טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentDmitry Yarmolich
SubjectPassive and Active Cathodes for High Current Electron Beam
Generation at Moderate Electric Fields
DepartmentDepartment of Physics
Supervisors Full Professor Krasik Yakov
Professor Emeritus Felsteiner Joshua


Abstract

The main purpose for studying a passive cathode made of velvet was to investigate the phenomenon responsible for the generation of high-current electron beams when a high-voltage pulse is applied to the cathode. We carried out experiments with a velvet-based cathode in a diode powered by a high-voltage pulse having an amplitude of ~ 200 kV and ~ 300 ns duration. Spectroscopic measurements demonstrated that the source of the emitted electrons is surface plasma with an electron density and temperature of ~ 4·1014 cm−3 and ~ 7 eV, respectively, in the case of an electron current density of ~ 50 A/cm2. It was found that bright spots on the velvet surface represent individual plasma sources that produce separate electron micro-beams. A satisfactorily uniform extracted electron beam is observed, in spite of the individual character of the cathode plasma sources. Also, it was shown that the CsI coating of the velvet cathode improves the uniformity of the electron emission. Finally, it was found that the interaction of the electron beam having a current density up to 50 A/cm2 with a stainless-steel anode does not cause formation of anode plasma.

As an active plasma source we investigated the operation of a hollow anode (HA) with a ferroelectric plasma source (FPS) incorporated in it. It was shown that a BaTiO3 sample (ε ≈ 1600) provides a more effective and fast plasma formation as compared with previously used FPS having ε ≈ 170. Three different electric schemes for igniting and sustaining the HA discharge were designed. The application of these schemes showed better reliability, an increase in lifetime and simplification of the HA operation at a background gas pressure ≤ 10−5 Torr. The main mechanism of the HA plasma production at low pressure of the background gas is the expansion of the plasma produced by incomplete discharges on the FPS front surface. It was shown that the use of an auto-bias resistor with resistance in the range of 35-100 Ω allows one on the one hand practically to avoid plasma pre-filling of the anode-cathode gap prior to the application of the accelerating pulse and on the other hand to achieve electron beam generation with current amplitude up to 1.2 kA. Finally, the electron beam current density radial distribution was improved and stabilized by the use of a new FPS front electrode design.