Fluctuation measurements of the ultrashort pulse reflectometry system on SSPX

Summary form only given, as follows. Summary form only given. New physics understanding of the ultrashort pulse reflectometry (USPR) diagnostic technique has been achieved with a series of receiver bandwidth studies on the Sustained Spheromak Physics Experiment (SSPX) device at the Lawrence Livermore National Laboratory. This new understanding, together with a number of transmitter and receiver modifications suggested by this new understanding, has resulted in significant increases in both dynamic range and signal-to-noise ratios of the USPR signals. This in turn has translated to a significant reduction in receiver timing jitter and the number of lost signals in which the received power has dropped below the detection threshold USPR involves the propagation of short duration (/spl sim/2-3 ns) chirps of moderate frequency bandwidth. These chirps axe upconverted to millimeter-wave frequencies, reflected from the plasma, and downconverted for detection and analysis. Each received chirp is analyzed at a number of simultaneous frequencies, with multiple chirps required to provide broad frequency coverage. Commercial time-to-amplitude converters convert the receiver time delays into easily acquired analog voltages with 25 ps resolution. On SSPX, four mixers (LO frequencies are 27, 44, 57 and 75 GHz) are utilized to up- and down-convert 7-16 GHz chirped waveforms to millimeter-wave frequencies (34-91 GHz). Four mixer operation provides 32 frequency channels of timing data with 12 /spl mu/s resolution, while operation with only the lowest frequency mixer provides 8 frequency data with 3 /spl mu/s resolution. In addition to edge density profile information, the system is also capable of simultaneously monitoring density fluctuations on as many as 32 distinct density layers. Details of the upgraded USPR system axe described together with USPR profile and fluctuation data.