Laser-assisted ablation and plasma formation of titanium explored by LIBS,QCM, optical microscopy and SEM analyses along with mechanical modifications under different pressures of Ar and Ne

This study deals with the investigation of Nd:YAG laser-assisted ablation and plasma formation of Ti at irradiance of 0.85 GW cm^-2 under Ar and Ne environment at various pressures ranging from 10-120 Torr.Laser-induced breakdown spectroscopy is used to evaluate plasma parameters,whereas quartz crystal microbalance is used for ablation yield measurements.The crater depth is evaluated by optical microscopy.The surface features are explored by scanning electron microscope(SEM) analysis a...     

Design of Thomson scattering diagnostic system on linear magnetized plasma device

In addition to the magnetic confinement fusion plasma, Thomson scattering has been applied to measure electron density and temperature of low-temperature plasmas. Based on a linear magnetized plasma device, a set of Thomson scattering diagnostic system is designed to diagnose the plasma with ${n}_{{\rm{e}}}={10}^{18}{\unicode{x02013}}{10}^{19}\,{{\rm{m}}}^{-3}$ and ${T}_{{\rm{e}}}=2{\unicode{x02013}}5$ eV. Due to low plasma temperature and density, this diagnostic system needs high spectral resolution and collection efficiency to meet the requirements of electron velocity distribution function measurements. Through the bench test, it is confirmed that the spectral resolution reaches 0.01 nm, and theoretical collection efficiency is high enough to obtain a Thomson scattering spectrum by 1000 accumulations.     

Microwave-assisted pre-ionization experiments on GLAST-III

GLAss Spherical Tokamak (GLAST-III) is a spherical tokamak with an insulating vacuum vessel that has a unique single-passage capability for incident microwaves. In this work, electron cyclotron resonance heating (ECRH)-assisted plasma pre-ionization in GLAST-III is explored for three radio-frequency (RF) polarizations (the O-, X-, and M-modes) at different toroidal-field (TF) strengths and filled gas pressures. The optimum hydrogen pressure is identified for efficient plasma pre-ionization. A comparison of the plasma pre-ionizations initiated by the O-, X-, and M-modes shows prominent differences in the breakdown time, location, and wave absorption. In the case of O-mode polarization, microwave absorption occurs for a relatively shorter duration, resulting in a bell-shaped electron-temperature $\left({T}_{{\rm{e}}}\right)$ temporal profile. Microwave absorption is dominant in the case of the X-mode, leading to a broader ${T}_{{\rm{e}}}$ temporal profile. The M-mode discharge contains features of both the X- and O-modes. Efficient plasma pre-ionization is achieved in the X-mode polarization for the intermediate TF strengths (with a central toroidal magnetic field ${B}_{{\rm{0}}}=0.075\,{\rm{{\rm T}}}$). Traces of the electron-number density show a similar tendency, as revealed by ${T}_{{\rm{e}}}.$ These results suggest that the X-mode is the best candidate for efficient plasma pre-ionization at low filled gas pressures $\left(\sim {{\rm{10}}}^{-2}\,{\rm{Pa}}\right)$ in small tokamaks.     

Estimation of China Fusion Engineering Test Reactor performance and burning fraction in different pellet fueling scenarios by a multi-species radial transport model

Tritium self-sufficiency in future deuterium–tritium fusion reactors is a crucial challenge. As an engineering test reactor, the China Fusion Engineering Test Reactor requires a burning fraction of 3% for the goal to test the accessibility to the future fusion plant. To self-consistently simulate burning plasmas with profile changes in pellet injection scenarios and to estimate the corresponding burning fraction, a one-dimensional multi-species radial transport model is developed in the BOUT++ framework. Several pellet-fueling scenarios are then tested in the model. The results show that the increased fueling depth improves the burning fraction by particle confinement improvement and fusion power increase. Nevertheless, by increasing the depth, the pellet cooling-down may significantly lower the temperature in the core region. Taking the density perturbation into consideration, the reasonable parameters of the fueling scenario in these simulations are estimated as pellet radius ${r}_{{\rm{p}}}=3\,{\rm{mm}},$ injection rate $=\,4\,\mathrm{Hz},$ and pellet injection velocity $=\,1000\mbox{--}2000\,{\rm{m}}\,{{\rm{s}}}^{-1}$ without drift or $450\,{\rm{m}}\,{{\rm{s}}}^{-1}$ with high-field-side drift.     

