Measurement of neutral beam profiles at DIII-D

The neutral beam systems of DIII-D, a National Fusion Facility at General A tomics, are used both for heating the plasma, and as tools for plasma diagn ostics. The spatial distribution (profile) and energy of the beam is used i n 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 recomb ination events between the plasma ions and the beam neutrals. As such, the spatial distribution of the beam is needed for an absolute calibration of t he CER diagnostic. The MSE diagnostic measures the internal poloidal field profile in the plasma. MSE measures the polarization angle of the Stark bro adned 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 ab solute calibration of the MSE diagnostic. In the past, the beam spatial profile used in these calibrations was derive d from beam divergence calculations and IR camera observations on the tokam ak inboard target tiles. Two experimental methods are now available to bett er determine the beam profile. In one method, the Doppler shifted D-alpha l ight from the energetic neutrals are measured, and the full-width at half-m aximum (FWHM) of the beam can be inferred from the measured divergence of t he D-alpha light intensity. The other method for determining the beam profi le uses the temperature gradients measured by the thermocouples mounted on the calorimeter. A new iterative fitting routine for the measured thermocou ple 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 n ew experimental verification of the beam profile.