Electron Internal Transport Barriers and Magnetic Topology in the Stellarator TJ-II

Abstract

In most helical systems electron Internal Transport Barriers (e-ITB) are observed in Electron Cyclotron Heated (ECH) plasmas with high heating power density. In the stellarator TJ-II, eITBs are easily achievable by positioning a low order rational surface close to the plasma core, because this increases the density range in which the e-ITB can form. Experiments with different low order rationals show a dependence of the threshold density and barrier quality on the order of the rational (3/2, 4/2, 5/3, ...). In addition, during the formation of e-ITB quasicoherent modes are frequently observed in the plasma core region. The mode can exist before or after the e-ITB phenomenon at the radial location of the transport barrier foot but vanishes as the barrier is fully developed. I. IntroductionElectron internal transport barriers (e-ITBs) are commonly observed in electron cyclotron heated (ECH) plasmas in stellarator devices such as CHS [1], W7-AS [2,3], LHD [4,5] and TJ-II [6,7]. e-ITBs are usually established in conditions of high ECH heating power density and are characterized by peaked electron temperature profiles with improved core electron heat confinement [1][2][3][4][5][6][7]. In addition, a large radial electric field and shear in the inner region is measured [1][2][3][4]7] accompanied by a reduction of fluctuations [1]. Most of these experimental results support the hypothesis that the mechanism for barrier formation is linked to a bifurcation of the radial electric field E r while the subsequent reduction of turbulence is due to an enhancement of E r xB shearing flow. A topical review on transport barriers has been published in [8] and a comparative study of transport barrier physics in different helical devices is reported in [9]. The specific characteristics of TJ-II, i.e. low magnetic shear and high flexibility with regard to the magnetic configuration allow us the control of low order rational surfaces within the rotational transform profile and, therefore, the study of how the magnetic topology affects e-ITB formation. Experimentally, a rational surface can be positioned in the plasma core region by selecting the appropriate magnetic configuration. In addition, the rotational transform profile can be modified dynamically during the discharge by inducing an ohmic current [10]. Experiments performed changing the magnetic configuration have shown that the presence of a low order rational surface (n=3/m=2) close to the plasma core triggers the e-ITB formation [7]. During the e-ITB formation, the electron temperature, measured by the ECE diagnostic

Core electron-root confinement (CERC) in helical plasmas

“…Systematic experiments with different low-order rationals have exhibited a dependence of the threshold density on the order of the rational surface (island width) [40]. The theoretical investigation for such experimental findings related to magnetic islands/rational surfaces are the future subject.…”

“…The impact of magnetic islands on the CERC establishment has also been recognized both at LHD [27,38] and TJ-II [39,40]. The low-order (ι/2π=1/2) rational surface is located at the central region in LHD.…”

Common Features of Core Electron-Root Confinement in Helical Devices

“…Both at LHD [41] and TJ-II [20,63] and neutral beam current drive, NBCD) must be calculated and the current diffusion must be taken into account. With the reasonable assumption that pronounced islands lead to a locally increased electron transport (parallel transport in the x-point region), their existence within the CERC region can be excluded.…”

Transitions to improved core electron heat confinement triggered by low order rational magnetic surfaces in the stellarator TJ-II

“…Finally, transitions triggered by the 3/2 rational have no effect on the ion temperature. Experiments with different low order rationals (n/m: 3/2, 4/2, 5/3…) have showed a dependence of the threshold density (and also of the confinement improvement) on the order of the rational [11]. In this work we report on experiments performed to study the influence of the island width keeping the order of the rational surface (n/m=3/2) and changing the magnetic shear by the induction of Ohmic current.…”