arXiv subjects

Ping Zhu

Publications and source records attributed to Ping Zhu.

At least 19 recordsLinked to original sources

arXiv2026

Two-fluid effects on the nonlinear dynamics of RFP relaxation

This study investigates the role of two-fluid effects during magnetic relaxation in reversed-field pinch (RFP) plasmas. Within the multiple-helicity (MH) regime, two-fluid simulations produce distinct sawtooth oscillations, in contrast to the sawtooth-free state obtained in single-fluid simulations. Analysis of the magnetic field aligned projection of Faraday's law reveals that, tearing modes collectively generate a dynamo electric field that sustains the magnetic relaxation, a process analogous to the flux-pumping in tokamaks. Despite the reduced linear tearing-mode growth rates, stronger two-fluid effects produce more pronounced sawtooth activity over the parameter range considered. Modal energy analysis shows that Hall-mediated nonlinear energy redistribution disrupts the coherent tearing-mode dynamics required to sustain steady flux-pumping, thereby facilitating intermittent reconnection. This transition is interpreted as a Hall-mediated dynamical bifurcation between steady flux-pumping and quasi-periodic sawtooth relaxation.

physics.plasm-ph↗

arXiv2026

Minimum-q induced alternation between infernal modes and EP-driven modes in advanced tokamak configurations

For an advanced tokamak configuration in the presence of energetic particles (EPs), the dominant instability is found to alternate between infernal modes and Alfv\'en eigenmodes with the variation of the minimum safety factor $q_{\min}$. For relatively high $q_{\min}$, the mode is identified as a reversed-shear Alfv\'en eigenmode (RSAE), characterized by its finite Alfv\'enic frequency and radial localization near the minimum of safety-factor profile. As $q_{\min}$ is further reduced, the dominant branch sequentially transitions through a low-frequency infernal-mode interval, then an energetic-particle-mode (EPM) regime, and finally another low-frequency infernal-mode interval. Increasing the EP beta fraction $\beta_h$ tends to destabilize the RSAE and EPM branches but to stabilize the infernal modes. Phase-space diagnostics further indicate that the destabilizing effects of EPs on the RSAE and EPM branches are mainly associated with trapped-particle drive, whereas the stabilizing effects of EPs on the infernal modes is dominated by passing particles.

physics.plasm-ph↗

arXiv2026

Hybrid kinetic-MHD simulation on the formation of runaway electron current plateau during a current quench process

Runaway electron (RE) generation is of great concern for high-current tokamak operations. A full-f particle-in-cell (PIC) model for RE dynamics has been developed and coupled with the 3D nonlinear extended magnetohydrodynamic (MHD) model implemented in the NIMROD code. Our model accounts for RE generation using analytical source terms and advances the RE motion along guiding-center (GC) orbits. The model was employed to simulate the formation of RE current plateau during a disruption, in which the plasma current becomes dominated by the RE current. The model agrees well with several RE codes based on fluid models when the RE GC drifts are ignored. For highly relativistic REs, the grad-$B$ and curvature drifts can play a significant role in the RE generation and motion due to the increase in the safety factor profile during the current quench process.

physics.plasm-ph↗

arXiv2026

Thickness-Driven Superconductor-Insulator Transition in (Cu,C)-1234 and Proximity-Induced Superconductivity Recovery in (Cu,C)-1234/YBCO Heterostructure

Superconducting proximity effect and related thickness-driven property evolution remain an important issue in understanding high temperature superconductors. Among proximity systems, superconductor-superconductor (S-S') is special for the existence of intrinsic superconductivity in both materials. Such platform allows the different superconducting orders to compete, couple and reconstruct at the interface. In this paper, (Cu,C)-1234/YBCO heterostructure grown on LAO (001) with fixed thickness of bottom YBCO layer as 150 nm and varied thickness of top (Cu,C)-1234 layer as 188nm, 87 nm, 18nm and estimated 1.2 nm were fabricated and component films were preserved. Electrical transport characterization indicated that as the thickness decrease the (Cu,C)-1234 film degrades and underwent the superconductor-insulator transition (SIT) from thicker to less than 18 nm. In contrast, superconductivity is re-established in transport measurements when the insulating (Cu,C)-1234 layer is coupled to superconducting YBCO As the (Cu,C)-1234 thickness is further reduced to approximately 1.2 nm, the recovered superconductivity is strongly suppressed. The observed thickness dependence is consistent with a scenario in which interfacial coupling restores superconductivity over a finite thickness range before increasing disorder and dimensional confinement dominate in the two-dimensional limit. This work establishes a promising platform for investigating interfacial coupling between cuprate superconductors and provides new insight into the superconducting proximity effect in high-temperature superconducting heterostructures.

