Edge-localized mode dynamics and transport in the scrape-off layer of the DIII-D tokamak

Abstract

High temporal and spatial resolution measurements in the boundary of the DIII-D tokamak show that edge-localized modes ͑ELMs͒ are produced in the low field side, are poloidally localized and are composed of fast bursts ͑ϳ20 to 40 s long͒ of hot, dense plasma on a background of less dense, colder plasma ͑ϳ5 ϫ 10 18 m −3 , 50 eV͒ possibly created by the bursts themselves. The ELMs travel radially in the scrape-off layer ͑SOL͒, starting at the separatrix at ϳ450 m / s, and slow down to ϳ150 m / s near the wall, convecting particles and energy to the SOL and walls. The temperature and density in the ELM plasma initially correspond to those at the top of the density pedestal but quickly decay with radius in the SOL. The temperature decay length ͑ϳ1.2 to 1.5 cm͒ is much shorter than the density decay length ͑ϳ3 to 8 cm͒, and the latter decreases with increasing pedestal ͑and SOL͒ density. The local particle and energy flux ͑assuming T i = T e ͒ at the midplane wall during the bursts are 10% to 50% ͑ϳ1 to 2ϫ 10 21 m −2 s −1 ͒ and 1% to 2% ͑ϳ20 to 30 kW/ m 2 ͒, respectively, of the LCFS fluxes, indicating that particles are transported radially much more efficiently than heat. Evidence is presented suggesting toroidal rotation of the ELM plasma in the SOL. The ELM plasma density and temperature increase linearly with discharge/pedestal density up to a Greenwald fraction of ϳ0.6, and then decrease resulting in more benign ͑grassier͒ ELMs.

Structure, stability and ELM dynamics of the H-mode pedestal in DIII-D

“…3 13 similar loss of impurities from the pedestal, a drop in pedestal toroidal and poloidal rotation, and the elimination of the pedestal electric field well by the ELM crash [63]. Scanning probe data near the separatrix showed large, rapid variations of both n e SOL and T e SOL during ELMs suggesting a filamentary structure of the perturbation [64]. This interpretation was supported by recent data from an ultra-fast radial interferometer chord (Fig.…”

Edge Localized Modes in Tokamaks

“…Another interesting observation, made on both MAST and DIII-D, is that the rotation shear (that is the differential rotation between different flux surfaces) is eliminated during the ELM event 34,35 (see Fig 12). Within the ideal MHD model of the non-linear ballooning mode, this must happen: the filament could not erupt out of the plasma if there is differential rotation.…”

Ion energies and currents of type I and mitigated ELMs in the ASDEX Upgrade far scrape-off layer

“…[4,10,23,[25][26][27][28][29][30][31] and references therein). Ion current observed by LPs during ELMs is found to be significantly higher compared to inter-ELM periods, with a somewhat lower increase of the electron temperature [29].…”