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The mean E × B shear in a stochastic magnetic field is calculated, using the radial force balance relation and transport equations. This analysis is relevant to the L → H transition with resonant magnetic perturbations, and special focus is placed upon the physics of non-ambipolar transport and radial current. The key physical process is the flow of fluctuating currents along wandering magnetic fields. The increments in poloidal and toroidal rotation, density and ion pressure are calculated. The radial envelope of the magnetic perturbations inside the plasma defines a new scale ℓ env , which is the characteristic scale of the magnetic fluctuation intensity profile. The net particle outflow due to stochastic magnetic fields is calculated and is determined by the net radial current through the separatrix. Implications for the L → H transition are discussed.

In order to understand plasma transport in the edge stochastic layers, a visible spectrometer has been developed. Two-dimensional impurity emission intensity distributions in the edge stochastic layer of LHD, including divertor plate, divertor legs, X-point, and the last closed flux surface, have been measured. Carbon emission intensities and their distributions are found to be clearly different depending on the different magnetic field configuration and on the charge state. Effects of the edge magnetic field structure on these results are discussed.

Space-resolved vacuum ultraviolet (VUV) spectroscopy using a 3 m normal incidence spectrometer has been developed in the large helical device (LHD) to study plasma transport in the ergodic layer by measuring the spatial profile of VUV lines from impurities emitted in the wavelength range of 300-3200 ?. Characteristics of the diagnostics system such as line dispersion, observable region and spatial resolution were evaluated. CIV spectra of 1548.20 × 2 ? were measured clearly. Intensity and ion temperature profiles were obtained simultaneously using CIV emissions in high-density discharges. Dependencies of the CIV intensity profiles on the electron density and magnetic configurations were observed.

Disruption-generated runaway electrons (RE) present an outstanding issue for ITER. The predictive computational studies of RE generation rely on orbit-averaged computations and, as such, they lack the effects from the magnetic field stochasticity. Since stochasiticity is naturally present in post-disruption plasma, and externally induced stochastization offers a prominent mechanism to mitigate RE avalanche, we present an advection-diffusion model that can be used to couple an orbit-following code to an orbit-averaged tool in order to capture the cross-field transport and to overcome the latter's limitation. The transport coefficients are evaluated via a Monte Carlo method. We show that the diffusion coefficient differs significantly from the well-known Rechester-Rosenbluth result. We also demonstrate the importance of including the advection: it has a two-fold role both in modelling transport barriers created by magnetic islands and in amplifying losses in regions where the islands are not present.