In this work,the effects of the frequency,pressure,gas composition,and secondary-electron emission coefficient on the discharge mode in capacitively coupled Ar/O_(2) plasmas were carefully studied through simulations....In this work,the effects of the frequency,pressure,gas composition,and secondary-electron emission coefficient on the discharge mode in capacitively coupled Ar/O_(2) plasmas were carefully studied through simulations.Three discharge modes,i.e.,α,γ,and drift-ambipolar(DA),were considered in this study.The results show that a mode transition from theγ-DA hybrid mode dominated by theγmode to the DA-αhybrid mode dominated by the DA mode is induced by increasing the frequency from 100 k Hz to 40 MHz.Furthermore,the electron temperature decreases with increasing frequency,while the plasma density first decreases and then increases.It was found that the electronegativity increases slightly with increasing pressure in the lowfrequency region,and it increases notably with increasing pressure in the high-frequency region.It was also observed that the frequency corresponding to the mode transition fromγto DA decreased when the secondary-electron emission coefficient was decreased.Finally,it was found that increasing the oxygen content weakens theγmode and enhances the DA mode.More importantly,the density of oxygen atoms and ozone will increase greatly with increasing oxygen content,which is of great significance for industrial applications.展开更多
The influence of driving frequency on the discharge regime of a homogenous dielectric barrier discharge in argon at atmospheric pressure is studied through a one-dimensional self-consistent fluid model. The simulation...The influence of driving frequency on the discharge regime of a homogenous dielectric barrier discharge in argon at atmospheric pressure is studied through a one-dimensional self-consistent fluid model. The simulation results show that the discharge exhibits five notable discharge modes, namely the Townsend mode, stable glow mode, chaotic mode, asymmetric glow, and multiple period glow mode in a broad frequency range. The transition mechanisms of these modes should be attributed to the competition between the applied voltage and the memory voltage induced by the surface charges.展开更多
Helicon wave plasma sources have the well-known advantages of high efficiency and high plasma density, with broad applications in many areas. The crucial mechanism lies with mode transitions, which has been an outstan...Helicon wave plasma sources have the well-known advantages of high efficiency and high plasma density, with broad applications in many areas. The crucial mechanism lies with mode transitions, which has been an outstanding issue for years. We have built a fluid simulation model and further developed the Peking University Helicon Discharge code. The mode transitions, also known as density jumps, of a single-loop antenna discharge are reproduced in simulations for the first time. It is found that large-amplitude standing helicon waves(SHWs) are responsible for the mode transitions, similar to those of a resonant cavity for laser generation.This paper intends to give a complete and quantitative SHW resonance theory to explain the relationship of the mode transitions and the SHWs. The SHW resonance theory reasonably explains several key questions in helicon plasmas, such as mode transition and efficient power absorption, and helps to improve future plasma generation methods.展开更多
A one-dimensional fluid model is employed to investigate the discharge sustaining mechanisms in the capacitively coupled argon plasmas, by modulating the driving frequency in the range of 40 kHz-613 MHz. The model inc...A one-dimensional fluid model is employed to investigate the discharge sustaining mechanisms in the capacitively coupled argon plasmas, by modulating the driving frequency in the range of 40 kHz-613 MHz. The model incorporates the density and flux balance of electron and ion, electron energy balance, as well as Poisson＇s equation. In our simulation, the discharge experiences mode transition as the driving frequency increases, from the γ regime in which the discharge is maintained by the secondary electrons emitted from the electrodes under ion bombardment, to the a regime in which sheath oscillation is responsible for most of the electron heating in the discharge sustaining. The electron density and electron temperature at the centre of the discharge, as well as the ion flux on the electrode are figured out as a function of the driving frequency, to confirm the two regimes and transition between them. The effects of gas pressure, secondary electron emission coefficient and applied voltage on the discharge are also discussed.展开更多
A pipe model with a mass ratio（mass/displaced mass） of 4.30 was tested to investigate the vortex-induced vibrations of submarine pipeline spans near the seabed.The pipe model was designed as a bending stiffness-domi...A pipe model with a mass ratio（mass/displaced mass） of 4.30 was tested to investigate the vortex-induced vibrations of submarine pipeline spans near the seabed.The pipe model was designed as a bending stiffness-dominated beam.The gap ratios（gap to diameter ratio） at the pipe ends were 4.0,6.0,and 8.0.The flow velocity was systematically varied in the 0-16.71 nondimensional velocity range based on the first natural frequency.The mode transition between the first and the second mode as the flow velocity increases was investigated.At various transition flow velocities,the research indicates that the peak frequencies with respect to displacement are not identical along the pipe,nor the frequencies associated with the peak of the amplitude spectra for the first four modes as well.The mode transition is associated with a continuous change in the amplitude,but there＇s a jump in frequency,and a gradual process along the pipe length.展开更多
A new type of two-dimensional self-consistent fluid model that couples an equivalent circuit module is used to in- vestigate the mode transition characteristics and hysteresis in hydrogen inductively coupled plasmas a...A new type of two-dimensional self-consistent fluid model that couples an equivalent circuit module is used to in- vestigate the mode transition characteristics and hysteresis in hydrogen inductively coupled plasmas at different pressures, by varying the series capacitance of the matching box. The variations of the electron density, temperature, and the circuit electrical properties are presented. As cycling the matching capacitance, at high pressure both the discontinuity and hysteresis appear for the plasma parameters and the transferred impedances of both the inductive and capacitive discharge components, while at low pressure only the discontinuity is seen. The simulations predict that the sheath plays a determi- native role on the presence of discontinuity and hysteresis at high pressure, by influencing the inductive coupling efficiency of applied power. Moreover, the values of the plasma transferred impedances at different pressures are compared, and the larger plasma inductance at low pressure due to less collision frequency, as analyzed, is the reason why the hysteresis is not seen at low pressure, even with a wider sheath. Besides, the behaviors of the coil voltage and current parameters during the mode transitions are investigated. They both increase （decrease） at the E to H （H to E） mode transition, indicating an improved （worsened） inductive power coupling efficiency.展开更多
