EXPERIMENTAL AND NEOCLASSICAL ELECTRON HEAT-TRANSPORT IN THE LMFP REGIME FOR THE STELLARATORS W7-A, L-2, AND W7-AS

The electron energy balance is analyzed for equivalent low-density ele ctron cyclotron resonance heated (ECRH) discharges with highly peaked central power deposition in the stellarators W7-A [Plasma Phys. Contro lled Fusion 28, 43 (1986)], L-2 [Proceedings of the 6th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, B erchtesgaden, 1976 (International Atomic Energy Agency, Vienna, 1977), Vol. 2, p. 115] and W7-AS [Proceedings of the 9th International Confe rence on Plasma Physics and Controlled Nuclear Fusion Research, Baltim ore, 1982 (International Atomic Energy Agency, Vienna, 1983), Vol. 3, p. 141]. Within the long mean-free path (LMFP) collisionality regime i n stellarators, the neoclassical electron heat diffusivity chi(e) can overcome the ''anomalous'' one. The neoclassical transport coefficient s are calculated by the DKES code (Drift Kinetic Equation Solver) [Phy s. Fluids 29, 2951 (1986); Phys. Fluids B 1, 563 (1989)] for these con figurations, and the particle and energy fluxes are estimated based on measured density and temperature profiles. Neoclassical transport in the LMFP regime is minimum in W7-A and maximum in L-2, the standard co nfigurations in W7-AS are in between. The radial electric field is est imated from the ambipolarity condition of only neoclassical particle f luxes. For these types of discharges in the quite different stellarato r configurations, only the ''electron root'' exists in the innermost r egion, and, at the outer radii, only the ''ion root.'' In the region w here both roots are found, a rather narrow shear layer in the poloidal plasma rotation is expected. Especially for W7-AS, a significant impr ovement of the neoclassical confinement is predicted in the ''electron root'' region. On the ''ion root'' side of the predicted ''shear laye r,'' both the neoclassical energy and particle fluxes agree quite well with the experimental findings. At outer radii, the neoclassical flux es are much lower. The predicted improvement for the ''electron root'' region is not found experimentally.