cond-mat.supr-con↗

arXiv2026

Identification of MHD equilibrium $\beta$ limits for CFQS plasmas

The magnetohydrodynamic (MHD) equilibrium $\beta$ limits in the Chinese First QuasiAxisymmetric Stellarator (CFQS) are investigated using the NTEC code, for both the standard and the magnetic island configurations. The equilibrium $\beta$ limit is identified upon the onset of the rapid destruction of nested flux surfaces by evaluating several numerical metrics, including the fractal dimension, weighted Birkhoff average, and effective volume of parallel diffusion. In the standard configuration, the net-current-free and the bootstrap-current-carrying equilibria can sustain well-ordered magnetic surfaces up to $\langle\beta\rangle\approx1.5\%$. The proliferation of stochastic field lines starts after the critical overlap between the internal major islands and the high-order island chains. Two types of divertor island configurations are studied based on net-current-free equilibria. It is found that the edge islands may transition into open field lines at low $\langle\beta\rangle$ values and lead to a drastic shrinkage of the last closed flux surface. Meanwhile, the threshold $\langle\beta\rangle$ value of the degradation of inner flux surfaces is similar to the standard configuration.

physics.plasm-ph↗

arXiv2026

Effects of external magnetic field ripple on FRC equilibrium

The two-dimensional equilibrium of Field-Reversed Configuration (FRC) plasma in presence of an external ripple magnetic field is computed to show the emergence of multiple magnetic axes for hollow equilibrium current profiles. An increase in ripple amplitude reduces the hollowness threshold required for the development of multiple magnetic axes. For an intermediate range of the ripple axial period, the formation of multiple magnetic axes becomes the most likely. The ripple's radial extension and the curvature of the axial field are the critical factors underlying the non-monotonic effect of ripple's axial period. When the axial period increases from the lower range, the ripple's radial extension gradually grows and enhances the chance of forming multiple magnetic axes. Once the ripple's radial extension covers the entire radial domain, further increasing the ripple axial period decreases the ripple field curvature, which becomes the dominant factor for lowering the hollowness threshold for the formation of multiple magnetic axes.

physics.plasm-ph↗

arXiv2026

Experiment-free disruption prediction for new devices enabled by synthetic diagnostic data augmentation

Deep learning based approaches have shown great promise in cross-device disruption prediction for tokamaks, however, the robustness of these models heavily relies on massive amounts of training data. For the upcoming ITER, to ensure the safety of the first plasma and subsequent operations, experimental data should be entirely unavailable initially, and disruptive discharges should be strictly avoided thereafter. This extreme data scarcity inherently conflicts with the data-intensive nature of deep learning algorithms. To address this challenge, we utilize synthetic diagnostic signals from the target device to supplement the experimental data from existing devices for the zero-shot disruption prediction on a new device. The detailed implementation pipeline of this scheme is presented. For experimental validation, a predictive model trained on data from the EAST tokamak is deployed for a zero-shot cross-device experiment on the J-TEXT tokamak. A synthetic diagnostic framework, configured with the diagnostic parameters of the target device, is developed to process NIMROD magnetohydrodynamic (MHD) simulation data based on the target device's magnetic configuration, thereby achieving effective data augmentation. Ultimately, the results demonstrate that by integrating the target device's synthetic diagnostic data with Fourier Domain Adaptation, the zero-shot accurate early warning rate of the model on 1,596 J-TEXT discharges is improved from 50% to 57%, while exhibiting enhanced predictive robustness.

