Relationship of mode transitions and standing waves in helicon plasmas

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.

Mode transition and oscillation suppression in supersonic cavity flow

Supersonic flows past two-dimensional cavities with/without control are investigated by the direct numerical simulation （DNS）. For an uncontrolled cavity, as the thickness of the boundary layer declines, transition of the dominant mode from the steady mode to the Rossiter Ⅱ mode and then to the Rossiter III mode is observed due to the change of vortex-corner interactions. Meanwhile, a low frequency mode appears. However, the wake mode observed in a subsonic cavity flow is absent in the current simulation. The oscillation frequencies obtained from a global dynamic mode decomposition （DMD） approach are consistent with the local power spectral density （PSD） analysis. The dominant mode transition is clearly shown by the dynamic modes obtained from the DMD. A passive control technique of substituting the cavity trailing edge with a quarter-circle is studied. As the effective cavity length increases, the dominant mode transition from the Rossiter Ⅱ mode to the Rossiter Ⅲ mode occurs. With the control, the pressure oscillations are reduced significantly. The interaction of the shear layer and the recirculation zone is greatly weakened, combined with weaker shear layer instability, responsible for the suppression of pressure oscillations. Moreover, active control using steady subsonic mass injection upstream of a cavity leading edge can stabilize the flow.

Mode transition in homogenous dielectric barrier discharge in argon at atmospheric pressure

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.

Transition from the Southern Mode of the Mei-yu Front Cloud System to Other Leading Modes

Based on normalized six-hourly black body temperature （TBB） data of three geostationary meteorological satellites,the leading modes of the mei-yu cloud system between 1998 and 2008 were extracted by the Empirical Orthogonal Function （EOF） method,and the transition processes from the first typical leading mode to other leading modes were discussed and compared.The analysis shows that,when the southern mode （EOF1） transforms to the northeastern mode （EOF3）,in the mid-troposphere,a low trough develops and moves southeastward over central and eastern China.The circulation pattern is characterized by two highs and one low in the lower troposphere.A belt of low pressure is sandwiched between the weak high over central and western China and the strong western North Pacific subtropical high （WNPSH）.Cold air moves southward along the northerly flow behind the low,and meets the warm and moist air between the WNPSH and the forepart of the low trough,which leads to continuous convection.At the same time,the central extent of the WNPSH increases while its ridge extends westward.In addition,transitions from the southern mode to the dual centers mode and the tropical-low-influenced mode were found to be atypical,and so no common points could be concluded.Furthermore,the choice of threshold value can affect the number of samples discussed.

Exact Solutions and Mode Transition for Out-of-Plane Vibrations of Non-uniform Beams with Variable Curvature

The two coupled governing differential equations for the out-of-plane vibrations of non-uniform beams with variable curvature are derived via the Hamilton’s principle.These equations are expressed in terms of flexural and torsional displacements simultaneously.In this study,the analytical method is proposed.Firstly,two physical parameters are introduced to simplify the analysis.One derives the explicit relations between the flexural and the torsional displacements which can also be used to reduce the difficulty in experimental measurements.Based on the relation,the two governing characteristic differential equations with variable coefficients can be uncoupled into a sixth-order ordinary differential equation in terms of the flexural displacement only.When the material and geometric properties of the beam are in arbitrary polynomial forms,the exact solutions with regard to the outof-plane vibrations of non-uniform beams with variable curvature can be obtained by the recurrence formula.In addition,the mode transition mechanism is revealed and the influence of several parameters on the vibration of the non-uniform beam with variable curvature is explored.

State and Mode Feedback Control for Discrete-time Markovian Jump Linear Systems With Controllable MTPM

In this note, the state and mode feedback control problems for a class of discrete-time Markovian jump linear systems(MJLSs) with controllable mode transition probability matrix(MTPM) are investigated. In most achievements, controller design of MJLSs pays more attention to state/output feedback control for stability, while the system cost in practice is out of consideration. In this paper, we propose a control mechanism consisting of two parts: finite-path-dependent state feedback controller design with which uniform stability of MJLSs can be ensured, and mode feedback control which aims to decrease system cost. Differing from the traditional state/output feedback controller design, the main novelty is that the proposed control mechanism not only guarantees system stability, but also decreases system cost effectively by adjusting the occurrence probability of system modes. The effectiveness of the proposed mechanism is illustrated via numerical examples.

