Pellet fueling experiments in Wendelstein 7-X

Abstract

During the two most recent experimental campaigns in the advanced stellarator Wendelstein 7-X (W7-X) (Klinger et al 2017 Plasma Phys. Control. Fusion 59 014018; Bosch et al 2017 Nucl. Fusion 57 116015; Wolf et al 2017 Nucl. Fusion 57 102020; Pedersen et al 2017 Phys. Plasmas 24 0555030) hydrogen ice pellet injection was performed for the first time. In order to investigate the potential of pellet fueling in W7-X and to study the particle deposition in a large stellarator, a blower-gun system was installed with 40 pellets capability. The experience gained with this system will be used for the specification of a future steady-state pellet injector system. One important motivation for a pellet injector (Dibon 2014 Master-Thesis Technical University Munich, Max-Planck Institut IPP) on W7-X is the mitigation of hollow density profiles expected in case of predominant neoclassical transport. For long-pulse operation of up to 30 min, only electron cyclotron resonance heating is available on W7-X. Hence, pellet injection will be the only source for deep particle fueling. Deep particle fueling by pellets in tokamaks is supported by a grad-B drift, if the pellets are injected from the magnetic high-field-side. This approach was tested in W7-X, as well. The injection of series of pellets was also tested. Here, deep fueling is supported for later pellets in the series by the plasma cooling following the initial pellets in the same series. As in earlier experiments in the heliotron LHD (Takeiri et al 2017 Nucl. Fusion 57 102023), deep and rapid fueling could be achieved successfully in W7-X.

Modelling of parallel dynamics of a pellet-produced plasmoid

“…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.…”

“…The pellet penetrates approximately into the plasma (Baldzuhn et al. 2019). At first, the ablated neutral material expands at the speed of sound in all three dimensions.…”

“…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.…”

Demonstration of reduced neoclassical energy transport in Wendelstein 7-X

“…In this regard, a substantial transient improvement of the energy confinement has been observed in certain W7-X experiments fuelled with hydrogen pellets 26,27 ; the discharge 20180918.045, performed in the W7-X standard configuration, provides such an example. Time traces of interest for this experiment are provided in Extended Data Fig.…”

Turbulence Mechanisms of Enhanced Performance Stellarator Plasmas

“…The recent campaign OP1.2 of the W7-X stellarator experiment has revealed a broad class of improved confinement discharges, for which the heat transport across some flux surfaces is largely neoclassical. Such discharges involve transient phases characterized by enhanced performance, following shortly after the injection of cryogenic hydrogen pellets into an electron cyclotron resonance (ECR)-heated plasma [5]. The energy confinement during these phases is notably better than that in gas fueled discharges at similar density and heating power, and exceeds the empirical stellarator scaling ISS04 [6], leading also to a high triple product [7].…”