Novel DARC sources

Summary form only given, as follows. In the DC to AC radiation converter (DARC source), a relativistic, underdense ionization front (plasma/vacuum boundary) directly converts the static electric field of an array of alternatively biased capacitors into a short pulse of tunable radiation. In previous experiments, frequencies continuously tunable from 6 to 21 GHz, and 39 to 93 GHz were produced with devices consisting of capacitor plates. In order to increase the power of the pulses produced by the DARC source, a new device in the microwave frequency range has been designed in which pins inserted in the X-band waveguide through its narrow side wall replace the capacitor of the previous devices. Cold tests show that between the stopbands created by the periodic pins the insertion loss of the structure is much smaller (<0.2 dB) than that measured with previous structures (>30 dB). The stopbands appear at frequencies given by /spl omega//sub sb,l/=(/spl omega//sup 2//sub c/+(l/sup 1//d)/sup 2/c/sup 2/)/sup 1/2/ l=1, 2, ... where /spl omega//sub c/=((m/sup 1//a)/sup 2/+(n/sup 1//b)/sup 2/)/sup 1/2/ is the cut-off frequency of the TE/sub m,n/ mode in a waveguide of transverse dimension a and b, and d is the distance between the pins along the waveguide axis. The stored electrostatic energy in the pin device is only a factor of 3 smaller than that of the plate case. Frequencies in the X-band and above (/spl omega/=(k/sub 0//sup 2/c/sup 2/+/spl omega//sub c//sup 2/+/spl omega//sub pe//sup 2/)/sup 1/2//2k/sub 0/c, k/sub 0/=1/d) can be generated with plasma densities in the 10/sup 11/ cm/sup -3/ range. We plan to operate a DARC source in the therahertz frequency range by using a capacitor array with d=300 /spl mu/m, and plasma densities in the 10/sup 17/ cm/sup -3/ range. Nitrogen or xenon gas will be used and will be field-ionized by a 150 fs laser pulse.