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The transition of a radial electric field from a negative to a positive value is observed in the compact helical system when the electron loss is sufficiently enhanced by the superposition of the off-axis second harmonic electron cyclotron heating on the neutral beam heated plasmas. Existence of the threshold for the enhanced particle flux required to cause the transition is experimentally certified. The observed threshold is compared with a theoretical prediction. PACS numbers: 52.55.Hc, 52.50.Gj A radial electric field near the plasma periphery has been found to play an important role in the improved confinement such as in the //-mode plasmas [1][2][3]. Theoretical models of the L/H transition in tokamaks have been proposed. It is predicted that the change in the radial electric field or in the plasma rotation has a strong influence on the transition [4][5][6][7][8]. In stellarator devices, the neoclassical theory suggests that the electric field reduces the helical ripple loss, and consequently improves the plasma confinement [9][10][11]. Multiple solutions of the electric field that satisfy the ambipolar constraint often arise when particle fluxes have a nonlinear dependence on the electric field. There are generally two stable states in the stellarator plasmas which are called the ion and the electron roots [11]. In a stellarator reactor, it is an important scenario to attain the electron root with higher energy confinement time through heating electrons in the start-up phase [9]. In a 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 (n e < lxlO 13 cm -3 ) and negative (the ion root) for the high density plasma (n e >2x\0 u cm " 3 ) [12]. In the Wendelstein VII-A stellarator, the observed electric field in the plasma with electron cyclotron heating (ECH) (n e~~5 xl0 13 cm -3 ) is consistent with a theoretical prediction [13]. In the advanced toroidal facility, the positive electric field is observed for the low density plasma with ECH (n e -5 xlO 12 cm -3 ) [14]. In the compact helical system (CHS) [15], the observed radial electric field is negative in the typical neutral beam (NB) heated plasmas [16]. 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 [17] and in stellarators [18,19]. 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 resonance [20]. One of the candidates to explain the mechanism of the density pump-out is the outward flux due to the poor confinement of perpendicularly accelerated electrons by ECH [21]. In this Letter, we present the transition of the radial electric field from the ion root to the electron root triggered by enhancing the electron particle flux with ECH.CHS is a heliotron/torsatron device with a pole number of /=2, a toroidal period number m=8, and an aspect r...

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Electron cyclotron heating that generates hot-electron plasma in a magnetic mirror trap by microwaves is studied experimentally. The evolution of the energy distribution functions for the high-energy electrons is observed in steps of milliseconds during 200 ms of the heating period from the initial stage at the microwave power input until the stationary final state. According to the proposed statistical model for the cyclotron heating, heating rates are estimated to be 10 MeV/s typically, in the three characteristic cases of mirror field configuration with heating microwave power as a parameter. Some problems associated with stochastic cyclotron heating are discussed in the light of the experiments.