves the dominant role of the electrodynamic system used in the vircator construction. Varying the cavity parameters allowed continuous frequency tuning within about 15% at half power. Sample batch operation of the system at 20 and 50 p.p.s. was demonstrated.

Observation of the Dependence of Backward Wave Oscillator's Microwave Output Power on Corrugation Number.

The dependence of the backward wave oscillator's (BWO's) microwave output power on the number of corrugations is observed experimentally. The slow-wave structure (SWS) of the BWO has been designed to operate at 9.8 GHZ . The SWS consists of detachable corrugated modules, and the corrugation number N can be changed from I up to I O. The BWO is driven by the high current electron accelerator LAX-1 (850 keV, 3 kA). The dependence ofpower on corrugation number is not monotonic; with 3 modules, a small output signal of 20kW Ievel is observed; with 4 modules, a rapid increase of output power of the level of 10 MW is observed, and from 5 to 10 modules the output power appears to saturate at the power level of I OOMW. These results are analyzed with a PIC Code called Magic.

Thermal-radiation processes in the MeIV–H system. Synthesis of hydrides of metals

A special hermetic chamber was elaborated and constructed to treat materials by the electron beam in vacuum and in hydrogen atmosphere ( P H=1–2 atm; 1 atm=101 325 Pa). The investigation was carried out on the high current electron accelerator LAE-5 using a focused electron beam. The system of temperature measurement during irradiation was developed. It allowed measurement of the thermal effects taking place under the action of irradiation, and the establishment of the main features of thermal-radiation processes in the Me–H system. As a result of experiments at different doses and power of dose (0.025–1 MRad/s) in vacuum and in the hydrogen atmosphere, the main peculiarities of the processes have been observed, such as: (i) the thermal-radiation synthesis of hydrides of IV group metals of stoichiometric composition – TiH 2, ZrH 2, and HfH 2; (ii) the phenomenon of ‘cold synthesis’ of hydrides. As a result of this phenomenon hafnium hydride of super-stoichiometric composition, HfH 2.4, has been obtained.

A strong electromagnetic field generated by a dipole

A strong electromagnetic field generated by a steep current pulse passing in a conductor dipole was measured. The current was created by an ultrashort voltage pulse formed in a coaxial transmitting line of a high-current picosecond electron accelerator.

Monitoring of the Changes in Chemical Composition of Gases in a Plasma Chemical Reactor During Condensation of the Reaction Products Using the Acoustic-Wave Frequency Data

A technique for monitoring the chemical transformation of a gas mixture in an enclosed volume using the frequency of acoustic waves propagating therein is proposed. A mathematical verification of this method is presented. The results of monitoring the plasmochemical reactions of direct recovery of metals from their fluorides in a high-current electron accelerator are reported. The error of the method under the given experimental conditions is estimated to be 2–3%.

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.

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.