When laser ablation is subjected to supersonic flow, the influence mechanism of airflow on laser ablation behavior is still unclear. A coupled thermal-fluid-structure model is presented to investigate the influence of...When laser ablation is subjected to supersonic flow, the influence mechanism of airflow on laser ablation behavior is still unclear. A coupled thermal-fluid-structure model is presented to investigate the influence of supersonic airflow on the development of a laser ablation pit. Results show that the aerodynamic convection cooling effect not only reduces the ablation velocity but also changes the symmetry morphology of the ablation pit due to the non-uniform convective heat transfer. Flow mode transition is also observed when the pit becomes deeper, and significant change in flow pattern and heat transfer behavior are found when the open mode is transformed into the closed mode.展开更多
A Langmuir probe and an ICCD are employed to study the discharge mode transition in Ar inductively coupled plasma. Electron density and plasma emission intensity are measured during the E （capacitive discharge） to H...A Langmuir probe and an ICCD are employed to study the discharge mode transition in Ar inductively coupled plasma. Electron density and plasma emission intensity are measured during the E （capacitive discharge） to H （inductive discharge） mode transitions at different pressures. It is found that plasma exists with a low electron density and a weak emission intensity in the E mode, while it has a high electron density and a strong emission intensity in the H mode. Meanwhile, the plasma emission intensity spatial （2D an asymmetric profile in the E mode. Moreover, the at high pressure, but increase almost continuously at image） profile is symmetrical in the H mode, but the 2D image is electron density and emission intensity jump up discontinuously the E to H mode transition under low pressure.展开更多
In this paper,E-H mode transition in magnetic-pole-enhanced inductively coupled neon-argon mixture plasma is investigated in terms of fundamental plasma parameters as a function of argon fraction(0%-100%),operating pr...In this paper,E-H mode transition in magnetic-pole-enhanced inductively coupled neon-argon mixture plasma is investigated in terms of fundamental plasma parameters as a function of argon fraction(0%-100%),operating pressure(1 Pa,5 Pa,10 Pa and 50 Pa),and radio frequency(RF)power(5-100 W).An RF compensated Langmuir probe and optical emission spectroscopy are used for the diagnostics of the plasma under study.Owing to the lower ionization potential and higher collision cross-section of argon,when its fraction in the discharge is increased,the mode transition occurs at lower RF power;i.e.for 0%argon and1 Pa pressure,the threshold power of the E-H mode transition is 65 W,which reduces to 20 W when the argon fraction is increased.The electron density increases with the argon fraction at afixed pressure,whereas the temperature decreases with the argon fraction.The relaxation length of the low-energy electrons increases,and decreases for high-energy electrons with argon fraction,due to the Ramseur effect.However,the relaxation length of both groups of electrons decreases with pressure due to reduction in the mean free path.The electron energy probability function(EEPF)profiles are non-Maxwellian in E-mode,attributable to the nonlocal electron kinetics in this mode;however,they evolve to Maxwellian distribution when the discharge transforms to H-mode due to lower electron temperature and higher electron density in H-mode.The tail of the measured EEPFs is found to deplete in both E-and H-modes when the argon fraction in the discharge is increased,because argon has a much lower excitation potential(11.5 eV)than neon(16.6 eV).展开更多
This paper presents the evolution of the electronegativity with the applied power during the E to H mode transition in a radio frequency(rf)inductively coupled plasma(ICP)in a mixture of Ar and O2.The densities of the...This paper presents the evolution of the electronegativity with the applied power during the E to H mode transition in a radio frequency(rf)inductively coupled plasma(ICP)in a mixture of Ar and O2.The densities of the negative ion and the electron,as well as their ratio,i.e.,the electronegativity,are measured as a function of the applied power by laser photo-detachment combined with a microwave resonance probe,under different pressures and O2 contents.Meanwhile,the optical emission intensities at Ar 750.4 nm and O 844.6 nm are monitored via a spectrograph.It was found that by increasing the applied power,the electron density and the optical emission intensity show a similar trench,i.e.,they increase abruptly at a threshold power,suggesting that the E to H mode transition occurs.With the increase of the pressure,the negative ion density presents opposite trends in the E-mode and the H-mode,which is related to the difference of the electron density and energy for the two modes.The emission intensities of Ar 750.4 nm and O 844.6 nm monotonously decrease with increasing the pressure or the O2 content,indicating that the density of high-energy electrons,which can excite atoms,is monotonically decreased.This leads to an increase of the negative ion density in the H-mode with increasing the pressure.Besides,as the applied power is increased,the electronegativity shows an abrupt drop during the E-to H-mode transition.展开更多
We study the quasinormal modes（QNMs） of massless scalar perturbations to probe the van der Waals like SBH/LBH phase transition of anti-de Sitter black holes in five-dimensional（5D） Gauss–Bonnet gravity. It is fou...We study the quasinormal modes（QNMs） of massless scalar perturbations to probe the van der Waals like SBH/LBH phase transition of anti-de Sitter black holes in five-dimensional（5D） Gauss–Bonnet gravity. It is found that the signature of this SBH/LBH phase transition is detected when the slopes of the QNMs frequency change drastically and differently in small and large black holes near the critical point. The obtained results further support that the QNMs can be a dynamic probe to investigate the thermodynamic properties in black holes.展开更多
