Speaker
Description
\noindent The STEP program run by UK Fusion Energy Ltd (UKFE, UKAEA group) aims to deliver a tritium self sufficient prototype fusion power plant generating 100 MW net electric power based on the spherical tokamak concept. The compact spherical geometry of STEP's SPP-2 design raises significant exhaust challenges, with powers crossing the separatrix reaching $P_\text{sep} \approx110 - 140\,\text{MW}$, corresponding to $P_\text{sep}/R_0 \approx 25-32\,\text{MW/m}$. To overcome these challenges advanced solutions, such as a double null with an extended outer leg divertor, will be employed to achieve sufficient detachment and protect plasma-facing components [1]. While these measures are predicted to achieve acceptable steady-state power to the divertor targets, transient power loads from fluctuations in core fusion/radiation power or vertical displacement-induced disconnected double null effects may lead to burn-through of the detachment front, and unacceptable power loading to material surfaces.
High fidelity (SOLPS-ITER) simulations of transient burn-through remain prohibitively expensive to explore the wide parameter space of reactor scale devices. The main cause of this expense is the kinetic treatment of neutral species. To provide initial screening for STEP and comparison with analytical models [2], it is attractive to adopt the more computationally tractable method offered by reducing dimensionality to 1D and adopting a fluid treatment of the neutral species.
We use the multi-fidelity Hermes-3 fluid code to study power transient burn-through in STEP-relevant 1D exhaust scenarios, including a netural reservoir model based on the form implemented in DIV1D [3] to capture neutral cross field transport. Results show that while the inclusion of neutral reservoirs significantly improves steady state agreement to 2D SOLPS-ITER simulations, strong reservoir action can lead to non-physical to detachment front transient response. In addition to this, at the point of reattachment, strong recycling leads to extreme buffering of the 5eV temperature front, requiring strong pumping to resolve. Achieving physical results with 1D fluid neutrals therefore requires careful consideration of both reservoir implementation and target recycling. We assess both effects against available MAST-U transient SOLPS-ITER simulations.