Cross-Field Transport by Instabilities and Blobs in a Magnetized Toroidal Plasma

Abstract

The mechanisms for anomalous transport across the magnetic field are investigated in a toroidal magnetized plasma. The role of plasma instabilities and macroscopic density structures (blobs) is discussed. Examples from a scenario with open magnetic field lines are shown. A transition from a main plasma region into a loss region is reproduced. In the main plasma, which includes particle and heat source locations, the transport is dominated by the fluctuation-induced particle and heat flux associated with a plasma instability. On the low-field side, the cross-field transport is ascribed to the intermittent ejection of macroscopic blobs propagating toward the outer wall. It is shown that instabilities and blobs represent fundamentally different mechanisms for cross-field transport. It is widely recognized that the transport of particles and heat across the magnetic field in magnetically confined plasmas is anomalous, i.e., much larger than the transport induced by collisional processes [1]. Similar statistical properties, observed in different physical systems and/or for a variety of experimental conditions [2], suggest that the mechanisms responsible for the anomalous cross-field transport may have a common fundamental character. In particular, the possible relationship with low-frequency electrostatic instabilities has been suggested [3]. Correlated fluctuations of density and potential associated with unstable modes can directly drive a cross-field flux [4]. In addition, losses are partly due to the ejection of blobs, i.e., macroscopic density perturbations stretched along the magnetic field [5]. Theoretical [6 -8] and experimental [9,10] investigations are shedding light upon the mechanisms governing the blob dynamics, hence the associated transport [1,11]. However, despite the fundamental difference in the properties of instabilities and blobs, no clear distinction is usually done between their contributions to the total transport, generically interpreted in terms of a turbulent flux.In this Letter, we address the question of the different contributions to the cross-field transport in a simple magnetized toroidal plasma from instabilities and blobs. It is shown that the associated mechanisms for cross-field transport present fundamental differences. Therefore, the corresponding transport rates should be measured through complementary techniques to fully characterize the turbulent flux. For the scenario investigated herein, a main plasma and an edge region, connected by a transition region, can be clearly separated [12]. An interchange instability develops in the main plasma [13]. Blobs originate from the instability in the transition region, then propagate toward the outer wall across the magnetic field [12,14]. Although * 90% of the total losses on TORPEX plasmas are due to losses along the open magnetic field lines [15], in this Letter we focus on cross-field transport induced by perturbations of the plasma parameters.In general, the instantaneous particle flux is nv, where n is the local plasma density and...

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Convective transport by intermittent blob-filaments: Comparison of theory and experiment

“…The local radial heat transport due to blobs Q ¼ hT e V r i can also be evaluated if the local T e in a blob can be measured, which has only rarely been attempted. 18,129,175,205 Another technique used to evaluate the blob radial velocity is "conditional sampling" or "conditional averaging," which is usually implemented by "triggering" on a blob as detected by a fixed probe, and then recording the signal in another probe for times close to the blob trigger in the first probe. For example, using a multi-tip probe, 18,129 the electron temperature in blobs in the SOL of DIII-D was estimated this way to be T e ¼ 150 eV, which was almost 3 times the background temperature.…”

“…131 Under some assumptions the particle transport C r ¼ hnV r i can be estimated from such 2D data. 199,205,236 More sophisticated statistical analysis techniques have been implemented to characterize the structure and motion of blobs. Wavelet analysis has been used to evaluate the blob size, lifetime, local intermittency measure, packing fraction, linear density of structures, and bicoherence in various devices.…”