The key plasma parameters under different discharge modes, such as heavy-particle and electron temperatures, electron number density, and nonequilibrium volume of plasmas, play important roles in various applications ...The key plasma parameters under different discharge modes, such as heavy-particle and electron temperatures, electron number density, and nonequilibrium volume of plasmas, play important roles in various applications of gas discharge plasmas. In this study, a self-consistent two-dimensional nonequilibrium fluid model coupled with an external circuit model is established to reveal the mechanisms related to the discharge modes, including the normal glow, abnormal glow,arc, and glow-to-arc transition modes, with an atmospheric-pressure direct-current(DC) argon discharge as a model plasma system. The modeling results show that, under different discharge modes, the most significant difference between the preceding four discharge modes lies in the current and energy transfer processes on the cathode side. On one hand, the current to the cathode surface is mainly delivered by the ions coming from the plasma column under the glow discharge mode due to the low temperature of the solid cathode, whereas the thermionic and secondary electrons emitted from the hot cathode surface play a very important role under the arc mode with a higher cathode surface temperature and higher ion flux toward the cathode. On the other hand, the energy transfer channel on the cathode side changes from mainly heating the solid cathode under the glow mode to simultaneously heating both the solid cathode and plasma column under the arc mode with an increase in the discharge current. Consequently, the power density in the cathode sheath(P_c) was used as a key parameter for judging different discharge modes, and the range of(0.28–1.2) × 10^(12) W m^(-3) was determined as a critical window of P_c corresponding to the glow-to-arc-mode transition for the atmospheric-pressure DC argon discharge, which was also verified by comparison with the experimental results in this study and the data in the previous literature.展开更多
The effect of a negative DC bias,|V_(dc)|,on the electrical parameters and discharge mode is investigated experimentally in a radiofrequency(RF)capacitively coupled Ar plasma operated at different RF voltage amplitude...The effect of a negative DC bias,|V_(dc)|,on the electrical parameters and discharge mode is investigated experimentally in a radiofrequency(RF)capacitively coupled Ar plasma operated at different RF voltage amplitudes and gas pressures.The electron density is measured using a hairpin probe and the spatio-temporal distribution of the electron-impact excitation rate is determined by phase-resolved optical emission spectroscopy.The electrical parameters are obtained based on the waveforms of the electrode voltage and plasma current measured by a voltage probe and a current probe.It was found that at a low|V_(dc)|,i.e.inα-mode,the electron density and RF current decline with increasing|V_(dc)|;meanwhile,the plasma impedance becomes more capacitive due to a widened sheath.Therefore,RF power deposition is suppressed.When|V_(dc)|exceeds a certain value,the plasma changes toα–γhybrid mode(or the discharge becomes dominated by theγ-mode),manifesting a drastically growing electron density and a moderately increasing RF current.Meanwhile,the plasma impedance becomes more resistive,so RF power deposition is enhanced with|V_(dc)|.We also found that the electrical parameters show similar dependence on|V_(dc)|at different RF voltages,andα–γmode transition occurs at a lower|V_(dc)|at a higher RF voltage.By increasing the pressure,plasma impedance becomes more resistive,so RF power deposition and electron density are enhanced.In particular,theα–γmode transition tends to occur at a lower|V_(dc)|with increase in pressure.展开更多
Dynamics of the axial mode transition process in a 0.33-THz second-harmonic gyrotron is investigated to reveal the physical mechanism of realizing broadband frequency tuning in an open cavity circuit. A new interactio...Dynamics of the axial mode transition process in a 0.33-THz second-harmonic gyrotron is investigated to reveal the physical mechanism of realizing broadband frequency tuning in an open cavity circuit. A new interaction mechanism about propagating waves, featured by wave competition and wave cooperation, is presented and provides a new insight into the beam-wave interaction. The two different features revealed in the two different operation regions of low-order axial modes （LOAMs） and high-order axial modes （HOAMs） respectively determine the characteristic of the overall performance of the device essentially. The device performance is obtained by the simulation based on the time-domain nonlinear theory and shows that using a 12-kV/150-mA electron beam and TE 3,4 mode, the second harmonic gyrotron can generate terahertz radiations with frequency-tuning ranges of about 0.85 GHz and 0.60 GHz via magnetic field and beam voltage tuning, respectively. Additionally, some non-stationary phenomena in the mode startup process are also analyzed. The investiga- tion in this paper presents guidance for future developing high-performance frequency-tunable gyrotrons toward terahertz applications.展开更多
Changes of the electron dynamics in hydrogen （H2） radio-frequency （RF） inductively coupled plasmas are investigated using a hairpin probe and an intensified charged coupled device （ICCD）. The electron density, p...Changes of the electron dynamics in hydrogen （H2） radio-frequency （RF） inductively coupled plasmas are investigated using a hairpin probe and an intensified charged coupled device （ICCD）. The electron density, plasma emission intensity, and input current （voltage） are measured during the E to H mode transitions at different pressures. It is found that the electron density, plasma emission intensity, and input current jump up discontinuously, and the input voltage jumps down at the E to H mode transition points. And the threshold power of the E to H mode transition decreases with the increase of the pressure. Moreover, space and phase resolved optical emission spectroscopic measurements reveal that, in the E mode, the RF dynamics is characterized by one dominant excitation per RF cycle, while in the H mode, there are two excitation maxima within one cycle.展开更多
A helicon wave plasma source in a tube of ring permanent magnets(PMs)has been constructed to study the effect of the conflguration of the magnetic fleld with zero magnetic points on plasma parameters.This device also ...A helicon wave plasma source in a tube of ring permanent magnets(PMs)has been constructed to study the effect of the conflguration of the magnetic fleld with zero magnetic points on plasma parameters.This device also serves as an exploration platform for a simple,compact helicon wave plasma source adaptable to engineering applications.A small-diameter(26 mm)highdensity(~10^(18)m^(-3))blue core plasma is produced in~1 Pa argon by helicon RF(radiofrequency)discharge using a NagoyaⅢantenna under magnetic fleld(~2 k G)of compact ring PMs(length~204 mm).Operational parameters,i.e.RF power and neutral gas pressure are scanned and plasma density is measured by an RF compensated probe to explore the operating characteristics of the device.Iconic feature of a helicon discharge,such as blue core plasmas and E-H-W mode transitions are well observed in the device,despite the wavelength calculated using the conventional dispersion relation of a bounded whistler waves(Chen 1991 Plasma Phys.Control.Fusion 33339)is order of magnitudes longer than the length of the plasma in this device which seems to suggest that such helicon device is impossible.Surprisingly,the wavelength calculated by the unbounded whistle wave dispersion formula in turn suggests the occurrence of a half wavelength resonance.展开更多