physics.plasm-ph↗

arXiv2026

Resistive wall mode induced disruptions in an advanced tokamak

Resistive wall mode is one of the leading causes for tokamak disruptions above the no-wall $\beta_N$ limit. This paper presents nonlinear three-dimensional resistive MHD simulations of an RWM-induced disruption in a CFETR baseline steady-state equilibrium using the NIMROD code. Linear calculations confirm the dominant presence of the $n=1$ RWM instability, whose growth rate is strongly sensitive to the wall response and becomes weakly dependent on plasma resistivity in the high-$S$ limit, along with a global external-kink-like structure. In the nonlinear phase, the RWM drives rapid flux surface stochastization and a thermal quench, followed by a current quench that is intensified by the post quench increase of Spitzer resistivity. The transient current spike before the current quench is shown to be the outcome of the conservation of poloidal flux and a rapid reduction of internal inductance. During the late current quench stage, closed flux surfaces partially reform from the core region to the edge, relaxing toward the force-free state. Toroidal mode coupling, parallel heat transport, plasma resistivity, and wall conductivity strongly modulate the disruption onset and the quench dynamics. Within the MHD model, these results provide a complete view on the RWM-driven disruption process in advanced tokamak configurations.

physics.plasm-ph↗

arXiv2026

Effects of neoclassical toroidal viscosity on plasma flow evolution in the presence of resonant magnetic perturbation in a tokamak

Effects of neoclassical toroidal viscosity (NTV) on plasma flow evolution in the presence of resonant magnetic perturbation (RMP) in a tokamak have been evaluated using a cylindrical theory model. Calculations show that the introduction of NTV has almost no effect on the flow on the resonant surface, so the locked or unlocked state on the resonant surface remains unchanged, but it impacts the rotation profile in the core region. The toroidal, poloidal, and parallel flows in the core region are slightly reduced with uniform pressure. For non-uniform pressure profiles, elevated $\beta$ enhances the global amplitude of NTV torque but suppresses that of electromagnetic (EM) torque. These two driving terms collectively maintain the locked mode state.

physics.plasm-ph↗

arXiv2026

Probing Internal Dynamics of Spatiotemporal Optical Vortex Strings: Spatiotemporal Attraction and Filament Stretching

Vortex dynamics are intriguing and challenging across multiple physics fields. In optics, customized spatiotemporally structured optical fields, especially spatiotemporal optical vortices (STOV), offer the potential to tailor light via coupled space-time degrees of freedom. However, the interaction mechanisms between multiple transverse orbital angular momentum singularities within a single wave packet remain elusive. This study explores the intrinsic dynamics of a STOV with three phase singularities, observing a pronounced vortex singularity oscillation phenomena by tuning the temporal dispersion. We show that these phenomena originate from the counterintuitive spatiotemporal attractive effect between vortices, which is closely related to the singularity distance. Furthermore, the stretching into filaments and annihilation behaviors is observed by introducing antivortex in the center of the wavepacket. Experimentally, we propose a Full Interferometric Retrieval of Spatiotemporal Tomography (FIRST) method that enables the complete, single-shot capture of wave packets, with excellent agreement between theoretical predictions and experimental results. To the best of our knowledge, the dynamics of transverse spatiotemporal singularities within a single wave packet are reported here for the first time. These findings confirm the existence of interesting interactions between STOV singularities, deepen our understanding of photonics and open a new direction for investigating the complex dynamics of vortex singularities in the spatiotemporal domain.

physics.optics↗

arXiv2026

Weak and reversed magnetic shear effects on internal kink and fishbone modes

Advanced tokamak scenarios often feature weak or reversed magnetic shear configurations. In this study, the hybrid kinetic-MHD model implemented in the NIMROD code is used to investigate the effects of reversed magnetic shear on internal kink and fishbone mode in a circular shaped limiter tokamak. In the absence of energetic particles (EPs), the mode growth rate initially increases and then decreases as the magnetic shear changes from positive to negative, indicating stabilizing effects of the reversed magnetic shear on the internal kink mode. In the presence of EPs, when the reversed magnetic shear region is sufficiently narrow, the transition from internal kink/fishbone modes to double kink/fishbone modes takes place, and the stabilizing effects of the reversed magnetic shear can significantly dominate the destabilization of EPs. For non-resonant modes, the EP beta fraction $\beta_f$ for excitation increases with $q_{min}$, concurrent with progressively lower growth rates in non-resonant fishbone modes. When the equilibrium profile has an internal transport barrier (ITB), broader ITB widths suppress internal kink modes more effectively, whereas steeper temperature gradients strengthen EP stabilization.

