Density Fluctuation Measurements with Reflectometry on LTX-β

The introduction of Li-coatings to plasma facing components has resulted in improved confinement and reduced edge turbulence in many fusion plasma devices. On the LTX and the LTX-β devices, Li evaporated onto a close-fitting shell surrounding the plas- mas has yielded discharges having a flat electron temperature profile with a hot edge, and a peaked density profile. The upgraded reflectometry system on LTX-β is uniquely equipped to measure low-k core and edge turbulence in this regime. This diagnostic consist of two frequency-modulated continuous-wave (FM-CW) channels (13.1−20.2 and 19.5−33.5 GHz) and two fixed-frequency channels (13.1−21 and 20.5−40 GHz) which can be scanned from shot to shot. Electron density profile measurements are possible in the range of 0.2−1.4 × 10¹³ cm⁻³ with a time resolution down to 4 μs. The fixed-frequency channels extend the density fluctuation coverage to 2.0 × 10¹³ cm⁻³ with a bandwidth ≤5 MHz. Since the FM-CW and fixed-frequency channels share a bistatic antenna array, measurements of the fluctuation level, spectrum, and radial correlation length are possible with a high degree of temporal and spatial localization. Initial measurements and analysis results will be presented at the meeting.