Comparison of heating mechanisms of argon helicon plasma in different wave modes with and without blue core

In this work,we investigated the discharge characteristics and heating mechanisms of argon helicon plasma in different wave coupled modes with and without blue core.Spatially resolved spectroscopy and emission intensity of argon atom and ion lines were measured via local optical emission spectroscopy,and electron density was measured experimentally by an RFcompensated Langmuir probe.The relation between the emission intensity and the electron density was obtained and the wavenumbers of helicon and’Trivelpiece-Gould’(TG)waves were calculated by solving the dispersion relation in wave modes.The results show that at least two distinct wave coupled modes appear in argon helicon plasma at increasing RF power,i.e.blue core(or BC)mode with a significant bright core of blue lights and a normal wave(NW)mode without blue core.The emission intensity of atom line 750.5 nm(lArⅠ750.5nm)is related to the electron density and tends to be saturated in wave coupled modes due to the neutral depletion,while the intensity of ion line 480.6 nm(IArⅡ480.6nm)is a function of the electron density and temperature,and increases dramatically as the RF power is increased.Theoretical analysis shows that TG waves are strongly damped at the plasma edge in NW and/or BC modes,while helicon waves are the dominant mechanism of power deposition or central heating of electrons in both modes.The formation of BC column mainly depends on the enhanced central electron heating by helicon waves rather than TG waves since the excitation of TG waves would be suppressed in this special anti-resonance region.

Observation of low-frequency oscillation in argon helicon discharge

We present here a kind of low-frequency oscillation in argon helicon discharge with a half helical antenna.This time-dependent instability shows a global quasi-periodic oscillation of plasma density and electron temperature,with a typical frequency of a few tens of Hz which increases with external magnetic field as well as radiofrequency(RF)power.The relative oscillation amplitude decreases with magnetic field and RF power,but the rising time and pulse width do not change significantly under different discharge conditions.The oscillation can only be observed in some specific conditions of low magnetic fields and low RF power when the gas flows in from one end of the discharge area and out from another end.This global instability is suggested to be attributed to the pressure instability of neutral depletion,which is the result of compound action of gas depletion by heating expansion and gas replenishment from upstream.There are two kinds of oscillations,large and small amplitude oscillations,occurring in different discharge modes.This study could be a good verification of and complement to earlier experiments.This kind of spontaneous pulse phenomenon is also helpful in realizing a pulsing plasma source without a pulsed power supply.

Influence of the pulse polarity on micro-hollow cathode helium plasma jet

Helium plasma jets generated by micro-hollow cathode discharge(MHCD) with the squarewave power source of different polarities are investigated in this work. The effects of positive and negative polarity pulses on the MHCD and plasma jet were compared, and the time-resolved optical emission spectra of excited species(N_(2)^(+) and O) were studied. The results confirm that the electric field is a key factor for the propagation of the jet during the rising edge of the positive current pulse, while the gas expansion is mainly important for the jet propagation during the current stable phase. The time-resolved spectra show that the generation of specie O in the jetdriven by the electric field is more efficient.

Experiments on the characteristics of underwater electrical wire explosions for reservoir stimulation

Underwater shock waves generated by pulsed electrical discharges are an effective,economical,and environmentally friendly means of stimulating reservoirs,and this technology has received much attention and intensive research in the past few years.This paper reviews the main results of recent work on underwater electrical wire explosion(UEWE)for reservoir stimulation.Aplatform is developed for microsecond singlewire explosions in water,and diagnostics based on a voltage probe,current coil,pressure probe,photodiode,and spectrometer are used to characterize the UEWE process and accompanying shock waves.First,the UEWE characteristics under different discharge types are studied and general principles are clarified.Second,the shock-wave generation mechanism is investigated experimentally by interrupting the electrical energy injection into the wire at different stages of the wire-explosion process.It is found that the vaporization process is vital for the formation of shock waves,whereas the energy deposited after voltage collapse has only a limited effect.Furthermore,the relationships between the electrical-circuit and shock-wave parameters are investigated,and an empirical approach is developed for estimating the shock-wave parameters.Third,how the wire material and water state affect the wire-explosion process is studied.To adjust the shock-wave parameters,a promising method concerning energetic material load is proposed and tested.Finally,the fracturing effect of the pulsed-discharge shock waves is discussed,as briefly are some of the difficulties associated with UEWE-based reservoir stimulation.