Axial uniformity diagnosis of coaxial surface wave linear plasma by optical emission spectroscopy

The coaxial surface wave linear plasma with preeminent axial uniformity is developed with the 2.45 GHz microwave generator. By optical emission spectroscopy, parameters of the argon linear plasma with a length over 600 mm are diagnosed under gas pressure of 30 and 50 Pa and different microwave powers. The spectral lines of argon and Hβ (486.1 nm) atoms in excited state are observed for estimating electron excitation temperature and electron density. Spectrum bands in 305–310 nm of diatomic OH (${{\rm{A}}}^{2}{{\rm{\Sigma }}}^{+}-{{\rm{X}}}^{2}{{\rm{\Pi }}}_{{\rm{i}}}$) radicals are used to determine the molecule rotational temperature. Finally, the axial uniformity of electron density and electron excitation temperature are analyzed emphatically under various conditions. The results prove the distinct optimization of compensation from dual powers input, which can narrow the uniform coefficient of electron density and electron excitation temperature by around 40% and 22% respectively. With the microwave power increasing, the axial uniformity of both electron density and electron excitation temperature performs better. Nevertheless, the fluctuation of electron density along the axial direction appeared with higher gas pressure. The axial uniformity of coaxial surface wave linear plasma could be controlled by pressure and power for a better utilization in material processing.     

Discharge characteristics and reactive species production of unipolar and bipolar nanosecond pulsed gas–liquid discharge generated in atmospheric N_2

In this paper,unipolar pulse (including positive pulse and negative pulse) and bipolar pulse voltage are employed to generate diffuse gas–liquid discharge in atmospheric N_2with a rumpetshaped quartz tube.The current–voltage waveforms,optical emission spectra of excited state active species,FTIR spectra of exhaust gas components,plasma gas temperature,and aqueous H_2O_2,NO_2~-,andNO_3~-production are compared in three pulse modes,meanwhile,the effects of pulse peak voltage and gas flow rate on the production of reactive species are studied.The results show that two obvious discharges occur in each voltage pulse in unipolar pulse driven discharge,differently,in bipolar pulse driven discharge,only one main discharge appears in a single voltage pulse time.The intensities of active species (OH(A),and O(3p)) in all three pulsed discharge increase with the rise of pulse peak voltage and have the highest value at 200 ml min~(-1)of gas flow rate.The absorbance intensities of NO_2and N_2O increase with the increase of pulse peak voltage and decrease with the increase of gas flow rate.Under the same discharge conditions,the bipolar pulse driven discharge shows lower breakdown voltage,and higher intensities of excited species (N_2(C),OH(A),and O(3p)),nitrogen oxides (NO_2,NO,and N_2O),and higher production of aqueous H_2O_2,NO_2~-,andNO_3~-compared with both unipolar positive and negative discharges.     

Quality enhancement and microbial reduction of mung bean (Vigna radiata) sprouts by non-thermal plasma pretreatment of seeds

