Plasma rotation and transport in MAST spherical tokamak

Abstract

The formation of internal transport barriers (ITBs) is investigated in MAST spherical tokamak (ST) plasmas. The roles of E×B flow shear, q-profile (magnetic shear) and MHD activity in their formation and evolution are studied using data from high-resolution kinetic-and q-profile diagnostics. In L-mode plasmas, with co-current directed NBI heating, ITBs in the momentum and ion thermal channels form in the negative shear region just inside q min. In the ITB region the anomalous ion thermal transport is suppressed, with ion thermal transport close to the neo-classical level, although the electron transport remains anomalous. Linear stability analysis with the gyro-kinetic code GS2 shows that all electrostatic micro-instabilities are stable in the negative magnetic shear region in the core, both with and without flow shear. Outside the ITB, in the region of positive magnetic shear and relatively weak flow shear, electrostatic micro-instabilities become unstable over a wide range of wave-numbers. At ITG length scales, flow shear reduces linear growth rates and narrows the spectrum of unstable modes, but flow shear suppression of ITG modes is incomplete. Flow shear has little impact on growth rates at ETG scales. This is consistent with the observed anomalous electron and ion transport in this region. With counter-NBI ITBs of greater radial extent form outside q min due to the broader profile of E×B flow shear produced by the greater prompt fast-ion loss torque.

Zero-Turbulence Manifold in a Toroidal Plasma

“…We note that the dependence of this optimum value of u ′ on q/ǫ is not as strong as the dependence of the optimum value of γ E on q/ǫ (clearly this must be so because u ′ = (q/ǫ)γ E ). In a device with an optimised value of q/ǫ, a near maximum critical temperature gradient would be achievable for u ′ > ∼ 5, shears comparable to those observed in experiment [3,16,17].…”

“…Firstly, and principally, we have calculated the shape of the zero-turbulence manifold, the surface that divides the regions in the parameter space (γ E , q/ǫ, R/L T ) where subcritical turbulence can and cannot be nonlinearly sustained. We have described the shape of this manifold and its physical origins, and presented its two implications for confinement in toroidal plasmas: that reducing the ratio q/ǫ, i.e., increasing the ratio of the poloidal to the toroidal magnetic field, improves confinement at ev- ery nonzero value of γ E , and that at fixed q/ǫ, there is an optimum value of γ E (that is, an optimum value of the toroidal flow shear u ′ = dRω/dr/(v thi /R)) at which the critical temperature gradient is maximised, in some instances to values comparable to those observed in internal transport barriers [3,17]. How to calculate the heat and momentum fluxes that would need to be injected in order for such optimal temperature gradients to be achieved was discussed in Ref.…”

Microstability analysis of pellet fuelled discharges in MAST

“…In this first analysis we have neglected the contributions of sheared equilibrium flows. However, estimates of the E × B shear on MAST seem to indicate that it could suppress turbulence with very long wavelengths (k y ρ i ∼ 0.1) but not the turbulence at k y ρ i ∼ 1.0 typical of TEMs, which are associated with particle transport [5,6,7].…”

Global gyrokinetic turbulence simulations of MAST plasmas

“…For the microstabilty analysis we choose a MAST L-mode discharge #22807 at 0.25 s that displays a steep ion temperature gradient at the location where there is also a steep gradient in the toroidal rotation [9]. The core has a negative magnetic shear up to s = 0.55 where s is the square root of the normalised poloidal flux.…”