Ph.D Thesis


Ph.D StudentShurik Yatom
SubjectNanosecond Discharge in Dense Gas
DepartmentDepartment of Physics
Supervisors Professor Emeritus Felsteiner Joshua
Full Professors Krasik Yakov
Full Thesis textFull thesis text - English Version


Abstract

The phenomenon of generation of high-energy electrons during a nanosecond time scale electrical discharge has attracted large attention due to the interesting physical phenomena involved and important practical applications. The plasma produced in such a discharge is widely used for pulsed laser pumping, effective release of energy from high-power microwave compressors, switching of low-inductance and high-current gas spark gaps, and has potential applications in biomedical treatments, fast combustion of gas mixtures and aerodynamics.

In spite of a long (around 40 years) history of experimental, theoretical and numerical simulation research of this type of discharge there was no clear knowledge concerning key parameters of the discharge, namely, the role of runaway electrons (RAE) in the initiation of the plasma generation, density and temperature of the plasma and time and space evolution of these parameters as well as the dynamics of the potential distribution in the plasma channel.

In the experimental part of the thesis, results of experimental studies of RAE formation and the role of these electrons in the discharge initiation and the properties of the plasma produced during the discharge in air, He and H2 gases are reported. Using a fast-framing optical photography the dynamics of the discharge and the propagation velocity of the plasma light emitting front were investigated, with a focus on the dependence on pressure and the differences between the discharge dynamics in air, He and H2 gases. The spectrum of RAE was studied with foil spectroscopy of the energetic RAE and x-ray radiation produced by the interaction of these electrons with the anode. The role of energetic RAE in the discharge initiation was discussed in the light of the results of time-resolved x-ray and optical images obtained and also using comparison with the spectra of RAE obtained by particle-in-cell simulations and x-ray foil spectroscopy. Optical emission spectroscopy (OES) was used to determine the plasma electron density and drawing constraints on the electron temperature in He and H2 gas discharges. Detailed fitting of the forbidden and allowed components of He lines revealed the existence of significant electric fields with root-mean-square (RMS) value of ~10 kV/cm2, which was further confirmed using coherent anti-Stokes Raman scattering diagnostics in H2 gas discharge. The presence of such significant electric fields in the plasma channel indicates on a low conductivity 22222

of the plasma produced in nanosecond discharges and this issue is discussed in detail.