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Abstract: Using high speed movies of edge/Scrape-Off-Layer emission, velocity fields of the turbulent fluctuations have been measured in a small region of the poloidal/radial plane at the plasma’s outboard midplane. Gas-puff-imaging is used to localize the emission in the toroidal dimension. The measured D α line emission responds to the underlying spatio-temporal dynamics of density and temperature. Time-delay cross-correlation analysis is applied to the 64×64 pixel movie images and yields time-averaged velocity fields for the emission perturbations. The spatial resolution of the velocity determinations is a few mm over the viewed region. It is found that the velocities inside the separatrix are almost purely poloidal. In the SOL the radial velocity component is outward, and significant radial acceleration is observed. The poloidal and radial components of the SOL velocities vary with the discharge conditions. The magnitudes of all the velocity components measured this way range from 0 to ∼1000m/s. [Copyright &y& Elsevier]

Measurements of toroidal flow velocities in the midplane of the inner and outer scrape-off layers (SOL) of Alcator C-Mod [I. H. Hutchinson et al., Phys. Plasmas 1, 1511 (1994)] plasmas are made using a high-resolution spectrograph. Doppler shifts of the line emissions from either He[sup +] at 468.6 nm or neutral deuterium at 656.1 nm from two local gas puffs are observed along radially separated, toroidally viewing chords to obtain the radial profiles of the toroidal ion and neutral flow velocities and of the temperatures in the midplane of the inner and outer SOL’s. In these experiments, the ion vertical drift due to the toroidal magnetic field gradient (ion B×∇B drift) is directed towards the lower X-point, and an outer SOL plasma flow with a velocity in the range of 5–10 km/s in the direction of the plasma current is observed at the midplane far above the divertor plates. Plasma flow in a similar direction with similar velocities is observed in the inner SOL midplane. This observation suggests a toroidal rotation of the edge plasma in a co-current direction, consistent with an E[sub r]×B[sub θ] motion that would arise from a radially outward electric field. Interestingly, the neutral atoms in these views are observed to flow in the opposite directions in the inner and outer SOL’s, i.e., the neutrals flow in the plasma current direction in the inner SOL and in a direction opposite to the plasma current in the outer SOL. Also, the neutrals move much slower (velocities ∼2–4 km/s) in the outer SOL compared to the inner SOL (velocities ∼4–10 km/s). Temperatures determined from the Doppler broadening of the spectral lines are found to be in the range of ∼15–25 eV for the He[sup +] ions in the inner and outer SOL’s. Much lower temperatures of ≤2–5 eV are indicated for the deuterium atoms in both SOL’s. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on the results obtained from Alcator C-Mod in which a high-confinement, clean core plasma has been maintained in conjunction with a dissipative divertor. Description of the divertor geometry and the diagnostics used in the study; Dissipative divertor H-mode operation with nitrogen puffing.

Discusses the injection of trace non-recycling impurities into Alcator C-Mod plasmas in order to determine impurity transport coefficients. Impurity diffusion and convection; Comparison of transport behaviors during low-confinement modes and high-confinement modes; Edge values of the transport coefficients.

Observes the particle and energy balance in the edge plasma of Alcator C-Mod. Measurement of neutral gas pressure and ionization source strength in the main chamber; Dominance of plasma flow in the scrape-off layer (SOL); Details on the diffusion coefficient of the cross-field particle transport in the SOL.

Determines the relative screening of the low recycling gaseous impurities, nitrogen and methane, in Alcator C-Mod tokamak discharges. Proportionality of the impurity density in the core plasma to the rate of impurity injection; Effects of plasma detachment; Evidence of impurity transport from the scrape-off layer into the divertor.

Abstract: IR thermography is used to measure the heat flux footprints on C-Mod’s outer target in I-mode and EDA H-mode plasmas. The footprint profiles are fit to a function with a simple physical interpretation. The fit parameter that is sensitive to the power decay length into the SOL, λSOL , is ∼1–3× larger in I-modes than in H-modes at similar plasma current, which is the dominant dependence for the H-mode λSOL . In contrast, the fit parameter sensitive to transport into the private-flux-zone along the divertor leg is somewhat smaller in I-mode than in H-mode, but otherwise displays no obvious dependence on Ip , Bt , or stored energy. A third measure of the footprint width, the “integral width”, is not significantly different between H- and I-modes. Also discussed are significant differences in the global power flows of the H-modes with “favorable” drift direction and those of the I-modes with “unfavorable” drift direction. [Copyright &y& Elsevier]

Abstract: Gas-puff imaging data is used to investigate the dynamics of blob-like structures in the scrape-off layer of Alcator C-Mod. A blob-tracking algorithm is developed and used to compute blob propagation velocities and cross-field sizes for a series of discharges where the line-averaged particle density is varied. We find that the mean radial blob velocity increases from for to for , where is the the Greenwald density. The cross-field size of the blobs is independent of the line-averaged particle densities. With increasing the observed blob velocities become larger than predicted by simple blob models for sheath-connected plasma filaments. [Copyright &y& Elsevier]

Abstract: The mitigation of divertor heat fluxes is an active topic of investigation on existing tokamaks. One approach uses radiation, both inside and outside the last closed flux surface (LCFS), to convert plasma thermal energy, usually directed towards dedicated plasma facing components, to soft X-ray and ultraviolet radiation, spread over a much larger surface area. Recent enhanced D-α H-mode experiments on Alcator C-Mod varied the ICRF input power and radiative power losses via impurity seeding to demonstrate that normalized energy confinement depends strongly on the difference between input power and the radiated power inside the LCFS. These investigations also show that when seeded with either Ne or N2, a factor of two and higher reduction in outer divertor heat flux is achieved while maintaining H98,y2 ∼1.0. Conversely, when seeding with Ar, confinement is limited to H98,y2 ∼0.8 for a similar level of exhaust power. [Copyright &y& Elsevier]