Identification of Mercier Instabilities in Alcator C-mod Tokamak
نویسندگان
چکیده
During current ramp-up discharges, highly localized magnetohydrody-namic (MHD) fluctuations were observed on the electron cyclotron emission (ECE) diagnostics of Alcator C-Mod tokamak [I. The electron temperature profile was hollow, while the density profile was weakly decreasing. Assuming that the equilibration time was short enough to quickly thermalize ions the pressure profile was also found to be hollow. Using this pressure profile as an additional constraint to the EFIT program, an equilibrium with reversed shear was constructed having a q(0) 1. The localized MHD activity was observed near the inner q=5 rational surface in this reconstructed equilibrium, where the Mercier criterion for ideal MHD stability was violated because of the reversed pressure gradient (dp/dr > 0), q > 1 and moderate shear. When kinetic effects were added, the ideal Mercier mode was finite ion Larmor radius (FLR) stabilized. However, ion Landau damping was found to be strong enough to drive a kinetic Mercier instability.
منابع مشابه
The Physics and Engineering of Alcator C - Mod
Alcator C-Mod is a new tokamak under construction at M.I.T. that promises to play an important and flexible role in the international fusion research effort. The physics and engineering features of the tokamak are described, giving an overview of the machine and plasma configurations. On the basis of empirical scaling laws, we predict the plasma confinement performance to be near DT equivalent ...
متن کاملTungsten Measurement on Alcator C-Mod and EBIT for Future Fusion Reactors
Tungsten will be an important element in nearly all future fusion reactors because of its presence in plasma facing components. This makes tungsten a good candidate for a diagnostic element for ion temperature and toroidal velocity measurement, and it makes understanding tungsten emissions important for tokamak power balance. The effect of tungsten on tokamak plasmas is investigated at the Alca...
متن کاملMagnetic topology effects on Alcator C-Mod scrape-off layer flow
Recent interest in the experimental study of tokamak plasma flow for different magnetic field geometries calls for theoretical understanding of the effects of tokamak magnetic topology changes on the flow. The consequences of total magnetic field reversal and/or X-point reversal on divergence-free plasma flow within magnetic flux surfaces is considered and the results are applied to interpret r...
متن کاملThe Ar 17+ Ly 2 /ly 1 Ratio in Alcator C-mod Tokamak Plasmas the Ar 17+ Ly Α2 /ly Α1 Ratio in Alcator C-mod Tokamak Plasmas
Spectra of hydrogen-like Ar have been obtained from Alcator C-Mod tokamak plasmas using a spatially imaging high resolution x-ray spectrometer system. The ratio of Lyα2 (1S1/2 2P1/2) to Lyα1 (1S1/2 2P3/2) was found to be ∼ 0.52 regardless of plasma parameters, which is somewhat greater than the ratio of the statistical weights of the upper levels, 1/2. This difference is partially due to the ef...
متن کاملObservation of edge instability limiting the pedestal growth in tokamak plasmas.
With fusion device performance hinging on the edge pedestal pressure, it is imperative to experimentally understand the physical mechanism dictating the pedestal characteristics and to validate and improve pedestal predictive models. This Letter reports direct evidence of density and magnetic fluctuations showing the stiff onset of an edge instability leading to the saturation of the pedestal o...
متن کامل