High Ion Temperature Mode in Heliotron-E

Abstract

A high ion temperature T i mode is observed for neutral beam heated plasmas in Heliotron-E. This high T i mode plasma is characterized by a peaked ion temperature profile and is associated with a peaked electron density profile produced by neutral beam fueling with low wall recycling. The observed improvement in ion heat transport can be related to the radial electric field shear rather than to the rotation velocity shear in the bulk plasma.PACS numbers: 52.55. Hc, 52.50.Gj The high ion temperature enhanced confinement regime has been observed for super shots in TFTR [1], hot ion modes in JET and in 3], and counter-NBI modes in ASDEX [4] and in JFT-2M [5]. These enhanced confinement regimes observed in many tokamaks are correlated with density peaking. However, no observation of the high ion temperature enhanced confinement regime has been reported in heliotron/torsatron or stellarator devices. Theoretically, radial electric field shear or plasma rotation shear is expected to improve confinement by suppressing microturbulence in the plasma. Then weaker particle pinch effects and a smaller improvement of energy confinement are predicted in heliotron/torsatron or stellarator devices than those in tokamaks because of strong damping of the plasma rotation due to helical ripple (only radial electric field shear due to plasma rotation shear is considered in this model) [6]. Experimentally, both bulk poloidal rotation and radial electric field were directly measured in an H-mode plasma in DIII-D [7]. These measurements clearly show that the bulk plasma is somewhat unrelated to the radial electric field due to a large diamagnetic drift (y u is in the ion diamagnetic direction which is not expected from E 3 B drift because E r is negative). However, which of these, the electric field shear or the velocity shear, suppresses the turbulence has not yet been clarified because both radial electric field and velocity shear unfortunately exist in the same region in the plasma in H mode but not in L mode. Therefore it is an important issue to investigate whether density peaking and improved confinement are realized and whether a radial electric field or plasma rotation shear can reduce heat transport in heliotron/torsatron and stellarator devices. A preliminary result of the high ion temperature mode in Heliotron-E has been reported [8]. In this Letter we present the observation of high ion temperatures and an enhanced confinement regime in the Heliotron-E device and discuss the mechanism of enhanced confinement.The Heliotron-E is an axially asymmetric heliotron/torsatron with poloidal period number l 2, toroidal period number m 19, major radius R 2.2 m, magnetic field B 1.9 T, and averaged minor radius a 21.4 cm [9]. Heliotron-E has several nearly perpendicular neutral beams with 0 ± (five ion sources, 1.5 MW), 11 ± (three ion sources, 1

Initial physics achievements of large helical device experiments

“…In this case, the pellet injection does not give rise to a peaked density profile although the trigger of inward pinch has been observed in other devices. 6 The decay time of the density after the pellet injection is the same as that before the injection, which may be consistent with the absence of an inward pinch. A full solid pellet penetrates to the center and forms a peaked density profile.…”

“…Clear evidence of further confinement improvement has also been observed. [4][5][6] However, the enhancement factor of confinement has been smaller than in large tokamaks. The reasons can be attributed to connection of the boundary where neutrals and atomic processes play an essential role and the core where high-temperature plasmas are contained.…”

Characteristics of transport in electron internal transport barriers and in the vicinity of rational surfaces in the Large Helical Device

“…In contrast to the electron thermal transport barriers in tokamak plasmas, in a stellarator where the magnetic shear is negative, the electron internal transport barrier ͑ITB͒ has been observed associated with the transition from ion root ͑large neoclassical flux with a small E r ) to the electron root ͑small neoclassical flux with a large positive E r ), when the collisionality becomes low enough for the transition. [9][10][11][12] Although the mechanism of ITB formation associated with the transition from ion root to electron root has been studied, 10,11 the quantitative study of the incremental electron thermal diffusivity, e inc ͓ϭd(Q/n e )/d(ٌT e )͔ has been scarce in ITB plasmas in helical devices in spite of its importance in understanding transport in toroidal devices. 13 In the L-mode plasma, an instability, such as the electron temperature gradient mode ͑ETG͒, 14,15 often results in the sharp increase of the thermal diffusivity above the critical electron temperature gradients and determines the upper limit of the electron temperature for the available heating power.…”

Measurements of poloidal rotation velocity using charge exchange spectroscopy in a large helical device

“…13 Therefore, the radial electric field is mainly determined by the poloidal rotation. [14][15][16] The poloidal rotation (v ) as well as ion temperature (T i ) of fully ionized carbon are measured with CXS using heating neutral beam injection ͑NBI͒ in a large helical device ͑LHD͒. Because of the negative ion source of the NBI, the operating energy range of heating neutral beam is relatively high (E NBI ϭ100-180 keV/amu͒.…”