This paper presents experimental studies of nonlinear processes between meso-scale structures and turbulent fluctuations, by use of bispectral analysis, in the edge region of toroidal plasmas. In ohmically heated plasmas of the JFT-2M tokamak, the drift wave-zonal flow system is investigated by observation of the potential fluctuations. The total bicoherence is composed of two components, i.e., the peak at the geodesic acoustic mode (GAM) frequency and broadband distribution. The biphase is also obtained, which indicates the phase angle of coherent interaction between GAM oscillations and broadband fluctuations. Bispectral functions (e.g., magnitude and phase) give an order-of-estimate agreement for the drift wave-zonal flow systems. Bicoherence analysis is also applied to the edge plasmas of the high-confinement mode (H-mode) of the Compact Helical System (CHS). In the case of CHS plasmas, nonlinear couplings in a system composed of coherent global magneto-hydrodynamic (MHD) oscillations and broadband fluctuations (up to hundreds kHz range) have been observed. Bispectral analysis enhances our understanding of the self-regulation systems of various toroidal plasmas in which strong turbulence and meso-scale structures coexist.

View full abstract

A new detection device for evaluating the ion energy distributions of a laser-plasma ion source is demonstrated. The Imaging Plate (IP) with no protective layer as ion detector device for the Thomson Parabola spectrometer is used. The Imaging Plate Thomson Parabola spectrometer (IPTPS) is applicable for the analysis of the energy distribution of the laser-driven ion beam using an opto-electric digitizing technique.

View full abstract

Steady state densities of up to 1.6 × 10²⁰ m^-3 have been sustained using gas puff fuelling and NBI heating up to 11 MW in the Large Helical Device (LHD). The density limit in LHD is observed to be greater than 1.6 times the Sudo limit. The density is ultimately limited by a radiating collapse which is attributed to the onset of a radiative thermal instability of the light impurities in the edge region of the plasma based on several recent observations in LHD. First of all the onset of the radiative thermal instability is tied to a certain edge temperature threshold. Secondly, the onset of the thermal instability occurs first in oxygen and then carbon as expected from their cooling rate temperature dependencies. Finally, radiation profiles show that as the temperature drops and the plasma collapses the radiating zone broadens and moves inward. In addition, comparison of impurity lines with the total radiated power behaviour suggests that carbon is the dominant radiator. Two dimensional tomographic inversions of Absolute eXtreme UltraViolet Diode (AXUVD) array data and comparison of modelling with images of radiation brightness from imaging bolometers indicate that the poloidal asymmetry which accompanies the radiating collapse is roughly toroidally symmetric.

View full abstract

A drop in divertor plasma density was observed during the transition to the ELM free H-mode phase as well as the drop in Dα emission intensity in JT-60U. The time scale for the drop in divertor plasma density ranged from a few ms to several tens of ms. 2-D fluid divertor code simulations indicated that the time scale for the divertor plasma density drop was ranged from several tens of ms to a hundred ms, when thermal and particle diffusivities were instantly reduced in the main plasma edge with a constant recycling coefficient. When it was assumed that the recycling coefficient decreased with a decrease in heat flux and increased with a decrease in particle flux, the time scale was reduced to a few ms in the simulations accompanied by a reduction of edge thermal diffusivity. These results indicate that the sharp drop of the divertor plasma density concomitant with the sharp drop of the Dα emission intensity on a time scale of a few ms can be ascribed to the change of plasma-wall interaction depending on the heat flux to the divertor plates.

View full abstract

New type of axisymmetric magnetohydrodynamic (MHD) equilibrium of toroidally flowing plasma, which cannot be found in the framework of the ideal MHD, emerges due to the Hall and toroidal effects. If either of these effects is neglected, only a conventional MHD-type equilibrium is obtained. Numerical solutions of both types are calculated in toroidal geometries to simulate the equilibrium states in the Ring Trap-1 device constructed at the University of Tokyo. Numerical analysis shows that when the toroidal flow is increased, the plasma shifts inward in the new-type equilibrium, while in the MHD type the outward shift occurs due to the centrifugal effect.

View full abstract