Comparison of double layer in argon helicon plasma and magnetized DC discharge plasma

We present in this paper the comparison of an electric double layer(DL)in argon helicon plasma and magnetized direct current(DC)discharge plasma.DL in high-density argon helicon plasma of 13.56 MHz RF discharge was investigated experimentally by a floating electrostatic probe and local optical emission spectroscopy(LOES).The DL characteristics at different operating parameters,including RF power(300-1500 W),tube diameter(8-60 mm),and external magnetic field(0-300 G),were measured.For comparison,DL in magnetized plasma channel of a DC discharge under different conditions was also measured experimentally.The results show that in both cases,DL appears in a divergent magnetic field where the magnetic field gradient is the largest and when the plasma density is sufficiently high.DL strength(or potential drop of DL)increases with the magnetic field in two different structures.It is suggested that the electric DL should be a common phenomenon in dense plasma under a gradient external magnetic field.DL in magnetized plasmas can be controlled properly by magnetic field structure and discharge mode(hence the plasma density).

Discharge and post-explosion behaviors of electrical explosion of conductors from a single wire to planar wire array

This work deals with an experimental study of a Cu planar wire array(PWA)in air and water under the stored energy 300-1200 J.A single Cu wire is adopted as a controlled trial.Four configurations of PWA and a wire with the same mass(cross-section area)but the different specific surface areas(15-223 cm^(2)g^(-1))are exploded.The transient process is analyzed using high-speed photography in combination with the results of optical emission and discharge.Discharge characteristics revealed that PWA always has a higher electric power peak,early but higher voltage peak,as well as faster vaporization and ionization process than the single-wire case.Two to three times stronger optical emission could be obtained when replacing the single-wire with PWA,indicating a higher energy-density state is reached.Phenomenologically,in both air and water,single-wire load tends to develop a transverse stratified structure,while PWA is dominated by the uneven energy deposition among wires.Finally,the synchronism and uniformity of the PWA explosion are discussed.

Dynamics of plasma bullets by nanosecond pulsed micro-hollow cathode discharge within air

In this paper,the air plasma jet produced by micro-hollow cathode discharge(MHCD)is investigated.The discharge is powered by a positive nanosecond pulse high voltage supply.The waveforms of the discharge,the images of the jet,the evolution of the plasma bullet and the reactive species are obtained to analyze the characteristics of the MHCD plasma jet.It is found that the length of the plasma jet is almost proportional to the air flow rate of 2–6 slm.Two plasma bullets appear one after another during a single period of the voltage waveform,and both of the two plasma bullets are formed during the positive pulse voltage off.The propagation velocity of the two plasma bullets is on the order of several hundred m/s,which is approximate to that of the air flow.These results indicate that the gas flow has an important influence on the formation of this MHCD plasma jet.

Observation and verification of surface electrical explosion driven by radial-distributed pulsed current in laboratory lightning strike test

The laboratory lightning test is essential for assessing the effectiveness of lightning strike protection(LSP).Particularly,direct lightning strike damage can be performed with pulsed current injection into the specimen.This paper focuses on the dynamic process of arc plasma and shock wave behaviour in the vicinity of the‘strike’point.A rod-plate discharge load is built for testing aluminium and coated plate under 40-kA-level pulsed current.The visualisation of the luminous discharge plasma and its flow field via high-speed photography(from different angles)is meticulously designed and implemented,synchronised with electro-physical diagnostics.The results indicate some new mechanisms for lightning strike damage,apart from the impulse heat loading from the thermal arc.The transient current injection through the arc root concentrates on a thin skin layer(skin-depth effect),with the radial-attenuated current density,driving asynchronously electrical explosions on the plate surface.The inhomogeneous Joule heating of the plate leads to outwardly propagating phase transition and shock wave along the conductive surface.In addition,the electro-thermal instability is observed and regarded as the seed of irregular erosion region.Spectroscopic information reveals two different plasma states of main discharge arc channel and adjacent surface electrical explosion.The correspondence of the physical mechanism of electrical explosion and optical radiation is established.Microscopic images for different regions depict erosion characteristics and summarise influencing factors,further confirming the mechanism above.The research clarifies the role of skin-depth effect in transaction arc erosion for electrode,complements the electrical explosion theory with unevenly distributed current and helps optimise strategies of LSP.

