Meytal Siman Tov - Video Lecture

Meytal Siman Tov - Video Lecture

IVS-IPSTA 2020 Online Conference December 13, 2020

Afternoon Plasma Session
Session chair:
Ido Barth 

The Hebrew University of Jerusalem


Generation of Periodic Bunches Produced by a Squeezed Electron Beam in a Resonant Cavity

Meytal Siman Tov

Pulse power and plasma laboratory, Faculty of Physics, Technion, Israel


Abstract

An experiment in which periodic high-current electron beam bunches are generated from a magnetized squeezed state of an electron beam is presented. An annular electron beam produced by a magnetically insulated co-axial foilless diode is injected into a drift tube of larger radius. In the larger radial tube, the space charge limits the current obtained in the smaller radius region. This leads to the formation of a virtual cathode (VC) near the transition point from where an upstream current flows toward the cathode to reduce the emitted current. In turn, this flow squeezes the beam between the cathode and the radial transition point, that is, the energy of the beam reduces and the charge density increases. Instead of squeezing, the beam between the VC and the cathode can oscillate if even more charge is added into it. To do this, we added a partially transparent screen kept at the same potential as the cathode by a connecting rod. This reflector, placed near the radial transition point, slows down the beam and reflects additional current upstream. A transit time oscillator forms, which emits a weekly oscillating current through the reflector to a nearby collector. When this structure is inside a cavity having a characteristic electro-magnetic frequency in resonance with the beam transit time a monotron type resonance forms which drives the beam to huge current oscillations.

We have set up an experiment to demonstrate this. We used a carbon fiber-based cathode placed in a 20 mm radius anode tube driven by a bipolar Marx generator (200 kV, 1000 A, 350 ns pulse duration). The drift had one radial transition point (from 20 mm to 60 mm). The reflector consisted from a 28% transparency perforated Mo foil connected by a 3 mm diameter central rod to the cathode was placed at a distance of 40mm from the radial transition point. A Faraday cup acting as a collector and measuring the output electron current followed the reflector at a distance of 20 mm. A 1T axial magnetic field produced by a solenoid magnetizes the electron beam. The experimental chamber was kept at a vacuum of 2×10-5 Torr. Distinct electron bunches of ~30 A appearing at a frequency of 200 MHz were observed for as long as the voltage was on. The discrepancy between the experimental results and the PIC simulated prediction is explained.