Measurements and scalings of the H-mode pedestal on Alcator C-Mod

Abstract

In this paper we summarize the results of recent measurements of the high confinement mode (H-mode) edge transport barrier in Alcator C-Mod. A number of new, high resolution edge diagnostics provide measurements of plasma parameters (electron density and temperature, and impurities density) in the pedestal region. Pedestal parameters are measured both at the top edge of the plasma and at the midplane. Very narrow (2 -5 mm) electron density and temperature and impurity density pedestals are observed. The impurity pedestal is located inside the density and temperature pedestals in the midplane measurements, but coincides with n e pedestal at the top edge. Correlation of both width and height of the pedestals at the midplane is observed with global plasma parameters and core confinement characteristics.At the same time there are no clear indications of scaling for the electron density and impurity pedestal widths measured at the top of the plasma.2

Multi-machine comparisons of H-mode separatrix densities and edge profile behaviour in the ITPA SOL and Divertor Physics Topical Group

“…doi:10.1016/j.jnucmat.2004. 10.099 measured by Thomson scattering in Alcator C-Mod [1], ASDEX Upgrade (AUG) [2], DIII-D [3], MAST [4], JET [5] and JT-60U [6]. The JET edge LIDAR data have been combined with Lithium beam measurements and other diagnostics and regularized by EDGE2D modelling in order to improve the spatial coverage and resolution [7].…”

Impurity toroidal rotation and transport in Alcator C-Mod ohmic high confinement mode plasmas

“…In particular, an edge Thomson scattering system, 48 two edge x-ray cameras 49 (ϰn e 2 ). The toroidal mid-plane views are abel-inverted to yield local emissivity profiles, with chordal resolution in the edge pedestal region of 0.6 mm, and time resolution of 4 ms. Correcting for the weak temperature dependence using measured profiles from electron cyclotron emission, and assuming Z eff is constant across the profile, the emissivity data are converted to equivalent electron density.…”

ELMs and the Role of Current-Driven Instabilities in the Pedestal

“…However, a simple model based entirely on pressure driven high-n ideal ballooning modes, without consideration of the edge current, appears to be ruled out by experiment. While the high-n ideal ballooning limit does correspond well to the observed type I ELM threshold in many regimes, recent experiments with high edge spatial resolution diagnostics indicate that the nominal ideal ballooning limit can be substantially exceeded, and that observed pressure gradient limits do not always scale as expected from ideal ballooning theory [12,13]. Figure 1 shows the results from a scan of the plasma triangularity in DIII-D, indicating both that the ideal ballooning limit can be substantially exceeded at high triangularity, and that the strong scaling of the pressure gradient limit with triangularity is not predicted by ideal ballooning theory [12].…”

“…Mossessian et al [13] studied the peeling-ballooning stability of EDA, ELM-free, and ELMy Alcator C-Mod discharges using the ELITE code. Intermediate 5 < n < 50 growth rates were found to increase strongly with edge current, and to be higher with broader mode structure in ELMy cases than in EDA or ELM-free.…”