Commissioning of the tracer-encapsulated solid pellet (TESPEL) injection system for Wendelstein 7-X and first results for OP1.2b

A new tracer-encapsulated solid pellet (TESPEL) injection system was successfully commissioned for the stellarator fusion experiment Wendelstein 7-X (W7-X) during its OP1.2b
operational campaign. TESPELs are polystyrene encapsulated solid pellets loaded with tracer
impurities that have been employed in other stellarator devices for impurity transport studies.
During the OP1.2b campaign approximately 140 pellet injections were performed with a
successful delivery rate of 89%, thus this system has proven to be very reliable. Here, the
experimental set-up and methodology are described first. In addition, it is outlined how, through the analysis of TESPEL time-of-flight signals and of the temporal evolution of line emissions originating from shell and tracer species as well as comparisons with ablation models, the radial localization of the deposited tracer is determined. This contribution also provides a general overview of the TESPEL injector performance during OP1.2b, discusses the global effects of TESPEL injections on W7-X plasmas and reports on first results in terms of a summary of TESPEL injections, plasma response to TESPELs, the post-deposition evolution of tracer spectral emission lines and soft x-ray emissions.