9 research outputs found
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Relating the L-H power threshold scaling to edge turbulence dynamics
Understanding the physics of the L-H transition power threshold scaling dependencies on toroidal field and density is critical to operating and optimizing the performance of ITER. Measurements of long-wavelength (k ρI < 1) turbulent eddy dynamics, characteristics, flows, and flow shear in the near edge region of DIIID plasmas have been obtained during an ion gyroradius scan (varying toroidal field and current) and density scan in a favourable geometry (ion ∇B drifts towards the X-point), in order to determine the underlying mechanisms that influence the macroscopic L-H power threshold scaling relations. It is found that the normalized integrated long wavelength density fluctuation amplitudes (ñ/n) in the pedestal increases with ρ* approaching the L-H transition. The turbulence poloidal flow spectrum evolves from geodesic acoustic mode dominant at lower power to low-frequency zonal flow (LFZF) dominant near the L-H transition, and the effective shearing rate correspondingly increases. An inferred Reynolds stress, , from BES velocimetry (inferring velocity field from imaging) measurements is found to significantly increase near the L-H transition. At lower electron density, a clear increase of the LFZF is observed prior to the L-H transition, which is not evident at higher density. Taken together, these results are qualitatively consistent with the electron density and toroidal field scaling of the L-H transition power threshold. © 2013 IAEA, Vienna
Dust studies in DIII-D and TEXTOR
Studies of naturally occurring and artificially introduced carbon dust are conducted in DIII-D and TEXTOR. In DIII-D, dust does not present operational concerns except immediately after entry vents. Submicrometre sized dust is routinely observed using Mie scattering from a Nd: Yag laser. The source is strongly correlated with the presence of type I edge localized modes (ELMs). Larger size (0.005-1 mm diameter) dust is observed by optical imaging, showing elevated dust levels after entry vents. Inverse dependence of the dust velocity on the inferred dust size is found from the imaging data. Heating of the dust particles by the neutral beam injection (NBI) and acceleration of dust particles by the plasma flows are observed. Energetic plasma disruptions produce significant amounts of dust; on the other hand, large flakes or debris falling into the plasma may induce a disruption. Migration of pre-characterized carbon dust is studied in DIII-D and TEXTOR by introducing micrometre-size particles into plasma discharges. In DIII-D, a sample holder filled with 30-40 mg of dust is inserted in the lower divertor and exposed, via sweeping of the strike points, to the diverted plasma flux of high-power ELMing H-mode discharges. After a brief dwell (similar to 0.1 s) of the outer strike point on the sample holder, part of the dust penetrates into the core plasma, raising the core carbon density by a factor of 2-3 and resulting in a twofold increase in the radiated power. In TEXTOR, instrumented dust holders with 1-45 mg of dust are exposed in the scrape-off-layer 0-2 cm radially outside of the last closed flux surface in discharges heated with 1.4 MW of NBI. Launched in this configuration, the dust perturbed the edge plasma, as evidenced by a moderate increase in the edge carbon content, but did not penetrate into the core plasma