Measurement of neutral beam profiles at DIII-D

The neutral beam systems of DIII-D, a National Fusion Facility at General Atomics, are used both for heating the plasma, and as tools for plasma diagnostics. The spatial distribution (profile) and energy of the beam is used in the absolute calibration of both the Charge Exchange Recombination (CER) and Motional Stark Effect (MSE) diagnostics. The CER diagnostic is used to make spatially and temporally resolved measurements of ion temperature and poloidal and toroidal rotational velocities. These measurements are made by visible spectroscopy of the Doppler shifted He II (468.6 nm), C VI (529.1 nm) and B V(494.5 nm) spectral lines, excited by the charge exchange recombination events between the plasma ions and the beam neutrals. As such, the spatial distribution of the beam is needed for an absolute calibration of the CER diagnostic. The MSE diagnostic measures the internal poloidal field profile in the plasma. MSE measures the polarization angle of the Stark broadned neutral beam D-alpha emission due to the V-beam x B motional electric field. Again, the spatial profile of the neutral beam is needed for the absolute calibration of the MSE diagnostic. In the past, the beam spatial profile used in these calibrations was derived from beam divergence calculations and IR camera observations on the tokamak inboard target tiles. Two experimental methods are now available to better determine the beam profile. In one method, the Doppler shifted D-alpha light from the energetic neutrals are measured, and the full-width at half-maximum (FWHM) of the beam can be inferred from the measured divergence of the D-alpha light intensity. The other method for determining the beam profile uses the temperature gradients measured by the thermocouples mounted on the calorimeter. A new iterative fitting routine for the measured thermocouple data has been developed to fit theoretical models on the dispersion of the beam. The results of both methods are compared, and used to provide a new experimental verification of the beam profile.