Shi-Jie Zhang

Energetic-particle-driven modes in magnetically confined fusion plasmas often exhibit nonlinear frequency sweeping (‘chirping’), reflecting complex wave–particle interactions near marginal stability. While the bump-on-tail (BOT) instability within the Berk–Breizman framework has served as a canonical model for understanding such phenomena, a unified nonlinear description remains incomplete when drag, diffusion, and Krook relaxation act simultaneously. In this work, we present a comprehensive numerical investigation of the BOT instability that explicitly retains all three collision operators together with external wave damping. Using a validated characteristic-based BOT code, we systematically scan the multi-dimensional collision parameter space and construct nonlinear regime maps and bifurcation diagrams. To organize the rich dynamics, we introduce a two-level categorization that combines global wave-energy evolution (damped, steady, periodic, and chaotic states) with chirping subtypes identified from spectral morphology. We find that diffusion and Krook relaxation regularize the nonlinear dynamics and promote ordered transitions from chaotic behaviors to periodic oscillations and steady saturation as collision strength increases, with the saturation level decreasing approximately exponentially with external damping. In contrast, drag alone does not admit steady solutions and instead drives persistent or chaotic chirping through convective deformation of resonant phase-space structures. When drag is combined with diffusion or Krook relaxation, clear transition sequences emerge: increasing drag breaks hole–clump symmetry, broadens the effective resonance region, and drives systematic transitions from transient to intermittent and persistent chirping. At fixed ratios of collision operators, higher absolute collision rates require larger wave amplitudes to balance collision-driven restoration against wave-induced phase-space flattening. Furthermore, extending our analysis into linearly stable regimes reveals robust subcritical nonlinear behavior, where finite-amplitude initial perturbations overcome background dissipation to trigger nonlinear states. These results provide unified nonlinear regime maps and mechanistic phase-space interpretations for energetic-particle-driven chirping, offering a predictive framework that is directly relevant to the diagnosis and control of chirping Alfvénic activity in fusion experiments.

This study explores the transitional characteristics of blue-core mode helicon discharge, which to our knowledge was not particularly focused before. Parameters are measured on a recently built advanced linear plasma device, i.e., a multiple plasma simulation linear device [Sun et al., Fusion Eng. Des. 162, 112074 (2021) and Wu et al., Plasma Sources Sci. Technol. 33, 085007 (2024)] by various diagnostics including the Langmuir probe, an optical emission spectrometer, and a standard high-speed camera. It is found that the jump direction of electron density (from low level to high level) is opposite to that of electron temperature (from a high level to a low level). Electron density increases significantly, and the radial profile becomes localized near the axis when the blue-core mode transition occurs. With increased field strength, electron density increases whereas electron temperature drops. The radial profile of electron temperature looks like a “W” shape, i.e., minimizing around the edge of the blue-core column. Electron density increases with background pressure, while electron temperature peaks around certain pressure value. High-speed videos show that the plasma column oscillates radially and experiences azimuthal instabilities with high rate once entered the blue-core mode. An electromagnetic solver based on Maxwell's equations and a cold-plasma dielectric tensor is also employed to compute the wave field and power absorption during blue-core mode transition, to provide more details that are valuable for understanding the transitional physics but not yet available in experiment. The results show that the wave field in both radial and axial directions changes significantly during the transition, its structure differs from antenna to downstream, and the power dependence of the wave magnetic field is overall opposite to that of a wave electric field. This work presents comprehensive characteristics of the transitional blue-core mode discharge and is important to both physics understanding and practical applications.

We investigate the spatial and temporal dynamics of blue-core helicon discharge using a novel planar antenna with four concentric rings on the LEAD (Linear Experimental Advanced Device, JINST 15: P11002, 2020). As RF power increases, the discharge transitions through E, H, W, and blue-core modes at 1100 W, exhibiting a hollow density profile peaking off-axis. High-speed imaging reveals counterclockwise rotation in the blue-core mode (800 rad/s at 2000 W), contrasting with clockwise rotation in non-blue-core modes, driven by a radial electric field reversal. A two-fluid flowing plasma model predicts a centrifugal instability that is strongest at small axial wavenumbers, weakening as the wavenumber increases. The blue-core mode diminishes beyond thresholds of 2.0 Pa or 800 G. This study highlights unique density profiles and rotational dynamics, advancing the understanding of helicon plasma behavior.