Confinement properties of the 'Advanced Stellarator' Wendelstein W VII-AS

An extended parameter range (Te ≤ 3 keV, Ti ≤ 0.7 keV, ne ≤ 2.5x1020m-3) was explored in the new modular stellarator Wendelstein W VII-AS using ECRH and NBI. At 2.5 T main field, central values of β up to 1.5% were achieved. A reduced Shafranov shift as consequence of the reduced Pfirsch-Schlueter currents for the advanced stellarator concept is verified. Depending on the plasma parameters, a net current is observed and identified as the bootstrap current. At the low shear stellarator WVII-AS, the magnetic configuration and the confinement depend sensitively on the plasma currents. Thus a control of the configuration by small currents (OH ECCD, Ohkawa current) is applied to maintain optimum confinement. On the basis of profile measurements the electron heat conduction and the ion particle diffusion coefficient were evaluated for ECRH plasmas. As a result of statistical analysis, a phenomenological expression for the energy confinement time τE is derived. After carbonization the high-Z impurities (Fe, Ti) and radiation losses were significantly reduced. In first experiments with NBI (PIN ≤ 1.5 MW), discharges at densities of ne ≤ 2.5x1020m-3 could be maintained and an energy replacement time of up to 30 ms was achieved. (author). 13 refs, 6 figs