Experimental and theoretical characterisation of He plasmas confinement in view of the ITER pre-fusion power operation phase

Helium plasmas are foreseen for the non--nuclear phase of ITER operation, in particular due to L-H power thresholds significantly lower than those in H. On the other hand, they are regularly observed to have lower confinement compared to deuterium plasmas. For the first time, in the ASDEX Upgrade tokamak the confinement of helium plasmas is experimentally demonstrated to increase with increasing fraction of electron heating, reaching values comparable to those of the D plasmas. These observations have been identified to be a combination of core and edge effects. Nonlinear electromagnetic gyrokinetic simulations show that the different impact of zonal flows in regulating the core turbulence in the limit of low electron heating in D and He plasmas breaks the gyro-Bohm scaling of transport, leading to higher levels of transport in He. Additionally, the thermal coupling between electrons and ions and stronger destabilization of electron temperature gradient modes lead to reduced confinement at the edge of He plasmas. This results in that regimes with large fraction of electron heating and low collisionality, as expected in the initial ITER pre-fusion power phase of operation, are found to be beneficial in terms of He plasma confinement.