The measurement of absolute helium ion density profiles on the DIII-D tokamak using charge exchange recombination spectroscopy

In future burning fusion devices, helium will be an unavoidable impurity component of the plasma. The overall performance of such devices is critically dependent on the helium transport properties of the confined plasma. Poor helium transport in reactors could lead to a buildup of fusion ash, causing fuel dilution and increased radiation that will result in degraded fusion power and possibly quench ignition altogether. As a result, it is essential to investigate the helium transport and exhaust properties of tokamak plasmas in order to design an optimized helium removal system for future reactors. These studies require accurate spatial and temporal measurements of the helium density profiles in tokamak plasmas.;The purpose of this work has been to instrument and calibrate the existing charge-exchange recombination (CER) spectroscopy diagnostic used to measure the ion temperature and plasma rotation speeds on the DIII-D tokamak for the simultaneous measurement of absolute helium density profiles as well. This has involved (1) absolutely calibrating the CER system using standard calibration lamps and two novel techniques involving neutral beam injection into a gas filled torus and a pure He plasma, (2) developing and testing a neutral beam attenuation code to calculate the local neutral beam density at each viewing location, (3) assembling a database of accurate atomic data, (4) analyzing the measured CER spectra with a multi-Gaussian fitting procedure, and (5) developing and testing a modeling code that distinguishes unwanted signal contributions from drifting hydrogen-like helium ions (the 'plume effect').;Absolute helium density profiles have been measured for a variety of tokamak operating conditions, including low confinement (L-mode) and high confinement (H-mode) discharges. Helium transport behavior has been observed by injecting helium gas puffs into DIII-D plasmas and measuring the He density profile evolution. The measured profiles are found to have a similar shape as the measured electron density profiles, thereby disputing the unfavorable prediction by neo-classical transport theory that there is preferential accumulation of helium in the plasma center. The effective use of edge-localized modes (ELMs) to purge helium from H-mode discharges has also been observed.