We present here an investigation of the self-pulsing phenomenon of negative corona and parallel-plate discharge in argon within one frame of a one-dimensional fluid model in cylinder–cylinder electrode geometry.The t...We present here an investigation of the self-pulsing phenomenon of negative corona and parallel-plate discharge in argon within one frame of a one-dimensional fluid model in cylinder–cylinder electrode geometry.The transition from corona to parallel-plate discharge is obtained by changing the inner and outer radii of the electrodes.The model reproduces the self-pulsing waveform well and provides the spatiotemporal behaviors of the charged particles and electric field during the pulse.The self-pulsing shows a common feature that occurs in various configurations and that does not depend on a specific electrode structure.The self-pulsing is the transformation between a weak-current Townsend mode and a large-current normal glow mode.The behavior of the positive ions is the dominant factor in the formation of the pulse.展开更多
The tuned substrate self-bias in a radio-frequency inductively coupled plasma is controlled by varying the impedance of an external tuning LCR （inductor, capacitor and resistor） network inserted between the substrat...The tuned substrate self-bias in a radio-frequency inductively coupled plasma is controlled by varying the impedance of an external tuning LCR （inductor, capacitor and resistor） network inserted between the substrate and the ground. In experiments, it was found that the variation of the tuned substrate self-bias with the tuning capacitance demonstrated three features, namely, continuity, instability and bistability. In this paper, a numerical study is focused on the elucidation of the physical mechanisms underlying continuity and bistability. For the sake of simplicity and feasibility to include the key factors influencing the tuned substrate bias, the tedious calculation of inductive-coupling to obtain the plasma density axtd electron temperature is omitted, and discussion of the tuned substrate self-bias is made under the prescribed plasma density and electron temperature. On the other hand, the parameters influencing capacitive- coupling are retained in modeling the system with an equivalent circuit. It is found that multi-stable state appears when one of the parameters, such as the resistance in LCR, substrate area and plasma density, decreased to its critical value, or the rf voltage or electron temperature increased to the critical value individually. In the reverse cases, the tuned substrate self-bias varies continuously with the tuning capacitance.展开更多
The radio frequency (rf) self-bias of the substrate in a rf inductively coupled plasma is controlled by means of varying the impedance of an external LC network inserted between the substrate and the ground. Experimen...The radio frequency (rf) self-bias of the substrate in a rf inductively coupled plasma is controlled by means of varying the impedance of an external LC network inserted between the substrate and the ground. Experimental studies were done on the relations of the tuned substrate self-bias with varying discharge and external circuit parameters. Under a certain discharge gas pressure, the curves of tuned substrate self-bias Vtsb versus tuning capacitance Ct demonstrate jumps and hysteresises when rf discharge power is higher than a threshold. The hysteresis loop in terms of △Ctcrit1(= Ccrit1-Ccrit2, here,Ccrit1, Ccrit2 are critical capacitance magnitudes under which the tuned substrate self-bias jumps) decreases with increasing rf discharge power, while the maximum |Vtsbimn| is achieved in the middle discharge-power region. Under a constant discharge power |Vtsb min|, Ccrit1 and Ccrit2 achieve their minimums in the middle gas-pressure region. When the tuning capacitance is pre-set at a lower value, Ttsb varies slightly with gas-flow rate; in the case of tuning capacitance sufficiently approaching Ctcriti, Vtsb undergoes the jump and hysteresis with the changing gas-flow rate. By inserting a resistor R into the external network, the characteristics of Vtsb-Ct curves are changed with the reduced quality factor Q depending on resistance values. Based on inductive- and capacitive-coupling characteristics of inductively coupled plasma, the dependence of a plasma sheath on plasma parameters, and the impedance properties of the substrate branch, the observed results can be qualitatively interpreted.展开更多
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 percenta...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 bothαregime and drift-ambipolar regime intoαregime 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 C_(2)H_(2)/He microplasmas.Finally,the growth of nanoparticles via coagulation is studied.展开更多
基金supported by National Natural Science Foundation of China(Nos.11805107 and 12275039)the Fundamental Research Funds in Heilongjiang Provincial Universities of China(No.145309625)。
文摘In this work,the effects of the frequency,pressure,gas composition,and secondary-electron emission coefficient on the discharge mode in capacitively coupled Ar/O_(2) plasmas were carefully studied through simulations.Three discharge modes,i.e.,α,γ,and drift-ambipolar(DA),were considered in this study.The results show that a mode transition from theγ-DA hybrid mode dominated by theγmode to the DA-αhybrid mode dominated by the DA mode is induced by increasing the frequency from 100 k Hz to 40 MHz.Furthermore,the electron temperature decreases with increasing frequency,while the plasma density first decreases and then increases.It was found that the electronegativity increases slightly with increasing pressure in the lowfrequency region,and it increases notably with increasing pressure in the high-frequency region.It was also observed that the frequency corresponding to the mode transition fromγto DA decreased when the secondary-electron emission coefficient was decreased.Finally,it was found that increasing the oxygen content weakens theγmode and enhances the DA mode.More importantly,the density of oxygen atoms and ozone will increase greatly with increasing oxygen content,which is of great significance for industrial applications.
