Microwave output power from a high-power heliumfilled backward wave oscillator (BWO) operating at X band is measured as a function of the strength of guiding magnetic field and helium gas pressure. Tenuous helium gas filling the slow wave structure is ionized by an injected electron beam with typical parameters of energy 100 keV and current 0.5 kA. The enhancement of output power from the present gas-filled BWO is less than those reported in some previous publications. Near = 0.8 T, we observe the minimum value of output that is judged to result from cyclotron absorption caused by resonant interaction of the fast electron cyclotron mode with the Cherenkov radiation. On both sides of the absorption, two peak outputs near = 0.5 and 1.0 T are measured. In the former case, the output is enhanced compared with vacuum case by the presence of helium gas with pressure of a few mtorr. In the latter case, on the other hand, the output monotonously decreases with the gas pressure. The output powers for the latter case are larger than those of the former case. Both cases are observed repeatedly for various experimental conditions. The physical reasons for the observed results are discussed.Index Terms-Backward wave oscillator (BWO), cyclotron absorption, enhanced radiation, high-power microwave, pulse shortening, relativistic electron beam, slow wave structure (SWS).

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.