Pellet fuelling model validation cuts risk for reactor-scale density control

Every density spike this pellet fuelling model predicts happens somewhere inside this chamber, within milliseconds of a pellet crossing the plasma edge

(Image courtesy of CEA/IRFM)

Researchers from DIFFER, the ITER Organization and CEA Cadarache have validated an upgraded WEST pellet fuelling model against real tokamak data, matching its predictions to measurements within roughly 10 percent and then extending the test into a full integrated simulation. For engineers designing pellet-based fuelling systems, the result offers a tested reference point for scaling density control from present machines toward reactor-class devices

Pellet fuelling model removes a guesswork parameter

The update centres on HPI2, a physics-based code used across the fusion sector to predict how cryogenic pellets ablate and deposit fuel inside magnetically confined plasma. In earlier versions, the spatial step used to track a pellet’s progress through the plasma was fixed by hand, a value the user chose rather than one derived from the physics itself. The revised model ties that step to the pellet’s own exit time from each ablation cycle, removing the manual parameter and, the authors report, improving numerical robustness across a wider range of injection conditions.

WEST pellet fuelling model matches discharge data within 10 percent

The researchers first tested the upgraded model in isolation against WEST discharge #58656, an ohmic plasma pulse with a toroidal field of 3.76 tesla, in which the team fired three deuterium pellets from the tokamak’s upper high-field-side injector. Matching the model’s predicted density changes against the tokamak’s interferometer measurements produced a mean error of around 10 percent across the three pellets.

No dedicated pellet diagnostics existed for this discharge, so the team worked from indirect evidence instead. They used interferometry to constrain pellet mass from the measured density rise, then estimated pellet velocity through a separate scan that weighed plausible penetration depths against observed ablation durations. The paper treats this as a transparent source of added uncertainty rather than a settled figure.

Coupled simulation captures density, temperature and radiation together

Beyond the stand-alone test, the researchers coupled the updated model to WEST’s wider plasma behaviour through the High Fidelity Pulse Simulator framework, linking it to codes that track turbulent transport and impurity radiation. WEST operates with a tungsten first wall, and radiated power from tungsten impurities strongly shapes core temperature, so the coupled simulation had to reproduce that interaction alongside the pellet response itself. The combined model matched measured electron temperature within around 13 percent and loop voltage within 12 percent, while radiated power proved harder to capture precisely, landing at roughly 22 percent error.

Radiation from this tungsten hardware, not the pellet source itself, set the limit on how precisely the model could reproduce WEST’s temperature evolution

(Image courtesy of CEA)

The team also identified a short density plateau appearing after the second and third pellets, lasting roughly 300 milliseconds, which the model could only reproduce by adding an extra edge fuelling source rather than by adjusting the pellet code itself. That distinction matters for engineering purposes, since the authors interpret it as pointing to edge recycling conditions, not the pellet deposition model, as the likely cause.

The road to ITER-scale validation remains open

This validation covers WEST alone, an ohmic pulse without auxiliary heating. Extending the same confidence to ITER and other larger devices remains future work, not something this study has already demonstrated. The authors point to testing against low-field-side pellet injection, extending validation to plasmas with auxiliary heating, and improving pellet-velocity measurements rather than inferring them.

For an industry weighing the industrial scaling of pellet-based fuelling toward ITER-class machines, a validated baseline paired with a clearly scoped set of open questions is arguably more useful than a premature claim of readiness.

Stay ahead in the fusion revolution explore more breakthroughs from leading innovators in clean energy technology.