Impact of wall conditioning and gas fuelling on the enhanced confinement modes in TJ-II

The TJ-II helical stellarator has been operated under full metallic first wall conditions until mid 2001. This scenario has proven fully compatible with the ECR production and heating of plasmas. Under those conditions, several modes with enhanced confinement have been found. In the last experimental campaigns, however, boronisation of the first wall of the TJ-II has been carried out. This has lead to strong changes in edge parameters and density control. Namely, a rise of the electron temperature at the limiter of a factor of ∼3, and a strong (<4×) increase in the required fuelling rate for the same density respect to the metallic case have been observed. According to the observation in the metal case, these two facts would in principle prevent the transition to the enhanced particle confinement mode. However, the same features associated with such mode have been reproduced. Moreover, a simultaneous increase in the total energy content, not achieved in the former wall scenario, was found for some conditions. The implications of these findings to the present understanding of the enhanced modes in TJ-II are addressed.