LNF:Fuelling and Impurity Control Studies in the stellarators TJ-II and W7-X using Cryogenic Pellets and Tracer-Encapsulated Solid Pellets (TESPEL): Difference between revisions

4. Pellet injection experiments are performed for several magnetic configurations of the TJ-II stellarator in order to increase our understanding of the role played by rational surfaces in plasmoid drift and deposition profiles in stellarators [7]. The analysis of plasmoid drifts during experiments is supported by simulations made with the code HPI2. Such plasmoid drifting is found to be significantly reduced, as is observed in tokamaks, in the vicinity of rational surfaces (rational surfaces have magnetic field lines that are periodic; i.e., the magnetic field lines close back on themselves). This is attributed to the fact that plasmoid external charge reconnection lengths become shorter close to rational surfaces, resulting in more effective damping of plasmoid drift. Although in stellarators, the effect of plasmoid external currents on drift is expected to be negligible, compared with plasmoid internal currents, this latter effect is clearly measurable in TJ-II. In addition, code simulations reveal that enhanced drift reductions near rational surfaces lead to significantly different deposition profiles for standard magnetic configurations in TJ-II. This implies that it should be possible to identify magnetic configurations that will result in more efficient pellet fuelling. In a further study in the area, a comparison was made on the influence of plasmoid-drift mechanisms on plasma fuelling by cryogenic pellets in ITER and Wendelstein 7-X [8].

6. Experiments in the LHD with continuous lithium power dropping have allowed the creation of a reactor-relevant high-density plasma regime [8, 9]. This is characterized by increased energy confinement as well as surpressed turbulence and reduced impurity confinement. The transition to this regime is driven by the continuous dropping of Li-powder grains into the plasma. When such plasmas are compared to plasmas without Li-powder the achieved high-performance characteristics include: increased plasma energy & core electron temperature, reduced plasma-wall interaction, and an up to one order of magnitude reduction in plasma turbulence across the whole plasma radius in the frequency range 5 to 500 kHz. In addition, and contrary to expectations for high-density plasmas in stellarators, it is seen, when injecting TESPELs to deposit tracers in the core of this high-performance phase, that impurity confinement is significantly reduced for plasmas with Li powder when compared to confinement in discharges without Li-powder. These new results demonstrate the potential of continuous dropping of Li-powder into stellarator plasmas for simultaneously accessing enhanced confinement regimes while avoiding impurity accumulation.