Electron internal transport barrier formation and dynamics in the plasma core of the TJ-II stellarator

Abstract

The influence of magnetic topology on the formation of electron internal transport barriers (e-ITBs) has been studied experimentally in electron cyclotron heated plasmas in the stellarator TJ-II. e-ITB formation is characterized by an increase in core electron temperature and plasma potential. The positive radial electric field increases by a factor of 3 in the central plasma region when an e-ITB forms. The experiments reported demonstrate that the formation of an e-ITB depends on the magnetic configuration. Calculations of the modification of the rotational transform due to plasma current lead to the interpretation that the formation of an e-ITB can be triggered by positioning a low order rational surface close to the plasma core region. In configurations without any central low order rational, no barrier is formed for any accessible value of heating power. Different mechanisms associated with neoclassical/turbulent bifurcations and kinetic effects are put forward to explain the impact of magnetic topology on radial electric fields and confinement.

Core electron-root confinement (CERC) in helical plasmas

“…Ion Beam Probe, HIBP) changes in an equivalent manner to the central T e (measured by Electron Cyclotron Emission) and anti-correlated to the central n e (proportional to the HIBP intensity) [37].…”

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

“…Electron 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].…”

“…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.…”

“…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 [11], and the plasma potential, measured by the HIBP diagnostic [12], increase in the plasma core region (ρ < 0.3), increasing substantially the radial electric field.…”

Electrostatic Potential Measurement by Using 6-MeV Heavy Ion Beam Probe on LHD

“…For example, the electron confinement is improved in the core region during electron cyclotron heating (ECH) [17][18][19][20][21][22]. This is considered to be related to the transition from the ion-root to the electron-root of E r predicted by the neoclassical theory, and called the Core Electron Root Confinement (CERC) [23].…”