Electron heat transfer and density fluctuations in the Wendelstein VII-A stellarator

The electron energy confinement in the W VII-A stellarator has been studied in Ohmically heated discharges. A low-density regime (nsub(e)(0) approximately <5x1013cm-3) has been found, where the coefficient of electron heat conductivity chisub(e) follows drift parameter scaling closely. This implies a reduction of chisub(e) with increasing nsub(e) and Tsub(e). An attempt is made to correlate density fluctuations, as measured by microwave scattering, with the observed losses. - With neutral-beam heating, a considerable improvement of confinement has been observed. The confinement time increases with temperature, supporting the results observed in Ohmic heating. - Furthermore, a sharp overall reduction in the density fluctuation level at the beginning of injection is observed and may also contribute to the improved confinement. A brief description of the microwave scattering system is given. - In the high-density regime (nsub(e)(0) approximately >5x1013cm-3), the energy loss is increased by MHD mode activity (the (2,1) mode for l bar(a) approximately <1/2). Changes in the temperature profile due to magnetic islands have been observed. (author)