טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentShafir Guy
SubjectThe Interaction of High Power, Sub Nanosecond Microwave Beam
with Plasma
DepartmentDepartment of Physics
Supervisors Professor Yakov Krasik
Dr. Amnon Fisher
Full Thesis textFull thesis text - English Version


Abstract

Wake-field generation characterizing by extremely high accelerating fields of 1011 V/m in plasma with density of ~5?1018 cm-3 was demonstrated using laser beams of several TW peak power and pulse duration of several tens of femtoseconds. These electric fields exceed 103 times fields in conventional charged particles accelerators. However diagnostics of this extremely short duration and small space scale plasma is very challenging.

In this research, we designed and characterized an experimental setup that enables the study of wake-field formation, guiding and self-focusing of high power microwave beam in a neutral gas and plasma. The results of this research can be applied in a wide range of physical systems such as particle acceleration, electromagnetic field propagation and astrophysical plasmas.

The experimental setup is composed of the following. A backward-wave oscillator as a high power microwave source, operated at frequency of 9.6 GHz, peak power ≤500 MW and pulse duration ≤0.6 ns. The oscillator is supplied by high-current electron beam (>250 keV, 1.2 kA, 8 ns) generated in a magnetically insulated foilless diode powered by all-solid state nanosecond high-voltage generator. The output of the oscillator is connected to a mode converter and cylindrical horn antenna, which radiates the microwave beam in the form of a linearly polarized Gaussian beam. This microwave beam is focused to a spot with diameter <2.2 cm and peak electric field ≤150 kV/cm by a specially designed hyperbolic dielectric ULTEM lens placed inside the Pyrex tube of 100 cm in length and 24 cm in diameter. This tube serves at the interaction chamber, which can be used as a neutral gas chamber or as a preformed plasma chamber. The latter is achieved with  RF discharge in the chamber by a quadruple antenna, which can generate stable, long (>60 cm) and uniform along the axis plasma with controllable density (1010-1013 cm-3) and temperature (1-5 eV). In our experiments, the microwave power is three orders of magnitude larger, and pulse duration is two orders of magnitude shorter than previously reported experiments of generating plasma waves by high power microwaves.

Carried out experimental research with time- and space-resolved optical, electrical, microwave and spectroscopic diagnostics showed for the first time that the plasma, generated by impact ionization of the gas by the microwave beam, has a radial density distribution reducing towards the beam axis, where the microwave field is highest, because the ionization rate is a decreasing function of the microwave amplitude. This forms a plasma channel, which prevents the divergence of the microwave beam. The experimental results were found in satisfactory agreement with the results of simplified analytical model and particle-in cell numerical simulations of microwave beam propagation in vacuum and gas. In addition, depending on gas pressure and type, diffuse and streamer-like plasma formation and generation of high-energy electrons were obtained. Analytical and numerical studies of the wakefield formation in preformed plasma showed that the plasma modulation could be close to 100% for our experimental conditions, either in free space, or inside a cylindrical waveguide.