Speaker
Description
Divertor detachment is mandatory for fusion reactors and must be reconciled with good confinement. While the H-mode remains attractive for its high confinement, heat and particle loads on plasma facing components must be mitigated, which result from ELM bursts and the narrow inter-ELM SOL width.
We present novel GRILLIX simulations of the Quasi-Continuous Exhaust (QCE) and detached X-Point Radiator (XPR) regimes, which reproduce and explain experimental measurements. Furthermore, dynamic regime transitions from L- to H-mode, EDA to QCE, and attached to detached divertor conditions are demonstrated.
In QCE, we find that the Quasi-Coherent Mode (QCM) which replaces ELMs is composed of a narrow kinetic ballooning mode (KBM) spectrum, destabilized linearly by high plasma shaping, and non-linearly by a reduction of the E×B shear by the Maxwell stress. At low density, this mode is coherent, and the SOL width remains narrow, consistent with EDA conditions. At high plasma density, additionally, the resistive X-point mode is destabilized and couples to the KBM. This results in decoherence of the QCM and production of plasma blobs in the near-SOL. The blobs increase SOL transport, increasing the heat channel width $λ_q$ by a factor 3.
In XPR conditions, we find fundamentally different mechanisms. The combination of turbulence and radiative condensation results in large fluctuations in the X-point region of up to 500% amplitude, as the plasma oscillates between ionizing and recombining conditions, and the spots move around. This has far reaching consequences for both detachment and transport. On one hand, the large and strongly anti-correlated density and temperature fluctuations locally reduce the ionization rate by a factor 2, and increase recombination by more than a factor 4, facilitating detachment. This is missed by mean-field simulations, e.g. with the SOLPS code, which evaluate reaction rates only from mean density and temperature. On the other hand, the radial transport coefficients increase by an order of magnitude, additionally to the formation a convective cell. This could explain the reduction of the pedestal by the XPR, and avoidance of ELMs.
Explaining these regimes and the dependence of their access on magnetic geometry and plasma composition bears promise for the design of fusion reactors.