基金supported by the National Natural Science Foundation of China(Grant No.11205044)the Hebei Natural Science Fund,China(GrantNos.A2012201015 and A2011201006)+1 种基金the Research Foundation of Education Bureau of Hebei Province,China(Grant No.Y2012009)the Postdoctoral Science Foundation and Foundation of Hebei University,China(Grant No.2010Q30)
文摘The influence of driving frequency on the discharge regime of a homogenous dielectric barrier discharge in argon at atmospheric pressure is studied through a one-dimensional self-consistent fluid model. The simulation results show that the discharge exhibits five notable discharge modes, namely the Townsend mode, stable glow mode, chaotic mode, asymmetric glow, and multiple period glow mode in a broad frequency range. The transition mechanisms of these modes should be attributed to the competition between the applied voltage and the memory voltage induced by the surface charges.
基金supported by the National Key R&D Program of China(No.2017YFE0301201)National Natural Science Foundation of China(No.11975038)the funding support from the State Key Laboratory of Nuclear Physics and Technology,Peking University(No.NPT2021ZZ03)。
文摘Helicon wave plasma sources have the well-known advantages of high efficiency and high plasma density, with broad applications in many areas. The crucial mechanism lies with mode transitions, which has been an outstanding issue for years. We have built a fluid simulation model and further developed the Peking University Helicon Discharge code. The mode transitions, also known as density jumps, of a single-loop antenna discharge are reproduced in simulations for the first time. It is found that large-amplitude standing helicon waves(SHWs) are responsible for the mode transitions, similar to those of a resonant cavity for laser generation.This paper intends to give a complete and quantitative SHW resonance theory to explain the relationship of the mode transitions and the SHWs. The SHW resonance theory reasonably explains several key questions in helicon plasmas, such as mode transition and efficient power absorption, and helps to improve future plasma generation methods.
基金Project supported by the National Natural Science Foundation of China (Grant No. 10775025)the Scientific Research Fund of Liaoning Provincial Education Department for Colleges and Universities (Grant No. 2008T229)the Program for New Century Excellent Talents in University (Grant No. NCET-08-0073)
文摘A one-dimensional fluid model is employed to investigate the discharge sustaining mechanisms in the capacitively coupled argon plasmas, by modulating the driving frequency in the range of 40 kHz-613 MHz. The model incorporates the density and flux balance of electron and ion, electron energy balance, as well as Poisson＇s equation. In our simulation, the discharge experiences mode transition as the driving frequency increases, from the γ regime in which the discharge is maintained by the secondary electrons emitted from the electrodes under ion bombardment, to the a regime in which sheath oscillation is responsible for most of the electron heating in the discharge sustaining. The electron density and electron temperature at the centre of the discharge, as well as the ion flux on the electrode are figured out as a function of the driving frequency, to confirm the two regimes and transition between them. The effects of gas pressure, secondary electron emission coefficient and applied voltage on the discharge are also discussed.
基金Supported by the National Natural Science Foundation of China（No.41176072） the Scientific Research Fund of Hunan Provincial Education Department（No.12C0030）+1 种基金 the Program for Hu’nan Province Key Laboratory of Water,Sediment Sciences and Flood Hazard Prevention（No.2012SS07） the National Natural Science Foundation for Youth of China（No.51109018）
文摘A pipe model with a mass ratio（mass/displaced mass） of 4.30 was tested to investigate the vortex-induced vibrations of submarine pipeline spans near the seabed.The pipe model was designed as a bending stiffness-dominated beam.The gap ratios（gap to diameter ratio） at the pipe ends were 4.0,6.0,and 8.0.The flow velocity was systematically varied in the 0-16.71 nondimensional velocity range based on the first natural frequency.The mode transition between the first and the second mode as the flow velocity increases was investigated.At various transition flow velocities,the research indicates that the peak frequencies with respect to displacement are not identical along the pipe,nor the frequencies associated with the peak of the amplitude spectra for the first four modes as well.The mode transition is associated with a continuous change in the amplitude,but there＇s a jump in frequency,and a gradual process along the pipe length.
