No abstract

The V3FIT code for performing equilibrium reconstruction in three-dimensional plasmas is described. It is a modular code that has the potential to be coupled with a variety of equilibrium solvers to compute the externally measured response to an arbitrary internal state of the plasma. Singular-value decomposition is used to identify the dominant components of the plasma state that can be accurately determined by the reconstruction process and to guide the minimization of the χ2 variance-normalized mismatch between the measured and computed signals. Comparison of a tokamak plasma equilibrium computed by V3FIT and by the axisymmetric equilibrium reconstruction code EFIT is presented. V3FIT is used to reconstruct an axisymmetric DIII-D equilibrium using experimentally observed magnetic diagnostic signals. Three-dimensional reconstructions of stellarator plasma equilibria in the CTH device show the code behaves as expected in the presence of experimental noise, appropriately ignores near-singular directions in parameter space and robustly reconstructs equilibria starting from substantially different initial parameter values.

Experimental verification of lower-hybrid RF current drive in the Versator II tokamak is presented. The experiments show that efficient current drive exists only in low density discharges (ffe < 6 X 101 2 cm-3) in the "slide-away" regime where a preformed supra-thermal electron tail exists prior to the application of the RF power.

High energy x-ray emission (E, > 20 keV) from superthermal plasma electrons during lower hybrid current drive on the Alcator C tokamak has been measured using Sodium Iodide (NaI) scintillation spectroscopy. The x-ray spectra are generally linear on a semi-log plot of count rate vs. photon energy and extend out to several hundred keV. For the range of densities (ft, , 0.3-0.8 x 10" cm-3 ) over which current drive was performed on Alcator, there was negligible emission before the injection of radiofrequency (rf) wave power. The radial profiles of the emission were also measured and indicate that the current carrying, high energy electrons exist primarily within the inner half (r/a < 1/2) of the plasma column. Plasma parameter scans produced variations in the x-ray emission profiles that are consistent with changes in the launched Fourier power spectrum, and the conditions imposed by lower hybrid wave accessibility. In addition, the velocity space distribution function of the energetic tail electrons has been determined using the angular variation in the x-ray emission.