Plasma Science and Technology

Explicit structure-preserving geometric particle-in-cell (PIC) algorithm in curvilinear orthogonal coordinate systems is developed. The work reported represents a further development of the structure-preserving geometric PIC algorithm achieving the goal of practical applications in magnetic fusion research. The algorithm is constructed by discretizing the field theory for the system of charged particles and electromagnetic field using Whitney forms, discrete exterior calculus, and explicit non-canonical symplectic integration. In addition to the truncated infinitely dimensional symplectic structure, the algorithm preserves exactly many important physical symmetries and conservation laws, such as local energy conservation, gauge symmetry and the corresponding local charge conservation. As a result, the algorithm possesses the long-term accuracy and fidelity required for first-principles-based simulations of the multiscale tokamak physics. The algorithm has been implemented in the SymPIC code, which is designed for high-efficiency massively-parallel PIC simulations in modern clusters. The code has been applied to carry out whole-device 6D kinetic simulation studies of tokamak physics. A self-consistent kinetic steady state for fusion plasma in the tokamak geometry is numerically found with a predominately diagonal and anisotropic pressure tensor. The state also admits a steady-state sub-sonic ion flow in the range of 10 km s⁻¹, agreeing with experimental observations and analytical calculations Kinetic ballooning instability in the self-consistent kinetic steady state is simulated. It is shown that high-n ballooning modes have larger growth rates than low-n global modes, and in the nonlinear phase the modes saturate approximately in 5 ion transit times at the 2% level by the E × B flow generated by the instability. These results are consistent with early and recent electromagnetic gyrokinetic simulations.

We performed an experimental investigation on the electromagnetic effect and the plasma radial uniformity in a larger-area, cylindrical capacitively coupled plasma reactor. By utilizing a floating hairpin probe, dependences of the plasma radial density on the driving frequency and the radio-frequency power over a wide pressure range of 5–40 Pa were presented. At a relatively low frequency (LF, e.g. 27 MHz), an evident peak generally appears near the electrode edge for all pressures investigated here due to the edge field effect, while at a very high frequency (VHF, e.g. 60 or 100 MHz), the plasma density shows a sharp peak at the discharge center at lower pressures, indicating a strong standing wave effect. As the RF power increases, the center-peak structure of plasma density becomes more evident. With increasing the pressure, the standing wave effect is gradually overwhelmed by the 'stop band' effect, resulting in a transition in the plasma density profile from a central peak to an edge peak. To improve the plasma radial uniformity, a LF source is introduced into the VHF plasma by balancing the standing wave effect with the edge effect. A much better plasma uniformity can be obtained if one chooses appropriate LF powers, pressures and other corresponding discharge parameters.

The physics of electrodeless electric thrusters that use directed plasma to propel spacecraft without employing electrodes subject to plasma erosion is reviewed. Electrodeless plasma thrusters are potentially more durable than presently deployed thrusters that use electrodes such as gridded ion, Hall thrusters, arcjets and resistojets. Like other plasma thrusters, electrodeless thrusters have the advantage of reduced fuel mass compared to chemical thrusters that produce the same thrust. The status of electrodeless plasma thrusters that could be used in communications satellites and in spacecraft for interplanetary missions is examined. Electrodeless thrusters under development or planned for deployment include devices that use a rotating magnetic field; devices that use a rotating electric field; pulsed inductive devices that exploit the Lorentz force on an induced current loop in a plasma; devices that use radiofrequency fields to heat plasmas and have magnetic nozzles to accelerate the hot plasma and other devices that exploit the Lorentz force. Using metrics of specific impulse and thrust efficiency, we find that the most promising designs are those that use Lorentz forces directly to expel plasma and those that use magnetic nozzles to accelerate plasma.

