V. G. Shpak

Supershort electron beam from air filled diode at atmospheric pressure

The properties of an electron beam (e-beam) formed in air under atmospheric pressure are reported. The nanosecond generators RADAN-303 (two devices) and RADAN-220, producing nanosecond voltage pulses with amplitude of up to 400 kV and subnanosecond rise time were used in the experiments. It was shown for the first time that the duration of e-beam current of gas diode behind the foil does not exceed 0.1 ns. The maximum amplitude of current of a supershort avalanche electron beam (SAEB) behind the foil was ∼400 A. The data on the influence of various parameters on e-beam current amplitude measured behind the foil were obtained. An electron beam with energy less than 60 keV and powerful X-ray radiation were formed in discharge gap simultaneously with SAEB.

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.

Dynamics of subnanosecond electron beam formation in gas-filled and vacuum diodes

The dynamic characteristics of a subnanosecond pulsed electron beam formation in the accelerating gap of a gas-filled or evacuated diode have been studied at a time resolution ∼10−11 s. In the air-filled gap, the electron beam pulse with a current amplitude of several amperes is formed up to about one hundred picoseconds earlier than the analogous pulse under vacuum conditions, and the measured pulse duration (∼10−10 s) is close to the electron flight time across a diode gap in the continuous acceleration regime. It is shown that a nanosecond prepulse plays an important role by initiating the emission of electrons that are subsequently accelerated by the high-voltage pulse with a subnanosecond front.

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.

Picosecond runaway electron beams in air

Experimental data on the generation of picosecond runaway electron beams in an air gap with an inhomogeneous electric field at a cathode voltage of up to 500 kV are presented. The methods and equipment developed for these experiments made it possible to measure the beam characteristics with a time resolution of better than 10−11 s, determine the voltage range and the beam formation time in the breakdown delay stage, and demonstrate the influence of the state of the cathode surface on the stability of runaway electron generation. It is demonstrated that the critical electron runaway field in air agrees with the classical concepts and that the accelerated beam can be compressed to ∼20 ps. It is unlikely that, under these conditions, the beam duration is limited due to the transition of field emission from the cathode to a microexplosion of inhomogeneities. The maximum energy acquired by runaway electrons in the course of acceleration does not exceed the value corresponding to the electrode voltage.

Compact High-Power Subnanosecond Repetitive-Pulse Generators (Review)

This review presents the results of investigations, design work, and tests of generators of subnanosecond pulses with amplitudes exceeding 100 kV. These generators were developed at the Institute of Electrophysics on the basis of compact nanosecond repetitive-pulse RADAN generators. Relatively long pulses (2–5 ns) were transformed into shorter ones (down to 150–200 ps) with the use of systems based on high-pressure sharpening and cutting gas spark gaps. This ensured stable operating modes for the generators at repetition frequencies of up to 100 Hz. Such spark gaps were utilized in systems of additional energy compression for enhancing the peak power of output subnanosecond pulses, as well as in devices producing high-power bipolar pulses. Some applications of short (<1 ns) powerful voltage pulses are considered.

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%.

Limitation of runaway electron beam duration in air-filled gap with inhomogeneous field

Alternative factors that account for a limitation of the period of injection of picosecond runaway electron bunches in air-filled diode with inhomogeneous electric field are analyzed. Experimental data on the characteristics of such electron beams have been obtained under the conditions with variable emissive properties of the cathode, time of the voltage prepulse action, and electric field strength in the region of electron injection. Based on these data, a hypothesis is formulated and justified that the mechanism of limitation related to a transition from the field electron emission to the explosion of microinhomogeneities is less probable than the mechanism of current limitation by a screening plasma cloud formed over the point electron emitters.

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.