Magnetic configuration and plasma parameter dependence of the energy confinement time in ECR heated plasmas from the TJ-II stellarator

In this work we study the influence of magnetic configuration-rotational transform and minor radius-and plasma parameters-mainly plasma density-on the confinement of low plasma pressure, electron cyclotron resonance (ECR) heated TJ-II plasmas, taking advantage of the TJ-II remarkable magnetic configuration flexibility. Previous discharges in all-metal wall conditions showed a positive exponential dependence of the energy confinement time on the rotational transform, with exponent 0.6, higher than the one deduced from the ISS95 database (0.4). A set of recent plasma discharges, produced in boronized wall conditions, yields different dependences on rotational transform and, above all, on plasma density. The rotational transform-dependence of the boronized data set, with exponent 0.35, might still be considered marginally compatible with the ISS95 prediction, but this is not the case with the density dependence. In this paper we describe the similarities and differences observed between all-metal and boronized data sets and we discuss their possible physical origins.