Description
In Wendelstein 7-X (W7-X) several plasma operation scenarios have been developed for high-β operation, e.g. (i) cryogenic pellet fueling in electron cyclotron resonance (ECR) heated plasmas, (ii) cryogenic pellet fueling in ECR and neutral beam injection (NBI) heated plasmas, (iii) ECR heating during NBI heated plasmas, and (iv) by method (iii) using magnetic field configurations with a reduced magnetic field strength. Along these operational approaches, volume-averaged β values in the range of 1-3 % could be achieved. This study investigates the dependence of β and the stability of the plasma on key operational parameters. The magnetic field configuration is one critical setting, affecting stability and confinement. The heating scenario — particularly the power ratio and timing between ECR heating and NBI heating — strongly influences β performance. Additionally, the details of the cryogenic pellet injection settings and the settings of control coils – which further shape the magnetic topology – plays a non-negligible role. Operational windows are discussed, regarding the presence or absence of MHD instabilities and plasma turbulence, both impacting the characteristics of the high-β phase.