Generation of microwave pulses from the static electric field of a capacitor array by an underdense, relativistic ionization front

The dc to ac radiation converter is a new device in which a relativistic ionization front directly converts the static electric field of an array of alternatively biased capacitors into a pulse of tunable radiation. In a proof-of-principle experiment frequencies between 6 and 21 GHz were generated with plasma densities in the ${10}^{12}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{-3}}$ range and a capacitor period $\text{2d=9.4\hspace{0.17em}}\mathrm{cm}.$ In the present experiment, short pulses with frequencies between 39 and 84 GHz are generated in a structure with $\text{2d=2\hspace{0.17em}}\mathrm{cm}.$ The frequency spectra of these pulses are measured using a diffraction grating. The spectra are discrete, and their center frequency varies linearly with the gas pressure prior to ionization (or plasma density), as expected from theory. Their relative spectral width is around 18%, consistent with the expected number of cycles (six) contained in the pulses. An upper limit of 750 psec (bandwidth detection limited) is placed on the pulses length. The emitted frequency increases from 53 to 93 GHz when the capacitors are connected by pair to obtain a effective array period of 4 cm.