Pellet-Size Scaling of Quasi-Steady-State Plasma Performance in Wendelstein 7-X

Abstract

A continuous cryogenic-pellet injector has been used to realize long-pulse high-performance plasmas in Wendelstein 7-X (W7-X). A hydrogen ice pellet deposits particles directly in the confined region. If the particles are deposited sufficiently far inside the plasma, they produce a density gradient that suppresses ion-temperature-gradient turbulence and improves plasma confinement. However, after multiple pellets have been injected and the plasma density and temperature have increased, the plasma performance begins to saturate. In this work, we analyze multiple pellet-injection experiments conducted in 2024 and 2025, during which the injected pellet sizes varied unintentionally. This analysis reveals a positive correlation between pellet size and the quasi-steady-state stored energy of W7-X plasmas. Although this pellet-size dependence can be understood qualitatively from pellet-ablation physics, the measured deposition position differs quantitatively from the neutral-gas-shielding (NGS) model prediction. This discrepancy suggests significant inward transport of the pellet cloud. The trend identified here suggests that injection of even larger pellets could further improve plasma performance.