arXiv subjects

Haomin Sun

Publications and source records attributed to Haomin Sun.

13 recordsLinked to original sources

arXiv2026

Radial transport of electric current by electromagnetic microturbulence in tokamaks

The turbulent transport of toroidal angular momentum helps determine the rotation profiles of tokamak plasmas, and thereby their confinement and stability. The electron contribution therein has an additional consequence: even a modest electron momentum flux can correspond to a substantial turbulent flux of toroidal current, whose divergence could in principle modify the safety-factor profile. Here, using nonlinear gyrokinetic simulations, we show that electromagnetic fluctuations qualitatively alter turbulent momentum transport. In microtearing-mode-driven turbulence, the total momentum transport is inefficient relative to that of heat, yet an electron contribution associated with the turbulent Maxwell stress dominates the momentum flux. We show that this contribution exceeds an estimated scale required for turbulent current redistribution to compete with the collisional processes maintaining the bootstrap current. In the case of kinetic-ballooning-mode-driven turbulence considered, the momentum transport is found to be stronger and remains dominated by the electrostatic ion contribution; nevertheless, retaining the Maxwell stress is essential for the electron momentum flux to exceed this bootstrap-based reference scale. To enable this study, we independently implemented complete electromagnetic momentum-flux diagnostics in the gyrokinetic codes GENE and CGYRO, and verified them through linear and nonlinear cross-code benchmarks. Taken together, these results suggest that electromagnetic momentum transport may potentially be important for the coupled evolution of the rotation, current, and safety-factor profiles in high-beta tokamak plasmas.

physics.plasm-ph↗

arXiv2026

Destabilization of temperature-gradient-driven plasma turbulence by equilibrium $\vec{E}\times \vec{B}$ flow shear

A novel physical mechanism whereby sheared equilibrium flow enables temperature-gradient-driven turbulence is identified. Gyrokinetic simulations of ion-scale plasma turbulence show that imposed equilibrium $\vec{E}\times \vec{B}$ flow shear can destroy the self-generated zonal flows that regulate the turbulence. This results in transport that increases sharply with flow shear. A reduced fluid model demonstrates that this is due to the spatial incompatibility of imposed and zonal shear layers. Simulations of spherical tokamak discharges place the inferred rotation shear at, or just below, the threshold of the sharp transport increase, implying that the toroidal rotation can be determined primarily by heat, rather than momentum, injection.

physics.plasm-ph↗

arXiv2025

Particle-In-Cell Informed Kinetic Modeling of Nonlinear Skin Effects in Low-Frequency Inductively Coupled Plasmas

We perform extensive 2D Particle-In-Cell (PIC) electromagnetic simulations of low pressure Inductively Coupled Plasma (ICP) discharges with various coil current and driving frequencies. Our simulations show that in low-frequency cases, electrons in the skin region near the coil can be predominantly magnetized by the Radio Frequency (RF) magnetic field. More specifically, the electrons are trapped in the combined potential well formed by the vector and electrostatic potentials, where they oscillate for most of the RF period while drifting perpendicular to the RF magnetic field. When the magnetic field weakens, electrons shortly demagnetize, leading to jet-like currents and periodic bursts of energy deposition. Based on the newly discovered electron trajectories, we develop a new kinetic theory for the plasma skin effect in low-frequency Inductively Coupled Plasma (ICP) discharges, incorporating nonlinear electron motion in an RF magnetic field by integrating Vlasov equation along the unperturbed particle trajectory. This theory successfully predicts the time evolution of electron currents in low-frequency ICP plasmas, as well as a nonlinear relation between electron current and the RF inductive electric field in the new regime we found. Furthermore, by coupling this new kinetic theory with a global model, we provide a straightforward method for estimating equilibrium electron temperature, plasma density and electron current. These analytical predictions match well with our 2D PIC simulations and can be validated through future experimental studies.

physics.plasm-ph↗

arXiv2025

Nonlinear skin effect regime when a radio frequency electromagnetic field penetrates into a background plasma

Two-dimensional, electromagnetic particle-in-cell simulations are employed to study particle kinetics and power deposition in the skin layer when a Radio Frequency (RF) electromagnetic field penetrates into a background plasma. We identify a new regime at low frequency ($\sim\mathrm{MHz}$) and low pressure, where the motion of electrons can be highly nonlinear in the skin region. Through most of the RF cycle, the electrons are trapped in the effective potential formed by the vector and electrostatic potentials, with energy deposition being small and magnetic moment $\mu$ no longer being an adiabatic invariant. However, for a brief period around the null of the oscillating magnetic field, the electrons get detrapped, causing a jet-like current penetrating into the bulk plasma. During these brief periods, the power deposition becomes high, exhibiting a periodic burst nature. Based on kinetic theory, we provide analytical expressions for the plasma current and energy deposition in the new regime. A criterion for transition between the newly identified low-frequency, periodic-burst regime and the usual anomalous non-local skin effect regime is proposed and verified.

