Energetic Particle Transport and Alfven Instabilities in Compact Stellarators

Stellarator design tools have evolved in recent years to allow the development of a number of new compact stellarator configurations (QPS, NCSX) that maintain good plasma neoclassical confinement while improving on various shortfalls of the tokamak (e.g., absence of disruptions, stability to neoclassical tearing instabilities, lowered poloidal flow damping). These improvements have resulted from the development of rapidly evaluated optimization targets for thermal plasma stability and transport. In the case of energetic particle confinement and stability, efficiently evaluated target functions remain to be developed and, as a result, energetic particle physics issues must be evaluated a posteriori. Significant issues include: classical confinement of energetic ions during slowing-down, Alfven gap modes, interaction of fast ions with plasma MHD modes, and impact of energetic ions on core transport properties (i.e., parallel viscosity, bootstrap current). We have developed tools to address a number of these issues. These include a parallel/vectorized fast particle Monte Carlo code (DELTA5D) and an Alfven gap stability code (STELLGAP). These codes have been applied both to compact stellarators (QPS, NCSX) and to a variety of existing experiments (CHS, LHD, W7-AS, TJ-II, HSX). This analysis can lead to the development of optimization target functions that can be useful in flexibility studies and in the design of future devices. A moments method analysis has also been developed for analyzing both the E x B shearing rates driven by ambient flows (diamagnetic and E x B) and modifications due to beam momentum sources. (author)