Transition of the radial electric field by electron cyclotron heating in the CHS heliotron/torsatron

Abstract

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...

The history of research into improved confinement regimes

“…In stellarators with 3-dimensional flux surface geometry, collisional transport depends explicitly on the radial electric field which acts as drive for the radial fluxes and determines the transport coefficients. Therefore, the role of the electric field has always been of interest in stellarator research and stellarator theory has demonstrated the existence of branches with different ambipolarity conditions -the electron and the ion root [50] before CHS [51] and W7-AS [52] could verify this in the experiment.…”

Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

“…40 On the other hand, it was experimentally observed that a transition was forced to occur when ECR-heating of 140 kW was applied at the plasma periphery. 24 The result was explained by electron loss cone current enhanced by the ECR-heating.…”

“…23 After the charge exchange recombination spectroscopy ͑CXRS͒ was used to measure the internal radial electric field, 24 advanced measurements were performed using a heavy ion beam probe ͑HIBP͒. 25,26 The high spatio-temporal resolution of the HIBP was sufficient to reveal the bifurcation nature of the radial electric field.…”

Reduction of Ion Thermal Diffusivity Associated with the Transition of the Radial Electric Field in Neutral-Beam-Heated Plasmas in the Large Helical Device

“…However, there has been no experimental study to test these issues on the neoclassical ion transport in stellarator plasmas. This is because the transition of the radial electric field from small negative (ion root) to large positive (electron root) was observed only in plasmas with the assistance of electron cyclotron heating (ECH), where electron heating is dominant [7][8][9][10][11]. The ion temperature is much lower than the electron temperature because ions are heated only by the energy exchange between ions and electrons.…”

“…In previous experiments [7][8][9][10][11], the transition from ion root to electron root was observed only in plasmas with ECH, therefore it has been also open to question as to whether the nonthermal electrons driven by ECH are required to achieve the transition. The transition of radial electric field from ion root to electron root clearly demonstrates that nonthermal electrons are not necessary for the transition, because there are no nonthermal electrons in NBI heated plasma.…”