The Impurity Powder Dropper program in LHD and W7-X

The Impurity Powder Dropper (IPD) is a device capable of injecting into the plasma controlled amounts of sub-millimeter powders under the action of gravity. Already operational on several tokamaks, in 2019 it was installed on the LHD stellarator, where several dedicated experiments were conducted, injecting boron and other low-Z powders into the plasma.

The main results of low-Z powder injections include improvement of wall conditions, such as decrease of wall recycling and intrinsic impurity concentration, both on a shot-to-shot basis and in real time. Furthermore, the access to a reduced-turbulence, improved-confinement regime was observed upon powder injection, increasing both T_e and T_i of about 25%, and up to 50% for T_i. The improvement is most likely caused by suppression of ion scale turbulence induced by the change in plasma profiles and increase of effective charge induced by powder injection.

In the neoclassically optimized W7-X stellarator, pulsed powder injection experiments were previously performed with a smaller powder injector, producing a similar confinement improvement, but only transiently. The installation of an IPD on W7-X, providing a continuous powder flow to the plasma, will allow to obtain a steady-state improvement alike to LHD, and performing real-time wall conditioning to complement standard glow discharge boronization. The installation of the IPD on W7-X, guided by predictive simulations of the powder injection with the EMC3-EIRENE and DIS codes, is planned for OP2.4.