Speaker
Description
Recent experiments on the WEST tokamak show that, with sufficient nitrogen seeding, a stable X-point radiator (XPR) can be sustained above the X-point for about 70s. This regime helps to reduce divertor heat loads by about 90% and tungsten divertor sources by up to 98% while enhancing core energy confinement by about 25%, highlighting the XPR’s potential for future fusion power plants.
For the first time, time-dependent simulations in SOLEDGE3X-EIRENE, including drifts, have reproduced experimentally observed millisecond-scale sequence of rapid divertor transitions from attached to XPR, with microsecond resolution, and achieved a stable XPR after the transition. This represents a major step beyond previous steady-state or drift-free models and demonstrates that self-consistent boundary plasma modeling can be a powerful tool for predictive control of XPR regimes in current and future fusion devices.
The simulations reveal that just before the onset of the XPR regime, a coherent X-point-centered vortex structure forms, connecting the common SOL, PFR, and closed-flux regions. This vortex is triggered by a transient electric potential well arising from steep temperature gradients in the LFS common SOL and is shaped by anomalous diffusion and drifts. It drives cross-field transport that rapidly increases the nitrogen concentration above the X-point, mixes cold and hot particles, steepens temperature gradients, and deepens the potential well (to –31 V), sustaining a self-amplifying cycle for several milliseconds until saturation and gradual decay.
The vortex plays a critical role in accessing a stable XPR: by advecting cold particles and providing ~91% of the total nitrogen source (~$1.1\times 10^{20}\rm{s}^{-1}$) into XPR region before XPR onset, it raises the neutral density and impurity concentration without causing edge over-cooling, thereby preventing disruptive MARFEs and bridging the hot and cold solutions. The vortex also balances steep impurity gradients (2–3% in the HFS common SOL/PFR vs. 0.2–0.5% in the LFS common SOL/closed-flux region), allowing the closed-flux nitrogen concentration to exceed the ~0.66% threshold required for XPR sustainment.
This vortex mechanism explains the clear hysteresis observed during XPR access. By effectively “locking in” a high impurity concentration, the resulting plasma — which subsequently evolves into a deeper, colder, and denser MARFE — requires the nitrogen seeding rate to be reduced by approximately 50% below the access rate before the plasma reverts to the attached state. Simulated radiation maps show good qualitative agreement with experimental fast-camera 2D reconstructions, indicating vortex-driven impurity transport as the underlying physical mechanism enabling stable XPR access.