Cross correlation refers to the technique when applied to signals which arise from different plasma volumes, or in this case different mixer channels. In ECE Imaging, the cross power spectral density and cross phases are calculated from two signals of vertically separated mixer channels.
Autocorrelation refers to the technique when applied to signals which arise from the same plasma volume. Signals from the same ECEI mixer channels that are split into two signals and filtered by non-overlapped bandpass filters, assuming that the two passbands are within the natural radiation linewidths of ECE, results in signals that correspond to the same plasma volume. The correlation between the resultant signals then yield the local power spectral density of the Te fluctuations.
Shown above right are coherence functions and corresponding cross phases measured by autocorrelation techniques at different plasma locations. The strong spatial dependence of the broadband Te fluctuation requires high resolution diagnostics. The zero cross phases verify that the signals are from the same plasma volume. The deviations from zero observed above 150 kHz occur when the signal coherence drops below the statistical noise level.
UC Davis has fabricated and installed multichannel ECE Imaging systems on a number of fusion plasma tokamaks across the world. Follow the links below for a description of the systems involved, and to sample data collected with these systems.
ECE Imaging on the TEXT-U tokamak in the U.S.A. ECE Imaging on the RTP tokamak in the Netherlands ECE Imaging on the TEXTOR tokamak in Germany