Density related operational limit in the limiter phase of Wendelstein 7-X

The stellarator Wendelstein 7-X (W7-X) has been designed to show that optimized stellarators can achieve and sustain fusion relevant plasma conditions. One of the main optimization criteria was the reduction of the neoclassical transport, which is considered as one of the most critical issues of the stellarator concept. While the demonstration of the neoclassical optimization will definitely be one of the most important aspects of W7-X, several additional topics are equally important due to the boundary condition that fusion relevant conditions should be demonstrated and sustained. Hence, low neoclassical transport needs to be achieved in a scenario which is compatible with divertor operation, high densities, and an acceptable impurity concentration without further accumulation during stable operation. So far, these issues have mostly not been tackled experimentally in the first experimental campaign. W7-X had its first plasma in December 2015 and the first operational campaign (OP1.1) was mainly intended for testing and commissioning purposes. In OP1.1, W7-X was operated in a limiter configuration without divertor and at low densities. Nevertheless, valuable insights could be gained which help to prepare and understand the first experiments with a test divertor planned to start in the second half of 2017. As will be shown in this contribution, one of the interesting observations in OP1.1 was the presence of an operational limit, where above a critical density the power balance seems to be dominated by radiative losses. Such a behavior is well-known from other stellarator experiments and while it can be expected that this critical density is particularly low for OP1.1 (high impurity concentration connected to the limiter operation and conservative wall conditioning), this observation also shows that impurity related radiation losses will be an important issue to keep track of as the density is progressively increased in the next experimental campaigns of W7-X.