Mung bean (Vigna radiata) sprouts are widely consumed worldwide due to their high nutritional value. However, the low yield and microbial contamination of mung bean sprouts seriously reduces their economic value. This study investigates the effects of non-thermal plasma on the quality and microbial reduction of mung bean sprouts by pretreatment of seeds in water for different times (0, 1, 3 and 6 min). The quality results showed that short-time plasma treatment (1 and 3 min) promoted seed germination and seedling growth, whereas long-time plasma treatment (6 min) had inhibitory effects. Plasma also had a similar dose effects on the total flavonoid and phenolic contents of mung bean sprouts. The microbiological results showed that plasma treatment achieved a reduction of native microorganisms ranging from 0.54 to 7.09 log for fungi and 0.29 to 6.80 log for bacteria at 96 h incubation. Meanwhile, plasma treatment could also efficiently inactivate artificially inoculated Salmonella typhimurium (1.83–6.22 log) and yeast (0.53–3.19 log) on mung bean seeds. The results of seed coat permeability tests and scanning electron microscopy showed that plasma could damage the seed coat structure, consequently increasing the electrical conductivity of mung bean seeds. The physicochemical analysis of plasma-treated water showed that plasma generated various long- and short-lived active species [nitric oxide radicals (NO·), hydroxyl radicals (·OH), singlet oxygen (1O2), hydrogen peroxide (H2O2), nitrate (${{\rm{NO}}}_{3}^{-}$), and nitrite (${{\rm{NO}}}_{2}^{-}$)] in water, thus the oxidizability, acidity and conductivity of plasma-treated water were all increased in a treatment time-dependent manner. The result for mimicked chemical mixtures confirmed the synergistic effect of activity of H2O2, ${{\rm{NO}}}_{3}^{-}$ and ${{\rm{NO}}}_{2}^{-}$ on bacterial inactivation and plant growth promotion. Taken together, these results imply that plasma pretreatment of mung bean seeds in water with moderate oxidizability and acidity is an effective method to improve the yield of mung bean sprouts and reduce microbial contamination.     

Effect of Various Parameters on Edge Plasma Stability in IR-T1 Tokamak

Edge plasma turbulence was investigated over a wide range of plasma and field parameters in the IR-T1 tokamak for the first time. Fluctuation levels and spectra were measured using two arrays of Langmuir probe in the region r/a = 0.75–1.2. Under almost all conditions the edge plasma was turbulently unstable, with a broadband fluctuation spectrum in the drift wave range of frequencies f = 10–1000 kHz. A stable state was observed only in the very cold, low-current discharge formed at unusually high neutral filling pressure. Otherwise, the relative fluctuation level as monitored by the ion saturation current was very high, in the range $ {{\tilde{\rm J}^{ + } } \mathord{\left/ {\vphantom {{\tilde{\rm J}^{ + } } {\bar{\rm J}^{ + } }}} \right. \kern-0pt} {\bar{\rm J}^{ + } }} \cong 0.2{-}0.8 $ , while the fluctuation power spectra were roughly invariant in shape. The relative fluctuation level was always highest near the wall and decreased monotonically toward the plasma centre.     

The application of a helicon plasma source in reactive sputter deposition of tungsten nitride thin films

A reactive helicon wave plasma (HWP) sputtering method is used for the deposition of tungsten nitride (WNx) thin films. N2 is introduced downstream in the diffusion chamber. The impacts of N2 on the Ar-HWP parameters, such as ion energy distribution functions (IEDFs), electron energy probability functions (EEPFs), electron temperature (Te) and density (ne), are investigated. With the addition of N2, a decrease in electron density is observed due to the dissociative recombination of electrons with ${{\rm{N}}}_{2}^{+}.$ The similar IEDF curves of Ar+ and N2+ indicate that the majority of ${{\rm{N}}}_{2}^{+}$ stems from the charge transfer in the collision between Ar+ and N2. Moreover, due to the collisions between electrons and N2 ions, EEPFs show a relatively lower Te with a depletion in the high-energy tail. With increasing negative bias from 50 to 200 V, a phase transition from hexagonal WN to fcc-WN0.5 is observed, together with an increase in the deposition rate and roughness     

Mode structure symmetry breaking of reversed shear Alfvén eigenmodes and its impact on the generation of parallel velocity asymmetries in energetic particle distribution

