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.
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Jianyuan XIAO and Hong QIN 2021 Plasma Sci. Technol. 23 055102
Tetsutarou OISHI et al 2021 Plasma Sci. Technol. 23 084002
An impurity powder dropper was installed in the 21st campaign of the Large Helical Device experiment (Oct. 2019–Feb. 2020) under a collaboration between the National Institute for Fusion Science and the Princeton Plasma Physics Laboratory for the purposes of real-time wall conditioning and edge plasma control. In order to assess the effective injection of the impurity powders, spectroscopic diagnostics were applied to observe line emission from the injected impurity. Thus, extreme-ultraviolet (EUV) and vacuum-ultraviolet (VUV) emission spectra were analyzed to summarize observable impurity lines with B and BN powder injection. Emission lines released from B and N ions were identified in the EUV wavelength range of 5–300 Å measured using two grazing incidence flat-field EUV spectrometers and in the VUV wavelength range of 300–2400 Å measured using three normal incidence 20 cm VUV spectrometers. BI–BV and NIII–NVII emission lines were identified in the discharges with the B and BN powder injection, respectively. Useful B and N emission lines which have large intensities and are isolated from other lines were successfully identified as follows: BI (1825.89, 1826.40) Å (blended), BII 1362.46 Å, BIII (677.00, 677.14, 677.16) Å (blended), BIV 60.31 Å, BV 48.59 Å, NIII (989.79, 991.51, 991.58) Å (blended), NIV 765.15 Å, NV (209.27, 209.31) Å (blended), NVI 1896.80 Å, and NVII 24.78 Å. Applications of the line identifications to the advanced spectroscopic diagnostics were demonstrated, such as the vertical profile measurements for the BV and NVII lines using a space-resolved EUV spectrometer and the ion temperature measurement for the BII line using a normal incidence 3 m VUV spectrometer.
Xiang GAO et al 2021 Plasma Sci. Technol. 23 092001
High fusion triple product has been obtained in the advanced scenarios with high normalized beta (βN) on the Experimental Advanced Superconducting Tokamak (EAST). A record value of ni0Ti0τE ∼ 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 ne, Te and Ti 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.
Weisheng CUI and Ruobing ZHANG 2024 Plasma Sci. Technol. 26 042001
Atmospheric pressure cold plasma jets (APCPJs) typically exhibit a slender, conical structure, which imposes limitations on their application for surface modification due to the restricted treatment area. In this paper, we introduce a novel plasma jet morphology known as the large-scale cold plasma jet (LSCPJ), characterized by the presence of both a central conical plasma jet and a peripheral trumpet-like diffuse plasma jet. The experimental investigations have identified the factors influencing the conical and the trumpet-like diffuse plasma jet, and theoretical simulations have shed light on the role of the flow field and the electric field in shaping the formation of the LSCPJ. It is proved that, under conditions of elevated helium concentration, the distributions of impurity gas particles and the electric field jointly determine the plasma jet's morphology. High-speed ICCD camera images confirm the dynamic behavior of plasma bullets in LSCPJ, which is consistent with the theoretical analysis. Finally, it is demonstrated that when applied to the surface treatment of silicone rubber, LSCPJ can achieve a treatment area over 28 times larger than that of APCPJ under equivalent conditions. This paper uncovers the crucial role of impurity gases and electric fields in shaping plasma jet morphology and opens up the possibility of efficiently diversifying plasma jet generation effects through external electromagnetic fields. These insights hold the promise of reducing the generation cost of plasma jets and expanding their applications across various industrial sectors.
Xiaofang XU et al 2024 Plasma Sci. Technol. 26 064005
Ammonia is one of the most important chemical raw materials in both manufacture and life of human. Traditionally Haber-Bosch method for ammonia synthesis involves high temperature and high pressure conditions, leading to significant energy consumption and environmental pollution. Non-thermal plasma (NTP) is a promising alternative approach to ammonia synthesis at low temperature and atmospheric pressure. In this study, the synergistic effect of nanosecond pulsed dielectric barrier discharge (np-DBD) and Ni-MOF-74 catalyst was investigated in ammonia synthesis by utilizing nitrogen and hydrogen as feedstock. The results demonstrated that the plasma catalytic-synthesis process parameters play a crucial role in the synthesis process of ammonia. The highest ammonia synthesis rate of 5145.16 μmol·g−1·h−1 with an energy efficiency of 1.27 g·kWh−1 was observed in the presence of the Ni-MOF-74 catalyst, which was 3.7 times higher than that without Ni-MOF-74 catalyst. The synergistic effect of Ni-MOF-74 catalyst and nanosecond pulsed plasma was explored by in-situ plasma discharge diagnostics.
