Performance of the Wendelstein 7-X stellarator during the first divertor operation phase*

Wendelstein 7-X is the most advanced stellarator put into operation to date. It is optimized with a number of goals in mind, in particular to demonstrate good confinement at plasma parameters relevant to a future stellarator power plant. Specifically, it aims to demonstrate good energy confinement at T_e=T_i~ 6 keV, n>10²⁰ m^‑3, and a volume averaged beta of 5 %. Plasma operation started in 2015-16 using a limiter configuration. In 2017, operation started with an uncooled magnetic island divertor, extending the integrated heating power from 4 to 80 MJ. For this phase, the electron cyclotron resonance heating (ECRH) capability was extended to 7 MW, and hydrogen pellet injection was implemented. All of the enhancements resulted in the highest triple product (6.5×10¹⁹ keV m^-3 sec) achieved in a stellarator up to now. Plasma conditions (T_e~T_i~3.8 keV,  tau_E > 200 ms) already were in the stellarator reactor-relevant plasma transport regime (so-called ion-root confinement), where optimization effects should become visible. Further important developments include the symmetrization of the heat loads between the 10 divertor modules by external resonant magnetic fields, observation of full divertor power detachment, and results that shed light on drift effects, plasma transport, inboard versus outboard pellet injection efficiency, and scrape-off-layer physics.  Stable operation above the 2^nd harmonic ECRH X-mode cut-off was also demonstrated, which is instrumental for achieving high plasma densities in Wendelstein 7-X. An update on the analysis of the limiter plasmas will also be given, comparing transport channels and bootstrap current predictions with theoretical expectations.