No abstract

As the finalization of the hydrogen experiment towards the deuterium phase, the exploration of the best performance of the hydrogen plasma was intensively performed in the Large Helical Device (LHD). High ion and electron temperatures, Ti, Te, of more than 6 keV were simultaneously achieved by superimposing the high power electron cyclotron resonance heating (ECH) on the neutral beam injection (NBI) heated plasma. Although flattening of the ion temperature profile in the core region was observed during the discharges, one could avoid the degradation by increasing the electron density. Another key parameter to present plasma performance is an averaged beta value . The high regime around 4 % was extended to an order of magnitude lower than the earlier collisional regime. Impurity behaviour in hydrogen discharges with NBI heating was also classified with the wide range of edge plasma parameters. Existence of no impurity accumulation regime where the high performance plasma is maintained with high power heating > 10 MW was identified. Wide parameter scan experiments suggest that the toroidal rotation and the turbulence are the candidates for expelling impurities from the core region.

[1] Southward-then-northward magnetic perturbations are often seen in the tail plasma sheet, along with earthward jets, but the generation mechanism of such bipolar B z (magnetic flux rope created through multiple X-line reconnection, transient reconnection, or else) has been controversial. At $2313 UT on 13 August 2002, Cluster encountered a bipolar B z at the leading edge of an earthward jet, with one of the four spacecraft in the middle of the current sheet. Application to this bipolar signature of GradShafranov (GS) reconstruction, the technique for recovery of two-dimensional (2D) magnetohydrostatic structures, suggests that a flux rope with diameter of $2 R E was embedded in the jet. To investigate the validity of the GS results, the technique is applied to synthetic data from a three-dimensional (3D) MHD simulation, in which a bipolar B z can be produced through localized (3D) reconnection in the presence of guide field B y (Shirataka et al., 2006) without invoking multiple X-lines. A flux rope-type structure, which does not in fact exist in the simulation, is reconstructed but with a shape elongated in the jet direction. Unambiguous identification of a mechanism that leads to an observed bipolar B z thus seems difficult based on the topological property in the GS maps. We however infer that a flux rope was responsible for the bipolar pulse in this particular Cluster event, because the recovered magnetic structure is roughly circular, suggesting a relaxed and minimum energy state. Our results also indicate that one has to be cautious about interpretation of some (e.g., force-free, or magnetohydrostatic) model-based results.

It is for the first time demonstrated by means of the three-dimensional electrostatic Particle-in-Cell (PIC) simulation that the impurity ion transport caused by blob and hole propagations might not be negligible as compared with other types of transport. The simulations have shown that the impurity ion density profile in the blob / hole structure becomes a dipolar profile and the dipolar profile of the impurity ion density propagates with the blob / the hole. Furthermore, the simulations in which the initial impurity ion density has a radial gradient have revealed that the estimated effective radial diffusion coefficient for impurity ions by a single blob / hole is comparable to the Bohm diffusion coefficient.