Herein, the generation and loss of runaway electrons (REs) during the plasma start-up phase are observed in the Experimental Advanced Superconducting Tokamak (EAST). A dataset of nearly 400 ohmic discharges is established, and statistical analysis is employed to investigate the relationship between RE generation and key plasma parameters. It is observed that generation of REs is considerably suppressed under conditions of high plasma density and low loop voltage. REs are detected to be considerably in number when the ratio of the loop electric field to critical electric field, Eloop/Ec exceed 10 during the start-up phase. In addition, analysis of the correlation between plasma streaming parameters and Eloop/Ecreveals that a substantial population of REs is generated when electron temperature exceeds 700 eV. Moreover, a sudden drop in plasma current is observed to be consistent with the loss of REs, which is primarily due to macroscopic magnetohydrodynamics.

The neoclassical impurities transport induced by 3D fields, known as the neoclassical toroidal viscosity (NTV) flux, is discussed in detail using NTVTOK. A substantial inward NTV flux of impurities is observed, which plays a key role in impurity transport and provides a mechanism for the enhancement of impurities intensity induced by resonant magnetic perturbations (RMP). The inward NTV impurities flux results from the negative gap between the E → × B → drift frequency and the diamagnetic drift frequency of the impurities, a condition that can occur when the impurities gradient is positive. Consequently, this effect is strongly dependent on the spatial distribution of the impurities, in particular the direction and magnitude of the impurities gradient. It is also influenced by the strength of the 3D fields and can be controlled by changing the RMP configuration. The complex dependence of the NTV impurity flux on the toroidal rotation, mass, charge and ion temperature of impurities is also presented and discussed in detail. These factors will alter the collisionality of the impurities, the gap between the E → × B → drift frequency and the diamagnetic drift frequency, the precession and the bounce-drift resonance, and consequently affect the NTV flux of the impurities. These results improve the understanding of RMP on impurities transport and provide a possible physical basis for the impurities control using RMP. © 2025 Author(s).

Multiple Beta-induced Alfv & eacute;n eigenmodes (BAEs) excited by runaway electrons (REs) have been observed in low-density Ohmic discharges in the EAST tokamak. The toroidal mode number is mainly n= -1 and they rotate in the electron diamagnetic drift direction. These modes in the low frequency range from 10 to 25 kHz, are found to be unstable when the electron density decreases to less than 0.65 x 1019 m-3. BAEs depend sensitively on the RE current fraction. By decreasing electron density, the resistive current is replaced by that carried by the REs. Then more branches of unstable BAEs, which located at different rational surfaces, are simultaneously observed, indicating that BAEs depend sensitively on the plasma beta contributed from REs. Simulation results provided by the global eigenvalue code magnetohydrodynamic Alfv & eacute;n spectra suggest trapped energetic particles are responsible for the destabilization of these BAEs, which match the resonance condition and the mode locations are close to the plasma boundary.

JOREK 3D non-linear magnetohydrodynamic simulations with non-equilibrium impurity treatment of thermal quench (TQ) triggered by a massive neon gas in EAST L-mode disruptions are first presented. Neon impurities are deposited at Psi N similar to 0.7, followed by asymmetrical parallel extension along magnetic lines driven by a parallel self-consistent electrical field induced by plasma cooling process. Both double-stage and single-stage TQ observed in EAST experiments are reproduced through simulations with varying impurity particle fluxes. In double-stage TQ, non-linear interactions among the m/n = 3/1, 4/1, and 5/1 modes primarily contribute to edge stochastic. Growth of m/n = 2/1 mode initiates core energy loss during the first temperature collapse. Subsequently, the m/n = 2/1 mode of comparable large amplitude, along with higher harmonics, couples with the 3/1 mode, resulting in a global stochastic and total energy loss in the second collapse. The transition between double-stage and single-stage TQ is primarily determined by the n = 1 mode growth rate. In our simulations, the longer duration of double-stage TQ offers benefits for reducing the peak power of outward energy flux. Additionally, deeper impurity injection enhances radiative power and reduces outward energy flow. Strike point splitting on the upper-outer target, observed experimentally, is also reproduced in the simulations.

Pedestal turbulence spreading into a crape-off layer (SOL) can be used to explain the experimentally observed strong pedestal-SOL coupling and is expected to be important for the broadening of divertor deposition profiles in future devices (Xu et al 2019 Nucl. Fusion 59 126039). In the EAST tokamak, it is found that an electromagnetic (EM) mode in the pedestal region can spread into the SOL and broaden the divertor particle flux width. Multi-channel fluctuation reflectometry is used to measure the density fluctuations at the plasma edge. The EM mode rotates in the electron diamagnetic drift direction in the lab frame with a frequency range of [40-90] kHz, toroidal mode number n= 12-13 and poloidal wavenumber k theta = 0.41 cm-1. The mode amplitude peaks around the maximum of the pedestal density gradient. As the mode amplitude increases, the reflectometry channel in the SOL can clearly capture the mode. This result suggests that the EM mode is excited in the pedestal gradient region and spreads into the SOL. It is further found that the particle flux deposition profile in the divertor is broadened as the EM mode appears.

A systematic numerical investigation is carried out to understand magnetohydrodynamic stability of the ideal infernal-kink instability in tokamak plasmas with both negative triangularity (neg-D) shaping and negative central shear for the equilibrium safety factor profile. The latter is motivated by the desire to form the internal transport barrier in the neg-D configuration, which is known to have difficulty in forming the edge transport barrier. The infernal-kink mode is generally found to be more unstable in neg-D plasmas as compared to their positive D-shaped (pos-D) counterpart. This is mainly due to less favorable (or even unfavorable) average magnetic curvature near the radial location of the minimum safety factor ( qmin) as compared to the pos-D configuration. The larger Shafranov shift associated with the neg-D shape helps the mode stabilization but is not sufficient to overcome the destabilizing effect due to bad curvature. Strong poloidal mode coupling due to plasma shaping (toroidicity, elongation, triangularity, etc.) helps explain the slight shift with respect to that predicted by the analytic theory of the peak location of the computed mode growth versus qmin.

Global magnetohydrodynamic (MHD) instabilities are assessed for a reference H-mode scenario designed for the China Fusion Engineering Demo Reactor (CFEDR) with a normalized beta of and a safety factor of . For this scenario, the Troyon no-wall beta limit and ideal-wall beta limit computed by MARS-F code are and , respectively. This means that it is stable against the external kink mode for the target plasma pressure. However, even with a stable classical tearing mode (TM) via optimization of the q-profile, the 2/1 neoclassical tearing mode (NTM) remains significantly unstable and can be triggered if the seed island exceeds 5 cm in width. Therefore, electron cyclotron current drive can be employed to stabilize this mode, with the necessary driven current amounting to 1% of the plasma current. Additionally, in both the plasma ramp-up and high-confinement flat-top stages, the error field tolerance of CFEDR ( 10−5 and 10−5) is larger than that of ITER ( 10−5 and 10−5), respectively, in Ohmic cases. The positional accuracy of the poloidal field coils and center solenoid coil should be controlled within 3.5 mm for shifts in order to reduce the intrinsic error field to within the specified tolerance. This reference H-mode scenario is being designed to operate in high-q 95 and high-plasma pressure, which will reduce the error field tolerance due to the resonant field amplification effect and NTM locking, as well as the actual beta limit. Therefore, further assessments for dynamic error field correction with higher plasma pressure are required. © 2025 Hefei Institutes of Physical Science, Chinese Academy of Sciences and IOP Publishing.