Neutral beam injection (NBI) has been proven as a reliable heating and current drive method for fusion plasma. For the high-energy NBI system (particle energy > 150 keV) of large-scale fusion devices, the negative ion source neutral beam injection (NNBI) system is inevitable, which can obtain an acceptable neutralization efficiency (> 55%). But the NNBI system is very complex and challengeable. To explore and master the key NNBI technology for future fusion reactor in China, an NNBI test facility is under development in the framework of the Comprehensive Research Facility for Fusion Technology (CRAFT). The initial goal of CRAFT NNBI facility is to achieve a 2 MW hydrogen neutral beam at the energy of 200–400 keV for lasting 100 s. In the first operation of the CRAFT NNBI facility, a negative ion source with dual RF drivers was developed and tested. By using the 50 keV accelerator, the long-pulse and high-current extractions of negative hydrogen ions have been achieved and the typical values were 55.4 keV, 7.3 A (~ 123 A/m²), 105 s and 55.0 keV, 14.7 A (~ 248 A/m²), 30 s, respectively. By using the 200 keV accelerator, the megawatt-class negative hydrogen beam has also been achieved (135.9 keV, 8.9 A, 8 s). The whole process of the gas neutralization of negative ion beam, electric removal of residual ions, and beam transport have been demonstrated experimentally.

In laser wakefield acceleration, injecting an external electron beam at a certain energy is a promising approach for achieving a high-quality electron beam with low energy spread and low emittance. In this paper, the process of laser wakefield acceleration with an external injection at 10 pC has been studied in simulations. A Bayesian optimization method is used to optimize the key laser and plasma parameters so that the electron beam is accelerated to the expected energy with a small emittance and energy spread growth. The effect of the rising edge of the plasma on the transverse properties of the electron beam is simulated and optimized in order to ensure that the external electron beam is injected into the plasma without significant emittance growth. Finally, a high-quality electron beam with an energy of 1.5 GeV, a normalized transverse emittance of 0.5 mm·mrad and a relative energy spread of 0.5% at 10 pC is obtained.

ENN is planning the next generation experimental device EHL-2 with the goal to verify the thermal reaction rates of p-¹¹B fusion, establish spherical torus/tokamak experimental scaling laws at 10's keV ion temperature, and provide a design basis for subsequent experiments to test and realize the p-¹¹B fusion burning plasma. Based on 0-dimensional (0-D) system design and 1.5-dimensional transport modelling analyses, the main target parameters of EHL-2 have been basically determined, including the plasma major radius, R₀, of 1.05 m, the aspect ratio, A, of 1.85, the maximum central toroidal magnetic field strength, B₀, of 3 T, and the plasma toroidal current, I_p, of 3 MA. The main heating system will be the neutral beam injection at a total power of 17 MW. In addition, 6 MW of electron cyclotron resonance heating will serve as the main means of local current drive and MHD instabilities control. The physics design of EHL-2 is focused on addressing three main operating scenarios, i.e., (1) high ion temperature scenario, (2) high-performance steady-state scenario and (3) high triple product scenario. Each scenario will integrate solutions to different important issues, including equilibrium configuration, heating and current drive, confinement and transport, MHD instability, p-¹¹B fusion reaction, plasma-wall interactions, etc. Beyond that, there are several unique and significant challenges to address, including

• establish a plasma with extremely high core ion temperature (T_i,0 > 30 keV), and ensure a large ion-to-electron temperature ratio (T_i,0/T_e,0 > 2), and a boron concentration of 10%‒15% at the plasma core;

• realize the start-up by non-inductive current drive and the rise of MA-level plasma toroidal current. This is because the volt-seconds that the central solenoid of the ST can provide are very limited;

• achieve divertor heat and particle fluxes control including complete detachment under high P/R (> 20 MW/m) at relatively low electron densities.

This overview will introduce the advanced progress in the physics design of EHL-2.

The application and development of pulsed plasma thrusters (PPTs) in recent years are reviewed in this paper. The advantages of PPTs are discussed. The schematics, propulsion performance parameters and key physical processes of PPTs are described. Some representative PPT products and flight systems developed in recent years are presented to show the performance of the PPT. Studies about how electrode structures, discharge circuits, propellant materials, energy discharge method, propellant feed method, ignition method and number of thruster heads influence the PPT performance are presented and analyzed. The ignitor design method, ignition process and propellant carbonization are introduced to discuss the reliability and lifetime issues in PPTs. The modeling methods of the discharge circuit, as well as ablation, ionization and acceleration in PPTs are presented. Finally, the application of PPTs in the future is analyzed and some suggestions for PPT development are proposed.