Analyses on an electronic voltage transformer's failure by its resonance with very fast transient overvoltage and suppression

As voltage measuring devices are widely used in the high voltage power system with a primary-secondary-fusion structure,electronic voltage transformers are directly connected to the primary conductor and would face much more serious electromagnetic environments than general secondary equipment.As a result,they would still experience failures even when general protection measures are adopted to suppress overvoltages'amplitudes.To address this issue,an innovative method is proposed after analyses are conducted on the resonance between a transformer and an overvoltage.Within this method,an air-cored coil is installed in a transformer to artificially shift its natural frequency and to dodge the high-frequency dominant component of a transient overvoltage,avoiding the aforementioned resonance and suppressing the secondary overvoltage.In addition,simulations and laboratory tests are conducted to prove this method's validity.

Application of an atmospheric pressure plasma jet in a rat model of ischaemic stroke:Design,optimisation,and characteristics

Recent studies have shown the in vitro neuro-protective functions of atmospheric pressure plasma(APP)against multiple pathological injuries during ischaemic stroke(IS).However,whether APP treatment exerts a therapeutic effect on a rat IS model remains unclear.Here,on the basis of needle-to-ring dielectric barrier discharge,an atmospheric pressure plasma jet(APPJ)was designed,with the Helium as the working gas which was driven by a sinusoidal voltage.Then,the treatment conditions were optimised for IS rat model treatment and the characteristics of this APPJ were further diagnosed.Subsequently,the rat IS model was established through 90 min middle cerebral artery occlusion(MCAO),and plasma was intermittently inhaled by rats via the nasal cavity for a 2 min period at 60 min of MCAO process.The therapeutic effects of this plasma jet device were then evaluated using biomedical analyses.According to our results,intermittent APP inhalation in the MCAO rats increased the serum NO content,improved the neurological function,enhanced regional cerebral blood flow,lowered brain infarction,and reduced the cell apoptosis in brain tissues of MCAO rats.Collectively,our data provides a novel potential strategy for IS treatment by using atmospheric-pressure plasma inhalation.

Experiments on plasma dynamics of electrical wire explosion in air

Besides a typical high‐density plasma source,electrical explosion of conductors is also indispensable in switches,nanomaterial synthesis,shock‐wave sources,etc.In this paper,an experimental study regarding plasma dynamics of electrical wire explosions(μs‐timescale)is presented,with spatiotemporal resolved diagnostics.Pure Cu/Ni wire and Cu‐Ni alloy wire were used and compared.The alloy wire usually has a higher resistivity,resulting in a higher initial energy deposition(heating)rate.Abel inverse transformation indicated that the plasma radiation focussed on the outer region of the discharge channel for the alloy wire.In addition,the metallic vapour determined by the material properties had a considerable influence on the plasma process and resulting nanomaterials.In particular,both transverse and axial‐layered structures were observed in alloy wire vapour.In addition,for the first time,the expanding arc‐like plasma of explosion products was understood and examined from aspects of material properties and energy relaxation.The later stage of wire explosion resembled the state of regular metal vapour arcs under 1 MPa pressure.Finally,the core factor for the fast energy deposition stage of wire explosion was ascertained.Correlations between pre‐exposition circuit parameters and post‐explosion dynamic effects were found,which is significant for practical applications.