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

2. New studies, performed with multiple pellet injections, have extended the TJ-II operational regime well beyond limits previously achieved in this device using NBI heating and gas puff [2]. In order to achieve best results, it has been determined that the plasma target electron density should to be in the range 1x10^19 m^-3 to 2.5x10^19 m^-3 and time separations between pellets should be close to energy confinement times, around 10 ms. It is also found, using a Charge Exchange Recombination Spectroscopy diagnostic, that while the plasma electron temperature is almost unaffected by such pellet injections, the majority ion temperature irises significantly due to reduced ion radial heat fluxes during the PiEC phase. It is also found that enhanced performance is independent of whether co- or counter-NBI heating beam is employed. Finally, record stored diamagnetic energy content and plasma beta values are achieved when the largest available pellets are employed. The results indicate that pellet injections extend the operational regime well beyond limits previously achieved in TJ-II without pellets.

3. As noted above, improvement confinement associated with the injection of pellets has been observed in TJ-II during NBI phase of its plasmas. Using a simple model, the modification of turbulent transport by a pellet injection and how this modification affects particle confinement time has been studied [3]. The results indicate a relationship between improved confinement and the evolution of shear flows due to turbulence, especially near low order rational surfaces. Furthermore, experiments show that an additional pellet, or pellets, may enhance the confinement improvement produced by the first. This effect is reproduced in the model when the second density pellet is launched soon after the first one. For this to occur, the second pellet must be injected in the transient period, before the plasma returns to the steady state.

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 [4]. 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.