M. I. Yalandin

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.

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.

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.

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.

Compact high-current accelerators based on the radan SEF-303 pulsed power source

Compact RADAN-series high-current accelerators with a Tesla transformer have already demonstrated their large capabilities in the construction of diverse electrophysical apparatus, such as relativistic millimeter-band microwave oscillators, battery-supplied X-ray apparatus and gas lasers. Such accelerators have been efficiently used for studying rapid processes in solid state physics, chemistry, and biology 111. Compactness, the absence of stringent requirements for vacuum conditions, rooms, and personnel qualification, low noise level, and compatibility with modern measuring equipment make an accelerator of this kind an inexpensive, efficient, and versatile research tool, especially in areas that are unconventional for pulsed power apparatus. A major drawback of such systems in that the gas discharge commutators and electronics tubes have a restricted life time, normally between lo5 and 106 pulses

High-power repetitive millimeter range back-wave oscillators with nanosecond relativistic electron beam

The present paper is devoted to some physical, technological and technical aspects, which facilitate creation of desk-top repetitive microwavc oscillators with relativistic electron beams. At an output power of microwave radiation of tens megawatts such oscillators as MG4R, MG5R and MG6 can operate at a repetition rate of 10 p.p.s . Burst repetitive operation mode for these oscillators is determined by heating of solenoid coils only.

Synphase operation of nanosecond relativistic 37-GHz backward-wave oscillators without electrodynamic coupling

The possibility of in-phase excitation of two independent nanosecond-pulsed relativistic 37-GHz backward-wave oscillators (BWOs) with high-current electron beams has been studied. This regime can be achieved using BWO switching with a picosecond precision. It is shown that long-term (up to 100–200 periods of the field) phase locking in each channel is stably reproduced from pulse to pulse, which ensures coherent summation of the output wave beams at a megawatt power.

Picosecond stability of injection of parallel high-current pulsed electron beams

The stability of operation of parallel explosive-emission cathodes driven by a split high-voltage pulse with a subnanosecond leading front has been studied. It is established that, upon the training of graphite cathodes in vacuum with up to ∼104 pulses, the current pulse fronts of injected high-current electron beams exhibit a mutual temporal dispersion not exceeding ten picoseconds. The dynamics of this parameter during the training stage, the variation of the absolute spread, and the growth of a relative delay of the moments of beam injection have been investigated.