Impurities in long-pulse operation of W7-X

The Wendelstein 7-X (W7-X) aims to demonstrate that the HELIAS line of stellarators can achieve high power and high performance under steady-state conditions. Such a scenario makes stellarators attractive candidates for fusion reactors offering potentially lower operating costs for continuously operated power plants. Therefore, a number of experiments have been performed at W7-X to demonstrate long-pulse operation. During the operational phase 1.2 (OP1.2) at W7-X, which took place in 2017–2018, a 100 s discharge with attached divertor plasmas was achieved, while the detached conditions were sustained for about 27 s. Performed experiments showed that a robust detachment scenario allows to reduce the peak heat flux by almost an order of magnitude and no significant increase of impurity concentration was observed (with Zeff < 1.5). The pulse duration was limited by the thermal limits of inertially (non active) cooled carbon divertor configuration. In operational phase OP2.1, the installation of a new actively water-cooled carbon-fiber composite divertor allowed a significant extension of pulse operation. The longest attached discharge lasted 8 min reaching 1.3 GJ energy throughput. In this experiment, attached plasma was heated with an average of 2.7 MW of ECRH power. The longest detachment phase in OP2.1 was achieved with feed-forward Ne seeding, which kept peak heat flux at the very low level, almost everywhere below 0.5 MW m−2. As a consequence, no significant increase of impurity concentration occurs with the cooler plasma boundary, and the Zeff stayed below 2. The spectroscopic observation confirms that there is no Ne accumulation in the plasma core, which is an important prerequisite for steady-state high power plasmas.