Turbulent particle transport in magnetized fusion plasma

Abstract

The understanding of the mechanisms responsible for particle transport is of the utmost importance for magnetized fusion plasmas. A peaked density profile is attractive to improve the fusion rate, which is proportional to the square of the density, and to self-generate a large fraction of non-inductive current required for continuous operation. Experiments in various tokamak devices (AUG, DIII-D, JET, TCV, TEXT, TFTR) have indicated the existence of an anomalous inward particle pinch. Recently, such an anomalous pinch has been unambiguously identified in Tore Supra very long discharges, in absence of toroidal electric field and of central particle source, for more than 4 minutes [1]. This anomalous particle pinch is predicted by a quasilinear theory of particle transport [2], and confirmed by non-linear turbulence simulations [3] and general considerations based on the conservation of motion invariants [4]. Experimentally, the particle pinch is found to be sensitive to the magnetic field gradient in many cases [5,6,7,8], to the temperature profile [5,9] and also to the collisionality that changes the nature of the microturbulence [10,11,12]. The consistency of some of the observed dependences with the theoretical predictions gives us a clearer understanding of the particle pinch in tokamaks, allowing us to predict more accurately the density profile in ITER.

Measurements of Impurity Transport Due to Drift-Wave Turbulence in a Toroidal Plasma

“…While there have been many investigations of plasma turbulence and transport in toroidal plasmas (e.g. [10][11][12][13][14][15][16]) and linear devices (e.g. [17,18]) a direct measurement of fluctuationinduced impurity ion transport has not been made to our knowledge (as opposed to inferred from analysis of particle balance).…”

Up-gradient particle flux in a drift wave-zonal flow system

“…3 In magnetically confined plasmas, up-gradient transport mechanisms have been invoked to explain the observed time-averaged plasma density and temperature profiles in astrophysical and near-Earth space plasmas [4][5][6][7][8][9] and in controlled fusion confinement experiments. [10][11][12][13][14][15] For fusion systems, inferred transport fluxes are often decomposed into an inward-directed turbulent pinch and down-gradient diffusive transport components (both of which are attributed to a combination of density and temperature gradients) that then compete to form the plasma equilibrium. In a few cases, direct measurements of net inward turbulent particle fluxes have been reported during or shortly after the formation of transport barriers associated with the formation of a suitably strong E Â B shear layer [16][17][18][19] or during the application of an externally forced E Â B shear flow.…”

Radial electric field generated by resonant trapped electron pinch with radio frequency injection in a tokamak plasma

“…It may include both diffusion and convection (pinch) through neoclassical and turbulence processes. The well-accepted conclusion [34][35][36] on the particle transport is that the curvature induced "turbulence-equipartition" effect 37,38 always induces an inward flux, but that the "thermo-diffusion" term [39][40][41][42] can either be inward when the ion temperature gradient mode (ITG) is dominant and outward when the TEM is dominant. Therefore, the reduction of particle transport can be realized by the weakness of outward transport or by the enhancement of inward transport, depending on different situations.…”