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.