Konstantin A. Sharypov

In a relativistic Ka-band backward-wave oscillator (BWO), the generation of doublets of superradiance (SR) pulses with multimegawatt power was achieved due to deep modulation of the current of an explosive-emission electron beam. A SR pulse interacting with the counter-propagating beam generates a strong electric field in the radial gap positioned in the region of the cylindrical insert incorporated in the BWO anode assembly instead of a commonly used resonant reflector. Here the beam is repetitively dumped onto the wall of the insert, so the SR generation fails and then the process repeats. Since the electrons reach their maximum energies within the leading edge of the subsequent segment of the total current pulse, the excitation of the next SR pulse is not significantly affected by residual slow electrons from the previous generation cycle. As a result, the time interval between measured successive SR pulses nearly matches the value of ≈1 ns obtained from numerical simulation, and their amplitudes are almost the same.

Application of 2 MV, 100 ps, 100 GW pulse from a solid-state driver for powering vacuum diodes with explosive emission cathodes is described. The first sealed-off diode with a “tube-plane” electrode geometry operated in a mismatched mode. A tantalum foil cathode emitted a divergent electron flow with the current of ≈ 12 kA, energy of ≈ 3.2 MeV and peak power of ≈ 40 GW through a beryllium window. Acceleration gradient of 500 МeV/m was achieved in the 6-mm long diode gap. Another diode with a tubular graphite cathode and a strong guiding magnetic field was closer to matching with the driver. The bunch current of 22–27 kA and energy of ≈ 2 MeV were estimated as it passed aluminum filters with thickness of 0.015–5.7 mm shielding the collector sensor. The charge of recorded bunch exceeded 2500 nC. These parameters do not contradict the ones obtained in diode simulation by particle-in-cell method.

An experimental study was performed to analyze the emission of runaway electrons in atmospheric gaps in the presence of an advanced pulse, preceding the subnanosecond leading edge of the main voltage pulse, during which the electric field at the cathode reaches a strength sufficient for the occurrence of runaway electrons. The pulses of the runaway electron current vary due to the appearance and dynamics of the cathode plasma layer from the boundary of which the electrons run away. Plasma is generated by ionization of the gas by field emission electrons as early as during the advance pulse. It is shown that the characteristics of the runaway electron flow depend on the amplitude and duration of this pulse, as well as on the presence of an interval between it and the leading edge of the main voltage pulse.

The paper presents experimental results on obtaining multigigawatt electron beams of picosecond duration using a serially produced sealed-off vacuum diode IMA3-150E with beam extraction into the atmosphere. The feeding pulse was supplied by an S-100 generator based on a solid-state SOS-diode driver and four pulse compression stages employing nonlinear ferrite transmission lines. The peak power of the driving pulse with a duration of ~100 ps reaches 100 GW. The experiment produced a beam with the following parameters: maximum energy of 3.2 MeV, peak current of ~12 kA, and current pulse duration of 85 ps. The peak power of the beam was ~40 GW. The cross-sectional distribution of beam current density, obtained by digitizing the beam autograph on a TsVID dosimetric film, and the angular current density distribution, measured using a collector current sensor, are presented. It was found that the diode lifetime in this mode is limited by intense erosion of cathode foil. Structural elements of the diode are able to withstand the voltage pulse without breakdown, developing electric fields of 5-10 MV/cm.