physics.plasm-ph↗

arXiv2026

Auroral signatures of ballooning instability and plasmoid formation processes in the near-Earth magnetotail

The nonlinear development of ballooning instability and the subsequently induced plasmoid formation in the near-Earth magnetotail demonstrated in MHD simulations has been proposed as a potential trigger mechanism for substorm onset over the past decade, and their connections to the in-situ satellite and ground all-sky auroral optical observations have been a subject of continued research. In this work, a set of THEMIS substorm onset events with good conjunction of auroral observations has been selected for comparative simulation study, whose pre-onset magnetotail configuration and conditions are inferred from in-situ data and compared with the onset conditions of ballooning instability obtained in our MHD simulations. The evolution of the near-Earth magnetotail is followed, where the signatures of ballooning instability and the plasmoid formation are extracted from simulations and compared with the magnetic fields and flow patterns within the magnetotail region from observation data. The field-aligned current (FAC) density is evaluated at the Earth side boundary of the magnetotail domain of simulation, which is further mapped along magnetic field lines to the auroral ionosphere and compared with the auroral pattern and evolution there in terms of growth rate, dominant wavenumber, and absolute auroral intensities. Such validation efforts are also the first step towards the development of a self-consistent coupling model that includes the magnetotail-ionosphere interaction in the substorm onset process.

physics.space-ph↗

arXiv2025

Asymptotic state of nonlinear Landau damping in one-dimensional plasma

In this work, the asymptotic state of nonlinear Landau damping in one-dimensional plasma has been examined using a quasi-linear model and a second-order symplectic integrator. The dispersion relation of the plateau distribution function for the steady-state solution of the quasi-linear mode is extended to the complex plane and compared with the nonlinear simulation. We determine that the asymptotic state of the collisionless plasma is a multi-wave BGK structure. This structure is characterized by multiple vortices in phase space, which correspond to distinct peaks in the frequency-wavenumber ({\omega}, k) spectrum of the electric field

physics.plasm-ph↗

arXiv2025

Influence of plasma shaping on the parity of core-localized toroidal Alfv\'{e}n eigenmode in an advanced tokamak configuration

Toroidal Alfv\'{e}n eigenmodes (TAEs) and energetic particle modes (EPMs) can both be excited by energetic particles from auxiliary heating and fusion-born alpha particles in a tokamak. Using the hybrid kinetic-MHD model implemented in the NIMROD code, the excitation of these modes and their properties are investigated in an advanced tokamak configuration with reversed magnetic shear in the core region. The dominant TAE/EPM is found to exhibit odd parity with an anti-ballooning structure when the plasma has elongated, non-circular two-dimensional shaping. As the plasma shaping becomes more circular with reduced elongation, the mode parity undergoes a transition to even parity accompanied by a ballooning structure. These results may help explain the dominant parity of TAE/EPMs observed in advanced tokamak configurations with different plasma shaping.

physics.plasm-ph↗

arXiv2025

MHD Simulation Study on Impurity Assimilation Efficiency and Disruption Dynamics during Shattered Pellet Injection

