Michael I. Yalandin

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.

EXPERIMENTAL OBSERVATION OF SUPERRADIANCE IN MILLIMETER-WAVE BAND

Abstract The first experimental results of the observation of superradiance from a single subnanosecond electron bunch are presented. Superradiance was associated with different varieties of stimulated emission (bremstruhlung, cyclotron, Cherenkov, etc). Unique megawatt power level microwave pulses of short duration (0.3–0.5 ns) have been obtained.

Coherent Summation of Power of Nanosecond Relativistic Microwave Oscillators

The possibility of developing a two-channel nanosecond relativistic microwave oscillator with a phase stability in each channel sufficient for coherent summation of their electromagnetic fields is demonstrated experimentally. In experiments, vacuum diodes of two independent superradiant backward wave oscillators operating in 10-GHz frequency range were connected to a common voltage source with a subnanosecond pulse rise time, which ensured the fixation of the initial phase of electromagnetic oscillations. The measured values of the phase difference jitter of the channel electromagnetic oscillations amount to several percent of the oscillation period.

Production of short microwave pulses with a peak power exceeding the driving electron beam power

This article presents results of theoretical and experimental studies on the production of ultrashort ~a few RF cycles duration! microwave pulses of gigawatt peak powers based on superradiance from high-current electron beams. With the Cherenkov backward-wave‐electron-beam interaction in a low-dispersion slow-wave structure, microwave pulses with a peak power greater than the peak power of the driving electron beam have been produced for the first time. In an experiment using the SINUS-150 compact high-current electron accelerator, with a 2.6-kA injected beam current and a 330-kV electron energy, microwave pulses of 1.2 GW peak power and;0.5 ns duration ~FWHM! were generated in the X-band. Production of superradiance pulses in a repetitive regime ~3500 Hz! in the Ka-band has been demonstrated using a compact hybrid SOS-modulator. The effect of spatial accumulation of microwave energy in extended slow-wave structures with substantially nonuniform coupling has been demonstrated. In an experiment using the SINUS-200 compact accelerator, X-band pulses of ;3 GW peak power and 0.6‐0.7 ns width~FWHM! were produced with a power conversion efficiency of 150‐180% and an energy efficiency of ;15%.

On the Nature of Picosecond Runaway Electron Beams in Air

Results of experiments on the generation of picosecond runaway electron (RE) beams in air gaps with a strongly nonuniform electric field distribution are discussed. The mechanism and conditions for the field emission initiation of beams of this type and possible reasons for the limitation of the beam duration are considered. REs are supposed to be the dominant factor in the development of an ionization wave in the gap. Estimates of the mean velocity of propagation of the ionization wave are given.

Generation of powerful subnanosecond microwave pulses in the range of 38-150 GHz

Experimental measurements of coherent stimulated radiation from intense, subnanosecond electron bunches moving through aperiodic waveguide and interacting with a backward propagating TM/sub 0.1/ wave are presented. The ultra-short microwave pulses in Ka, W, and G band were generated with repetition frequencies of up to 25 Hz. Observation of RF breakdown of ambient air, as well as direct measurements by hot-carrier germanium detectors, gives an estimate of the peak power up to 140 MW for the 300-400 ps pulses at 38 GHz. The initial observation of 75 GHz 10-15 MW radiation pulses with duration less than 150 ps, and of 150 GHz microwave spikes with a risetime of 75 ps are also reported.

Desktop subnanosecond pulser: research, development, and applications

The studies on production of high-power high voltage pulses of subnanosecond duration having been performed at the Laboratory of Electron Accelerators are reviewed. The subnanosecond pulser we have created is a supplementary device for the earlier developed nanosecond repetitive pulsed power source RADAN 303. The principle of operation of the pulser is successive formation of the leading and trailing edges of the high voltage pulse with the use of high-pressure gas switches. Design versions with a different number of electrodes in the peaking spark gaps have been examined. With a four-electrode spark gap, a pulse of peak voltage 170 kV and FWHM duration 150 ps was produced across a 50 (Omega) load. The processes occurring in transportation of high voltage pulses through the coaxial section of the pulser have been analyzed. The data on the control of the output pulse parameters and typical jitter values are presented. The fields of possible applications of the pulser are discussed. The pulser has been proved in short-run tests at a pulse repetition rate of 100 pps.

High-power relativistic microwave sources based on the backward wave oscillator with a modulating resonant reflector

Results of theoretical and experimental investigations into a relativistic backward wave oscillator with a modulating resonant reflector are generalized. The modulating resonant reflector is used to reflect a counter propagating wave and guide it toward an electron collector. It is shown that premodulation of the electron beam near the reflector may have a significant effect on the starting conditions of oscillation; selective properties of the oscillator; and its efficiency, which may reach 40% when a high-current beam is transported by a strong magnetic field. In the reduced magnetic fields that were employed in the pulsed-periodic regime and were 1.5–2.0 times lower than those at which cyclotron resonance with the counter propagating wave is observed, the oscillator efficiency (30–35% at a wavelength of 8 mm) is limited by position and velocity spreads of particles. Mechanical pulsewise frequency tuning within about 10% at a repetition rate of 1–50 Hz and a multigigawatt microwave power, as well as a rise in the power and energy of microwave pulses via an increase in the cross-sectional dimensions of the slow-wave structure, are demonstrated to be feasible.

Degradation and Recovery of the Emission From a Graphite Cathode in Relation to the Repetition Frequency of Nanosecond Accelerating Pulses

A study has been performed on the emissive characteristics of a cold graphite cathode in a magnetically insulated coaxial diode under the action of nanosecond accelerating pulses at a pulse repetition frequency (PRF) of up to 3.5 kHz. Emission was observed to degrade at PRF < 1 kHz and recover at PRF ~3.5 kHz. Estimates of the temperature conditions in the region of an explosive electron emission (EEE) center have shown that the pulse interval t~1 ms suffices for this region to cool down to 300 K. The cooling occurs predominantly by heat conduction. For t~0.3 ms, the residual heat is substantial. It has been proposed that there exists a frequency limit for the cathode microrelief polishing effect. The results of an experiment on the study of the mechanism of cathode emission recovery with increasing PRF are presented. Micrographs of the cathode taken after aging, photographs of the cathode in operation, and analyses of the fractional composition of the material removed from the cathode suggest that heating of some regions of the cathode emitting edge to the melting point of graphite plays an important role in the recovery of emission. This counts in favor of the hypothesized dominant contribution of thermoelectronic emission to the initiation of EEE due to the residual heat remaining in regions that have not cooled off during the pulse interval

Review of studies of superradiative microwave generation in X band and Ka band relativistic BWOs

This paper presents a review of recent studies of superradiant (SR) regime of high-power microwave generation of relativistic backward-wave tubes. The fundamental possibility of producing microwave pulses with peak power substantially exceeding the driving electron beam power was demonstrated in theory and simulation. In experiments, beam-to-microwaves power conversion factors of 1.5 in the Ka band and up to 1.8 in the X band were achieved. High-efficiency Ka band generation was obtained at a low B field (2 T) below the cyclotron resonance. With the use of a dc solenoid, SR pulses were generated in a batch mode at a pulse repetition frequency of 1000-3500 Hz.