Dependence on axial magnetic field of the radiation from a high power gas-filled backward wave oscillator

Abstract

Previously, we observed enhanced microwave radiation from gas-filled backward wave oscillator (BWO) at a particular gas pressure of helium and argon; the radiated power was enhanced by approximately ten times compared with the vacuum case.[I] Also, we observed a resonant enhanced radiation at certain value of axial magnetic field. Recently, Grabowski et al. examined in detail a plasma-filled BWO using a cathode-mounted plasma prefill source.[2] The plasma prefill was found to either enhance or reduce the microwave generation efficiency depending on the plasma density. Stimulated by their work, we re-examine the gas-filled BWO in more detail. A 100 ns high voltage pulse (70 to 130 kV) is applied to an annular cold cathode to produce electron beam with current on the order of 500 A. The electron beam is guided by an axial magnetic field B = 0.2 to 1.16 T. The slow wave structure (SWS) is designed so that operation is in the pi-mode resulting in TMOl radiation. The length of the SWS is increased up to 21.5 periods in steps of 0.5 periods. The X-band microwave output versus B for various beam energies is measured. In the vacuum case, the output is relatively large near B = 0.5 and 1.0 T, and output at 1.0 T is larger than at 0.5 T. On the other hand, the ten times enhanced radiation for helium gas pressure of 10 mTorr is observed only for the case of B = 0.5 T. Tentatively, the 0.5 T and the 1.0 T cases are identified, respectively, as a plasma-BWO and a vacuum-BWO, as were previously analyzed [3]. Frequency up-shift with gas pressure is observed for the plasma-BWO. It is found that gas-filled BWO has complicated characteristics as will be reported.Footnotes:[l] K. Minami et al.