Lower hybrid modeling and experiments on Alcator C-Mod

Abstract

A highly flexible phase control system was developed in order to dynamically adjust the parallel wave number spectrum of driven LH waves on Alcator C-Mod. Wave coupling between the antenna and the plasma was studied using this system. A coupling code was modified to allow arbitrary phase and amplitude in each waveguide, which was required for agreement between the theory and experiment. Remarkable agreement was achieved when a small vacuum gap of 0.6 to 1.4 mm was included in the model, similar to results obtained on ASDEX. The location of the fast electron population is critical in diagnosing the performance of the lower hybrid system and can be inferred by measuring nonthermal bremsstrahlung emission. A state-of-the-art diagnostic for hard x-ray (HXR) emission was designed and used to make measurements of fast electrons bremsstrahlung during LHCD experiments on C-Mod. The HXR diagnostic consists of an array of 32 CdZnTe detectors to image energies in the 20-200 keV range. Detectors and pulse processing electronics were integrated into a compact and modular package. The system also makes use of fast digitization and software signal processing techniques allowing a maximum counting rate of 1 MHz. HXR experimental results indicate a spatially broad and centered fast electron population. HXR profiles predicted by a synthetic diagnostic in the Fokker-Planck code CQL3D are narrower than the observed profiles, suggesting the importance of spatial diffusion of fast electrons in the experiment. Lower hybrid experiments on C-Mod represent the first results obtained with density, magnetic field, plasma shape, and source frequency all near conditions expected on ITER. In initial experiments, about 400 kW of LH power was coupled for 400 ms into low density L-mode discharges.

(cont.) Clear evidence of current drive was seen as the loop voltage decreased by about 50%. Studies were performed by varying the phase, density, and magnetic field as well as changing the direction that the waves were launched. Analysis using CQL3D indicates that a current drive efficiency of 0.15 (1020 m-2A/W) was obtained by driving 167 kA of LH current with 410 kW at a line-average density of 5.5 x 1019 m-3, without an electric field. This efficiency exceeds what was observed on Alcator C, but is within the! range observed on FTU for similar densities. Including the residual electric field increased the LH driven current to 308 kA, corresponding to an increased effective efficiency of 0.28. In addition to experiments, extensive modeling of current profile control through phase variation was studied, including compound spectra, using CQL3D. It was found that over 200 kA of off-axis LH current can be generated in a variety of profile shapes in an H-mode target plasma. Finally, time dependent modeling of an integrated scenario was performed using the transport code TRANSP to explore what performance can be ultimately achieved on C-Mod. Results indicate that fully non-inductive, quasi-steady-state plasmas are possible with bootstrap fractions as high as 75%.