Local transport barrier formation and relaxation in reverse-shear plasmas on the TFTR tokamak

Abstract

The roles of turbulence stabilization by sheared ExB flow and Shafrunov-shift gradients are examined for TFTR [Plasma Phys. Controlled Nucl. Fusion Res. 26, 11 (1984)l Enhanced Reverse-Shear plasmas. Both effects in combination provide the basis of a positive-feedback model that predicts reinforced turbulence suppression with increasing pressure gradient. Local fluctuation behavior at the onset of ERS confinement is consistent with this framework. The power required for transitions into the ERS regime are lower when high power neutral beams are applied earlier in the current profile evolution, consistent with the suggestion that both effects play a role. Separation of the roles of ExB and Shafranov shift effects was performed by varying the ExB shear through changes in the toroidal velocity with nearly-steady-state pressure profiles. Transport and fluctuation levels increase only when ExB shearing rates are driven below a critical value that is comparable to the fastest linear growth rates of the dominant instabilities. While a turbulence suppression criterion that involves the ratio of shearing to linear growth rates is in accord with many of these results, the existence of hidden dependencies of the criterion is suggested in experiments where the toroidal field was varied. The forward transition into the ERS regime has also been examined in strongly rotating plasmas. The power threshold is higher with unidirectional injection than with balanced injection.

Tests of causality: Experimental evidence that sheared E×B flow alters turbulence and transport in tokamaks

“…Because of the need to use several phase contrast chords to obtain the radial correlation length, the value is plotted as the average value over the region sampled. Plasma conditions are 1.5 MA plasma current, 2.2 T toroidal field, 8.6 MW injected deuterium neutral beam power and 3.6ϫ10 19 the H-mode, the EϫB shear hypothesis could be disproven by finding L to H transitions in which turbulence and transport change first and EϫB shear changes significantly later. Correspondingly, finding cases where E r changes first would be consistent with causality.…”

“…8,18,19,132,133 The spatial correlation is illustrated in Figs. 15 and 16, which show that turbulent density fluctuations decrease in the same region where the local ion thermal diffusivity improves and where the EϫB shearing rate increases.…”

“…There is also a significant amount of data from DIII-D 7,126,128-131 and TFTR 8,18,19,132,133 showing that EϫB is comparable to ␥ MAX prior to the formation of the transport barrier and significantly exceeds it after formation. An example of the DIII-D data is shown in Fig.…”

“…Over the past five years, there have been four clear demonstrations of causality performed in tokamak plasmas, both at the plasma edge on Doublet III-D ͑DIII-D͒ 5,[11][12][13] and Tokamak Experiment for Technologically Oriented Research ͑TEXTOR͒ 14,15 and further into the plasma core on DIII-D 4,5,16,17 and Tokamak Fusion Test Reactor ͑TFTR͒. 10,18,19 The main goal of the present paper is to discuss these causality tests in detail.…”

The effect of safety factor and magnetic shear on turbulent transport in nonlinear gyrokinetic simulations

“…For axisymmetric toroidal geometry, the magnetic shear is a measure of the twist ͑2 / q͒ from one magnetic flux surface to another. It has been shown both in nonlinear simulations 11,12 and in tokamak experiments [13][14][15][16][17] that negative central shear ͑NCS͒ is stabilizing for turbulent transport and has proven to be a useful mechanism for the creation of internal transport barriers ͑ITBs͒. A simple physical explanation of the stabilizing effect of negative shear on curvature-driven instabilities is given in Ref.…”