Reflectometer density fluctuation measurements on DIII-D

An O-mode, homodyne reflectometer system utilizing seven discrete channels spanning 15–75 GHz is in operation on DIII-D. A frequency-tunable X-mode system is also used to take fluctuation data from reflecting layers lying between those of the fixed frequency, O-mode system. In order to demonstrate the power and flexibility of this diagnostic, results are presented from several different plasma regimes, including L to H transitions, ELMS, and high beta operation. One specific example is that at all L–H transitions, high-frequency (≥50 kHz) fluctuations are dramatically suppressed in a localized region near the edge of the plasma on a timescale of ≊100 μs.