基金supported by the National Natural Science Foundation of China(Grant Nos.11175034,11205025,11305023,and 11075029)
文摘A new type of two-dimensional self-consistent fluid model that couples an equivalent circuit module is used to in- vestigate the mode transition characteristics and hysteresis in hydrogen inductively coupled plasmas at different pressures, by varying the series capacitance of the matching box. The variations of the electron density, temperature, and the circuit electrical properties are presented. As cycling the matching capacitance, at high pressure both the discontinuity and hysteresis appear for the plasma parameters and the transferred impedances of both the inductive and capacitive discharge components, while at low pressure only the discontinuity is seen. The simulations predict that the sheath plays a determi- native role on the presence of discontinuity and hysteresis at high pressure, by influencing the inductive coupling efficiency of applied power. Moreover, the values of the plasma transferred impedances at different pressures are compared, and the larger plasma inductance at low pressure due to less collision frequency, as analyzed, is the reason why the hysteresis is not seen at low pressure, even with a wider sheath. Besides, the behaviors of the coil voltage and current parameters during the mode transitions are investigated. They both increase （decrease） at the E to H （H to E） mode transition, indicating an improved （worsened） inductive power coupling efficiency.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11332011 and 11472276the Project of the Chinese Academy of Sciences,and the Defense Industrial Technology Development Program
文摘When laser ablation is subjected to supersonic flow, the influence mechanism of airflow on laser ablation behavior is still unclear. A coupled thermal-fluid-structure model is presented to investigate the influence of supersonic airflow on the development of a laser ablation pit. Results show that the aerodynamic convection cooling effect not only reduces the ablation velocity but also changes the symmetry morphology of the ablation pit due to the non-uniform convective heat transfer. Flow mode transition is also observed when the pit becomes deeper, and significant change in flow pattern and heat transfer behavior are found when the open mode is transformed into the closed mode.
基金Project supported by the National Natural Science Foundation of China (Grant Nos. 11075029 and 11175034)the Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20090041110026)the Fundamental Research Funds for Central Universities of China(Grant No. DUT11ZD109)
文摘A Langmuir probe and an ICCD are employed to study the discharge mode transition in Ar inductively coupled plasma. Electron density and plasma emission intensity are measured during the E （capacitive discharge） to H （inductive discharge） mode transitions at different pressures. It is found that plasma exists with a low electron density and a weak emission intensity in the E mode, while it has a high electron density and a strong emission intensity in the H mode. Meanwhile, the plasma emission intensity spatial （2D an asymmetric profile in the E mode. Moreover, the at high pressure, but increase almost continuously at image） profile is symmetrical in the H mode, but the 2D image is electron density and emission intensity jump up discontinuously the E to H mode transition under low pressure.
基金partially supported by Quaid-i-Azam University URF for the year 2019-2020Higher Education Commission(HEC)P.No.820 for Plasma Physics Gomal University(D I Khan)。
文摘In this paper,E-H mode transition in magnetic-pole-enhanced inductively coupled neon-argon mixture plasma is investigated in terms of fundamental plasma parameters as a function of argon fraction(0%-100%),operating pressure(1 Pa,5 Pa,10 Pa and 50 Pa),and radio frequency(RF)power(5-100 W).An RF compensated Langmuir probe and optical emission spectroscopy are used for the diagnostics of the plasma under study.Owing to the lower ionization potential and higher collision cross-section of argon,when its fraction in the discharge is increased,the mode transition occurs at lower RF power;i.e.for 0%argon and1 Pa pressure,the threshold power of the E-H mode transition is 65 W,which reduces to 20 W when the argon fraction is increased.The electron density increases with the argon fraction at afixed pressure,whereas the temperature decreases with the argon fraction.The relaxation length of the low-energy electrons increases,and decreases for high-energy electrons with argon fraction,due to the Ramseur effect.However,the relaxation length of both groups of electrons decreases with pressure due to reduction in the mean free path.The electron energy probability function(EEPF)profiles are non-Maxwellian in E-mode,attributable to the nonlocal electron kinetics in this mode;however,they evolve to Maxwellian distribution when the discharge transforms to H-mode due to lower electron temperature and higher electron density in H-mode.The tail of the measured EEPFs is found to deplete in both E-and H-modes when the argon fraction in the discharge is increased,because argon has a much lower excitation potential(11.5 eV)than neon(16.6 eV).
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11675039,11875101,and 11935005)the Fundamental Research Founds for the Central Universities,China(Grant Nos.DUT18TD06 and DUT20LAB201).
文摘This paper presents the evolution of the electronegativity with the applied power during the E to H mode transition in a radio frequency(rf)inductively coupled plasma(ICP)in a mixture of Ar and O2.The densities of the negative ion and the electron,as well as their ratio,i.e.,the electronegativity,are measured as a function of the applied power by laser photo-detachment combined with a microwave resonance probe,under different pressures and O2 contents.Meanwhile,the optical emission intensities at Ar 750.4 nm and O 844.6 nm are monitored via a spectrograph.It was found that by increasing the applied power,the electron density and the optical emission intensity show a similar trench,i.e.,they increase abruptly at a threshold power,suggesting that the E to H mode transition occurs.With the increase of the pressure,the negative ion density presents opposite trends in the E-mode and the H-mode,which is related to the difference of the electron density and energy for the two modes.The emission intensities of Ar 750.4 nm and O 844.6 nm monotonously decrease with increasing the pressure or the O2 content,indicating that the density of high-energy electrons,which can excite atoms,is monotonically decreased.This leads to an increase of the negative ion density in the H-mode with increasing the pressure.Besides,as the applied power is increased,the electronegativity shows an abrupt drop during the E-to H-mode transition.
