S. Yatom

Numerical simulations of runaway electron generation in pressurized gases

The results of a numerical simulation of the generation of runaway electrons in pressurized nitrogen and helium gases are presented. It was shown that runaway electrons generation occurs in two stages. In the first stage, runaway electrons are composed of the electrons emitted by the cathode and produced in gas ionization in the vicinity of the cathode. This stage is terminated with the formation of the virtual cathode, which becomes the primary source of runaway electrons in the second stage. Also, it was shown that runaway electrons current is limited by both the shielding of the field emission by the space charge of the emitted electrons and the formation of a virtual cathode. In addition, the influence of the initial conditions, such as voltage rise time and amplitude, gas pressure, and the type of gas, on the processes that accompany runaway electrons generation is presented.

Space- and time-resolved characterization of nanosecond time scale discharge at pressurized gas

The phenomenon of ultra-fast electrical gas breakdown was investigated. Nanosecond high-voltage pulses with durations of 1 and 5 ns and amplitudes of 100 and 200 kV, respectively, were used to study the parameters of the discharge in a pressured (1-7) × 105 Pa air-filled gap. The development of the discharge and the plasma propagation velocity was examined using optical fast frame imaging. The generation of runaway electrons in the breakdown process was confirmed by electron imaging and time-resolved x-ray diagnostics. Runaway electron beam energy distribution was obtained for a 1 ns duration high-voltage pulse. The origin and the role of runaway electrons in the discharge initiation are also discussed.

Effect of explosive emission on runaway electron generation

The results of numerical simulations of the generation of runaway electrons in a nitrogen-filled coaxial diode with electron emission governed by field emission that transfers to explosive emission with a variable time delay are presented. It is shown that the time when the explosive emission turns on influences significantly the generation of runaway electrons. Namely, an explosive emission turn-on prior to the formation of the virtual cathode leads to an increase in the current amplitude of the runaway electrons and a decrease in its duration. Conversely, an explosive emission turn-on after the formation of the virtual cathode and during the high-voltage pulse rise time does not influence the generation of runaway electrons significantly. When the explosive emission turns on during the fall of the high-voltage pulse and after the virtual cathode formation, one obtains additional runaway electron generation. Finally, a comparison between electron energy distributions obtained with and without explosive e...

Time-resolved investigation of nanosecond discharge in dense gas sustained by short and long high-voltage pulse

The results of experimental and numerical studies of the generation of runaway electrons (RAE) in a pressurized air-filled diode under the application of 20 ns, 5 ns and 1 ns duration high-voltage pulses with an amplitude up to 160 kV are presented. It is shown that with a 1 ns pulse, RAE with energy ≥20 keV reach the anode prior to the formation of the plasma channel between the cathode and anode. Conversely, with 20 ns or 5 ns pulses, RAE with energy ≥20 keV were obtained at the anode only after the formation of the plasma channel. In addition, the high- and low-impedance stages of the development of the discharge were found. Finally, a comparison between experimental and numerical simulation results is presented.

High-current carbon-epoxy capillary cathode

The experimental results of a promising pulsed plasma source, producing electron beams with a current density of up to 10 kA/cm² are presented. The beam duration was up to ~1 μs, in an accelerating voltage of up to ~350 kV, without shorting of the cathode-anode gap by the cathode plasma. This plasma electron source is proven to sustain hundreds of pulses without degradation in its emission properties. The cathode plasma electron density n_e ≤ 10¹⁵ cm^-3, electron T_e ≤ 13 eV and ion T_i ≤ 4 eV temperatures, and expansion velocity V_pl 1.6 × 10⁶ cm/s were determined using time- and space-resolved spectroscopy and light emission.