High fusion triple product has been obtained in the advanced scenarios with high normalized beta (β_N) on the Experimental Advanced Superconducting Tokamak (EAST). A record value of n_i0T_i0τ_E ∼ 1.0 × 10¹⁹ m⁻³ keV s for EAST deuterium plasma has been achieved, which is due to the formation of strong and broad internal transport barriers (ITBs) in n_e, T_e and T_i profiles. Analysis shows that the strong ITB formation could be attributed to the reduction of transport from ITG modes. Based on the analysis, the physical mechanisms and methods to further improve the plasma performance are discussed.

A new method for directly injecting plasma into the central cell of the Keda Mirror with AXisymmetricity (KMAX) device by using a compact toroid (CT) has been developed. This radial injection approach overcomes the limitations of conventional plasma initiation methods, which typically rely on injecting plasma from one or both ends in a magnetic mirror configuration. The radial injection method aims to produce high-density plasma and facilitate studies of mirror-confined plasma. This paper presents its latest results and provides a detailed description of the injection system design. Our findings confirm the theoretical prediction that the injected plasma must achieve sufficient speed to penetrate the magnetic field and reach the chamber's center. Experimental observations show that, at medium voltage, the plasmoid may linger near the chamber edge, marking the first experimental identification of the conditions required for plasma penetration.

This study investigated the effect of cold helium plasma treatment on seed germination, growth and yield of wheat. The effects of different power of cold plasma on the germination of treated wheat seeds were studied. We found that the treatment of 80 W could significantly improve seed germination potential (6.0%) and germination rate (6.7%) compared to the control group. Field experiments were carried out for wheat seeds treated with 80 W cold plasma. Compared with the control, plant height (20.3%), root length (9.0%) and fresh weight (21.8%) were improved significantly at seedling stage. At booting stage, plant height, root length, fresh weight, stem diameter, leaf area and leaf thickness of the treated plant were respectively increased by 21.8%, 11.0%, 7.0%, 9.0%, 13.0% and 25.5%. At the same time, the chlorophyll content (9.8%), nitrogen (10.0%) and moisture content (10.0%) were higher than those of the control, indicating that cold plasma treatment could promote the growth of wheat. The yield of treated wheat was 7.55 t · ha⁻¹, 5.89% more than that of the control. Therefore, our results show that cold plasma has important application prospects for increasing wheat yield.

Neutral beam injection (NBI) has been proven as a reliable heating and current drive method for fusion plasma. For the high-energy NBI system (particle energy > 150 keV) of large-scale fusion devices, the negative ion source neutral beam injection (NNBI) system is inevitable, which can obtain an acceptable neutralization efficiency (> 55%). But the NNBI system is very complex and challengeable. To explore and master the key NNBI technology for future fusion reactor in China, an NNBI test facility is under development in the framework of the Comprehensive Research Facility for Fusion Technology (CRAFT). The initial goal of CRAFT NNBI facility is to achieve a 2 MW hydrogen neutral beam at the energy of 200–400 keV for lasting 100 s. In the first operation of the CRAFT NNBI facility, a negative ion source with dual RF drivers was developed and tested. By using the 50 keV accelerator, the long-pulse and high-current extractions of negative hydrogen ions have been achieved and the typical values were 55.4 keV, 7.3 A (~ 123 A/m²), 105 s and 55.0 keV, 14.7 A (~ 248 A/m²), 30 s, respectively. By using the 200 keV accelerator, the megawatt-class negative hydrogen beam has also been achieved (135.9 keV, 8.9 A, 8 s). The whole process of the gas neutralization of negative ion beam, electric removal of residual ions, and beam transport have been demonstrated experimentally.

When discharge faults occur in dry air switchgear, the air decomposes to produce diverse gases, with NO₂ reaching the highest levels. Detecting the NO₂ level can reflect the operation status of the equipment. This paper proposes to combine ZnO cluster with MoS₂ to improve the gas-sensitive properties of the monolayer. Based on the Density Functional Theory (DFT), the effect of (ZnO)_n size on the behavior of MoS₂ is considered. Key parameters such as adsorption energy and band gap of (ZnO)_n-MoS₂/NO₂ system were calculated. The ZnO-MoS₂ heterojunction was successfully synthesized by a hydrothermal method. The gas sensor exhibits a remarkable response and a fast response-recovery time to 100 ppm NO₂. In addition, it demonstrates excellent selectivity, long-term stability and a low detection limit. This work confirms the potential of the ZnO-MoS₂ composite structure as a highly effective gas sensor for NO₂ detection, which provides valuable theoretical and experimental insights for fault detection in dry air switchgear.

