High power, broadband, microwave gas discharge switch tube

Abstract

This paper describes an electronically controllable, broadband gas discharge switch tube capable of rapidly switching high power pulsed microwaves at S-band. The tube consists of a section of rectangular waveguide sealed off at both ends by low-loss vacuum windows and containing a rare gas at a pressure of the order of 0.1 mm of Hg. The tube is 'fired' by the incident microwave power when the proper magnetic field is applied to it and the high electron density of the discharge acts like a short circuit at the input window of the tube. The tube's operation is based on the phenomenon of electron cyclotron resonance. An electron in a magnetic field B spirals around the flux lines with an orbital angular frequency given by \omega_{B} = eB/m , where e and m are the charge and mass of the electron. When a magnetic field is applied in a direction perpendicular to the electric field in the switch tube, two principal effects occur. First, the energy transferred from the microwave electric field to an electron exhibits a sharp resonance when \omega_{B} = \omega , where \omega is the angular frequency of the microwaves. The magnetic field for which this occurs is called the cyclotron resonant field and has a value of around 1200 gauss at S-band. Secondly, the electron's diffusive motion transverse to the magnetic field is slowed down by the factor where (\omega_{B}^{2} + \nu_{c}^{2}) /\nu_{c}^{2} ,where \nu_{c} is the electron-neutral atom collision frequency. These effects determine the breakdown power vs. magnetic field characteristic. This characteristic was studied both theoretically and experimentally and has been used to develop a controllable high power switch tube. The tube was operated with a manually controlled magnetic field, but in principle can be made electronically controllable. The switching time depends upon the design of a modulated magnetic field source. Switching times of the order of 1 millisecond should be possible. The cold bandwidth of the tube depends upon the type of windows used and their separation. A 25/32 long tube sealed off with Sylvania ATR 788 windows had a VSWR less than 1.25:1 over a 37% frequency band. Since the cyclotron resonant magnetic field is directly proportional to \omega , the frequency variation of the switch parameters at constant magnetic field is essentially equivalent to their magnetic field variation at a constant frequency. Both the theoretical and experimental magnetic field dependence of the switch tube parameters were investigated. It was found that an overall bandwidth of more than 30% can be achieved without having to vary the magnetic field as the frequency is changed. A theoretical analysis was carried out to determine how the switching parameters, such as breakdown time, breakdown power, recovery time and arc loss vary with the type of gas filling, pressure, input power, magnetic field and geometry. The experimental data are in qualitative agreement with the theory. The following experimental results have been achieved in an argon filled switch tube operating at a frequency of 2.85 kmc: a) Power Switched: 250 KW peak; 250 W ave.; 2 \mu sec Pulse Width. b) High Level Firing Time: 2 \times 10^{-8} seconds. c) Arc Loss: 0.4 db. d) Low Level Insertion Loss: < 0. 1 db. e) Isolation: 60 db.