Enhanced energy confinement after series of pellets in Wendelstein 7-X

Baldzuhn, J.; Damm, H.; Beidler, C. D.; McCarthy, K. J.; Panadero, N.; Biedermann, C.; Bozhenkov, S.; Dinklage, A.; Brunner, K. J.; Fuchert, G.; Kazakov, Yevgen; Beurskens, M.; Dibon, M.; Geiger, J.; Grulke, O.; Höfel, U.; Klinger, T.; Köchl, F.; Knauer, J.; Kocsis, G.; Kornejew, P.; Lang, P. T.; Langenberg, A.; Laqua, H. P.; Pablant, N.; Pasch, E.; Pedersen, T. S.; Ploeckl, B.; Schlisio, G.; Scott, E. R.; Stange, T.; Stechow, A. von; Szepesi, T.; Turkin, Y.; Wagner, F.; Winters, V.; Wurden, G. A.; Zhang, D.; Team, W X

doi:10.1088/1361-6587/ab8112

Cited by 23 publications

(34 citation statements)

References 37 publications

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“…Spectroscopic measurements of the pellet plasmoid in the Large Helical Device suggest that the plasmoid size in the direction perpendicular to the magnetic field reaches approximately 10 cm (Motojima et al 2010). A similar size is recorded in the W7-X experiments (Baldzuhn et al 2019). However, after the 3-D expansion phase, the plasmoid expands along the field lines to much greater length.…”

Section: General Considerationssupporting

confidence: 64%

“…In the W7-X, one cryogenic hydrogen pellet typically contains approximately N total = 10 20 electrons, and is injected at a speed of 100-300 m s −1 by a blower gun. The pellet penetrates approximately L p = 5-15 cm into the plasma (Baldzuhn et al 2019). At first, the ablated neutral material expands at the speed of sound in all three dimensions.…”

Section: General Considerationsmentioning

confidence: 99%

“…Because the stopping power of the hot ions on cold electrons is very high, the cold plasmoid is not transparent for the ambient ions before it is heated, while it is always transparent for the ambient electrons. In the high-performance pellet experiments in the W7-X, pellets are injected into a plasma with an electron temperature of 4 keV and an ion temperature of 2 keV (Bozhenkov et al 2020). Figure 1 shows attenuation s = L/2 −L/2 (1/λ) dx of a hot particle flux in the plasmoid of T cold temperature.…”

Section: General Considerationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

“…In the W7-X, pellets are used for plasma fuelling (Baldzuhn et al 2019). One of the most prominent features observed in pellet experiments in the W7-X is a significant enhancement of global energy confinement time (Baldzuhn et al 2020;Bozhenkov et al 2020). In these experiments, the ion temperature exceeds 3 keV with 5 MW of electron heating after injection of a series of cryogenic hydrogen pellets.…”

Section: Introductionmentioning

confidence: 99%

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Modelling of parallel dynamics of a pellet-produced plasmoid

et al. 2021

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The parallel expansion of a dense, pellet-produced plasmoid is modelled with parameters relevant to pellet fuelling experiments in the Wendelstein7-X stellarator. Good agreement is found between the analytical theory and more detailed modelling. In particular, much of the energy deposited in the pellet by the ambient plasma is transferred to the pellet ions by the ambipolar electric field during the expansion. The validity of the hydrodynamic treatment of the plasmoid and the ambient plasma is discussed.

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Section: General Considerationssupporting

confidence: 64%

Section: General Considerationsmentioning

confidence: 99%

Section: General Considerationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Modelling of parallel dynamics of a pellet-produced plasmoid

et al. 2021

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Electron kinetics in a high-Z plasmoid

2023

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The problem of the electron dynamics on a closed magnetic field line passing through a high- $Z$ plasmoid is considered. The electron kinetic equation is integrated over bounce motion and pitch angle, reducing the independent variables to a single adiabatic invariant plus time. Integration of the full Landau self-collision operator is carried out exactly, resulting in a nonlinear integro-differential operator in the new invariant. Conservation laws and the $H$ theorem of the integrated self-collision operator are proven. Numerical solutions of the integrated kinetic equation are obtained with a self-consistent quasineutral electric potential, given the initial condition of a cold plasmoid immersed in a hot ambient plasma. The fact that cold electrons are deeply trapped in a potential with a parabolic peak leads to exactly 3/4 the usual rate of collisional heating by the ambient plasma, independent of any other parameters.

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Parallel expansion of a fuel pellet plasmoid

Arnold,

Aleynikov,

Breizman

2024

J. Plasma Phys.

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The problem of the assimilation of a cryogenic fuel pellet injected into a hot plasma is considered. Due to the transparency to ambient particles of the plasmoid, the localised region of high-density plasma created by ionisation of the ablated pellet material, electrons reach a ‘quasiequilibrium’ (QE) state which is characterised by a steady-state on the fastest collisional time scale. The simplified electron kinetic equation of the QE state is solved. Taking a velocity moment of the higher-order electron kinetic equation, which is valid on the expansion time scale, permits a fluid closure, yielding an evolution equation for the macroscopic parameters describing the QE distribution function. In contrast to the Braginskii equations, the closure does not require that electrons have a short mean free path compared with the size of density perturbations, and permits an anisotropic and highly non-Maxwellian distribution function. As the QE distribution function accounts for both trapped and passing electrons, the self-consistent electric potential that causes the expansion can be properly described, in contrast to earlier models of pellet plasmoid expansion with an unbounded potential. The plasmoid expansion is simulated using both a Vlasov model and a cold-fluid model for the ions. During the expansion plasmoid ions and electrons obtain nearly equal amounts of energy; as hot ambient electrons provide this energy in the form of collisional heating of plasmoid electrons, the expansion of a pellet plasmoid is expected to be a potent mechanism for the transfer of energy from electrons to ions on a time scale shorter than that of ion–electron thermalisation.

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Enhanced energy confinement after series of pellets in Wendelstein 7-X

Cited by 23 publications

References 37 publications

Modelling of parallel dynamics of a pellet-produced plasmoid

Modelling of parallel dynamics of a pellet-produced plasmoid

Electron kinetics in a high-Z plasmoid

Parallel expansion of a fuel pellet plasmoid

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