In laser wakefield acceleration, injecting an external electron beam at a certain energy is a promising approach for achieving a high-quality electron beam with low energy spread and low emittance. In this paper, the process of laser wakefield acceleration with an external injection at 10 pC has been studied in simulations. A Bayesian optimization method is used to optimize the key laser and plasma parameters so that the electron beam is accelerated to the expected energy with a small emittance and energy spread growth. The effect of the rising edge of the plasma on the transverse properties of the electron beam is simulated and optimized in order to ensure that the external electron beam is injected into the plasma without significant emittance growth. Finally, a high-quality electron beam with an energy of 1.5 GeV, a normalized transverse emittance of 0.5 mm·mrad and a relative energy spread of 0.5% at 10 pC is obtained.

The phenomenon of shock/shock interaction (SSI) is widely observed in high-speed flow, and the double wedge SSI represents one of the typical problems encountered. The control effect of single-pulse plasma synthetic jet (PSJ) on double wedge type-VI and type-V SSI was investigated experimentally and numerically, and the influence of discharge energy was also explored. The findings indicate that the interaction between PSJ and the high-speed freestream results in the formation of a plasma layer and a jet shock, which collectively governs the control of SSI. The control mechanism of single-pulse PSJ on SSI lies in its capacity to attenuate both shock and SSI. For type-VI SSI, the original second-wedge oblique shock is eliminated under the control of PSJ, resulting in a new type-VI SSI formed by the jet shock and the first-wedge oblique shock. For type-V SSI, the presence of PSJ effectively mitigates the intensity of Mach stem, supersonic jet, and reflected shocks, thereby facilitating its transition into type-VI SSI. The numerical results indicate that the peak pressure can be reduced by approximately 32.26% at maximum. Furthermore, the development of PSJ also extends in the Z direction. The pressure decreases in the area affected by both PSJ and jet shock due to the attenuation of the SSI zone. With increasing discharge energy, the control effect of PSJ on SSI is gradually enhanced.

The electromagnetic turbulence in reversed field pinch (RFP) plasmas exhibits three-dimensional characteristics. Suppression of this turbulence is crucial for enhancing plasma confinement, necessitating control over the electric field or the current profile. To this end, two sets of electrodes have been designed and installed on the Keda Torus eXperiment (KTX) RFP device to manipulate the edge electric field and the edge parallel current profile. Subsequently, the edge radial electric field and edge parallel current profile control experiments are conducted. In the edge radial electric field control experiments, the edge radial electric field is altered under bias, accompanied with an increase in the electron density and plasma duration. However, under bias, both electrostatic and magnetic fluctuations are enhanced. In the edge parallel current profile control experiments, the results indicate that bias modifies the edge parallel current profile locally, leading to a localized increase in the field reversal depth and electron density. Additionally, a reduction in magnetic fluctuations is observed within the reversed field enhanced region under bias, suggesting that the bias suppresses magnetic perturbations.

Hydrogels are biomaterials with 3D networks of hydrophilic polymers. The generation of hydrogels is turning to the development of hydrogels with the help of enabling technologies. Plasma can tailor the hydrogels' properties through simultaneous physical and chemical actions, resulting in an emerging technology of plasma-activated hydrogels (PAH). PAH can be divided into functional PAH and biological tissue model PAH. This review systematically introduces the plasma sources, plasma etching polymer surface, and plasma cross-linking involved in the fabrication of PAH. The 'diffusion-drift-reaction model' is used to study the microscopic physicochemical interaction between plasma and biological tissue PAH models. Finally, the main achievements of PAH, including wound treatment, sterilization, 3D tumor model, etc, and their development trends are discussed.

