Evidence for electromagnetic fluid drift turbulence controlling the edge plasma state in the Alcator C-Mod tokamak

Abstract

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 e) 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 e 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', d), electron pressure gradients in the near SOL increase with plasma current squared, holding the MHD ballooning parameter, 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 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 MHDd 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.

Experimental studies of edge turbulence and confinement in Alcator C-Mod

“…If the measured velocities were simply interpreted as E × B drifts, E r would have to change sign in less than the 3.7mm spot size, and attain its maximal values. This is in disagreement with both previous probe results [13] and measurements of the radial electric field by charge-exchange recombination spectroscopy [23]. Interpretation of the edge propagation velocity solely as v E in these regimes also contradicts the existence of the E rwell found in ref.…”

“…Nevertheless, recent experiments on Alcator C-Mod [13] have revealed a number of connections between global operating parameters (density and temperature profiles, plasma currents) and dimensionless scaling parameters of electromagnetic turbulence. These advances, together with the above transport phenomenology have led to the hypothesis that edge transport physics is described as a system near marginal stability [14,15].…”

Theory of the scrape-off layer width in inner-wall limited tokamak plasmas

“…figure 4) is different respect to what has been observed for diverted L-mode discharges in C-Mod. In this configuration, LaBombard et al [29,30] found that the relevant dimensionless parameters are α d and α. It is conjectured that a different saturation mechanism could be at play in diverted shots.…”

Advances in measurement and modeling of the high-confinement-mode pedestal on the Alcator C-Mod tokamak

“…Examination of probe data taken approximately 2 mm outside the LCFS has provided evidence for electromagnetic fluid drift turbulence [31] [32] setting the plasma transport state at this location. The study demonstrated experimentally both a local minimum in the pressure gradient scale length L nT = |nT /∇(nT )| and a functional dependence of normalized pressure gradient α MHD = 2µ 0 q 2 Rp /B 2 on a diamagnetic parameter α d , which is an inverse collisionality reflecting the ratio of diamagnetic drift to resistive ballooning times [33] [34].…”