Review of supershort avalanche electron beam during nanosecond-pulse discharges in some gases

Supershort avalanche electron beam(SAEB)plays an important role in nanosecond-pulse discharges.This paper aims at reviewing ex-periments results on characteritics of SAEB and its spectra in different gases in nanosecond-pulse discharges.All the joint experiments were carried in the Institute of High Current Electronics of the Russian Academy of Sciences and the Institute of Electrical Engineering of the Chinese Academy of Sciences.In these experiments,the generation of a SAEB in SF 6 in an inhomogeneous electric field was studied on three generators with pulse rise times of 0.3,0.5 and~2 ns.Firstly,the comparison of SAEB parameters in SF 6 with those obtained in other gases(air,nitrogen,argon,and krypton)is introduced.Secondly,the SAEB spectra in SF 6 and air at pressures of 10 kPa(75 torr),and 0.1 MPa(750 torr)are reviewed and discussed.Finally,1.5-D theoretical simulation of the supershort pulse of the fast electron beam in a coaxial diode filled with SF 6 at atmospheric pressure is described.The simulation was carried out in the framework of hybrid model for discharge and runaway electron kinetics.The above research progress can provide better understanding of the investigation into the mechanism of nanosecond-pulse discharges.

Passive Quenching Electronics for Geiger Mode 4H-SiC Avalanche Photodiodes

We design and fabricate 4H-SiC UV avalanche photodiodes （APDs） ~qth positive beveled mesa, which exhibit low leakage current and high avalanche gain when working in the Geiger mode. The single photon counting performance of the SiC APDs is studied by using a passive-quenching circuit. A new method to determine the exact breakdown voltage of the APD is proposed based on the initial emergence of photon count pulses. The photon count rate and dark count rate of the APD are also evaluated as a function of quenching resistance.

Ion-focused propagation of a relativistic electron beam in the self-generated plasma in atmosphere

It is known that ion-focused regime(IFR)can effectively suppress expansion of a relativistic electron beam(REB).Using the particle-in-cell Monte Carlo collision(PIC-MCC)method,we numerically investigate the propagation of an REB in neutral gas.The results demonstrate that the beam body is charge neutralization and a stable IFR can be established.As a result,the beam transverse dimensions and longitudinal velocities keep close to the initial parameters.We also calculate the charge and current neutralization factors of the REB.Combined with envelope equations,we obtain the variations of beam envelopes,which agree well with the PIC simulations.However,both the energy loss and instabilities of the REB may lead to a low transport efficiency during long-range propagation.It is proved that decreasing the initial pulse length of the REB can avoid the influence of electron avalanche.Using parts of REB pulses to build a long-distance IFR in advance can improve the beam quality of subsequent pulses.Further,a long-distance IFR may contribute to the implementation of long-range propagation of the REB in space environment.

Influence of non-uniform electric field distribution on the atmospheric pressure air dielectric barrier discharge

The dielectric barrier discharge(DBD)in air at atmospheric pressure is not suitable for industrial applications due to its randomly distributed discharge filaments.In this paper,the influence of the electric field distribution on the uniformity of DBD is theoretically analyzed and experimentally verified.It is found that a certain degree of uneven electric field distributions can control the development of electron avalanches and regulate their transition to streamers in the gap.The discharge phenomena and electrical characteristics prove that an enhanced Townsend discharge can be formed in atmospheric-pressure air with a curved-plate electrode.The spectral analysis further confirms that the gas temperature of the plasma produced by the curved-plate electrode is close to room temperature,which is beneficial for industrial applications.This paper presents the relationship between the electron avalanche transition and the formation of a uniform DBD,which can provide some references for the development and applications of the DBD in the future.

GaN based transfer electron and avalanche transit time devices

A new model is developed to study the microwave/mm wave characteristics of two-terminal GaNbased transfer electron devices（TEDs）,namely a Gunn diode and an impact avalanche transit time（IMPATT） device.Microwave characteristics such as device efficiency and the microwave power generated are computed and compared at D-band（140 GHz center frequency） to see the potentiality of each device under the same operating conditions.It is seen that GaN-based IMPATT devices surpass the Gunn diode in the said frequency region.