Ph.D Thesis


Ph.D StudentVladislav Vekselman
SubjectPlasma Sources for Pulsed High-Current Relativistic
Electron Beam Generation
DepartmentDepartment of Physics
Supervisors Full Professors Krasik Yakov
Professor Emeritus Felsteiner Joshua
Full Thesis textFull thesis text - English Version


Abstract

Results of an experimental study of pulsed electron beams generation by a plasma electron sources serving as a cathode in planar high current diode operating in the moderate pressure range (10-4-10-5 Torr) are presented. Current research considers two types of plasma cathodes - passive and active, which are extensively used in various applications, for instance, generation of strong shock waves, high power pulses of microwave and X-ray radiation, pumping of gaseous lasers, electron beam welding, etc. They are distinguished by the inception of the plasma formation - before or simultaneously with the application of the accelerating pulse.

As passive electron sources the carbon-fiber cathodes with and without CsI coating have been investigated. Cathodes showed nanosecond timescale turn-on, long lifetime, reliable and reproducible generation of electron beam in a planar diode under accelerating pulse of 200 kV and duration ~250 ns. All investigated carbon-fiber cathodes form the dense surface plasma (flashover mechanism) from which the electron beam is extracted. The plasma parameters were obtained by space- and time- resolved spectroscopic diagnostics. The plasma expansion velocity of 1.5 cm/?s was estimated using time of flight technique and density and temperature of the plasma were determined using spectroscopic diagnostics.

The active plasma electron source - ferroelectric plasma source (FPS)-assisted hollow anode (HA) discharge allows reliable and reproducible generation of electron beams with total current up to 3 kA, current density ≤30 A/cm2, electron energy ≤400 keV, pulse duration ≤350 ns and cross-sectional area up to 130 cm2. Laser induced fluorescence (LIF) technique was applied to obtain data on plasma generation and evolution and to develop models describing the plasma formation inside the HA and the plasma prefilled mode of the diode operation. LIF diagnostics revealed that the plasma filling of the HA cavity occurs due to expansion of the plasma flows generated by the FPS. The obtained LIF data were verified by the results of 1D modeling of the plasma evolution and time-dependent collision-radiative modeling which includes the presence of the non-thermal electron beam. Two different modes of the diode operation (“low impedance” and “high impedance”) were revealed and explained.

Finally, the application of the FPS-assisted HA discharge as a source of low-energy electron beam (hundreds keV) and cross-sectional area up to 100 cm2 was realized and studied. A simple method to control the electron beam energy spectrum was proposed and confirmed.