Formation Of Electron Internal Transport Barrier Research Articles

Dynamics of electron internal transport barrier formation at the H–L transition on EAST

The dynamics of electron-internal transport barrier (e-ITB) formation is studied in the hybrid plasma of neutral beam injection and lower hybrid wave heating on EAST. The e-ITB is observed to be repetitively formed at the H–L transition, where the relaxation process of core radiation triggers an increase in neutron yield and core electron temperature. A reversed q-profile is beneficial for generating the barrier, identified by the appearance of reversed shear Alfvén eigenmodes (RSAEs). The dimensionless parameter of the normalized Larmor radius is estimated to visualize the dynamic behavior of the e-ITB. It is found that the e-ITB forms once the electron exceeds 0.014, and the RSAEs are accompanied by the e-ITB formation. The barrier foot moves inwards until the L–H transition occurs, where the H-mode pedestal or type-I edge localized mode (ELM) strongly influences the e-ITB intensity. The results of thermal transport modeling show a significant reduction in the core thermal transport coefficient in the electron channel while maintaining a nearly unchanged ion transport level as the e-ITB evolves.

Isotope effect in transient electron thermal transport property and its impact on the electron internal transport barrier formation in LHD

In this study, we perform a comprehensive comparison of the transport hysteresis width in deuterium (D) plasmas, hydrogen (H) plasmas, and D-H mixed plasmas. The core focused modulation electron cyclotron resonance heating (MECH) is applied as the heat source perturbation, and the heat flux is evaluated using the energy conservation equation with the measured electron temperature response and the ECH deposition profile calculated by the ray-tracing scheme. Systematic density scan in plasmas with different ion mass reveals that there is no significant isotope effect in their hysteresis width. It is found that plasmas with heavier isotope mass can easily form the electron internal transport barrier. As the hysteresis width is insensitive to the isotope mass, the classical part of the diffusivity is considered to be responsible for the isotope effect in the transport barrier formation.

Rapid electron internal transport barrier formation during magnetic shear reversal in fully non-inductive TCV discharges

The rapid formation of an electron internal transport barrier (eITB) is observed during a slow evolution (≃200 ms) from a centrally peaked to a hollow current density profile, while all external actuators remain constant. The time constant for the barrier formation appears to be shorter than the electron energy confinement time. The improved confinement associated with the barrier formation occurs first in a localized region off-axis. Then the effects propagate to inner and outer flux surfaces on a confinement time scale. The temporal and spatial localizations of the barrier formation suggest a threshold in the magnetic shear profile that triggers the onset of the eITB. The threshold-like behaviour can be attributed to the sudden appearance of a minimum q (or zero-shear) flux surface in the safety factor profile. Based on modelling of the current profile evolution using the ASTRA transport code, the appearance of the zero-shear flux surface is correlated with the temporal and spatial formation of the barrier.

Electron ITB Formation with Combination of NBI and ECH in LHD

The electron internal transport barrier (ITB) is formed with centrally focused electron cyclotron resonance heating superposed on plasmas heated by neutral beam injection in the Large Helical Device. The electron transport is investigated for the electron ITB plasmas observed in various magnetic axis positions of Rax = 3.6, 3.75, and 3.9 m, and it turns out that the core electron transport is reduced with suppression of the anomalous transport in all three magnetic axis positions. In the theoretical calculations, positive radial electric fields are generated in the improved transport region, implying that the electron ITB formation is correlated with the neoclassical electron root. At an outer-shifted configuration of Rax = 3.9 m, where the helical ripple is large, the thermal diffusivity is decreased with decreasing collisionality, suggesting the reduction of the ripple transport by the radial electric field. The temperature and density conditions for the ITB formation are consistent with the theoretical density dependence of the transition temperature to the neoclassical electron root from the ion root.

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

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.