S. A. Shunailov

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.

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.

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.

Electron source and acceleration regime of a picosecond electron beam in a gas-filled diode with inhomogeneous field

It is experimentally demonstrated that, upon the application of a subnanosecond high-voltage pulse to the gap of a diode filled with air at atmospheric pressure, a bunch of runaway electrons is formed in a sharply inhomogeneous electric field near the cathode. The bunch duration does not exceed 50 ps, which is shorter than the electron flight time through the interelectrode gap in the continuous acceleration regime. This duration remained unchanged when the gap width was varied between 6 and 26 mm. The electron energy in the picosecond electron beam, as determined from the time-of-flight measurements in the drift channel behind the anode foil of the diode, agree with the results of numerical calculations of the electron acceleration dynamics in the vacuum diode approximation.

Generation of gigawatt 10-GHz pulses with stable phase

The generation of microwave pulses in a 10-GHz range has been studied in a nonstationary relativistic backward wave oscillator (BWO) operating at a pulse train repetition rate of up to 300 Hz. Regimes with a stabilized phase of the high-frequency filling of pulses with respect to the accelerating voltage pulse front have been observed at a BWO peak output power of ∼1 and 3 GW. In pulse trains with a length of 10–100 s, the average output microwave power reached ∼1 kW.

High-power subnanosecond 38 GHz microwave pulses generated at a repetition rate of up to 3.5 kHz

An original relativistic backward tube (BWT) for a 38 GHz range is developed and tested. The BWT is capable of generating stable pulses of ∼250 ps duration and a peak power of ∼250 MW in trains with a length of up to 1 s at a repetition rate of 1–3.5 kHz. The BWT design implements an inhomogeneous slow-wave structure of increased cross section with a band reflector. A pulsed electron beam (∼270 keV, ∼2 kA, 0.9 ns) was injected by a high-current accelerator based on a high-voltage generator with an inductive energy store, a semiconductor current interrupter, and a pulse-shaping hydrogen-filled discharge gap. A focusing magnetic field of 2 T was generated by a cooled pulsed solenoid power-supplied from a special stabilized current source.

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.

Experimental observation of wiggler superradiance under group synchronism condition

Abstract The first results of the observation of superradiance from a single, subnanosecond, high current, electron bunch passing through a wiggler immersed in a guide magnetic field are presented. The 300–500 ps microwave pulses were generated in the high gain regime for both the conventional and reverse directions of the guide magnetic field. The dependence of the radiation power on the interaction length as well as the absolute value of the power, 100–200 kW, were related with the development of self-bunching and consequently with coherent emission.