The most advanced concept for magnetic-confinement fusion is a tokamak which confines plasmas in a toroidally symmetric geometry with the poloidal field generated by the large plasma currents. Another solution is applied in stellarators – devices which receive the poloidal field from external, helical magnetic coils. This concept is, after the tokamak, the most promising alternative to fusion power plant.
One of the main goals of the largest advanced stellarator in the world, Wendelstein 7-X (W7-X), operated since 2015 in Greifswald, Germany, is to demonstrate the high performance quasi-stationary long-pulse operation with a magnetic island divertor designed to accommodate many magnetic field configurations.
The W7-X operation has been divided into three main phases: the first, OP1.1 with carbon limiter configuration, the second - OP1.2 with graphite uncooled divertor configuration and the third, current operational phase, OP2 with actively cooled graphite divertor configuration. Understanding of impurity behaviour in fusion devices is a key task as they are responsible for enhancing radiation losses and plasma dilution which in consequence may lead even to plasma termination. The main impurities at W7-X are carbon (from the divertor), oxygen (from the inner wall, adsorbed during venting of the machine) and boron (from boronisation process) which are intrinsically produced by surface material erosion in the divertor and machine’s wall. There are many diagnostics dedicated for impurity transport study which deliver information about the impurity behaviour during the W7-X discharge, impurity concentration and effective charge Zeff. One of the aims of the performed experiments is to study the impurity confinement in dependence of heating power, electron density and magnetic configuration.
During the seminar, the stellarator concept on the example of the W7-X, as well as, Polish involvement in the W7-X programme including the participation in the design, development and exploitation will be discussed. The main results obtained so far in recent experimental campaigns and plans for further exploitation will be presented.