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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
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LNF:Fuelling and Impurity Control Studies in the stellarators TJ-II and W7-X using Cryogenic Pellets and Tracer-Encapsulated Solid Pellets (TESPEL) (view source)
Revision as of 11:07, 7 November 2024
, 7 November→Main Results
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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. | 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. | 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. 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 seen 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 near such rational surfaces, resulting in more effective damping of the plasmoid drift. Although the effect of plasmoid external currents on the drift is expected to be negligible in stellarators, compared with those caused by plasmoid internal currents, the effect observed in TJ-II is clearly measurable. In addition, simulations show that enhanced drift reductions near rational surfaces lead to significantly different deposition profiles for the magnetic configurations included in this study. This implies that it should be possible to select the magnetic configurations to obtain more efficient pellet fuelling. | ||
== References == | == References == | ||