In this study, high-frequency instabilities driven by runaway electrons in the EXL-50 spherical torus have been reported using a high-frequency magnetic pickup coil. The frequency of these instabilities is found to be power function dependent on the plasma density, similar to the dispersion relation of the whistler wave. The observed instability seems to exhibit a fluctuating pattern, resembling frequency chirping behavior, which appears to align with the expected outcomes predicted by the Berk-Breizman model. Theoretically, the excitation threshold of the instability driven by runaway electrons is related to the ratio of the runaway electron density to the background plasma density, and the stability criterion is first demonstrated qualitatively in this work. The instability can be stabilized by the spontaneous rise of plasma density, consistent with the wave-particle resonance mechanism. This investigation demonstrates the excitation characteristic of chirping instabilities in a tokamak plasma and reveals new features of these instabilities, thereby advancing the understanding of the mechanisms for controlling and mitigating runaway electrons.

Due to the discrete nature of the saddle coils for the active feedback control of the plasma instabilities, the emergence of the sideband modes is inevitable. In the Keda Torus eXperiment (KTX), the edge Mirnov coils are unevenly distributed on the inner surface of the vacuum vessel to suppress the sideband modes. These arrays of the Mirnov coils are used for the feedback control system of KTX. They are also used for the magnetohydrodynamics (MHD) mode identification, which is a fundamental and significant method to distinguish and describe the plasma instabilities. A set of suitable MHD mode analysis methods has been utilized to complete the mode detection, including the spatial discrete Fourier transform (SDFT) method, the singular value decomposition (SVD) method and the Lomb periodogram method. These methods can obtain comprehensive mode information of the plasma instabilities and their results can be used as the feedback of the feedback control system. These methods have been successfully applied to detect and characterize an impulsive mode of (m = 1, n = 0) and rotating modes of (m = 2, 3, 4, n = 1) in the tokamak plasma of KTX, while the matrix decomposition technique is not applicable for KTX.

We report, for the first time, non-disruptive central fueling experiments utilizing Compact Torus (CT) plasma injection in the medium-sized fully-superconducting tokamak, EAST (R ₀ = 1.9 m, a = 0.45 m, B _t = 1.5–2.5 T). In these experiments, the CT plasma achieved velocities near 150 km s⁻¹, with electron densities of 1.6 × 10²² m⁻³ and particle numbers reaching 1.3 × 10²⁰. After traversing approximately 1300 mm through the drift tube, the CT plasma was successfully injected into the normalized poloidal flux surface at Ψ _N = 0.8–0.85, corresponding to a magnetic field strength of 2.05–2.02 T. This direct injection depth exceeds predictions from empirical formulas and represents the highest magnetic field strength at which CT injection has been demonstrated to date. Following the direct injection, CT particles underwent further indirect inward diffusion toward the core region. Density measurements revealed that CT deposition disrupted the density profile in the core of the tokamak, resulting in a hollow distribution that ultimately evolved into a more peaked density profile. These findings have important implications for improving plasma confinement in tokamaks.

In this study, the first observation of high-frequency instabilities driven by runaway electrons has been reported in the EXL-50 spherical torus using a high-frequency magnetic pickup coil. The central frequency of these instabilities is found to be exponentially dependent on the plasma density, similar to the dispersion relation of the whistler wave. The instability frequency displays chirping characteristics consistent with the Berk-Breizman model of beam instability. Theoretically, the excitation threshold of the instability driven by runaway electrons is related to the ratio of the runaway electron density to the background plasma density, and such a relationship is first demonstrated experimentally in this study. The instability can be stabilized by increasing the plasma density, consistent with the wave-particle resonance mechanism. This investigation demonstrates the controlled excitation of chirping instabilities in a tokamak plasma and reveals new features of these instabilities, thereby advancing the understanding of the mechanisms for controlling and mitigating runaway electrons.