Generation of Kilojoule Microwave Pulses in a Plasma Assisted Slow-Wave Oscillator

Summary form only given. Some of the goals that DoD agencies have set for high power microwave (HPM) research programs are to increase the radiated energy, improve efficiency and decrease the size and weight of UPM sources and systems. Generation of kJ microwave pulse has been, for many years, an important requirement for many defense directed energy (DE) applications. Progress has been hindered by pulse shortening caused by various breakdown mechanisms that have plagued many HPM devices. We report recent experimental results on the plasma-assisted slow wave oscillator (pasotron) where several important milestones have been achieved. The electron beam in the pasotron is extracted from a plasma filled hollow cathode. Argon gas, which is injected into the cathode region by means of a puff valve, becomes ionized by a plasma keep-alive electrode. A positive voltage (with respect to cathode) applied to a control grid drives ion current back into the cathode plasma and extract electrons forward into the accelerating region. Recent improvements in the control of the gas flow have substantially improved the performance of the pasotron. Sufficient ion densities exist in the transport region ahead of the cathode to provide good ion focusing without the need for a magnetic guiding field; however, the interaction circuit can be maintained at nearly vacuum conditions. Stable operation of the pasotron in L-band with pulses as long as 1 msec, output power as high as 2 MW and efficiencies between 33 and 50% were recently reported. Subsequent experiments on the long-pulse pasotron, where an improved RF transition between the helix and the TE10 rectangular output waveguide was implemented, yielded radiated RF energies of over 1.5 kJ per pulse. We discuss the basic physics, present the results of the most recent experiments and show the design improvements that led to high energy operation of the pasotron.