基金Supported by the National Natural Science Foundation of China under Grant Nos 11647050,11675139 and 51575420the Scientific Research Program Funded by Shaanxi Provincial Education Department under Grant No 16JK1394
文摘We study the quasinormal modes（QNMs） of massless scalar perturbations to probe the van der Waals like SBH/LBH phase transition of anti-de Sitter black holes in five-dimensional（5D） Gauss–Bonnet gravity. It is found that the signature of this SBH/LBH phase transition is detected when the slopes of the QNMs frequency change drastically and differently in small and large black holes near the critical point. The obtained results further support that the QNMs can be a dynamic probe to investigate the thermodynamic properties in black holes.
基金supported by National Natural Science Foundation of China (No. 12075132)。
文摘The key plasma parameters under different discharge modes, such as heavy-particle and electron temperatures, electron number density, and nonequilibrium volume of plasmas, play important roles in various applications of gas discharge plasmas. In this study, a self-consistent two-dimensional nonequilibrium fluid model coupled with an external circuit model is established to reveal the mechanisms related to the discharge modes, including the normal glow, abnormal glow,arc, and glow-to-arc transition modes, with an atmospheric-pressure direct-current(DC) argon discharge as a model plasma system. The modeling results show that, under different discharge modes, the most significant difference between the preceding four discharge modes lies in the current and energy transfer processes on the cathode side. On one hand, the current to the cathode surface is mainly delivered by the ions coming from the plasma column under the glow discharge mode due to the low temperature of the solid cathode, whereas the thermionic and secondary electrons emitted from the hot cathode surface play a very important role under the arc mode with a higher cathode surface temperature and higher ion flux toward the cathode. On the other hand, the energy transfer channel on the cathode side changes from mainly heating the solid cathode under the glow mode to simultaneously heating both the solid cathode and plasma column under the arc mode with an increase in the discharge current. Consequently, the power density in the cathode sheath(P_c) was used as a key parameter for judging different discharge modes, and the range of(0.28–1.2) × 10^(12) W m^(-3) was determined as a critical window of P_c corresponding to the glow-to-arc-mode transition for the atmospheric-pressure DC argon discharge, which was also verified by comparison with the experimental results in this study and the data in the previous literature.
基金financially supported by National Natural Science Foundation of China(NSFC)(Nos.12275043 and 11935005)the Fundamental Research Funds for the Central Universities(No.DUT21TD104)China Scholarship Council(No.202106060085)。
文摘The effect of a negative DC bias,|V_(dc)|,on the electrical parameters and discharge mode is investigated experimentally in a radiofrequency(RF)capacitively coupled Ar plasma operated at different RF voltage amplitudes and gas pressures.The electron density is measured using a hairpin probe and the spatio-temporal distribution of the electron-impact excitation rate is determined by phase-resolved optical emission spectroscopy.The electrical parameters are obtained based on the waveforms of the electrode voltage and plasma current measured by a voltage probe and a current probe.It was found that at a low|V_(dc)|,i.e.inα-mode,the electron density and RF current decline with increasing|V_(dc)|;meanwhile,the plasma impedance becomes more capacitive due to a widened sheath.Therefore,RF power deposition is suppressed.When|V_(dc)|exceeds a certain value,the plasma changes toα–γhybrid mode(or the discharge becomes dominated by theγ-mode),manifesting a drastically growing electron density and a moderately increasing RF current.Meanwhile,the plasma impedance becomes more resistive,so RF power deposition is enhanced with|V_(dc)|.We also found that the electrical parameters show similar dependence on|V_(dc)|at different RF voltages,andα–γmode transition occurs at a lower|V_(dc)|at a higher RF voltage.By increasing the pressure,plasma impedance becomes more resistive,so RF power deposition and electron density are enhanced.In particular,theα–γmode transition tends to occur at a lower|V_(dc)|with increase in pressure.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.61471007,61531002,61522101,and 11275206)the Seeding Grant for Medicine and Information Science of Peking University,China(Grant No.2014-MI-01)
文摘Dynamics of the axial mode transition process in a 0.33-THz second-harmonic gyrotron is investigated to reveal the physical mechanism of realizing broadband frequency tuning in an open cavity circuit. A new interaction mechanism about propagating waves, featured by wave competition and wave cooperation, is presented and provides a new insight into the beam-wave interaction. The two different features revealed in the two different operation regions of low-order axial modes （LOAMs） and high-order axial modes （HOAMs） respectively determine the characteristic of the overall performance of the device essentially. The device performance is obtained by the simulation based on the time-domain nonlinear theory and shows that using a 12-kV/150-mA electron beam and TE 3,4 mode, the second harmonic gyrotron can generate terahertz radiations with frequency-tuning ranges of about 0.85 GHz and 0.60 GHz via magnetic field and beam voltage tuning, respectively. Additionally, some non-stationary phenomena in the mode startup process are also analyzed. The investiga- tion in this paper presents guidance for future developing high-performance frequency-tunable gyrotrons toward terahertz applications.
基金Project supported by the National Natural Science Foundation of China(Grant Nos.11075029,11175034,and 11205025)the Fundamental Research Funds for Central Universities,China(Grant No.DUT12RC(3)14)
文摘Changes of the electron dynamics in hydrogen （H2） radio-frequency （RF） inductively coupled plasmas are investigated using a hairpin probe and an intensified charged coupled device （ICCD）. The electron density, plasma emission intensity, and input current （voltage） are measured during the E to H mode transitions at different pressures. It is found that the electron density, plasma emission intensity, and input current jump up discontinuously, and the input voltage jumps down at the E to H mode transition points. And the threshold power of the E to H mode transition decreases with the increase of the pressure. Moreover, space and phase resolved optical emission spectroscopic measurements reveal that, in the E mode, the RF dynamics is characterized by one dominant excitation per RF cycle, while in the H mode, there are two excitation maxima within one cycle.
