Plasma profile development with pellet injection on Wendelstein 7-X

The ability of a magnetically-confined fusion plasma to confine energy and reach ignition conditions depends substantially on the shapes of the plasma profiles. This dependence arises from the influence of spatial gradients in temperature and density on energy and particle loss mechanisms such as turbulent transport. Hence, the ability to develop and sustain plasma scenarios with profiles favorable to good confinement is likely to be essentially to the success of tokamaks and stellarators alike. Wendelstein 7-X (W7-X), for example, is known to achieve substantially better confinement and triple products when its density profiles are peaked. One key way to achieve such profiles is from core fueling via pellet injection. In this talk, we present the results of experiments on W7-X that utilized its new Continuous Pellet Fueling System to obtain plasma profiles amenable to high performance. One especially promising method for scenario development entails initializing a pellet train in a low-power ECRH heated discharge and steadily increasing the heating power during the first few pellets. We also briefly discuss preparations of real-time diagnostics to enable control and sustainment of the profiles in future experiment campaigns.