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

Abstract

A prime goal in physics research is the development of theories which have the universality needed to explain a wide range of observations. Developed over the past decade, the model of turbulence decorrelation and stabilization by sheared E×B flow has the universality needed to explain the turbulence reduction and confinement improvement seen in the edge and core of a wide range of magnetic confinement devices. Because the E×B shear, turbulence, and transport are all intimately intertwined in multiple feedback loops, devising experiments to test whether E×B shear causes a change in turbulence and transport has been a major challenge for experimentalists. Over the past five years, there have been at least four clear demonstrations of causality performed in tokamak plasmas, both at the plasma edge on Doublet III-D (DIII-D) [Plasma Physics and Controlled Fusion Research 1985 (International Atomic Energy Agency, Vienna, 1986) Vol. I, p.159] and Tokamak Experiment for Technologically Oriented Research (TEXTOR) [Plasma Physics and Controlled Nuclear Fusion Research 1990 (International Atomic Energy Agency, Vienna, 1991) Vol. I, p. 473] and further into the plasma core on DIII-D and Tokamak Fusion Test Reactor [Phys. Plasmas 5, 1577 (1998)]. This paper discusses these tests in detail; the results agree with the expectations from the E×B shear model. This paper also discusses similarities between flow shear effects in plasmas and in neutral fluids and provides examples of flow shear reduction of turbulence in neutral fluids under the proper conditions.

The role of turbulence–flow interactions in L- to H-mode transition dynamics: recent progress

“…The turbulence-suppression criterion was found to be satisfied in tokamak plasmas just prior to the L-H transition and in H-mode [10,11,14], and also in internal transport barriers maintained via E × B shear primarily resulting from toroidal or poloidal ion rotation [16][17][18]. L-H transitions exhibiting LCO are particularly well suited to uncover the dynamics of shear-flow decorrelation due to the slow transition timescale.…”

“…A physics-based model of the L-H transition threshold power is therefore needed to confidently extrapolate to auxiliary heating requirements for ITER and future burning plasma experiments. It was recognized early on that the H-mode edge barrier forms as fluctuations are suppressed due to E × B flow shear [9][10][11] in a narrow (few cm wide) radial layer just inside the last closed flux surface (LCFS) [12][13][14][15]. While the paradigm of flow-shear suppression has been experimentally verified in the H-mode edge transport barrier as well as in internal transport barriers [16][17][18], the sequence of events leading to the formation of a highly sheared E × B jet flow layer has been the subject of intensive research.…”

Plasma Turbulence Imaging via Beam Emission Spectroscopy in the Core of the DIII-D Tokamak

“…The poloidal velocity of turbulence is governed primarily by the radial electric field, which naturally drops to zero at the magnetic axis. The v E×B profile is simultaneously measured using charge exchange recombination spectroscopy [24] and is overlaid in Fig. 7 as a dashed line.…”

Bifurcation theory of a one-dimensional transport model for the L-H transition

“…21 The turbulence quenching sheared flows in a plasma are identified as the E Â B-flows. [22][23][24] These flow shears are indeed observed as driven by Reynolds stress in the form of zonal flows, 3,25 and externally driven by probes generating a radial electric field, 26 and by various other mechanisms. This quenching mechanism is frequently modelled 13,14 as an effective diffusivity depending on the E Â Bflow shear,…”