DEMONSTRATION OF MICROWAVE GENERATION FROM A STATIC-FIELD BY A RELATIVISTIC IONIZATION FRONT IN A CAPACITOR ARRAY

We present the results of a proof-of-principle experiment to demonstra te the generation of tunable radiation from a laser-ionized gas-filled capacitor array. This scheme directly converts a static electric fiel d of wave number k(0) into coherent radiation pulses of frequency omeg a(p)(2)/2k(0)c, where omega(p) is the plasma frequency. The radiation frequency can be tuned by varying gas pressure and/or capacitor spacin g. In this experiment, well-polarized, short (less than 5 ns) microwav e pulses have been generated over a frequency range of 6 to 21 GHz. Th e frequency of the detected signal, as measured with cut-off waveguide s, scales linearly with the plasma density, and the relative power of the signal scales quadratically with the de bias voltage in agreement with the theory.