|M.Sc Student||Siman-Tov Meytal|
|Subject||Explosive Emission Cathodes as a Source for Hollow|
High-Current Electron Beam; Applied for Microwaves
Generation in a Backward Wave
|Department||Department of Physics||Supervisor||Professor Yakov Krasik|
|Full Thesis text|
High current electron beams are useful for many applications such as X-ray generation, electron beam welding, high power microwave devices, etc. In particular, this research is the first step of the investigation of powerful microwave beam interaction with plasma for formation of wake-field. In order to generate powerful microwave (power up to 100 MW and pulse duration of 1 ns), one requires high-current electron beam which transfers its potential energy to microwave during its transport in Backward Wave Oscillator (BWO).
This research investigates the operation of different types of cathodes which are used as plasma electron source for generation of high power electron beam under application of high voltage with pulse duration of several nanoseconds. Electron beam was generated in a magnetically insulated foilless diode powered by semiconductor opening switches (SOS) generator (200 kV, 1 kA, either 30 ns or 5 ns pulse duration). The electrons are extracted from the boundary of the plasma that formed at the surface of the cathode as a result of explosive plasma formation which requires electric field >107 V/cm, dE/dt ≥1014 V/(cm?s) and fast ignition.
The amplitude of the current, the electron energy, the diode impedance and the azimuthal uniformity of the beam which govern the efficiency of the BWO operation, were investigated. Several types of cathodes were investigated: carbon/epoxy fibers based cathode, graphite cathode, edged stainless steel cathode and surface discharge cathode. The graphite cathode showed the best performances: ignition was fast (≤1ns), the diode impedance was almost constant, life time was sufficiently long, the annular beam was uniform and thin (less than 0.3mm).
The microwave beam generated in the BWO and transmitted through the horn antenna to the free space should be focused inside a plasma chamber. In the second part of the research a beam propagation method (BPM) code was written. The code was used to investigate the propagation of the microwave and to design a dielectric lens that focuses the microwave beam to a given position in order to achieve high power density. The field has a Gaussian profile with frequency of 10 GHz, wavelength and waist of 3 cm. Characterization of the field and its matching to the Gaussian profile was done.
Preliminary research of microwave beam focusing using a dielectric lens (diameter of 32 cm, thickness of 6cm, refractive index: 31/2 ) was carried out. Using a network analyzer connected to the horn antenna, the beam profile in a free space without the lens and with the lens placed at the several distances from the antenna was studied. The value and the position of the maximum beam power at the axis was found versus the distance from the antenna and a comparison between the simulation and the experiments data was done.