Xingyu CHEN et al 2024 Plasma Sci. Technol. 26 045403
The discharge morphology of pulsed dielectric barrier discharge (PDBD) plays important roles in its applications. Here, we systematically investigated the effects of the voltage amplitude, discharge gap, and O2 content on the PDBD morphology, and revealed the possible underlying mechanism of the U-shaped formation. First, the morphological evolution under different conditions was recorded. A unique U-shaped region appears in the middle edge region when the gap is larger than 2 mm, while the entire discharge region remains columnar under a 2 mm gap in He PDBD. The width of the discharge and the U-shaped region increase with the increase in voltage, and decrease with the increase of the gap and O2 content. To explain this phenomenon, a two-dimensional symmetric model was developed to simulate the spatiotemporal evolution of different species and calculate the electric thrust. The discharge morphology evolution directly corresponds to the excited-state atomic reduction process. The electric thrust on the charged particles mainly determines the reaction region and strongly influences the U-shaped formation. When the gap is less than 2 mm, the electric thrust is homogeneous throughout the entire region, resulting in a columnar shape. However, when the gap is larger than 2 mm or O2 is added, the electric thrust in the edge region becomes greater than that in the middle, leading to the U-shaped formation. Furthermore, in He PDBD, the charged particles generating electric thrust are mainly electrons and helium ions, while in He/O2 PDBD those that generate electric thrust at the outer edge of the electrode surface are mainly various oxygen-containing ions.
Meng SUN et al 2024 Plasma Sci. Technol. 26 064006
The characteristics of the blue core phenomenon observed in a divergent magnetic field helicon plasma are investigated using two different helical antennas, namely right-handed and left-handed helical antennas. The mode transition, discharge image, spatial profiles of plasma density and electron temperature are diagnosed using a Langmuir probe, a Nikon D90 camera, an intensified charge-coupled device camera and an optical emission spectrometer, respectively. The results demonstrated that the blue core phenomenon appeared in the upstream region of the discharge tube at a fixed magnetic field under both helical antennas. However, it is more likely to appear in a right-handed helical antenna, in which the plasma density and ionization rate of the helicon plasma are higher. The spatial profiles of the plasma density and electron temperature are also different in both axial and radial directions for these two kinds of helical antenna. The wavelength calculated based on the dispersion relation of the bounded whistler wave is consistent with the order of magnitude of plasma length. It is proved that the helicon plasma is part of the wave mode discharge mechanism.
Zhiyuan XU et al 2024 Plasma Sci. Technol. 26 044001
The environmental contamination caused by antibiotics is increasingly conspicuous due to their widespread manufacture and misuse. Plasma has been employed in recent years for the remediation of antibiotic pollution in the environment. In this work, a falling-film dielectric barrier discharge was used to degrade the antibiotic tetracycline (TC) in water. The reactor combined the gas-liquid discharge and active gas bubbling to improve the TC degradation performance. The discharge characteristics, chemical species' concentration, and degradation rates at different parameters were systematically studied. Under the optimized conditions (working gas was pure oxygen, liquid flow rate was 100 mL/min, gas flow rate was 1 L/min, voltage was 20 kV, single treatment), TC was removed beyond 70% in a single flow treatment with an energy efficiency of 145 mg/(kW·h). The reactor design facilitated gas and liquid flow in the plasma area to produce more ozone in bubbles after a single flow under pure oxygen conditions, affording fast TC degradation. Furthermore, long-term stationary experiment indicated that long-lived active species can sustain the degradation of TC. Compared with other plasma treatment systems, this work offers a fast and efficient degradation method, showing significant potential in practical industrial applications.
S N BATHGATE et al 2017 Plasma Sci. Technol. 19 083001
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.
