Sergei A. Shunailov

Generation of a Picosecond Runaway Electron Beam in a Gas Gap With a Nonuniform Field

The formation of a picosecond beam of runaway electrons in a gas-filled acceleration gap with a cold cathode and a strongly nonuniform electric field was investigated. The experimental data obtained were compared to those characteristic of the mode of generation and acceleration of electrons in vacuum. Voltage pulses of amplitudes up to -300 kV (in a traveling wave) whose minimum rise time and FWHM did not exceed 100-150 ps were applied to the cathode. The duration and amplitude of the current pulse of the picosecond runaway electron beam behind the anode foil were measured with high time resolution. The emission region of the beam in a gas-filled diode was determined experimentally. The time-of-flight method was used to investigate the acceleration mode of particles in the gap. Information about the part played by field emission in the initiation of the runaway electron beam has been obtained. It has been demonstrated that the point within the rise time of the accelerating voltage pulse at which the beam is injected into the gap correlates with the magnitude of the macroscopic electric field at the cathode emitting edge.

High peak power and high average power subnanosecond modulator operating at a repetition frequency of 3.5 kHz

This paper presents results of tests of a hybrid subnanosecond modulator with an output resistance of 45 /spl Omega/. The modulator comprises an all-solid-state nanosecond charger, which is equipped with an inductive energy store and a semiconductor opening switch, and a pulse peaker with hydrogen spark gaps. The modulator generates stable pulses-(180 to 200) kV in amplitude and 400 to 700 ps long at a pulse repetition rate of up to 3.5 kHz. An average output power of 1.5 kW was achieved under the pulse burst mode.

Phase Control in Parallel Channels of Shock-Excited Microwave Nanosecond Oscillators

The theoretical premises and experimental results of phase control in high-power microwave oscillators with nanosecond pulse duration are presented. In experiments, two-channel backward wave oscillators (BWOs) for both steady state (100-150 cycles) and super-radiance (SR) mode operation (10-20 cycles) are discussed. For the phase control, the shift of the moment with fastest current rise is provided in the sections of nonlinear transmission lines with axially biased ferrites. The voltage pulse sharpening and shift of group velocity depend on the dc axial magnetic field. In SR mode, two-channel source is capable of producing 2 × 0.3 GW pulses with duration of 2 ns and the center frequency of 10 GHz. The source operates at the repetition rate up to 100 pps with electronic control of the phase in one channel relative to another. The last experiment is carried out using two synchronized compact RADAN-type drivers with two parallel Ka-band BWOs (100 MW, 2 ns, 37 GHz). The controllable shift of interference picture is a proof of the coherency in the aggregated radiation. At the maximum of the pattern in the far zone, the detector indicateds fourfold increase in power density over that measured from single channel.

Nanosecond hybrid Modulator for the fast-repetitive driving of X-band, gigawatt-power microwave source

Results of tests of a fast-repetitive nanosecond modulator with a peak output power of 3 GW are presented. It comprises of a type S-5N charging driver, a power compressor based on a pulsed forming line, and a gas spark gap. The modulator has been designed for the formation of high-current electron beams and high-power microwave generation in a pulsed-repetitive regime. Excitation of 10-GHz microwave pulses /spl sim/1 ns long in a relativistic backward-wave oscillator with an elongated periodic slow-wave system has been studied. Optimization of the regime of interaction between electromagnetic fields and particles provided an average power of microwave radiation in the burst-repetitive mode (1 s; /spl sim/700 Hz) of up to 2.5 kW at a focusing magnetic field (/spl sim/0.6 T) below the cyclotron resonance region. The peak output power of the oscillator exceeded 2 GW.

Highly efficient generation of subnanosecond microwave pulses in Ka-band relativistic BWO

This paper presents the results of investigations of the mode of nonsteady-state oscillations with a short-time power burst which is characteristic of the initial stage of the transient process in a backward wave oscillator when the beam operating current is far in excess of the starting current. Numerical simulations have yielded the conditions under which the efficiency of the power transfer from an electron beam with a particle energy of 300 keV, a current of 2 kA, and a duration of 1 ns into a microwave pulse containing 8-10 high-frequency field periods approaches 90%. Experimentally, it has been demonstrated that the production of pulses like these with a duration of 200-250 ps, a power of up to /spl sim/400 MW, and a central frequency of about 38 GHz is feasible.

Compact Ka-Band Backward-Wave Generator of Superradiative Pulses Operating at Reduced Guiding Magnetic Field

A compact repetitive superradiative Ka-band BWO is described, and the results of a study of its characteristics are presented. For an electron beam transported in a magnetic field of 2 T, which is less than the value corresponding to a cyclotron resonance, the efficiency of the conversion of electron beam power to pulsed electromagnetic radiation is close to unity. The peak power of an electromagnetic superradiative pulse of 250-ps duration is ~ 500 MW.

Control and Stabilization of Runaway Electron Emission at the Delay Stage of Pulsed Breakdown in an Overvolted Atmospheric Gap

In this paper, we present investigations of picosecond effects in atmospheric gaps at the stage of a pulsed breakdown delay. It is shown that in centimeter gaps with rapid achievement of multiple overvoltages, the breakdown point, its stability, and development time are determined by the advance emission of runaway electrons. Methods for controlling and stabilizing the temporary and energy characteristics of the flows of these particles are proposed.

Control of the Operation Mode of a Relativistic Ka -Band Backward-Wave Oscillator

Variations in the transient time and in the microwave pulse duration and peak power have been demonstrated for a relativistic subgigawatt Ka -band backward-wave oscillator (BWO) with the slow-wave structure geometry, the accelerating voltage, and the electron beam current remained unchanged. This was done taking into account that the BWO operation mode depends on the starting-current-to-beam-current ratio. Efficient variation of the starting current was attained by changing the beam path in view of a significant difference between the times of the guide pulsed magnetic field diffusion into the electron injection region and beam-wave interaction space.

Four channel high power rf source with beam steering based on gyromagnetic nonlinear transmission lines

The synchronized operation of four gyromagnetic nonlinear transmission lines (NLTLs) was tested with a pulse repetition frequency up to 1 kHz during 1 s bursts. High voltage pulses with a duration of ∼5 ns from the solid state driver S-500 were split into four 48 Ω channels reaching about -200 kV in each channel with ∼10% variation in the amplitude. The maximum peak voltage at the NLTL output was within 220-235 kV with the maximum modulation depth of decaying oscillations up to 90% at the center frequency near 2.1 GHz. The relative delay between channels reached the half-period of the center frequency of oscillations. The associated beam steering by four element array of conical helical antennas was demonstrated in a horizontal plane at 17°. The effective potential of radiation reached 360 kV at the radiation axis. The effect of ferrite temperature on the shock wave velocity in gyromagnetic NLTL is observed.

Coherent Summation of Radiation From Four-Channel Shock-Excited RF Source Operating at 4 GHz and a Repetition Rate of 1000 Hz

Testing results of a generator based on the parallel gyromagnetic nonlinear transmission lines with saturated ferrite are presented. Practically identical and stable RF-modulated high-voltage nanosecond pulses were shaped in each of the four channels. The pulse amplitude reaches −175 kV at a modulation depth of RF oscillations to 50% and an effective frequency ~4 GHz. Power in the packet operation mode with a 1-s packet duration at pulse repetition frequencies up to 1000 Hz (in packet) was supplied by a solid-state driver. The electric field strength achieved 250 kV/m at a distance of 3 m from antennas.