physics.plasm-ph↗

arXiv2024

Reducing turbulent transport in tokamaks by combining intrinsic rotation and the low momentum diffusivity regime

Based on the analysis of a large number of high-fidelity nonlinear gyrokinetic simulations, we propose a novel strategy to improve confinement in spherical tokamak plasmas by combining up-down asymmetric flux surface shaping with the Low Momentum Diffusivity (LMD) regime. We show that the intrinsic momentum flux driven by up-down asymmetry creates strong flow shear in the LMD regime that can significantly reduce energy transport, increasing the critical gradient by up to $25\%$. In contrast to traditional methods for generating flow shear, such as neutral beam injection, this approach requires no external momentum source and is expected to scale well to large fusion devices. The experimental applicability of this strategy in spherical tokamaks is addressed via simulations by considering actual equilibria from MAST and a preliminary equilibrium from SMART.

physics.plasm-ph↗

arXiv2024

Physics of the low momentum diffusivity regime in tokamaks and its experimental applicability

Strong $E\times B$ plasma flow shear is beneficial for reducing turbulent transport. However, traditional methods of driving flow shear do not scale well to large devices such as future fusion power plants. In this paper, we use a large number of nonlinear gyrokinetic simulations to study a novel approach to increase flow shear: decreasing the momentum diffusivity to make the plasma ``easier to push''. We first use an idealized circular geometry and find that one can obtain low momentum diffusivity at tight aspect ratio, low safety factor, high magnetic shear and low temperature gradient. This is the so-called Low Momentum Diffusivity (LMD) regime. To drive intrinsic momentum flux, we then tilt the flux surface, making it up-down asymmetric. In the LMD regime, this intrinsic momentum flux drives strong flow shear that can significantly reduce the heat flux and increase the critical temperature gradient. We also consider the actual experimental geometry of the MAST tokamak to illustrate that this strategy can be practical and create experimentally significant flow shear. Lastly, a preliminary prediction for the SMART tokamak is made.

physics.plasm-ph↗

arXiv2024

Prevalence of non-standard collapsing of strong Langmuir turbulence in solar corona plasmas

We present a fully-kinetic simulation of the full life cycle of strong Langmuir turbulence (SLT) excited by electron beams that are accelerated under the solar corona conditions. We find that (1) most packets ($\sim$80%) are affected by their neighbors during their collapse, as a result, their spatial scale variations present non-standard evolutionary features, i.e., deviating away from what was predicted by the Zakharov model; (2) the collapsing cavity is too shallow to trap the wave packet due to the growth of the Coulomb force, as a result a majority ($\sim$70%) of the packet energy runs away and a secondary localization may occur. The study indicates that the non-standard Langmuir collapse may play an important role in coronal plasmas interacting with an intense electron beam, that may be eventually confirmed by humanity's first mission to fly through the corona.

astro-ph.SR↗

arXiv2023

A new quasilinear model for turbulent momentum transport in tokamaks with flow shear and plasma shaping

In tokamak experiments, sufficiently strong $E\times B$ flow shear reduces turbulent transport, thereby improving the prospects for fusion power plants. It is therefore of great importance to efficiently explore parameter space to find where strong plasma flow can be achieved. To this end, we propose a new, physically motivated quasi-linear model for estimating momentum transport from turbulence in the presence of toroidal flow shear and plasma shaping. The method gives good estimates of momentum transport for up-down asymmetric geometries as well as low magnetic shear and tight aspect ratio. The results are benchmarked with high-fidelity nonlinear GENE simulations, demonstrating that it provides a fast and accurate estimate of momentum transport.

physics.plasm-ph↗

arXiv2023

Direct Implicit and Explicit Energy-Conserving Particle-in-Cell Methods for Modeling of Capacitively-Coupled Plasma Devices

Achieving large-scale kinetic modelling is a crucial task for the development and optimization of modern plasma devices. With the trend of decreasing pressure in applications such as plasma etching, kinetic simulations are necessary to self-consistently capture the particle dynamics. The standard, explicit, electrostatic, momentum-conserving Particle-In-Cell method suffers from restrictive stability constraints on spatial cell size and temporal time step, requiring resolution of the electron Debye length and electron plasma period respectively. This results in a very high computational cost, making the technique prohibitive for large volume device modeling. We investigate the Direct Implicit algorithm and the explicit Energy Conserving algorithm as alternatives to the standard approach, both of which can reduce computational cost with a minimal (or controllable) impact on results. These algorithms are implemented into the well-tested EDIPIC-2D and LTP-PIC codes, and their performance is evaluated via 2D capacitively coupled plasma discharge simulations. The investigation revels that both approaches enable the utilization of cell sizes larger than the Debye length, resulting in reduced runtime, while incurring only minor inaccuracies in plasma parameters. The Direct Implicit method also allows for time steps larger than the electron plasma period, however care must be taken to avoid numerical heating or cooling. It is demonstrated that by appropriately adjusting the ratio of cell size to time step, it is possible to mitigate this effect to an acceptable level.