Weisheng CUI et al 2021 Plasma Sci. Technol. 23 075402
The dielectric barrier discharge (DBD) in air at atmospheric pressure is not suitable for industrial applications due to its randomly distributed discharge filaments. In this paper, the influence of the electric field distribution on the uniformity of DBD is theoretically analyzed and experimentally verified. It is found that a certain degree of uneven electric field distributions can control the development of electron avalanches and regulate their transition to streamers in the gap. The discharge phenomena and electrical characteristics prove that an enhanced Townsend discharge can be formed in atmospheric-pressure air with a curved-plate electrode. The spectral analysis further confirms that the gas temperature of the plasma produced by the curved-plate electrode is close to room temperature, which is beneficial for industrial applications. This paper presents the relationship between the electron avalanche transition and the formation of a uniform DBD, which can provide some references for the development and applications of the DBD in the future.
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Ruchen SHU et al 2024 Plasma Sci. Technol. 26 075502
The ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate treated with radiofrequency plasma is proposed for functionalization and immobilization on polyethersulfone supports to form supported ionic liquid membranes for CO2 separation. The effects of treatment time and transmembrane pressure difference on CO2 permeance were evaluated. The best gas permeation performance was obtained with a treatment time of 10 min and the transmembrane pressure difference was 0.25 MPa. Characterization of the materials by Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy and nuclear magnetic resonance spectroscopy demonstrates that the IL is grafted with carboxyl groups and deprotonated through plasma treatment. A preliminary mechanism for the plasma treatment and facilitated transport of CO2 has been proposed on this basis.
Liang QIN et al 2024 Plasma Sci. Technol. 26 075401
In this paper, self-designed multi-hollow needle electrodes are used as a high-voltage electrode in a packed bed dielectric barrier discharge reactor to facilitate fast gas flow through the active discharge area and achieve large-volume stable discharge. The dynamic characteristics of the plasma, the generated active species, and the energy transfer mechanisms in both positive discharge (PD) and negative discharge (ND) are investigated by using fast-exposure intensified charge coupled device (ICCD) images and time-resolved optical emission spectra. The experimental results show that the discharge intensity, number of discharge channels, and discharge volume are obviously enhanced when the multi-needle electrode is replaced by a multi-hollow needle electrode. During a single voltage pulse period, PD mainly develops in a streamer mode, which results in a stronger discharge current, luminous intensity, and E/N compared with the diffuse mode observed in ND. In PD, as the gap between dielectric beads changes from 0 to 250 μm, the discharge between the dielectric bead gap changes from a partial discharge to a standing filamentary micro-discharge, which allows the plasma to leave the local area and is conducive to the propagation of surface streamers. In ND, the discharge only appears as a diffusion-like mode between the gap of dielectric beads, regardless of whether there is a discharge gap. Moreover, the generation of excited states and is mainly observed in PD, which is attributed to the higher E/N in PD than that in ND. However, the generation of the radical in ND is higher than in PD. It is not directly dominated by E/N, but mainly by the resonant energy transfer process between metastable and . Furthermore, both PD and ND demonstrate obvious energy relaxation processes of electron-to-vibration and vibration-to-vibration, and no vibration-to-rotation energy relaxation process is observed.
Yang ZHAO et al 2024 Plasma Sci. Technol. 26 075402
Enhancing plasma uniformity can be achieved by modifying coil and chamber structures in radio frequency inductively coupled plasma (ICP) to meet the demand for large-area and uniformly distributed plasma in industrial manufacturing. This study utilized a two-dimensional self-consistent fluid model to investigate how different coil configurations and chamber aspect ratios affect the radial uniformity of plasma in radio frequency ICP. The findings indicate that optimizing the radial spacing of the coil enhances plasma uniformity but with a reduction in electron density. Furthermore, optimizing the coil within the ICP reactor, using the interior point method in the Interior Point Optimizer significantly enhances plasma uniformity, elevating it from 56% to 96% within the range of the model sizes. Additionally, when the chamber aspect ratio k changes from 2.8 to 4.7, the plasma distribution changes from a center-high to a saddle-shaped distribution. Moreover, the plasma uniformity becomes worse. Finally, adjusting process parameters, such as increasing source power and gas pressure, can enhance plasma uniformity. These findings contribute to optimizing the etching process by improving plasma radial uniformity.
