arXiv subjects

Zi-Chen Kan

Publications and source records attributed to Zi-Chen Kan.

5 recordsLinked to original sources

arXiv2026

First reduced model for integrated computations of helicon wave heating and current drive in magnetic fusion plasmas

Fast predictive modelling of radio-frequency heating and current drive is important for integrated tokamak scenario design, yet kinetic calculations of helicon-wave absorption remain too computationally expensive for large-scale parameter scans. We present a reduced model for helicon-wave heating and current drive that retains the dominant parallel electron Landau-damping channel. The wave response is evaluated on the cold-plasma dispersion root, and a single-Landau-pole correction is introduced to obtain compact expressions for the local damping rate and current-drive efficiency. The model is benchmarked against the Chiu-Chan heating model using approximately 1.6 million samples covering representative conditions of EAST, HL-3, DIII-D and KSTAR. The reduction error is found to be governed primarily by the electron Landau parameter and electron beta. Within an identified sub-lower-hybrid-frequency validity window, results from different devices collapse onto a common error curve, which enables an empirical correction that is further tested using ITER-like and BEST-like extrapolation cases. Near and above the lower-hybrid frequency, the agreement deteriorates rapidly owing to changes in the cold-dispersion root structure and the breakdown of the single-branch WKB description. When coupled to a reduced current-drive source, the corrected heating model gives a median deviation of 10.8 percent from the Landau-channel Ehst-Karney reference and reproduces published CFETR current-density profiles. The resulting model provides a computationally efficient reduced closure for helicon-wave heating and current-drive calculations, together with physically interpretable limits on its range of validity.

physics.plasm-ph↗

arXiv2026

Fully nonlinear phenomenology of the bump-on-tail (BOT) instability with drag, diffusion and Krook relaxation

Energetic-particle-driven modes in magnetically confined fusion plasmas often exhibit nonlinear frequency sweeping, 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 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.

physics.plasm-ph↗

arXiv2025

Plasma fibre using bright-core helicon plasma

This paper reports an innovative concept of ``plasma fibre" using bright-core helicon plasma, inspired by its spatial and spectral similarities to the well-known optical fibre. Theoretical analyses are presented for both ideal case of step-like density profile and the realistic case of Gaussian density profile in radius. The total reflection of electromagnetic waves near the sharp plasma density gradient and consequently the wave-guide feature could indeed happen if the incident angle is larger than a threshold value. Numerical computations using electromagnetic solver that based on Maxwell's equations and cold-plasma dielectric tensor yield consistent results. The experimental verification and prospective applications are also suggested. The ``plasma fibre" could be functional component that embedded into existing communication systems for special purpose based on its capability of dynamic reconfiguration.

physics.plasm-ph↗

arXiv2025

The bump-on-tail instability excited by energetic electrons in helicon plasma

This work explores for the first time bump-on-tail (BOT) instability excited by energetic electrons in helicon plasma. The Berk-Breizman model that developed for the wave-particle interaction and resulted instability in magnetic fusion is used. Details of the BOT instability are computed referring to typical helicon discharge conditions. Parameter studies are also conducted to reveal the effects of collisionality and energetic drive, to account for high-pressure and high-power senarios respectively. It is found that under the HXHM (high magnetic field helicon experiment) experimental parameters, the disturbed distribution function oscillates explosively at the initial stage of BOT instability excitation, and the wave frequency shift does not appear, i.e., the steady-state solution always exists under this mode. In the process of restoring stability, the exchange of energetic particles and wave energy is concurrent with the change of wave amplitude. As the Krook operator increases (i.e., from 0.1 to 1), the saturation level of the electric field and the instability enhance. Additionally, there have a bigger disturbance for the initial EEDF (electron energy distribution function) in high-power helicon devices, so that the energy exchange between waves and energetic particles is stronger as well. Moreover, BOT instability effects the density and flux of bulk plasma, and the flux increases with the Krook operator. The effect of BOT instability is one order of magnitude larger on rotating plasma than that on stationary plasma.These findings present a full picture of BOT instability in helicon plasma and are valuable to controlling it for efficient and safe applications, e.g., high-power space plasma propulsion and plasma material interactions using helicon source.

physics.plasm-ph↗

arXiv2025

Spatial and temporal evolutions of blue-core helicon discharge driven by planar antenna with concentric rings

The spatial and temporal evolutions of blue-core helicon discharge driven by a planar antenna with four concentric rings are explored on the Linear Experimental Advanced Device (LEAD). The discharge experiences distinct density jumps from E mode to H mode, W mode, and blue-core mode, when RF input power increases. This is similar to previous observations using other typical helicon antennas; however, this special antenna could drive modes of even higher levels for which the blue-core plasma column is actually hollow in radius, i.e. peaking off-axis, which was not presented before. The column shows counterclockwise rotation for blue-core mode and clockwise rotation for non-blue-core mode. The reason could be attributed to the radial electric field differenceses for both modes which reverses the rotation direction via ExB drive. Moreover, the centrifugal instability of blue-core helicon plasma is computed using a two-fluid flowing plasma model. It shows that the instability is strong for small axial wave number but becomes weak for large axial wave number. Perturbed density peaks at radius of 0.045 m, while the equilibrium density gradient peaks at radius of 0.055 m. The coincidence of their radial locations suggests that it is a resistive drift mode driven by density gradient. The blue-core mode weakens once the magnetic field or flow rate exceeds the threshold value. Increasing power further leads to a smoother plasma density gradient. The electron temperature profiles decrease with increased power, and the radial gradient of the electron temperature inside the core is smaller as the magnetic field changes. To our best knowledge, it is the first detailed characterization of blue-core helicon plasma driven by planar antenna, especially in terms of azimuthal rotation and centrifugal instability.

physics.plasm-ph↗