Preliminary electron density profile and fluctuation measurements on GAMMA10 using ultrashort-pulse reflectometry

The use of ultrashort pulses (full width at half-maximum = 1 - 100 ps) as a source of broadband, time-resolved radiation has high potential for applic ation in microwave diagnostics for fusion plasmas. Here we report on the ul trashort-pulse reflectometry system developed for electron density profile and fluctuation measurements on the GAMMA 10 tandem mirror. The four-channe l system (selectable frequencies of 8-17 GHz at 1 GHz intervals) measures t he double-pass time-of-flight from the vacuum window to the cutoff layer. T he hardware has an uncertainty of +/-40 ps and can be operated at a repetit ion rate of 400 kHz. The system is operational, however, data from plasma e xperiments is unavailable at the moment. Results from a single-channel O-mo de system show time-of-flight measurements consistent with values calculate d from profiles obtained by a scannable single-channel interferometer. Fluc tuation measurements are also consistent with results from a Fraunhofer dif fraction diagnostic. These results suggest that with further refinements, u ltrashort-pulse reflectometry will yield a simple yet reliable diagnostic f or fusion devices. (C) 1999 American Institute of Physics. [S0034-6748(99)5 3701-5].