Bifurcation of radial electric field in the low-aspect-ratio heliotron/torsatron CHS

There are generally two stable states in the stellarator plasmas which are called the ion and the electron roots. In a stellarator reactor, it is an important scenario to attain the electron root with higher energy confinement time through heating electrons in the startup phase. In Heliotron-E device, the radial electric field at r ≅ 0.7-0.9a is found to be positive (the electron root) for the low density plasma (ne<1x1013 cm-3), and negative (the ion root) for the high density plasma (ne>2x1013 cm-3). In Wendelstein VII-A stellarator, the observed electric field in the plasma with electron cyclotron heating (ECH) (ne ∼ 5x1013 cm-3) is consistent with a theoretical prediction. In CHS, the observed radial electric field is negative in the typical neutral beam (NB) heated plasmas. The electric field becomes more negative near the plasma edge for the higher electron density. It is generally observed that ECH has an effect of density pump-out both in tokamaks and stellarators. In CHS, it is observed that the particle confinement becomes worse in the plasma with second harmonic ECH at low field side resonance than at high field side one. One of the candidates for the mechanism of the density pump-out is the outward flux due to the poor confinement of perpendicularly accelerated electrons by ECH. In this paper, we present the bifurcation of the radial electric field from negative to positive value triggered by enhancing the electron particle flux with ECH. (author) 12 refs., 3 figs