Tatiana HABIB et al 2024 Plasma Sci. Technol. 26 075505
Homogeneous gold nanoparticles were synthesized under atmospheric pressure using a non-thermal helium plasma jet in a single-step process. A current power supply was used to generate the plasma discharge rich in diverse reactive species. These species induce rapid chemical reactions responsible for the reduction of the gold salts upon contact with the liquid solution. In this study, spherical and monodispersed gold nanoparticles were obtained within 5 min of plasma exposure using a solution containing gold (III) chloride hydrate (HAuCl4) as a precursor and polyvinylpyrrolidone (PVP) as a capping agent to inhibit agglomerations. The formation of these metal nanoparticles was initially perceptible through a visible change in the sample's color, transitioning from light yellow to a red/pink color. This was subsequently corroborated by UV-vis spectroscopy, which revealed an optical absorption in the 520‒550 nm range for Au NPs, corresponding to the surface plasmon resonance (SPR) band. An investigation into the impact of various parameters, including plasma discharge duration, precursor and capping agent concentrations, was carried out to optimize conditions for the formation of well-separated, spherical gold nanoparticles. Dynamic light scattering (DLS) was used to measure the size of these nanoparticles, transmission electron microscopy (TEM) was used to observe their morphology and X-ray diffraction (XRD) was also employed to determine their crystallographic structure. The results confirm that homogeneous spherical gold nanoparticles with an average diameter of 13 nm can be easily synthesized through a rapid, straightforward, and environmentally friendly approach utilizing a helium atmospheric pressure plasma.
Min YANG et al 2024 Plasma Sci. Technol. 26 075001
During spacecraft re-entry, the challenge of measuring plasma sheath parameters directly contributes to difficulties in addressing communication blackout. In this work, we have discovered a phenomenon of multiple peaks in reflection data caused by the inhomogeneous plasma. Simulation results show that the multi-peak points fade away as the characteristic frequency is approached, resembling a series of gradually decreasing peaks. The positions and quantities of these points are positively correlated with electron density, yet they show no relation to collision frequency. This phenomenon is of significant reference value for future studies on the spatial distribution of plasmas, particularly for using microwave reflection signals in diagnosing the plasma sheath.
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Jiacheng LI et al 2023 Plasma Sci. Technol. 25 093001
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.
Heping LI et al 2022 Plasma Sci. Technol. 24 093001
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.
Zhengxiong WANG et al 2022 Plasma Sci. Technol. 24 033001
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.
Zimu XU et al 2020 Plasma Sci. Technol. 22 103001
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.
Min JIANG et al 2020 Plasma Sci. Technol. 22 080501
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.
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Ye et al
A non-contact method for millimeter-scale inspection of materials' surface flatness via Laser-Induced Breakdown Spectroscopy (LIBS) is investigated experimentally. The experiment is performed using a planished surface of an alloy steel sample to simulate its various flatness, ranging from 0 to 4.4 mm, by adjusting the laser focal plane to the surface distance with a step length of 0.2 mm. It is found that LIBS measurements are successful in inspecting the flatness differences among these simulated cases, implying that the method investigated here is feasible. It is also found that, for achieving the inspection of surface flatness within such a wide range, when univariate analysis is applied, a piecewise calibration model has to be constructed due to the complex dependence of plasma formation conditions on the surface flatness, which inevitably complicates the inspection procedure. To solve the problem, a multivariate analysis with the help of Back-Propagation Neural Network (BPNN) algorithms is applied to further construct the calibration model. By detailed analysis of the model performance, we demonstrate that a unified calibration model can be well established based on BPNN algorithms for unambiguous millimeter-scale range inspection of surface flatness with a resolution of about 0.2 mm.
Dong et al
A three-fluid equilibrium plasma with bulk plasma and energetic electrons has been observed on the Xuanlong-50 (EXL-50) spherical torus, where the energetic electrons play a crucial role in sustaining the plasma current and pressure. In this study, the equilibrium of a multi-fluid plasma was investigated by analyzing the relationship between the external vertical magnetic field (B<sub>V</sub >), plasma current (Ip), the poloidal ratio (βp) and the Shafranov formula. Remarkably, our research demonstrates some validity of the Shafranov formula in the presence of multi-fluid plasma in EXL-50 spherical torus. This finding holds significant importance for future reactors as it allows for differentiation between alpha particles and background plasma. The study of multi-fluid plasma provides a significant reference value for the equilibrium reconstruction of burning plasma involving alpha particles.