Cold atmospheric plasmas (CAPs) have shown great applicability in agriculture. Many kinds of CAP sources have been studied in agricultural applications to promote plant growth and cure plant diseases. We briefly review the state-of-the-art stimulating effects of atmospheric-pressure dielectric-barrier-discharge (AP-DBD) plasmas, after the direct or indirect treatment of plants for growth promotion and disease control. We then discuss the special demands on the characteristics of the CAP sources for their applications in plant mutation breeding. An atmospheric and room temperature plasma (ARTP) jet generator with a large plasma irradiation area, a high enough concentration of chemically reactive species and a low gas temperature is designed for direct plant mutagenesis. Experimental measurements of the electrical, thermal and optical features of the ARTP generator are conducted. Then, an ARTP-P (ARTP for plant mutagenesis) mutation breeding machine is developed, and a typical case of plant mutation breeding by the ARTP-P mutation machine is presented using Coreopsis tinctoria Nutt. seeds. Physical and agricultural experiments show that the newly-developed ARTP-P mutation breeding machine with a large irradiation area can generate uniform CAP jets with high concentrations of chemically reactive species and mild gas temperatures, and have significant mutagenesis effects on the Coreopsis tinctoria Nutt. seeds. The ARTP-P mutation breeding machine may provide a platform for systematic studies on mutation mechanisms and results for various plant seeds under different operating conditions in future research.

This paper reviews the effects of resonant magnetic perturbation (RMP) on classical tearing modes (TMs) and neoclassical tearing modes (NTMs) from the theory, experimental discovery and numerical results with a focus on four major aspects: (i) mode mitigation, where the TM/NTM is totally suppressed or partly mitigated by the use of RMP; (ii) mode penetration, which means a linearly stable TM/NTM triggered by the externally applied RMP; (iii) mode locking, namely an existing rotating magnetic island braked and finally stopped by the RMP; (iv) mode unlocking, as the name suggests, it is the reverse of the mode locking process. The key mechanism and physical picture of above phenomena are revealed and summarized.

Atmospheric pressure cold plasma, with advantages such as high particle activity, no thermal damage, high efficiency and direct and friendly contact with human tissues, is considered to have great potential in biomedical applications. Therefore, 'plasma medicine' as a new interdiscipline has been developed in the past two decades. This review first briefly describes the development of typical plasma sources suitable for biomedical applications, and those with different discharge forms are simply compared, evaluated and summarized. Subsequently, measurement of the crucial gaseous reactive particles (e.g. OH and O) and their spatio-temporal distributions are introduced. Meanwhile, the generation and variation rules and the related critical macroscopic parameters of the plasma-induced aqueous reactive species are summarized. Finally, related studies in the last ten years on the mechanisms of the plasma-driven microbial inactivation and plasma-induced apoptosis of cancer cells are introduced. Moreover, some scientific problems that need to be urgently solved in the field of plasma medicine are also discussed. This review will provide useful guidance for future related research.

The influence of m/n = 2/1 (m and n are poloidal and toroidal mode numbers) tearing modes on plasma perpendicular flows and micro-fluctuations has been investigated in HL-2A neutral beam injection heated L-mode plasmas. It is found that the local perpendicular rotation velocity and turbulence energy are modulated by the alternation between the island X-point and O-point of the naturally rotating tearing modes. Cross-correlation analysis indicates that the modulation of density fluctuations by the tearing mode is not only limited to the island region, but also occurs in the edge region near the last closed flux surface. The turbulence exhibits distinct spectral characteristics inside and outside the island region. In addition, it is observed that the particle flux near the strike point is also significantly impacted by the tearing modes. The experimental evidence reveals that there are strong core-edge interactions between the core tearing modes and the edge transport.

Tungsten erosion and re-deposition has been simulated by the Monte Carlo code ERO for EAST upper outer (UO) divertor, with a comparative study specifically addressing the effects of carbon and lithium impurities on tungsten target erosion under helium versus deuterium plasma discharge conditions. The simulations indicate that tungsten erosion in helium discharges is significantly higher than that in deuterium discharges due to higher sputtering yield by helium ions. In deuterium discharges, the tungsten gross erosion rate initially rises with increasing carbon concentration in background plasma due to enhanced tungsten sputtering by carbon, and then decreases due to protective effects by carbon deposition on tungsten surface. In contrast, in helium discharges, an increase in background carbon concentration directly leads to a lower tungsten gross erosion rate, as helium sputtering dominates the tungsten erosion process. Lithium impurities play a notable protective role against tungsten erosion in both helium and deuterium discharges. Assuming suitable lithium and carbon concentrations, the modeling results align well with experimental data. The simulations offer key insights into tungsten erosion processes in deuterium and helium discharges.

