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, E r , from the ambipolarity condition taken into account. Strongly positive E r values significantly reducing the electron neoclassical transport coefficients are found in the central region with highly peaked T e -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.