physics.plasm-ph↗

arXiv2022

Physical Regimes of Electrostatic Wave-Wave nonlinear interactions generated by an Electron Beam Propagating in a Background Plasma

Electron-beam plasma interaction has long been a topic of great interest. Despite the success of Quasi-Linear (QL) theory and Weak Turbulence (WT) theory, their validities are limited by the requirement of sufficiently dense mode spectrum and small wave amplitude. In this paper, we extensively studied the collective processes of a mono-energetic electron beam emitted from a thermionic cathode propagating through a cold plasma by performing a large number of high resolution two-dimensional (2D) particle-in-cell (PIC) simulations and using analytical theories. We confirm that the initial stage of two-stream instability is saturated due to well-known wave-trapping mechanism. Further evolution occurs due to strong wave-wave nonlinear processes. We show that the beam-plasma interaction can be classified into four different physical regimes in the parameter space for the plasma and beam parameters. The differences between the different regimes are analyzed in detail. For the first time, we identified a new regime in strong Langmuir turbulence featured by what we call Electron Modulational Instability (EMI) that could create a local Langmuir wave packet growing faster than the ion plasma frequency. Ions do not have time to respond to EMI in the initial growing stage. On a longer timescale, the action of the ponderomotive force produces very strong ion density perturbations, and eventually the beam-plasma wave interaction stops being resonant due to strong ion density perturbations. Consequently, in this EMI regime, electron beam-plasma interaction occurs in a periodic (intermittent) process. The beams are strongly scattered by waves, and the Langmuir wave spectrum is significantly broadened, which in turn gives rise to strong heating of bulk electrons. A resulting kappa distribution and a wave-energy spectrum, E^2 (k)~k^(-5), are observed in the strong turbulent regime.

physics.plasm-ph↗

arXiv2022

Electron Modulational Instability in the Strong Turbulent Regime for an Electron Beam Propagating in Background Plasma

We study collective processes for an electron beam propagating through a background plasma using simulations and analytical theory. A new regime where the instability of a Langmuir wave packet can grow locally much faster than ion frequency is clearly identified. The key feature of this new regime is an Electron Modulational Instability that rapidly creates a local Langmuir wave packet, which in its turn produces local charge separation and strong ion density perturbations because of the action of the ponderomotive force, such that the beam-plasma wave interaction stops being resonant. Three evolution stages of the process and observed periodic burst features are discussed. Different physical regimes in the plasma and beam parameter space are demonstrated for the first time.

physics.plasm-ph↗

arXiv2022

RFI Identification Based On Deep-Learning]{A Robust RFI Identification For Radio Interferometry based on a Convolutional Neural Network

The rapid development of new generation radio interferometers such as the Square Kilometer Array (SKA) has opened up unprecedented opportunities for astronomical research. However, anthropogenic Radio Frequency Interference (RFI) from communication technologies and other human activities severely affects the fidelity of observational data. It also significantly reduces the sensitivity of the telescopes. We proposed a robust Convolutional Neural Network (CNN) model to identify RFI based on machine learning methods. We overlaid RFI on the simulation data of SKA1-LOW to construct three visibility function datasets. One dataset was used for modeling, and the other two were used for validating the model's usability. The experimental results show that the Area Under the Curve (AUC) reaches 0.93, with satisfactory accuracy and precision. We then further investigated the effectiveness of the model by identifying the RFI in the actual observational data from LOFAR and MeerKAT. The results show that the model performs well. The overall effectiveness is comparable to AOFlagger software and provides an improvement over existing methods in some instances.

astro-ph.IM↗

arXiv2019

High Resolution Finite Volume Method for Kinetic Equations with Poisson Brackets

Simulation of plasmas in electromagnetic fields requires numerical solution of a kinetic equation that describes the time evolution of the particle distribution function. In this paper we propose a finite volume scheme based on integral relation for Poisson brackets to solve the Liouville equation, the most fundamental kinetic equation. The proposed scheme conserves the number of particles, maintains the total-variation-diminishing (TVD) property, and provides high-quality numerical results. Other types of kinetic equations may be also formulated in terms of Poisson brackets and solved with the proposed method including the transport equations describing the acceleration and propagation of Solar Energetic Particles (SEPs), which is of practical importance, since the high energy SEPs produce radiation hazards. The proposed scheme is demonstrated to be accurate and efficient, which makes it applicable to global simulation systems analyzing space weather.

physics.comp-ph↗