Interplay between magnetic topology, density fluctuations and confinement in high-β Wendelstein 7-AS plasmas

Previous high-β experiments in the Wendelstein 7-AS (W7-AS) stellarator have shown that it was not possible to reach the stability β -limit; the discharges remained either power or radiation limited. A central β (β0) of 4 % and an average β ([β ]) of between 1.7 % and 1.8 % was achieved. To ameliorate this situation, the W7-AS neutral beam system has been reconfigured from balanced to unbalanced injection, totalling 4 MW power. We analyse experiments that were performed at half field (1.25 T) with full beam power to ascertain the effects of the increased β-value. A moderate increase of [β ] up to 2.2 % was obtained in this series, which was later improved to 3.1 %. The main diagnostic used in the present paper is the CO2 laser based LOcalised TUrbulence Scattering (LOTUS) instrument. This measures electron density fluctuations in two toroidally displaced vertical measurement volumes - here we will only treat a single volume. The wavenumber of the fluctuations measured (κperpendicular) was set to 20 cm-1. Copyright (2002) Australian National University- Research School of Physical Sciences and Engineering