Current profile measurements using MSE on Alcator C-Mod

Abstract

A Motional Stark Effect (MSE) diagnostic system has been installed on the Alcator C-Mod tokamak to measure the plasma internal magnetic pitch angle profile. The diagnostic utilizes polarization patterns from Doppler-shifted Balmer-alpha decay emission from an energetic neutral beam injected into a magnetically confined plasma. This dissertation consists of three parts: (1) the current status of the C-Mod MSE diagnostic which includes major upgrades in the hardware and calibration techniques; (2) the elimination of the spurious drift in the polarization measurements due to thermal-stress induced birefringence; and (3) the measurement of current density profiles in Lower Hybrid Current Drive (LHCD) experiments. The major hardware upgrades include replacement of photomultiplier tubes (PMT's) with avalanche photodiodes (APD's) which enhanced the quantum efficiency; installation of a wire-grid polarizer to verify small Faraday rotation in the diagnostic; installation of steep edge filters to minimize pollution by the thermal Balmer-alpha signals; rotation of the Diagnostic Neutral Beam (DNB) which significantly reduced the anomalous effect from the secondary beam neutrals during the beam-into-gas calibrations. The new calibration techniques include two plasma calibrations: plasma current sweeping and the plasma size sweeping whose feasibility was experimentally proven; and an absolute intensity calibration which measured the real optical throughput of the system. A large database study indicates the signal-to-background ratio larger than 100 is required to have the measurement uncertainty under 0.1 degrees.

(cont.) The spurious drift in the measurement has been identified as the thermals tress induced birefringence imposed on the in-vessel lenses. By modeling this effect as a single wave plate, an in-situ calibration method has been proposed and its feasibility was experimentally verified. Based on the experiments that characterized the thermal response of the system, a single-layer heat shield with gold plating and a lens holder which reduces the thermal conduction path to the lens have been designed and fabricated. A more rigorous model that includes an intrinsic phase shift by mirrors reveals the thermal phase shift can be greatly magnified by the intrinsic phase shift. The current density profiles from LHCD experiments have been obtained from the MSE data corrected by a baseline magnetic equilibrium whose internal profile is constrained by the sawtooth inversion radius. The resultant profiles successfully demonstrate several standard predictions of LHCD theory such as the dependence of efficiency on the parallel refractive index and the off-axis current drive.