Plasma density evolution during nanosecond discharge in hydrogen gas at (1-3) × 10 5 Pa pressure

The results of a study of the nanosecond discharge in H2 gas at pressures of (1–3) × 10 5 Pa using fast-framing photography and space- and time-resolved spectroscopy are presented. The discharge is initiated by the application of a high-voltage pulse with an amplitude of ∼100 kV and duration of ∼5 ns to a blade cathode placed at a distance of 20 mm from the anode. The results show the dynamics of the discharge formation and the build-up of the plasma electron density in the discharge channels close to and at a distance from the edge of the cathode. The results obtained are compared to those obtained in recent studies of similar discharges in air and He gas. It was shown that the time and space evolution of the plasma light emission in the H2 gas discharge is very similar to that in air. Namely, the generation of the plasma is mainly confined to the plasma channels initiated at the top and bottom edges of the cathode electrode and that there are no new plasma channels formed from the explosive emission centres along the blade as it was obtained in earlier experiments with He gas. Spectroscopic measurements showed that the plasma density reaches 2 × 10 17 cm −3 and 1.6 × 10 16 cm −3 in the vicinity of the cathode and the middle of the anode–cathode gap, respectively, for a plasma electron temperature of <1.5 eV. The values of plasma electron density and the previously presented results of electric field measurements allow calculation of the resistance of the plasma channels.

Time evolution of nanosecond runaway discharges in air and helium at atmospheric pressure

Time- and space-resolved fast framing photography was employed to study the discharge initiated by runaway electrons in air and He gas at atmospheric pressure. Whereas in the both cases, the discharge occurs in a nanosecond time scale and its front propagates with a similar velocity along the cathode-anode gap, the later stages of the discharge differ significantly. In air, the main discharge channels develop and remain in the locations with the strongest field enhancement. In He gas, the first, diode “gap bridging” stage, is similar to that obtained in air; however, the development of the discharge that follows is dictated by an explosive electron emission from micro-protrusions on the edge of the cathode. These results allow us to draw conclusions regarding the different conductivity of the plasma produced in He and air discharges.

Electron emission mechanism during the nanosecond high-voltage pulsed discharge in pressurized air

A comparison between the results of x-ray absorption spectroscopy of runaway electrons (RAEs) generated during nanosecond timescale high-voltage (HV) gas discharge and the simulated attenuation of the x-ray flux produced by the runaway electron spectrum calculated using particle-in-cell numerical modeling of such a type of discharge is presented. The particle-in-cell simulation considered the field and explosive emissions (EEs) of the electrons from the cathode. It is shown that the field emission is the dominant emission mechanism for the short-duration ( 5 ns) high-voltage pulses, the explosive emission is likely to play a significant role.

X-ray diagnostics of runaway electrons generated during nanosecond discharge in gas at elevated pressures

The properties of high-energy runaway electrons generated during a nanosecond discharge in an air filled diode at pressures up to 3 × 105 Pa were studied using x-ray absorption spectroscopy. The results of studies of the discharge at different pressures and with different lengths of cathode-anode gap allow an insight into the factors that influence the energy distribution of runaway electrons. Energy distribution functions for runaway electrons produced in particle-in-cell simulation were used to create the x-ray attenuation curves via a computer-assisted technique simulating the generation of x-ray by energetic electrons. The simulated attenuation curves were compared to experimental results.

Recent studies on nanosecond-timescale pressurized gas discharges

The results of recent experimental and numerical studies of nanosecond high-voltage discharges in pressurized gases are reviewed. The discharges were ignited in a diode filled by different gases within a wide range of pressures by an applied pulsed voltage or by a laser pulse in the gas-filled charged resonant microwave cavity. Fast-framing imaging of light emission, optical emission spectroscopy, x-ray foil spectrometry and coherent anti-Stokes Raman scattering were used to study temporal and spatial evolution of the discharge plasma density and temperature, energy distribution function of runaway electrons and dynamics of the electric field in the plasma channel. The results obtained allow a deeper understanding of discharge dynamical properties in the nanosecond timescale, which is important for various applications of these types of discharges in pressurized gases.