The ‘electron-root’ feature in the WENDELSTEIN-7-AS stellarator with ECRH in O1-mode compared to X2-mode

At the W7-AS stellarator, the neoclassical 'electron-root' was found in discharges with electron cyclotron resonance heating (ECRH) in both O1- and X2-mode. The electron heat diffusivity estimated from the power balance is compared with neoclassical predictions with the radial electric field, Er, from the ambipolarity condition taken into account. Strongly positive Er values significantly reducing the electron neoclassical transport coefficients are found in the central region with highly peaked Te-profiles. Furthermore, the 'electron-root' feature in the ECE time-traces is analysed in the phase when the ECRH is switched off. The power deposition in phase space is analysed with ray-tracing and bounce-averaged Fokker-Planck calculations. For the O1-mode, the power is almost completely absorbed by passing electrons. These findings are different from the previous X2 results (Maassberg et al 2000 Phys. Plasmas 7 295) for the magnetic configuration with a significant fraction of ripple-trapped electrons in the ECRH launching plane. For these X2 scenarios, fast `convective' contributions to the radial fluxes had been identified. For the O1-scenarios, a purely `diffusive' neoclassical modelling of the power threshold for obtaining the `electron-root' in the different magnetic configurations is described based on the concept of the `effective helical ripple'. Finally, a simulation of the ECRH switch-off by solving the time-dependent electron energy balance is performed. The results show that the `electron-root' feature at W7-AS is consistent with the neoclassical predictions.