Laboratory generation of strong magnetic fields opens new frontiers in plasma and beam physics, astro- and solar-physics, materials science, and atomic and molecular physics. Although kilotesla magnetic fields have already been produced by magnetic flux compression using an imploding metal tube or plasma shell, accessibility at multiple points and better controlled shapes of the field are desirable. Here we have generated kilotesla magnetic fields using a capacitor-coil target, in which two nickel disks are connected by a U-turn coil. A magnetic flux density of 1.5 kT was measured using the Faraday effect 650 μm away from the coil, when the capacitor was driven by two beams from the GEKKO-XII laser (at 1 kJ (total), 1.3 ns, 0.53 or 1 μm, and 5 × 1016 W/cm2).
Divertor plasma characteristics in the Large Helical Device (LHD) have
been investigated mainly by using Langmuir probes. The three-dimensional
structure of the helical divertor, which is naturally produced in the
heliotron-type magnetic configuration, is clearly seen in the measured
particle and power deposition profiles on the divertor plates. These
observations are consistent with the numerical results of field line
tracing. The particle flux to the divertor plates increases almost linearly
with the line averaged density. The high-recycling regime and divertor
detachment, which are observed in tokamaks, have not been observed even
during high density discharges with low input power. Both electron density
and temperature decrease with increasing radius in the stochastic layer with
open field lines, and at the divertor plate they become fairly low compared
with those at the last closed flux surface. This means the reduction of
pressure along the magnetic field lines occurs in the open field line region
in LHD.
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.
Different characteristics of ion acoustic waves were experimentally observed in two types of Xe+–F− double plasmas at different electron temperatures. For the lower electron temperature (around 0.15 eV), the slow mode, which had been considered not to dominate the wave propagation, was found to be dominant rather than the fast mode, which was observed to be dominant for the higher electron temperature (around 1.5 eV). According to the previous numerical investigation [Phys. Plasmas 8, 4275 (2001)], the new wave characteristic appeared when the ratio of negative ion mass to positive ion mass and to the ratio of electron temperature to ion temperature are lower than certain critical values. Further, a method of evaluating both the positive ion temperature and the negative ion temperature in a negative ion plasma by observing the dominant slow mode is described. Using this method, the positive and negative ion temperatures in the former plasma were estimated to be 0.075 eV at the highest and 0.1 eV at the lowest, respectively.
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