Divertor dynamics of pellet-fuelled discharges in DIII-D

Edge plasma transport simulations and experiments in DIII-D are used to explore the divertor dynamics in pellet-fuelled discharges. Core-edge integration is a key challenge of future reactors, where a high-performance core plasma is required for maximizing fusion gain while simultaneously ensuring well-protected material surfaces. It is widely accepted that some degree of divertor detachment will be required for such protection. A high-performance core scenario typically necessitates a scrape-off layer (SOL) that is opaque to neutrals, requiring pellet injection for effective core fuelling¹. With edge density profiles that differ from gas fuelling, pellet fuelling alters cross-field transport into the SOL² and subsequently alters divertor conditions³, while the inherently transient nature of pellet fuelling threatens detachment stability².

DIII-D experiments have been performed with purely core fuelling, injecting up to 4.5x10²¹ atoms/s of D pellets into H-mode plasmas (B_t~2.1 T, I_p~1.3 MA, P_inj~6 MW, ∇B ion drift towards the X-point), with an increase in core Greenwald fraction from 0.56 to 0.71. While this density rise results in a 40% reduction in peak target heat flux, the plasma remains in the high recycling regime, with no clear observation of outer target ion flux rollover or CIII front movement off the plate. During upstream density relaxation that follows each pellet injection, the target ion flux transiently increases, accompanied by a transient reduction in peak heat flux.

Time-dependent SOLPS-ITER simulations are underway to study the role of the edge particle source distribution and transient fuelling on divertor conditions. Preliminary results in the high recycling regime indicate an increase in target ion flux following pellet injection, in agreement with experimental observations, with the target cooling to below the pre-pellet conditions. Future work looks to perform pellet-fuelled experiments under detached conditions and use interpretative simulations to study the detachment stability of pellet-fuelled discharges.

¹Kukushkin et al., Nucl. Fus. 43 (2003) 716

²Wiesen et al., Nucl. Fus. 57 (2017) 7

³Polevoi et al., Nucl. Fus. 58 (2018) 056020