Operation of a relativistic backward wave oscillator filled with a pre-ionized, high density, radially inhomogeneous plasma

A modified relativistic (500 kV, 500 A) backward wave oscillator filled a radially non-uniform, pre-ionized plasma of high peak density (peak plasma frequency>operating frequency) is studied experimentally. The effects of high plasma density in the interaction region as well as the relativistic diode and the output horn regions were studied. By protecting both the diode and output regions against plasma penetration, the plasma effects on the BWO was studied up to a peak plasma density>10¹³ cm^-3. It was demonstrated that filling the BWO structure with high density radially non-uniform plasma leads to: (a) suppression of high order modes. This is a promising approach for implementing single mode operation of high power, large diameter overmoded devices. (b) frequency upshift of up to 100% (from 8.5 to 16 GHz). This demonstrates the possibility of electronic frequency tunability by controlling the plasma density. (c) substantial increase in the measured microwave output power for n_p>10¹³ cm^-3 over the corresponding vacuum value.