Shattered Pellet Injection (SPI) has become a critical technique for mitigating plasma disruptions in fusion devices, yet optimizing its efficiency demands a proper understanding of the interaction between impurity dynamics and MHD response. We perform 3D nonlinear MHD simulations of SPI-induced disruption in a J-TEXT-like tokamak using the NIMROD code, systematically examining key parameters: fragment velocity and fineness, injection quantity, impurity composition, injection location and multiple injectors, resistivity, and parallel thermal conductivity. We find that slower fragment velocity enhances impurity assimilation and amplifies MHD activity. Finer fragments significantly increase impurity ablation and cooling efficiency. Mixed deuterium-neon pellets effectively elevate electron density without compromising radiative cooling efficiency. Plasma poloidal rotation affects ablation and cooling efficiency, whereas toroidally uniform multi-pellet injection enhances impurity ablation by nearly a factor equal to the number of pellets and lowers radiation asymmetry. Higher plasma parallel thermal conductivity results in higher radiation cooling efficiency in parallel directions, enhances impurity transport, and reduces toroidal peaking factor (TPF) of radiation. Variations in resistivity significantly influence Ohmic heating, impurity deposition and current dynamics after TQ, with higher resistivity leading to stronger magnetic perturbations and more pronounced current spikes. These findings provide physical bases for optimizing SPI schemes in future tokamak devices.

physics.plasm-ph↗

arXiv2025

End-to-end PDDL Planning with Hardcoded and Dynamic Agents

We present an end-to-end framework for planning supported by verifiers. An orchestrator receives a human specification written in natural language and converts it into a PDDL (Planning Domain Definition Language) model, where the domain and problem are iteratively refined by sub-modules (agents) to address common planning requirements, such as time constraints and optimality, as well as ambiguities and contradictions that may exist in the human specification. We support two categories of agents: hardcoded, which are informed by logs and error traces and have a pre-defined goal (e.g., fix issues with PDDL syntax, check temporal constraints), and dynamic, which have no predefined goal but adapt to the specific domain and revise the latent planning abstraction. The validated domain and problem are then passed to an external planning engine to generate a plan. The orchestrator and agents are powered by Large Language Models (LLMs) and require no human intervention at any stage of the process. Finally, a module translates the final plan back into natural language to improve human readability while maintaining the correctness of each step. We demonstrate the flexibility and effectiveness of our framework on GPT-\{4o, 5-mini, 5.4\}, and Gemini-\{2.5, 3\}-flash across more than ten domains and tasks, including the Google NaturalPlan benchmark, Planbench, and classic planning problems like Sokoban, Blocksworld and the Tower of Hanoi, where LLMs are known to struggle even with small instances. Our framework can be integrated with any PDDL planning engine and validator (we successfully tested Fast Downward, LPG, POPF, VAL, and uVAL) and represents a significant step toward end-to-end planning aided by LLMs.

cs.AI↗

arXiv2025

Hamiltonian estimation of island width threshold for stochasticity onset on edge pedestal top in presence of a resonant magnetic perturbation

This study applies the Hamiltonian method to analyze the nonlinear magnetic topology induced by Resonant Magnetic Perturbations (RMPs) in tokamaks. We investigate the system's chaotic behavior by comparing three methods: the renormalization method, Lyapunov exponents (LE), and weighted Birkhoff average (WBA). A strong consistency is found among these methods in predicting the large scale stochasticity threshold. The magnetic island width threshold provides a quantitative criterion for optimizing RMP-based ELM control, bridging a critical gap in plasma control strategies.

physics.plasm-ph↗

arXiv2025

Visual Anomaly Detection for Reliable Robotic Implantation of Flexible Microelectrode Array

Flexible microelectrode (FME) implantation into brain cortex is challenging due to the deformable fiber-like structure of FME probe and the interaction with critical bio-tissue. To ensure reliability and safety, the implantation process should be monitored carefully. This paper develops an image-based anomaly detection framework based on the microscopic cameras of the robotic FME implantation system. The unified framework is utilized at four checkpoints to check the micro-needle, FME probe, hooking result, and implantation point, respectively. Exploiting the existing object localization results, the aligned regions of interest (ROIs) are extracted from raw image and input to a pretrained vision transformer (ViT). Considering the task specifications, we propose a progressive granularity patch feature sampling method to address the sensitivity-tolerance trade-off issue at different locations. Moreover, we select a part of feature channels with higher signal-to-noise ratios from the raw general ViT features, to provide better descriptors for each specific scene. The effectiveness of the proposed methods is validated with the image datasets collected from our implantation system.

cs.CV↗