Ji et al
A vacuum ultraviolet (VUV) spectroscopy with a focal length of 1 m has been engineered specifically for observing edge impurity emissions in Experimental Advanced Superconducting Tokamak (EAST). In this study, wavelength calibration for the VUV spectroscopy is achieved utilizing a zinc lamp. The grating angle and CCD position are carefully calibrated for different wavelength positions. The wavelength calibration of the VUV spectroscopy is crucial for improving the accuracy of impurity spectral data, and is required to identify more impurity spectral lines for impurity transport research. Impurity spectra of EAST plasmas have also been obtained in the wavelength range of 50–300 nm with relatively high spectral resolution. It is found that the impurity emissions in the edge region are still dominated by low-Z impurities, such as carbon, oxygen, and nitrogen, albeit with the application of full-tungsten divertors on the EAST tokamak.
Zhang et al
The linear and nonlinear simulations are carried out using the gyrokinetic code NLT for the electrostatic instabilities in the core region of a deuterium plasma based on the ITER baseline scenario. The kinetic electron effects on the linear frequency and nonlinear transport are studied by adopting the adiabatic electron (ae) model and the fully drift-kinetic electron (ke) model in the NLT code, respectively. The linear simulations focus on the dependence of linear frequency on the plasma parameters, such as the ion and electron temperature gradients κTi,e≡R/LTi,e, the density gradient κn≡R/Ln, and the ion-electron temperature ratio τ=Te/Ti with Te and Ti the electron and ion temperatures, respectively. Here, R is the major radius, LA=(-∂rln A)-1 denotes the gradient scale length. In the ke model, the ion temperature gradient (ITG) instability and the trapped electron mode (TEM) dominate in the small and large kθ region, respectively, where kθ is the poloidal wavenumber. The TEM-dominant region becomes wider by increasing (decreasing) κTe (κTi) or by decreasing κn. For the nominal parameters of ITER baseline scenario, the maximum growth rate of dominant ITG instability in the ke model is about 3 times larger than that in the ae model. The normalized linear frequency depends on the value of τ, instead of the value of Te or Ti in both the ae and ke models. The nonlinear simulation results show that the ion heat diffusivity in the ke model is quite larger than that in the ae model, the radial structure is finer and the time oscillation is more rapid. Besides, the magnitude of fluctuated potential at the saturated stage peaks in the ITG-dominated region, contributions from the TEM dominated in higher kθ region to the nonlinear transport can be neglected. The zonal radial electric field is found to be mainly driven by the turbulent energy flux, the contribution of turbulent poloidal Reynolds stress is quite small due to the toroidal shielding effect. The mechanism of turbulence-driven zonal radial electric field is not affected by the kinetic electron effects.
Wang et al
The first mirror (FM) cleaning, using radio frequency plasma, has been proposed to recover the FM reflectivity in nuclear fusion reactors such as ITER. To investigate the influence of simultaneous cleaning of two mirrors on mirror cleaning efficiency and uniformity, experiments involving single-mirror cleaning and dual-mirror cleaning were conducted using radio frequency (RF) capacitively coupled plasma in the laboratory. For the test and simultaneous cleaning of two mirrors, the FM and second mirror (SM), both measuring 110 × 80 mm2, were placed inside the first mirror unit (FMU). They were composed of 16 mirror samples with a dimension of 27.5 × 20 mm2 each. These mirror samples consist of a titanium-zirconium-molybdenum (TZM) alloy substrate, a 500 nm molybdenum intermediate layer, and a 30 nm aluminum oxide (Al2O3) surface coating as a proxy for Be impurities. The cleaning of a single first mirror (SFM) and the simultaneous cleaning of FM and SM (DFM and DSM) were lasted for 9 h using argon (Ar) plasma at a pressure of 1 Pa. The total reflectivity of mirror samples on the DSM did not fully recover and varied with locations with a self-bias of −140 V. While with a self-bias of −300 V, the total reflectivity of mirror samples on SFM and DFM was fully recovered. The energy dispersive spectrometer (EDS) results demonstrated that the Al2O3 coating had been completely removed from these mirror samples. However, the mass loss of each mirror sample on SFM and DFM before and after cleaning varied depending on their locations, with higher mass loss observed for mirror samples located in the corners and lower for those in the center. Compared to the single mirror cleaning, the simultaneous cleaning of two mirrors reduced the difference of the mass loss between the highest and lowest. Furthermore, this mass loss for the mirror samples of DFM facing the DSM was increased. It indicated that the simultaneous cleaning of mirror samples face to face in the FMU could influence each other, highlighting the necessity for special attention in future studies.