Keyhole formation and its characteristics in laser welding mode transition

Keyhole is the most important characteristic for laser deep penetration welding, and its formation indicates the beginning of laser deep penetration welding mode. The keyhole developing process was analyzed and the keyhole formation time was calculated according to welding speed and the length of weld bead formed in the keyhole formation process. The results showed that the keyhole forms in 40 -70 ms at different rate of change of laser power. In laser deep penetration welding process, the variation of light intensity radiated by laser induced plasma can identify the keyhole formation, but it can not be used to estimate the keyhole formation time because of delay effect.

Heat Transfer and Mode Transition for Laser Ablation Subjected to Supersonic Airflow

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.

Discontinuity of mode transition and hysteresis in hydrogen inductively coupled plasma via a fluid model

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.

Investigation of E-H mode transition in magnetic-pole-enhanced inductively coupled neon-argon mixture plasma

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).

Spatial variation behaviors of argon inductively coupled plasma during discharge mode transition

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.

Driving frequency effects on the mode transition in capacitively coupled argon discharges

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.

Measurement of electronegativity during the E to H mode transition in a radio frequency inductively coupled Ar/O2 plasma

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.

Welding mode transition and process stability in high power laser welding

For high-power CO2 laser welding, besides two well known stable welding processes, i.e. stable deep penetration welding (DPW) and stable heat conduction welding (HCW), the authors have found the third welding process, i.e. unstable-mode welding (UMW) under the certain condition. UMW has its basic feature that the two welding modes (DPW and HCW) appear intermittently, with jumping of penetration depth and weld width between large and small levels. In this paper, effects of welding parameters (focal position, laser power and traveling speed) on laser welding mode and weld appearance have been comprehensively studied. Double-U curves of laser welding mode transition have been obtained, which indicate the ranges of the three mentioned welding processes. This work affords science foundation for selecting the welding process parameters correctly and obtaining stable laser welding.

Phase Transition and Quasinormal Modes for Spherical Black Holes in 5D Gauss–Bonnet Gravity

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.

Numerical study of the self-pulsing of DC discharge:from corona to parallel-plate configurations

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.

All-fiber ellipsometer for nanoscale dielectric coatings

Multiple mode resonance shifts in tilted fiber Bragg gratings(TFBGs)are used to simultaneously measure the thickness and the refractive index of TiO_(2) thin films formed by Atomic Layer Deposition(ALD)on optical fibers.This is achieved by comparing the experimental wavelength shifts of 8 TFBG resonances during the deposition process with simulated shifts from a range of thicknesses(T)and values of the real part of the refractive index(n).The minimization of an error function computed for each(n,T)pair then provides a solution for the thickness and refractive index of the deposited film and,a posteriori,to verify the deposition rate throughout the process from the time evolution of the wavelength shift data.Validations of the results were carried out with a conventional ellipsometer on flat witness samples deposited simultaneously with the fiber and with scanning electron measurements on cut pieces of the fiber itself.The final values obtained by the TFBG(n=2.25,final thickness of 185 nm)were both within 4%of the validation measurements.This approach provides a method to measure the formation of nanoscale dielectric coatings on fibers in situ for applications that require precise thicknesses and refractive indices,such as the optical fiber sensor field.Furthermore,the TFBG can also be used as a process monitor for deposition on other substrates for deposition methods that produce uniform coatings on dissimilar shaped substrates,such as ALD.

Development of a compact high-density blue core helicon plasma device under 2000 G magnetic field of ring permanent magnets

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.

Changes of the electron dynamics in hydrogen inductively coupled plasma

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.

Fault on-off versus strain rate and earthquakes energy

We propose that the brittle-ductile transition （BDT） controls the seismic cycle. In particular, the movements detected by space geodesy record the steady state deformation in the ductile lower crust, whereas the stick-slip behavior of the brittle upper crust is constrained by its larger friction. GPS data allow analyzing the strain rate along active plate boundaries. In all tectonic settings, we propose that earthquakes primarily occur along active fault segments characterized by relative minima of strain rate, segments which are locked or slowly creeping. We discuss regional examples where large earthquakes happened in areas of relative low strain rate. Regardless the tectonic style, the interseismic stress and strain pattern inverts during the coseismic stage. Where a dilated band formed during the interseismic stage, this will be shortened at the coseismic stage, and vice-versa what was previously shortened, it will be dilated. The interseismic energy accumulation and the coseismic expenditure rather depend on the tectonic setting （extensional, contractional, or strike-slip）. The gravitational potential energy dominates along normal faults, whereas the elastic energy prevails for thrust earthquakes and performs work against the gravity force. The energy budget in strike-slip tectonic setting is also primarily due elastic energy. Therefore, precursors may be different as a function of the tectonic setting. In this model, with a given displacement, the magnitude of an earthquake results from the coseismic slip of the deformed volume above the BDT rather than only on the fault length, and it also depends on the fault kinematics.