Numerical simulations of edge plasmas are essential for optimizing divertor designs in fusion reactors. The neutral source terms in plasma fluid simulations are typically computed using Monte Carlo method, which is computationally expensive and constitutes a bottleneck for simulation efficiency. To accelerate edge plasma simulations, this study investigates the feasibility of applying deep learning on the calculation of neutral source terms. Transformers excel at capturing sequence relationships and performing parallel computations, making them well-suited for modeling interactions between various nodes in complex data structures. A Transformer-based neural network (NN) is thus employed to learn the mapping from plasma backgrounds to neutral source terms. The model is trained and evaluated on a dataset of approximately 600 samples obtained by SOLPS-ITER simulations of pure deuterium under a typical EAST upper single-null configuration. Subsequent tests show it achieves relative errors of ~5% and ~3% at the peak values of particle and electron energy sources, respectively, and relatively large errors for momentum and ion energy sources, reaching up to ~20%. Coupled B2.5-NN simulations demonstrate acceptable accuracy for preliminary divertor design assessments, with relative errors below 5% in peak particle and heat flux densities at divertor targets. The computational time per simulation time step is reduced by 80%–90%, underscoring the potential of deep learning in enhancing the efficiency of edge plasma simulations. Nevertheless, the present NN model does not yet provide physical understanding or reliable extrapolation. This remains an important direction for future research.

The limited research on kilovolt switches utilizing sealed gas has impeded the advancement of medium-voltage direct current (MVDC) power grids, particularly in aeronautic and astronautic applications. A self-initiation methodology was introduced in the magnetohydrodynamic (MHD) model, which defines a negative correlation between the minimum conductivity of the gas medium and time. This methodology enables the spontaneous initiation of the arc in the contact gap, eliminating the need to specify the arc's location and temperature. The proposed methodology was validated experimentally via a multicontact contactor, and two related characteristic parameters were simulated to be independent of the gas-phase arcing voltage, current, and time. Furthermore, the primary characteristics and mechanisms of arc extinction, restrike–extinction, and restrike–sustained arcing were comprehensively analyzed, with the voltage-breaking capacity at 450 A determined to be 1050 V at an opening distance of 1.5 mm in a 4 atm H2-N2 mixture (molar ratio of 7:3). The mechanisms of motion and deformation during gas-phase arcing were interpreted based on characteristic times. Finally, the temperature of the conductive branch wake at time t5 under the condition of restrike-sustained arcing was analyzed, and its variation with gas composition and pressure was determined.

Recently many research teams have been endeavoring to develop plasma sources to generate ultrahigh electron densities for cancer treatment and this paper presents a theoretical framework by calculating the entropy change from mixing different gases to determine extra energy dissipated to system to create even higher additional electrons. Because the formulation proposed conforms to all the established physical and chemical principles and it agrees perfectly with experimental results so this theoretical framework can be utilized to evaluate the performance of gaseous mixing in a wide variety of fields, not limited to plasma applications. The theoretical framework is a breakthrough in associated technologies for which most electrons are needed to optimize their efficacies so it has the potential to act as a guideline for future clinic and scientific explorations.

Artificial intelligence driven by large data is becoming increasingly important in magnetic fusion research. Here we scan the plasma gradient space for Cyclone Base case parameters using nonlinear gyrokinetic simulations to generate data for typical electrostatic drift wave turbulences. The main candidates, ion temperature gradient mode (ITG), and trapped electron mode (TEM), are then classified and labeled by conventional methods for the datasets in the linear stage. We then apply a classical machine learning algorithm, namely the support vector machine (SVM), and use plasma gradients or turbulent transport coefficients as the input to classify the type of the drift wave turbulence. Simple distance formulae are derived for rapid classification of the turbulence type, demonstrating effectiveness suitable for future theoretical analyses and real-time experimental applications.