In this work, the gyrokinetic eigenvalue code LIGKA, the drift-kinetic/MHD hybrid code HMGC and the gyrokinetic full-f code TRIMEG-GKX are employed to study the mode structure details of reversed shear Alfvén eigenmodes (RSAEs). Using the parameters from an ASDEX-Upgrade plasma, a benchmark with the three different physical models for RSAE without and with energetic particles (EPs) is carried out. Reasonable agreement has been found for the mode frequency and the growth rate. Mode structure symmetry breaking (MSSB) is observed when EPs are included, due to the EPs' non-perturbative effects. It is found that the MSSB properties are featured by a finite radial wave phase velocity, and the linear mode structure can be well described by an analytical complex Gaussian expression ${\rm{\Phi }}(s)={{\rm{e}}}^{-\sigma {\left(s-{s}_{0}\right)}^{2}}$ with complex parameters σ and s0, where s is the normalized radial coordinate. The mode structure is distorted in opposite manners when the EP drive shifted from one side of ${q}_{\min }$ to the other side, and specifically, a non-zero average radial wave number 〈ks〉 with opposite signs is generated. The initial EP density profiles and the corresponding mode structures have been used as the input of HAGIS code to study the EP transport. The parallel velocity of EPs is generated in opposite directions, due to different values of the average radial wave number 〈ks〉, corresponding to different initial EP density profiles with EP drive shifted away from the ${q}_{\min }$.     

Effect of rotating liquid samples on dynamic propagation and aqueous activation of a helium plasma jet

This paper aims to explore the effects of a rotating plasma-activated liquid on the dynamic propagation and biomedical application of a helium plasma jet. The spatial distribution of reactive species and the associated physico-chemical reactions are altered by the rotating liquid, which shows a significant weakening in the axial propagation of the plasma bullet and a strengthening in its radial expansion at the liquid surface. The phenomenon is prompted by the nonzero rotational velocity of the liquid and is regulated by airflow, target distance and liquid permittivity. The concentrations of aqueous reactive species, especially OH and ${{\rm{O}}}_{2}^{-},$ and the inactivation effectiveness on cancer cells are weakened, indicating that a rotating liquid is not conducive to water treatment of the plasma jet although the treatment area of the plasma jet increases dynamically. This finding is of significance for the plasma–liquid interaction and the biomedical-related applications of plasma jets.     

Influence of anode temperature on ignition performance of the IRIT4-2D iodine-fueled radio frequency ion thruster

This paper reports the ignition performance of the iodine-fueled radio frequency (RF) ion thruster (IRIT) at different anode temperatures (${T}_{{\rm{a0}}}$). The experimental results show that the anode temperature plays important role on the ignition process of the IRIT. There were two characteristic temperatures related to the anode: the minimum ignition temperature (${T}_{{\rm{i0}}}$) and the stable ignition temperature (${T}_{{\rm{is}}}$), which were much lower than the pipeline temperature and the storage tank temperature. At ${T}_{{\rm{a0}}}$ < ${T}_{{\rm{i0}}},$ it failed to discharge. When ${T}_{{\rm{i0}}}$ ≤ ${T}_{{\rm{a0}}}$<${T}_{{\rm{is}}},$ it was ignited with dramatical oscillations. At ${T}_{{\rm{a0}}}$ ≥ ${T}_{{\rm{is}}},$ the discharge was stable in a large anode temperature range. The analysis showed that the different discharge phenomena at different anode temperatures were attributed to the change of iodine flow rate during the process of the iodine deposition-clogging and sublimation-clearing inside the thruster. The research helps improve the preheating design of the iodine-fueled electric thruster.     

