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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 14:54, 11 November 2024
, 11 November→Main Results
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1. A pellet-induced enhanced confinement regime (or PiEC) has been identified in Neutral Beam Injection (NBI) heated discharges made in TJ-II after the injection of a single cryogenic fuel pellet into its plasma core [1]. In addition to the expected increase in core electron density, the plasma diamagnetic energy content, as determined using a diamagnetic loop, is seen to rise by up to 40%, with respect to reference discharges without pellet injection. Furthermore, the energy confinement time is significantly enhanced when compared to predictions obtained using the 2004 International Stellarator Scaling law, ISS04. Indeed, the operational regimes of other stellarator devices, such as LHD and W7-X, can be similarly extended to performance well beyond those obtainable with gas puffing alone. | 1. A pellet-induced enhanced confinement regime (or PiEC) has been identified in Neutral Beam Injection (NBI) heated discharges made in TJ-II after the injection of a single cryogenic fuel pellet into its plasma core [1]. In addition to the expected increase in core electron density, the plasma diamagnetic energy content, as determined using a diamagnetic loop, is seen to rise by up to 40%, with respect to reference discharges without pellet injection. Furthermore, the energy confinement time is significantly enhanced when compared to predictions obtained using the 2004 International Stellarator Scaling law, ISS04. Indeed, the operational regimes of other stellarator devices, such as LHD and W7-X, can be similarly extended to performance well beyond those obtainable with gas puffing alone. | ||
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, 3]. In order to achieve best results, it has been determined that the plasma target electron density should to be in the range <math>1 | 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, 3]. In order to achieve best results, it has been determined that the plasma target electron density should to be in the range <math>1 10^{19} {\mathrm m}^{-3}</math> to <math>2.5 \times 10^{19} {\mathrm m}^{-3}</math> 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. An important inter-machine study of cryogenic-pellet fueling in helical devices has also been made [4]. This was done to evaluate controlling performance bifurcations in stellarators. | ||
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 [5]. 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. In a separate new study on enhanced confinement for a specific magnetic configuration, 100-48-65 (comparison with with and without pellets), it is found that enhanced confinement can depend strongly on plasma currents, which in turn, indicates a dependence on rotational transform (location of low-order rational surfaces in gradient region) [6]. | 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 [5]. 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. In a separate new study on enhanced confinement for a specific magnetic configuration, 100-48-65 (comparison with with and without pellets), it is found that enhanced confinement can depend strongly on plasma currents, which in turn, indicates a dependence on rotational transform (location of low-order rational surfaces in gradient region) [6]. | ||