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

'''Title''': '''Fuelling and Impurity Control Studies in the stellarators TJ-II and W7-X using Cryogenic Pellets and Tracer-Encapsulated Solid Pellets (TESPEL)e'''

'''Financing granted (direct costs)''': 130000 €

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 \times 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].

4. Pellet injection experiments have been performed for a range of magnetic configurations of TJ-II in order to increase our understanding of pellet deposition profiles and of the role of rational surfaces in plasmoid drift in stellarators [7, 8, 9]. In a first instance, it is found that fast-electron impacts on a pellet can lead to ice destruction, this leading to enhanced fuelling efficiency. It is considered that a sudden pellet destruction inhibits the development of normal outward drifting of plasmoids that occurs when pellets are ablated by thermal electrons only [7]. In a separate study, 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) [8]. This is attributed to the fact that plasmoid external charge reconnection lengths shorten when 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 [9].

5. A tracer-encapsulated solid pellet (TESPEL) system was commissioned successfully for the stellarator Wendelstein 7-X (W7-X) during its OP1.2b experimental campaign [10, 11, 12, 13]. TESPELs are polystyrene encapsulated solid pellets loaded with a single tracer or multiple tracers that are employed for impurity transport studies. During the OP1.2b campaign approximately 140 pellet injections were performed with successful delivery rate of 89%, this result showing that TESPEL production is very reliable. A significant fraction of those TESPELs were fabricated at Ciemat. A large number of TESPELs have been produced for the 2024 SOII experimental campaign on W7-X and for the 2024 campaign on the Large Helical Device (LHD) stellarator. The results for these experiments will be published in the near future.

6. Experiments in the LHD with continuous lithium power dropping have allowed the creation of a reactor-relevant high-density plasma regime [14, 15]. This is characterized by increased energy confinement as well as suppressed 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.

[6] Impact of the rotational transform on enhanced confinement in the TJ-II stellarator. Part 1: experimental results, B. Ph. van Milligen, I. García-Cortés, K. J. McCarthy, T. Estrada, A. Cappa, P. Pons-Villalonga, B. A. Carreras, L. García, O. S. Kozachok, O. O. Chmyga, J. L. de Pablos, J. M. Barcala, A Molinero, D. Tafalla, I. Pastor, A. de la Peña, F. Lapayese, (...) and the TJ-II Team, in preparation for Nucl. Fusion.

[11] Additional ECRH mitigates thermal quenches induced by tungsten TESPEL injection in LHD , H. Bouvain, A. Dinklage, N. Tamura, H. Igami, H. Kasahara, K. J. McCarthy, D. Medina Roque, I. García-Cortés and the LHD Experiment Group, sent to Nuclear Fusion for publication, NF-106861.  

[12] Experimental confirmation of efficient island divertor operation and successful neoclassical transport optimization in Wendelstein 7-X, T. S. Pedersen, ..., I. García-Cortés, K. J. McCarthy, …, N. Panadero Alvarez, ..., N. Tamura, ..., Nucl Fusion 62, 042022 (2022). DOI: 10.1088/1741-4326/ac2cf5.

12) Influence of the magnetic well on pellet fuelling in the TJ-II stellarator, N. Panadero, K. J. McCarthy, I. García Cortés, B. López Miranda, et al., and TJ-II Team, Poster presentation at the 24th International Stellarator Helitotron Workshop, Hiroshima, Japan (2024).