Density Profiles in the Levitated Dipole Experiment

A multi-chord interferometer (center frequency 60 GHz) has been constructed to measure the electron density profiles of plasmas in the Levitated Dipole Experiment (LDX). Theoretical considerations suggest that the density of a dipole-confined plasma will vary with radius as 1/r ⁴. Measurements have been made for LDX plasmas, where the dipole-field was produced by a coil that was not levitating but rather suspended from a central column by thin supports. A ray-tracing code has been written to fit the chord data, which are line-integrated densities, to azimuthally-symmetric electron density profiles n _e (r, t). Initial analysis has focused on the model , where A(t) and α(t) are free parameters; α(t) is referred to as the “steepness exponent.” The density profiles are observed to exhibit dynamics due to ECR heating and neutral-gas fueling. The model-fit, meant only to serve as a rough approximation, suggests that LDX plasmas have steepness exponents in the range of 1.5–4. The density at the location of the 2.45 GHz ECRH resonance is reconstructed and found to be of the same order of magnitude as the heating cut-off .     

The effects of dilution gas on nanoparticle growth in atmospheric-pressure acetylene microdischarges

A two-dimensional multi-fluid model is developed to investigate the effects of dilution gas on microplasma properties and nanoparticle behavior in atmospheric-pressure radio-frequency acetylene discharges. The percentage of dilution gases (argon and helium) percentage varied from 0% to 90%, with the pressure kept constant. Simulation results show that the dilution gas percentage has a significant influence on the spatial distributions of the electron density and temperature, as well as on the formation of nanoparticles in acetylene microplasmas. With increasing dilution gas percentage, the electron density profile changes continuously from being high at the edge to high in the center. A mode transition from a mixed discharge mode with both α regime and drift-ambipolar regime into α regime occurs, which is associated with a sudden decrease in the electron density of the presheaths and an increase in the electron temperature of the bulk plasma. The mode transition point corresponds to the lowest number density ratio of hydrocarbon ions to acetylene. The highest number density ratio is observed at a dilution percentage of 90%, and causes more effective nucleation and coagulation of nanoparticles. Furthermore, owing to the high ionization potential of helium, the transition point moves to a larger dilution gas percentage in ${{\rm{C}}}_{2}{{\rm{H}}}_{2}$/He microplasmas. Finally, the growth of nanoparticles via coagulation is studied.     

The Effect of Plasma Profiles on the Critical Value of $$n\uptau_{E}$$ for Ignition

The effect of plasma profiles for ignition condition in a stationary D–T plasma is investigated using the energy conservation equations for ions and electrons, assuming that steady state fusion power is produced with no external power. The alpha power heating is sufficiently large to sustain the plasma and to balance the combined Bremsstarhlung and thermal conduction losses. The space dependent Lawson criteria is derived and critical condition is identified. As a result of this analysis we have shown that the optimum temperature might be \(\bar{T} \approx 26\,{\text{keV}}\) and that the peaked profiles with \(n\sim\left( {1 - \frac{{r^{2} }}{{a^{2} }}} \right)^{{v_{n} }}\), ν _n  = 1, and \(T\sim\left( {1 - \frac{{r^{2} }}{{a^{2} }}} \right)^{{v_{T} }} ,\,v_{T} = 2\) are good to minimizing \(\bar{n}\uptau_{E}\) for ignition. The results for these profiles show the critical value of \((\bar{n}\uptau_{E} )_{min} = 0.08 \times 10^{20 } \,{\text{m}}^{ - 3} \,{\text{s}}\) showing the reduction by 1/3 from the reference value limit ν _n  = ν _T  = 0. For a 26 keV plasma with an energy confinement time of 1 s, a pressure of about 6.24 atm is required for the plasma to be ignited; that is, it is sustained purely by the self-heating of the fusion alpha particles.     

