Integrated full pulse modeling for pellet injection in tokamaks: HPI2 model improvement and validation in WEST

TL;DR

This paper presents an improved, validated physics-based pellet injection model (HPI2) for tokamaks, enhancing predictive capabilities for core density control in fusion reactors like WEST and ITER.

Contribution

The upgraded HPI2 model self-consistently determines plasmoid release from ablation physics, improving robustness and accuracy in integrated tokamak simulations.

Findings

HPI2 model validated against WEST discharge with ~10% error.

Coupled simulations reproduce main density and temperature transients post-injection.

Supports use of pellet modeling in integrated simulations for ITER.

Abstract

Reliable modeling and control of core density is essential for reactor-relevant magnetic confinement fusion operation, motivating cryogenic pellet injection as a primary fueling actuator and the need for predictive pellet source models in integrated modeling. Here we present an upgrade of the physics-based pellet code HPI2 in which the plasmoid release spatial step is determined self-consistently from ablation physics, $dx_{var}=v_{\mathrm{pel}}\,t_{\mathrm{exit}}$ (optionally rescaled to trade accuracy for computational cost), removing an ad-hoc discretization parameter and improving numerical robustness across injection conditions. The upgraded model is first validated in stand-alone against a high-field-side pellet-fueled, ohmic, WEST discharge (#58656) by comparing synthetic and measured interferometry line-integrated density increments, obtaining a mean error of $\sim 10\%$. We then perform full-radius, time-dependent integrated modeling validation by coupling the new HPI2 within the High Fidelity Pulse Simulator (HFPS) workflow (JINTRAC/IMAS), combining JETTO with SANCO for the impurity/radiation evolution and TGLF-SAT2 for the turbulent transport. The coupled simulations reproduce the main density rise and relaxation after pellet injection and the associated electron-temperature transient, while taking into account the strong influence of tungsten radiation in WEST, supporting the consistency of HPI2 as a predictive pellet particle source in integrated modeling frameworks. Ultimately, this validation study supports the use of pellet modeling tools in integrated modeling studies for larger devices such as ITER.