Speaker
Description
Particle recycling and wall conditions play a crucial role in plasma confinement and long-pulse operation in tokamaks, yet their quantitative impact has not been clearly established. In EAST superconducting tokamak with full metal wall, a systematic statistical analysis was performed to assess the dependence of plasma stored energy on density, heating power, wall conditions, and recycling.
The analysis, with all double-null divertor configuration, shows that the plasma stored energy follows a multi-parameter scaling of the form
$$ W_{\mathrm{MHD}} \;\propto\; n_e^{0.67}\, \mathrm{Li}^{0.11}\, P_{\mathrm{heat}}^{0.28}\, p_{div}^{-0.11}, $$ where $n_e$ is the line-averaged electron density, $P_{\mathrm{heat}}$ is the total auxiliary heating power, $\mathrm{Li}$ represents lithium emission intensity, and $p_{div}$ is the lower divertor neutral pressure characterizing recycling. This scaling demonstrates a clear and independent negative dependence of confinement on recycling, in contrast to the positive dependences on density, heating power, and lithium wall conditioning. The relatively weak power exponent indicates that, once particle-related effects are explicitly included, confinement degradation previously attributed to heating power is largely driven by recycling-related particle processes. Wall conditioning is found to be an effective means to regulate recycling and improve confinement. Compared with boron-coated conditions, lithium conditioning systematically reduces the recycling level and enhances effective wall pumping, leading to improved stored energy under otherwise similar operational parameters. These statistically identified confinement trends are directly demonstrated in the recent achievement of a **1066 s H-mode discharge** in EAST. Particle balance analysis during the stationary phase shows stable density control at approximately $3.0\times10^{19}\,\mathrm{m^{-3}}$, a nearly constant global recycling coefficient of $R_{\mathrm{global}}\approx0.95$, and a dominant contribution from wall pumping, accounting for about 70% of the total particle exhaust. The results confirm that controlled recycling and enhanced wall pumping are essential for maintaining both improved confinement and long-pulse H-mode operation.