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

Abstract

Recent large helical device experiments revealed that the transition from ion root to electron root occurred for the first time in neutral-beam-heated discharges, where no nonthermal electrons exist. The measured values of the radial electric field were found to be in qualitative agreement with those estimated by neoclassical theory. A clear reduction of ion thermal diffusivity was observed after the mode transition from ion root to electron root as predicted by neoclassical theory when the neoclassical ion loss is more dominant than the anomalous ion loss. Neoclassical ion transport is important in stellarator plasmas, because the helical ripple losses are comparable to or sometimes even higher than the anomalous losses in contrast to those in tokamaks. The crucial issues of neoclassical ion transport are (1) the reduction of ion thermal diffusivity due to a large positive radial electric field in the electron root [1][2][3], and (2) the reduction of ion thermal diffusivity due to the optimization of the magnetic field structure (s optimization) [3][4][5][6]. 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 these experiments, the significant increase of electron temperature and a clear reduction of electron thermal diffusivity were observed in the plasma core in the electron root. However, no reduction of ion thermal diffusivity was observed because of the lack of direct ion heating. There have been no experimental results to show the improvement of ion transport in the electron root, although a significant improvement of ion transport (rather than the electron transport) is predicted by the neoclassical theory [6]. This paper describes the experimental results of the neoclassical feature of ion transport for the first time, the reduction of ion thermal diffusivity due to the transition to the large positive electric field (electron root), and/or the optimization of the magnetic field structure (s optimization).The large helical device (LHD) [12] is a Heliotron device (poloidal period number L 2, and toroidal period number M 10) with a major radius of R ax 3.5 4.1 m, an average minor radius of 0.6 m, and magnetic field up to 3 T. The radial electric field ͑E r ͒ is derived from the poloidal and toroidal rotation velocity and pressure gradient of neon impurity measured with charge exchange spectroscopy [13] at the midplane in LHD (vertically elongated cross section) using a radial force balance. The radial force balance equation can be expressed as E r ͑en I Z I ͒ 21 ͑≠p I ͞≠r͒ 2 ͑y u B f 2 y f B u ͒, where B f and B u are toroidal a...

Observation of Plasma Flow at the Magnetic Island in the Large Helical Device

“…0 and dE r ͞dr . 0͒ [18]. Since the flow along the magnetic flux surface inside the magnetic island is driven by the imbalance of viscous forces between the inside and outside of the magnetic island, its direction is considered to be determined by the sign of the velocity shear.…”

Implementation of Neoclassical Effects in Momentum Transport Analysis at LHD

“…Therefore the module registers both the electron and ion roots of the neoclassical electric field and is zero if no corresponding root is found in the original output. There exists a transition from the small negative electron root to the positive ion root when the density (and consequently plasma collisionality) decreases beyond a certain critical value [18]. However, this was not implemented into the module and the physical root is ought to be selected manually.…”

Ion kinetic transport in TJ-II

“…In stellarator devices, transitions to improved core electron-root confinement (CERC) are established in conditions of high Electron Cyclotron Heating (ECH) absorbed power density (see the review [14] and references therein). These regimes have been found in different hehcal devices: the Compact Hehcal System [15], Wendelstein 7-AS [16], the Large Hehcal Device [17] andTJ-II [18] (in the latter, provokedby the introduction of a rational value of rotational transform [19]). These transitions are characterized by the appearance of a peaked electron temperature profile and a large positive electric field in the core plasma region.…”