A stochastic magnetic boundary, produced by an applied edge resonant magnetic perturbation, is used to suppress most large edge-localized modes (ELMs) in high confinement (H-mode) plasmas. The ...resulting H mode displays rapid, small oscillations with a bursty character modulated by a coherent 130 Hz envelope. The H mode transport barrier and core confinement are unaffected by the stochastic boundary, despite a threefold drop in the toroidal rotation. These results demonstrate that stochastic boundaries are compatible with H modes and may be attractive for ELM control in next-step fusion tokamaks.
In reduced recycling discharges in the Large Helical Device, a super dense core plasma develops when a series of pellets are injected. A core region with density as high as 4.5 x 10(20) m(-3) and ...temperature of 0.85 keV is maintained by an internal diffusion barrier with very high-density gradient. These results may extrapolate to a scenario for fusion ignition at very high density and relatively low temperature in helical devices.
The Large Helical Device (LHD) is a heliotron-type device employing large-scale superconducting magnets to enable advanced studies of net-current-free plasmas. The major goal of the LHD experiment is ...to demonstrate the high performance of helical plasmas in a reactor-relevant plasma regime. Engineering achievements and operational experience greatly contribute to the technological basis for a fusion energy reactor. Thorough exploration for scientific and systematic understanding of the physics in the LHD is an important step to a helical fusion reactor. In the 12 years since the initial operation, the physics database as well as operational experience has been accumulated, and the advantages of stable and steady-state features have been demonstrated by the combination of advanced engineering and the intrinsic physical advantages of helical systems in the LHD. The cryogenic system has been operated for 56 000 h in total without any serious trouble and routinely provides a confining magnetic field up to 2.96 T in steady state. The heating capability to date is 23 MW of neutral beam injection, 3 MW of ion cyclotron resonance frequency, and 2.5 MW of electron cyclotron resonance heating. Highlighted physical achievements are high beta (5.1%), high density (1.2 × 10
21
m
−3
), and steady-state operation (3200 s with 490 kW).
Associated with the transition from ion root to electron root, an electron internal transport barrier (ITB) appears in the large helical device, when the heating power of electron cyclotron resonance ...heating exceeds the threshold power. The incremental thermal diffusivity of electron heat transport chi(inc)(e) in the ITB plasma is much lower than that in the plasma with the heating power below the threshold, and the thermal diffusivity chi(e) decreases with increasing of heating power dchi(e)/d(P/n(e))<0 in helical ITB plasmas.
Particle flux profiles on the divertor plates and the electron temperature profiles in the scrape-off layer (SOL) in the Large Helical Device (LHD) heliotron were investigated with EMC3-EIRENE code. ...These profiles are modified during a discharge due to the changes of the edge plasma density and temperature those can cause the change of transport coefficient. Comparison of the edge electron temperature profiles between the measurements and the simulations revealed that the cross field transport coefficients in the LHD scrape-off layer depend on plasma parameters, especially electron temperature. For the particle flux profile on the divertor plates, the absolute value of the simulation results with the transport coefficient consistent with the edge temperature profile analysis were well agree with the experimental data, though the profile shapes of experimental data were not necessarily reproduced well by the simulation.
Radial profiles of ion temperature and plasma flow are measured at the n/m = 1/1 magnetic island produced by external perturbation coils in the Large Helical Device. The sheared poloidal flows and ...sheared radial electric field are observed at the boundaries of the magnetic island, because the poloidal flow vanishes inside the static magnetic island. When the width of the magnetic island becomes large, the flow along the magnetic flux surface inside the magnetic island appears around the O point in the direction which reduces the shear of the poloidal flow at the boundary of the magnetic island.
PDF
