Argon doped pellets for fast and efficient radiative power removal in ASDEX Upgrade

Argon doped deuterium pellets with about 1 per mille Ar atomic fraction have been injected into ASDEX Upgrade H-mode discharges and demonstrate fast and efficient power removal from the outer plasma region. Bolometer views directly on the pellet and in its very close vicinity observe a strong, non-toroidally symmetric radiation pattern on a fast, sub-ms time scale. This pattern is explained by the ionisation process of the delivered Ar neutrals, leading to non-coronal radiation by intermediate ionisation stages until the equilibrium He like stage is reached. Subsequently, a toroidally symmetric radiation increase is observed with a fast initial decay time of about 20 ms consistent with the radial particle transport time scale for a source located in the outer plasma. In the following, a radiation pattern related to Ar recycling develops comparable to the pattern observed with Ar midplane gas puffing which decays with the Ar divertor pumping time of about 200 ms. The energy radiated around each pellet is described by the radiative potential of Ar evaluated for non-coronal conditions with the ablation time as the relevant time scale. A factor 4 too low radiated energy is obtained by this simple model, which could be explained by stronger deviations from corona equilibrium and plasma parameter variations in the vicinity of the pellet. Injection of Ar-doped pellets into a no-ELM quasi-continuous exhaust H-mode discharge showed no triggering of ELMs by the pellet. Advantages of the pellet seeding scheme are the faster reaction time of the radiation compared to gas puffing, which may be important to counter-act transient re-attachment in ITER or in a reactor and an elevated radiative efficiency in the outer core plasma due to non-corona effects.