Electron transport in tokamaks has many different features which are briefly reviewed. The paper is focused on electron heat transport in conventional tokamak plasmas. An inter-machine comparison indicates that the non-dimensional gradient length of the electron temperature profiles R/L Te is almost independent of the devices and varies little with plasma parameters. This strongly suggests that electron heat transport is governed by turbulence with a threshold in R/L Te . This is confirmed by modulation experiments using electron cyclotron heating. Simulations with empirical and physics-based transport models confirm this assumption.

Experimental studies of electron transport

ICRF long-pulse discharge and interaction with a chamber wall and antennas in LHD

The Large Helical Device is a heliotron device with L = 2 and M = 10 continuous helical coils with a major radius of 3.5–4.1 m, a minor radius of 0.6 m and a toroidal field of 0.5–3 T, which is a candidate among toroidal magnetic confinement systems for a steady state thermonuclear fusion reactor. There has been significant progress in extending the plasma operational regime in various plasma parameters by neutral beam injection with a power of 13 MW and electron cyclotron heating (ECH) with a power of 2 MW. The electron and ion temperatures have reached up to 10 keV in the collisionless regime, and the maximum electron density, the volume averaged beta value and stored energy are 2.4 × 1020 m−3, 4.1% and 1.3 MJ, respectively. In the last two years, intensive studies of the magnetohydrodynamics stability providing access to the high beta regime and of healing of the magnetic island in comparison with the neoclassical tearing mode in tokamaks have been conducted. Local island divertor experiments have also been performed to control the edge plasma aimed at confinement improvement. As for transport study, transient transport analysis was executed for a plasma with an internal transport barrier and a magnetic island. The high ion temperature plasma was obtained by adding impurities to the plasma to keep the power deposition to the ions reasonably high even at a very low density. By injecting 72 kW of ECH power, the plasma was sustained for 756 s without serious problems of impurities or recycling.

Overview of confinement and MHD stability in the Large Helical Device

An internal transport barrier (ITB) was observed in the electron temperature profile in the Large Helical Device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)] with a centrally focused intense electron cyclotron resonance microwave heating. Inside the ITB the core electron transport was improved, and a high electron temperature, exceeding 10 keV in a low density, was achieved in a collisionless regime. The formation of the electron-ITB is correlated with the neoclassical electron root with a strong radial electric field determined by the neoclassical ambipolar flux. The direction of the tangentially injected beam-driven current has an influence on the electron-ITB formation. For the counter-injected target plasma, a steeper temperature gradient, than that for the co-injected one, was observed. As for the ion temperature, high-power NBI (neutral beam injection) heating of 9 MW has realized a central ion temperature of 5 keV with neon injection. By introducing neon gas, the NBI absorption power was incr...

Formation of Electron Internal Transport Barrier and Achievement of High Ion Temperature in Large Helical Device

The local and global fluid magnetohydrodynamic stability properties of anisotropic pressure plasmas are investigated with the Kruskal-Oberman and rigid hot particle Johnson et al energy principles. A Heliotron configuration that models the Large Helical Device with finite pressure anisotropy driven by neutral beams at beta = 4% shows that the Kruskal-Oberman model predicts stability when beta(h)/beta similar to 1/3 provided that the hot particle pressure profile is sufficiently peaked. The rigid hot particle model, on the other hand, is stable to local and global modes for broad and peaked profiles. For central deposition, the marginal pressure profiles are somewhat broader for p(parallel to) > p(perpendicular to) than for p(perpendicular to)> p(parallel to). Global n = 3 modes produce stricter stability criteria than n = 1,2 and 4 modes. Off-axis hot particle deposition yields more unstable conditions with respect to global and local modes than on-axis deposition. The mode structures localize near the plasma periphery according to the Kruskal-Oberman model and near the plasma core according to the Johnson et al model. This observation could help resolve the appropriate model to apply to the experimental conditions in Heliotron devices.

Fluid magnetohydrodynamic stability in a Heliotron with anisotropic fast particle species

Recent progress in the Large Helical Device (LHD) experiment during the last 2 yr is reviewed The LHD has been extending its operational regime toward fusion-relevant conditions while taking advantage of the net-current-free heliotron concept employing a superconducting coil system Heating capability has exceeded 10 MW, and the central ion and electron temperatures have reached 7 and 10 keV, respectively The maximum values of beta and pulse length have been extended to 32% and 150 s, respectively Several encouraging physics observations have been obtained, ie, simultaneous achievement of the mitigation of the magnetohydrodynamic instability criteria and good confinement, and formation of an internal transport barrier The initial results have been obtained using a local island divertor, which shows the possibility of particle control at the plasma edge

Review on the Progress of the LHD Experiment

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Review on the Progress of the LHD Experiment