Improved cross-calibration of Thomson scattering and electron cyclotron emission with ECH on DIII-D

Thomson scattering produces n_e profiles from measurement of scattered laser beam intensity. Rayleigh scattering provides a first calibration of the relation n_e ∝ I_TS, which depends on many factors (e.g., laser alignment and power, optics, and measurement systems). On DIII-D, the n_e calibration is adjusted against an absolute n_e from the density-driven cutoff of the 48 channel 2nd harmonic X-mode electron cyclotron emission system. This method has been used to calibrate Thomson n_e from the edge to near the core (r/a > 0.15). Application of core electron cyclotron heating improves the quality of cutoff and depth of its penetration into the core, and also changes underlying MHD activity, minimizing crashes which confound calibration. Less fueling is needed as "ECH pump-out" generates a plasma ready to take up gas. On removal of gyrotron power, cutoff penetrates into the core as channels fall successively and smoothly into cutoff.