Experimental observations and nonlinear numerical simulations strongly suggest that the radial transport of plasma through the scrape-off layer (SOL) is strongly related to turbulent fluctuation structures, called `blobs', which propagate coherently into the far SOL. These intermittent transport events are not only important for confinement properties of fusion devices but have also a strong impact on divertor heat loads and wall recycling phenomena. Thus, a detailed understanding of the dynamical properties and scaling dependencies of blobs is required. The present work characterizes turbulent SOL fluctuations using turbulence imaging in Alcator C-Mod, along with comparisons to similar imaging data from NSTX. A fast camera system, which records Dalpha light fluctuations of the edge and SOL plasma in the poloidal plane, shows that blobs form close to the separatrix in the high pressure gradient region and propagate into the SOL, thereby causing high radial correlation lengths. The optical measurement in combination with Langmuir probe measurements shows that the radial propagation is a result of the self-consistent plasma potential structure associated with the blobs. Correlations are found not to be limited to the blob propagation region but extend across the separatrix into the edge plasma. The experiments are compared to the results of nonlinear global numerical simulations covering the edge plasma and SOL [1]. The phase relation between plasma density and potential of blobs is consistent with the experimental findings. In the simulation radial correlations across the separatrix are found to be a result of the global edge plasma respond to the blob transport showing a close interplay between the edge and SOL plasma.
[1] O. E. Garcia et al., Phys. Rev. Letters 92(16), 2004.
[CI2A.002] Edge Impurity Transport Dynamics during the ELM Cycle in DIII-D
M.R. Wade (Oak Ridge National Laboratory)
High time resolution measurements of the edge carbon density
(n_c), temperature, and rotation profiles are revealing
never before observed details of the impurity and radial
electric field (E_r) dynamics associated with the edge
localized mode (ELM) cycle. The ELM event has three distinct
phases: the ELM crash, the recovery phase, and the improved
transport phase. The ELM crash is characterized by a rapid
(<0.3~ms), localized (<4~cm on the outboard midplane)
expulsion of impurity density, energy, and momentum from the
edge region and a marked increase in n_c in the
scrape-off-layer (SOL). In addition, the E_r well that is
generally present in H-mode plasmas is obliterated by the
ELM event. The transition between the recovery phase and the
improved transport phase occurs as the E_r well begins to
redevelop approximately 10~ms after each ELM and is
characterized by a rapid steepening in the profiles
(particularly in n_c and poloidal rotation) near the
separatrix. Transport analysis indicates a 50% reduction in
impurity diffusivity and a large increase in inward
convection across this transition, providing further
evidence of the role of the E_r well in the reduction of
radial transport evident in the edge of H-mode plasmas. The
observed n_c perturbation increases with the total ELM
energy loss and is consistent with the envelope of the
eigenfunction of the most unstable mode calculated using the
ELITE code, supporting the notion that that these ELMs are
medium-n peeling/ballooning instabilities. These studies
were made possible by recent upgrades to the DIII-D
charge-exchange recombination system to allow
sub-millisecond time resolution (>0.274~ms) at excellent
spatial resolution (<3~mm in the edge).
[CI2A.003] Characteristics of confinement and stability in edge plasmas of Large Helical Device
Akio Komori (National Institute for Fusion Science)
Recent progress in the heating capability in the Large Helical Device (LHD) enables exploration of MHD stability in the beta range up to 4%. This high beta value was also attributed to the features of MHD instabilities in the edge region, that is, its control was found to be very important, since there is a magnetic hill in the edge region of the heliotron configuration. MHD modes excited at m/n = 1 and m/n < 1 resonant surfaces were observed to grow with the increase of the pressure gradient and the average-beta in previous experiments. Recent high-beta experiments with higher heating power of NBI, however, show that MHD activities spontaneously become stable from the inner region to the outer region when the average-beta exceeds a certain value, and then a flattening of the electron temperature, Te, profile is observed around the resonant surface. Such a flattening can be formed externally by producing an m/n = 1/1 magnetic island, and the complete stabilization of the m/n = 1/1 mode is demonstrated by the moderate island width. Furthermore, m/n > 1 resonant modes as well as the m/n = 1/1 mode are completely suppressed by the Local Island Divertor (LID), which is a divertor utilizing the m/n = 1/1 island. The LID was found to be able to control the edge plasma. More specifically, with the LID, the Te gradient inside the island separatrix becomes larger, compared with that of the standard helical divertor (HD) configuration. In addition, with the LID a factor of about 1.2 improvement of the energy confinement time was observed over the International Stellarator Scaling 95. The stabilization of peripheral modes by the LID may contribute to the improvement of global energy confinement, in addition to the particle control in the edge region.