The impact of impurities and fast particles on STEP confinement*

Multiple flux-tube gyrokinetic codes - GS2, CGYRO, GENE and GKW - applied to a high β~9% STEP flat top operating point reveal the presence of a dominant hybrid-KBM (coupled to a TEM) and a subdominant MTM. In these regimes, the inclusion of δB_∥ is critical to recovering the dominant instability [1].  Nonlinear flux-tube gyrokinetic simulations including only thermal components of the plasma, and neglecting equilibrium  flow-shear, indicate that the transport fluxes may be very large, but including equilibrium flow shear (at the level of the diamagnetic flow), reduces the heat flux considerably to a value that is more compatible with the assumed sources [2]. While this is encouraging, there is a large uncertainty on the flow shear value:  to bound this more accurately, it is important to include the missing physics.

It is seen that additional stabilization of hybrid KBMs is achieved, with a corresponding reduction in turbulent fluxes, when a self-consistent equilibrium that includes the fast-alpha pressure is considered. The inclusion of kinetic impurities (He, Ar, Xe) to the main ions has a further stabilizing impact. Finally, detailed studies of multiple STEP operating points (e.g. electron cyclotron vs electron Bernstein wave heated) which include the kinetic fast-alphas are underway, and we will report on these findings at the meeting.

[1] D. Kennedy et al, submitted to Nucl. Fusion

[2] M. Giacomin et al, submitted to Nucl. Fusion