When excited by beams of runaway electrons with a duration of 100 ps, the pulsed cathodoluminescence of alkali halide crystals, cerium-doped yttrium-aluminum garnet ceramics, and synthetic gems (ruby, sapphire and diamond) was studied. The runaway electron beams were generated in the cathode-anode gap filled with atmospheric pressure air and focused by a non-uniform magnetic field with an amplitude of B ≈ 0.6 T into an area with a diameter of 15 mm. A comparative analysis was carried out between the findings and pulsed cathodoluminescence parameters of the same samples excited by an electron beam with a duration of 2 ns and a current density of 130 A/cm² generated in a vacuum diode. In the case of irradiation with the runaway electron beams, the formation of F-centers has not been observed in alkali halide crystals, that arises out of a reduced radiation impact on samples. Thus, the spectral information is not distorted. When excited by a picosecond runaway electron beam, the decay kinetics of the Ce³⁺ luminescence band in yttrium-aluminum garnet ceramics is described by an exponential function with a characteristic time of 100 ns. When the ceramics are irradiated with a nanosecond electron beam with a higher fluence, the kinetic curve has a more complex shape with an additional maximum in the microsecond range due to the appearance of an extra pumping mechanism for the cerium emissive level. The luminescence spectra and kinetic parameters of the synthetic gems are identical in both cases. The difference is manifested only in the emission intensity, which is much weaker when samples are exposed to irradiation with picosecond runaway electron beams.

The RADAN pulsed power generators were developed in the 1980s as portable pulsed X-ray sources. However, the main goal of that work was to develop a series of multi-purpose compact high voltage generators for laboratory research. The successful combination of a high-voltage coaxial capacitive energy store and a built-in Tesla transformer, the low-jitter connection to a load, and the possibility to shorten and compress the output pulses made these generators immediately in demand, mainly in physical experiments. They were used most successfully in the studies of fast emission and discharge processes, as well as for the generation of runaway electron flows in gases and formation of short beams of explosively emitted electrons in vacuum. These devices also served as the base equipment for experiments on generating high-power coherent and ultra-wideband microwave pulses, pumping semiconductor lasers, driving pulsed cathodoluminescence, etc. To date, more than 200 research facilities available to solve complex research problems have been designed based on RADAN generators. This paper provides a brief overview of some of the results obtained in recent years.

A novel experimental approach to estimating the kinetic energy of a picosecond runaway electron bunch formed in an air electrode gap is proposed. When such an electron bunch travels in a longitudinal magnetic field through a hollow circular waveguide or a waveguide partially loaded with a dielectric, it emits microwave radiation having spectral peaks associated with cyclotron or Cherenkov excitation of the waveguide modes. The first mechanism cannot be invoked to estimate the bunch energy in a single shot, since such an estimation requires identification of the excited modes. The Cherenkov mechanism of excitation of slow modes in a dielectric-loaded waveguide, on the contrary, allows one to estimate the electron energy, since the dispersion curves of the lowest, predominantly excited TE11 and TM01 modes, degenerate starting from a certain frequency. Therefore, when radiation of a higher frequency is excited, the electron energy of hundreds of kiloelectronvolts can be determined for a single radio frequency pulse both at the axis and in the periphery of its radiation pattern. Application of a strong guide magnetic field makes it possible to tune out the parasitic electron cyclotron interaction and discriminate the spectral peaks of Cherenkov radiation, which has been proven to be coherent in this case.

A comprehensive analysis of the physical processes occurring in high-voltage nonlinear ferrite transmission lines in the microwave generation mode is presented. The nonlinear lines of the coaxial type, based on the microwave ferrite rings with different combinations of the magnetic parameters and hysteresis loop shapes, have been investigated for this purpose. A complex comparative analysis of the dependence of microwave generation modes on the magnetic properties, the length of the ferrite ring assembly, and the magnitude of the external bias field enables the processes observed in the nonlinear ferrite lines to be related to the properties of the spin waves. The proposed new approaches make it possible to associate, in the most complete and harmonious way the specific manifestation of the ferrite properties under the condition of ferromagnetic resonance in the oscillation modulation mode, with the classical statements of the theory of ferrites that refer to the relaxation processes involved in the propagation mode of powerful microwave (high alternating magnetic fields). The microwave generation modes observed for the first time and a detailed analysis of their efficient implementation are discussed. In addition, a promising frequency analysis based on the wavelet transform is proposed. The experimental data obtained indicate that microwaves can be generated in the absence of an external biasing field. The effect of secondary modulation with a frequency ∼10 times that of the main oscillations also was discussed. The findings of the study have a great potential as a basis for a wide range of further research and may, therefore, be of interest to researchers engaged in a variety of fields.