基金supported by National Natural Science Foundation of China(No.U19A20113)。
文摘A helicon wave plasma source in a tube of ring permanent magnets(PMs)has been constructed to study the effect of the conflguration of the magnetic fleld with zero magnetic points on plasma parameters.This device also serves as an exploration platform for a simple,compact helicon wave plasma source adaptable to engineering applications.A small-diameter(26 mm)highdensity(~10^(18)m^(-3))blue core plasma is produced in~1 Pa argon by helicon RF(radiofrequency)discharge using a NagoyaⅢantenna under magnetic fleld(~2 k G)of compact ring PMs(length~204 mm).Operational parameters,i.e.RF power and neutral gas pressure are scanned and plasma density is measured by an RF compensated probe to explore the operating characteristics of the device.Iconic feature of a helicon discharge,such as blue core plasmas and E-H-W mode transitions are well observed in the device,despite the wavelength calculated using the conventional dispersion relation of a bounded whistler waves(Chen 1991 Plasma Phys.Control.Fusion 33339)is order of magnitudes longer than the length of the plasma in this device which seems to suggest that such helicon device is impossible.Surprisingly,the wavelength calculated by the unbounded whistle wave dispersion formula in turn suggests the occurrence of a half wavelength resonance.
基金supported in part by the Electrostatic Research Foundation of Liu Shanghe Academicians and Experts Workstation,Beijing Orient Institute of Measurement and Test(No.BOIMTLSHJD20221002)。
文摘We present here an investigation of the self-pulsing phenomenon of negative corona and parallel-plate discharge in argon within one frame of a one-dimensional fluid model in cylinder–cylinder electrode geometry.The transition from corona to parallel-plate discharge is obtained by changing the inner and outer radii of the electrodes.The model reproduces the self-pulsing waveform well and provides the spatiotemporal behaviors of the charged particles and electric field during the pulse.The self-pulsing shows a common feature that occurs in various configurations and that does not depend on a specific electrode structure.The self-pulsing is the transformation between a weak-current Townsend mode and a large-current normal glow mode.The behavior of the positive ions is the dominant factor in the formation of the pulse.
基金supported by National Science Foundation of China (No.10175014)
文摘The tuned substrate self-bias in a radio-frequency inductively coupled plasma is controlled by varying the impedance of an external tuning LCR （inductor, capacitor and resistor） network inserted between the substrate and the ground. In experiments, it was found that the variation of the tuned substrate self-bias with the tuning capacitance demonstrated three features, namely, continuity, instability and bistability. In this paper, a numerical study is focused on the elucidation of the physical mechanisms underlying continuity and bistability. For the sake of simplicity and feasibility to include the key factors influencing the tuned substrate bias, the tedious calculation of inductive-coupling to obtain the plasma density axtd electron temperature is omitted, and discussion of the tuned substrate self-bias is made under the prescribed plasma density and electron temperature. On the other hand, the parameters influencing capacitive- coupling are retained in modeling the system with an equivalent circuit. It is found that multi-stable state appears when one of the parameters, such as the resistance in LCR, substrate area and plasma density, decreased to its critical value, or the rf voltage or electron temperature increased to the critical value individually. In the reverse cases, the tuned substrate self-bias varies continuously with the tuning capacitance.
文摘The radio frequency (rf) self-bias of the substrate in a rf inductively coupled plasma is controlled by means of varying the impedance of an external LC network inserted between the substrate and the ground. Experimental studies were done on the relations of the tuned substrate self-bias with varying discharge and external circuit parameters. Under a certain discharge gas pressure, the curves of tuned substrate self-bias Vtsb versus tuning capacitance Ct demonstrate jumps and hysteresises when rf discharge power is higher than a threshold. The hysteresis loop in terms of △Ctcrit1(= Ccrit1-Ccrit2, here,Ccrit1, Ccrit2 are critical capacitance magnitudes under which the tuned substrate self-bias jumps) decreases with increasing rf discharge power, while the maximum |Vtsbimn| is achieved in the middle discharge-power region. Under a constant discharge power |Vtsb min|, Ccrit1 and Ccrit2 achieve their minimums in the middle gas-pressure region. When the tuning capacitance is pre-set at a lower value, Ttsb varies slightly with gas-flow rate; in the case of tuning capacitance sufficiently approaching Ctcriti, Vtsb undergoes the jump and hysteresis with the changing gas-flow rate. By inserting a resistor R into the external network, the characteristics of Vtsb-Ct curves are changed with the reduced quality factor Q depending on resistance values. Based on inductive- and capacitive-coupling characteristics of inductively coupled plasma, the dependence of a plasma sheath on plasma parameters, and the impedance properties of the substrate branch, the observed results can be qualitatively interpreted.
基金supported by the Fundamental Research Funds in Heilongjiang Provincial Universities of China (No. 135509124)National Natural Science Foundation of China (No. 11805107)the Graduate Innovation Foundation of Qiqihar University, China (No. YJSCX2021017)
文摘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 bothαregime and drift-ambipolar regime intoαregime 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 C_(2)H_(2)/He microplasmas.Finally,the growth of nanoparticles via coagulation is studied.