Saturation characteristics of low voltage ionization chamber filled with argon or xenon

Argon and xenon are both attractive working gas for low voltage ionization chamber (LVIC), which is a promising candidate for ITER X-ray detectors. In this work, the performances of Ar-filling LVIC (Ar-LVIC) and Xe-filling LVIC (Xe-LVIC) as well as the impacts of operation parameters were investigated. Saturation curves of Ar-LVIC and Xe-LVIC with pressure from 0.4 to 1.2 bar were measured with a tungsten X-ray source. The minimum voltage of saturation region (${V}_{{\rm{\min }}}$) of Ar-LVIC and Xe-LVIC, the relationship between ${V}_{{\rm{\min }}}$ and saturation current, the ideal operating voltage in ITER and impacts of pressure on saturation current were studied. It was found that Ar-LVIC had smaller ${V}_{{\rm{\min }}}$ and saturation currents which decreased with the drop of pressure from 1.2 to 0.4 bar; Xe-LVIC had larger ${V}_{{\rm{\min }}}$ and saturation currents which did not obviously decrease with the same pressure drop. It is envisaged that ITER can take advantage of the larger saturation current and lower pressure of Xe-LVIC in the non-nuclear operation phase, and flexibility of pressure and low sensitivity to neutron/gamma radiation of Ar-LVIC in the nuclear operation phase.     

Scaling Laws of Nitrogen Soft X-ray Yields from 1 to 200 kJ Plasma Focus

Numerical experiments are carried out systematically to determine the nitrogen soft X-ray yield for optimized nitrogen plasma focus with storage energy E₀ from 1 to 200 kJ. Scaling laws on nitrogen soft X-ray yield, in terms of storage energies E₀, peak discharge current I_peak and focus pinch current I_pinch were found. It was found that the nitrogen X-ray yields scales on average with $ {\text{Y}}_{\text{sxr,N}} = 1.93 \times {\text{E}}_{0}^{1.21} {\text{J}} $ (E₀ in kJ) with the scaling showing gradual deterioration as E₀ rises over the range. A more robust scaling is $ {\text{Y}}_{\text{sxr}} = 8 \times 10^{ - 8} {\text{I}}_{\text{pinch}}^{3.38} $ . The optimum nitrogen soft X-ray yield emitted from plasma focus is found to be about 1 kJ for storage energy of 200 kJ. This indicates that nitrogen plasma focus is a good water-window soft X-ray source when properly designed.     

Bright γ-ray source with large orbital angular momentum from the laser near-critical-plasma interaction

We propose a new laser-plasma-based method to generate bright γ-rays carrying large orbital angular momentum by interacting a circularly polarized Laguerre–Gaussian laser pulse with a near-critical hydrogen plasma confined in an over-dense solid tube. In the first stage of the interaction, it is found via fully relativistic three-dimensional particle-in-cell simulations that high-energy helical electron beams with large orbital angular momentum are generated. In the second stage, this electron beam interacts with the laser pulse reflected from the plasma disc behind the solid tube, and helical γ beams are generated with the same topological structure as the electron beams. The results show that the electrons receive angular momentum from the drive laser, which can be further transferred to the γ photons during the interaction. The γ beam orbital angular momentum is strongly dependent on the laser topological charge l and laser intensity a0, which scales as ${L}_{\gamma }\propto {a}_{0}^{4}$. A short (duration of 5 fs) isolated helical γ beam with an angular momentum of −3.3 × 10⁻¹⁴ kg m² s⁻¹ is generated using the Laguerre–Gaussian laser pulse with l = 2. The peak brightness of the helical γ beam reaches 1.22 × 10²⁴ photons s⁻¹ mm⁻² mrad⁻² per 0.1% BW (at 10 MeV), and the laser-to-γ-ray angular momentum conversion rate is approximately 2.1%.     

Experimental and numerical simulation study of the effect for the anode positions on the discharge characteristics of 300 W class low power Hall thrusters

Low-power Hall thruster(LHT) generally has poor discharge efficiency characteristics due to the large surface-to-volume ratio.Aiming to further refine and improve the performance of 300 W class LHT in terms of thrust and efficiency,and to obtain the most optimal operating point,the experimental study of the discharge characteristics for three different anode positions was conducted under the operation of various discharge voltages(100-400 V) and anode mass flow rates(0.65 mg·s^-1 and 0...