Capabilities of nitrogen admixed cryogenic deuterium pellets

Operation at high core density with high energy confinement - as foreseen in a future fusion reactor like DEMO - is being investigated at ASDEX Upgrade tokamak. The efficiency of pellet fuelling from the high-field side usually increases with increasing injection speed. Due to the fragile nature of the deuterium ice, however, the increment of pellet mass losses and subsequent pellet fragmentations take place when the speed is increased. Studies show, that admixing of a small amount of nitrogen (N₂) into D₂ gas can be favorable for the mechanical stability of pellets. This might be helpful for deeper pellet penetration. Besides, seeding by N₂ can enhance plasma performance due to both increasing the energy confinement time and reducing the divertor heat load in the envisaged ELMy H-mode plasma scenario. Fuelling efficiency of N₂-admixed solid D₂ pellets and their nitrogen seeding capabilities were investigated. It was found that both the overall plasma density increase and the measured averaged pellet penetration depth were smaller in case of the admixed (1% mol. in the gas resulting in about 0.8% in the ice) pellet fuelling. Possibility of the N₂-seeding by admixed pellets was confirmed by CXRS measurements of N⁷⁺ content in plasma.