A. G. Reutova

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.

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.

Stability of Injection of a Subnanosecond High-Current Electron Beam and Dynamic Effects Within Its Rise Time

The stability of the injection of short electron beams and the dynamic processes that occur during their transport were experimentally studied. Beams of energy 200-300 keV, current of 1-1500 A, and duration of 0.05-3 ns with a current rise time of 30-300 ps were formed in a cold-cathode electrode gap. The distribution of the accelerating electric field was highly nonuniform. The cases of vacuum and air insulation of the electron diode were considered. The shortest beams with currents of a few amperes were generated in the mode of continuous acceleration of electrons in atmospheric air. For measuring beam currents, special collector probes were used which ensured a picosecond resolution.

Moment of injection of runaway electrons at the front of accelerating pulse in air-filled diode with inhomogeneous field: From instability to determinacy

A simple method is proposed that makes it possible to determine the moment of injection of a bunch of runaway electrons relative to a subnanosecond leading front of the accelerating high-voltage pulse applied to the cathode in air-filled diode with inhomogeneous field. The moment of injection is determined by finding a minimum in dispersion of the time delay between the variable point of oscilloscope sweep triggering on the instable front of the accelerating voltage pulse and the detected front of the electron current pulse.

Microwave Breakdown of Air by Nanosecond and Subnanosecond Ka-Band Pulses

The initial stage of atmospheric air breakdowns in Ka-band pulsed microwave fields was observed for the microwave power increasing to a maximum within a characteristic time of ~300 ps. Pulsed microwaves were produced by relativistic BWOs with output powers of ~170 and ~500 MW (4-ns and 300-ps FWHMs, respectively). The effects of air breakdown in the fields of microwave pulses of this type are pronounced when the radiation is extracted through the vacuum window of a horn antenna and when it is channeled in passing through waveguides and quasi-optical lines. Of particular interest are modes in which repetitive pulsed microwaves are generated.

A setup for recording the picosecond dynamics of radiation of semiconductor targets in a gas diode

On the basis of a РАДАН generator of high-voltage subnanosecond pulses, an experimental setup for recording the picosecond dynamics of excitation of optical radiation in semiconductor targets (plates) in strong electric fields has been developed. Semiconductor targets were placed between the electrodes to which voltage pulses with an amplitude of up to 150 kV were applied. These pulses were delayed by a coaxial transmission line by 20–30 ns. This technique allowed realization of triggering in advance the sweep of an electron-optical camera and studying of the dynamics of emission from targets with a resolution of 5 ps at a synchronization accuracy close to the rise time of an acting high-voltage pulse.

Generator of picosecond laser pulses

The design of the generator of picosecond laser pulses and the results on semiconductor target (ZnSe, CdS, and others) excitation by electric field and electron beam pulses are presented. The maximum power of laser radiation reached 10 kW at pulse durations of 100–200 ps.

An experimental setup for exciting semiconductors and dielectrics with picosecond electron-beam and electric-field pulses

An experimental facility for forming high-voltage pulses with amplitudes of 30–250 kV and durations of 100–500 ps and electron beams with a current density of up to 1000 A/cm2 is described. The facility was built using the principle of energy compression of a pulse from a nanosecond high-voltage generator accompanied by the subsequent pulse sharpening and cutting. The setup is equipped with two test coaxial chambers for exciting radiation in semiconductor crystals by an electron beam or an electric field in air at atmospheric pressure and T = 300 K. Generation of laser radiation in the visible range under field and electron pumping was attained in ZnSSe, ZnSe, ZnCdS, and CdS (462, 480, 515, and 525 nm, respectively). Under the exposure to an electric field (up to 106 V cm−1), the lasing region was as large as 300–500μm. The radiation divergence was within 5°. The maximum integral radiation power (6 kW at λ = 480 nm) was obtained under field pumping of a zinc selenide sample with a single dielectric mirror.

Novel schemes of production and amplification of superradiance pulses by short intense electron beams

A novel mechanism of superradiance (SR) in the process of stimulated backscattering of powerful pump wave by intense electron bunch has been studied. Using a relativistic 38 GHz BWO as a pump wave source, the short SR pulses of scattered radiation were observed experimentally. Due to the Doppler up-shift, spectrum of scattered pulse included the frequencies up to 150 GHz. After modifications the same experimental set-up will be used for observation of effect of amplification of subnanosecond SR pulse propagating along nanoseconds electron beam. According to PIC-code simulations under experimental conditions output pulse peak power can exceed the power of driving electron beam.