Pellet ELM triggering with low collisionality, peeling-limited pedestals in DIII-D*

ELMs triggered using large deuterium pellets injected from the low-field side of the DIII-D tokamak into ITER-relevant low collisionality plasmas have a peak ELM energy fluence that is reduced by as much as 50% relative to a natural ELM, as measured by infrared camera. ELM energy deposition is reduced with respect to naturally occurring ELMs at the inner strike point (ISP), where it is naturally largest, while energy deposition is less affected at the outer strike point (OSP). The reduction of peak ELM energy fluence to the divertor may enable a reduction in the required pellet pacing frequency and an increase in the peak and time-averaged pedestal pressure during pellet ELM pacing in ITER and next-step devices with low collisionality pedestals. D-alpha fast camera images show that the ISP ELM particle flux is reduced in the pellet-triggered case as well, suggesting that the reduction in heat flux is not simply due to pellet mass being carried by divertor drifts to the inner leg and protecting the ISP by enhanced ionization / radiation. ELITE calculations show that without pellet injection, these low collisionality pedestals were limited by current density (peeling-limited) rather than pressure gradient (ballooning-limited), which is the condition expected in ITER and any other next-step tokamak with high edge temperatures and large bootstrap currents. These are novel conditions for pellet ELM triggering experiments in DIII-D, as it has been historically difficult to achieve low collisionality while injecting pellets that raise the density. In many cases here, a large pellet (1.8 mm) triggered an ELM shortly after a smaller pellet (1.3 mm) failed to do so, suggesting that a larger pressure perturbation is required to trigger an ELM in low collisionality discharges that are far from the ballooning stability boundary.