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Plasma profiles across the separatrix and scrape-off layer
(SOL) in Alcator C-Mod are examined for a range of plasma densities, currents
and magnetic fields in ohmic L-mode discharges and for a subset of conditions
in ohmic H-mode discharges. In all plasmas, electron pressure gradient scale
lengths (Lp) exhibit a minimum value just outside the separatrix (i.e.,
in the near SOL), forming the base of a weak (strong) pedestal in L-mode (H-mode)
plasmas. Over a wide range of conditions in ohmic L-mode discharges, Lp
at this location are found to track with a monotonic function of electron
collision frequency, when this quantity is normalized according to the framework
of electromagnetic fluid drift turbulence theory. Moreover, at fixed values
of normalized collisionality (characterized as the ‘diamagnetic parameter’,
alpha_d), electron pressure gradients in the near SOL increase with plasma
current squared, holding the MHD ballooning parameter, alpha_MHD, unchanged.
Thus, the state of the near SOL is restricted to a narrow region within this
two-parameter phase-space. An implication is that cross-field heat and particle
transport are strong functions of these parameters. Indeed, as alpha_d
is decreased below ~0.3, cross-field heat convection increases sharply and
competes with parallel heat conduction along open field lines, making high
plasma density regions of alpha_MHD-alpha_d space energetically inaccessible.
These observations are consistent with the idea that the operational space
of the edge plasma, including boundaries associated with the tokamak density
limit, are controlled by electromagnetic fluid drift turbulence. |
