Investigation on pulsed discharge mode in SF6-C2H6 mixtures

Nannan YANG (杨楠楠),Guofu LI (李国富),Yongli ZHAO (赵永利),,Jialiang ZHANG (张家良) and Xiaoqiong WEN (温小琼)

1 State Key Laboratory for Material Modification,Dalian University of Technology,Dalian 116024,People’s Republic of China

2 State Key Laboratory of Chemical Lasers,Dalian Institute of Chemical Physics,Chinese Academy of Sciences,Dalian 116023,People’s Republic of China

Abstract

Keywords:pulsed chemical HF laser,SF6-C2H6 mixtures,discharge mode,α ionization avalanche

1.Introduction

A self-sustained volume discharge (SSVD) in high gas pressure is widely used for pumping gas lasers,which have been the subject of many studies.Kamyushin [1]and Levatter[2]determined not only the minimum of the initial electron concentration for SSVD,but also the upper limits of the electric field and voltage pulse rise time.Mesyats et al[3]pointed out that a certain concentration of the initial electrons formed a series of diffuse channels due to their avalanche multiplication,which overlapped in space and thereby formed a homogeneous discharge,rather than owing to the transformation of an individual avalanche to a streamer channel followed by the growth of its conduction.Additions into gas mixtures of the non-chain HF/DF lasers or replacement of H2(D2)with hydrocarbons and deuteron–carbons [4–6]and optimization of the pre-ionization [7–9]can increase discharge homogeneity and stability.Regarding the mechanism of SSVD,Osipov [10]confirmed that SSVD was not classical low-pressure glow discharge.As opposed to glow discharge,SSVD usually exists at gas pressure of tens to hundreds of Torr and requires preionization to trigger from cathode,moreover,SSVD has no high voltage negative sheath region that can cause secondary electron emission near the cathode surface and the specific power of discharge is much superior to that of glow discharge.Belevtsev et al [11–15]investigated the stability of SSVD in different mixtures for non-chains HF laser and concluded that the collision dissociation of electron-anion may lead to SSVD instability because the electron collision dissociation rate and electron-ion recombination rate limited the SSVD current density.Apollonov et al [16–19]named the self-initiated volume discharge (SIVD) mode,which was no need to pre-ionize the gas whatsoever when smallscale (～50 μm) inhomogeneities were deposited on the cathode surface,that greatly improved the pulsed output energy of HF laser.And SIVD was not dissimilar to an ordinary SSVD,which was a form of SSVD.They also pointed out that the stability and uniformity of SIVD were determined by the surface density of cathode spots and that the SF6dissociation was the main factor limiting discharge current density.Panchenko et al[20–22]studied the effects of different excitation parameters on the discharge stability of SF6mixtures and the efficiency of the discharge nonchain HF/DF laser and determined the optimal experimental conditions for the discharge non-chain HF/DF laser.All these investigations targeted on the ignition and evolution of SSVD.Although the mechanism of limiting discharge current density and SSVD instability has been explored experimentally and theoretically,the investigations of the SSVD current generation kinetics are still to be expected,which is necessary for effective control of SSVD homogeneity and stability.

The present work is aimed at the dynamic mechanism of discharge current formation in SF6-C2H6mixtures for nonchain pulsed HF laser system using pulse peaking circuit in parallel plate electrode configuration.Two modes have been distinguished,one of them is SSVD.The SSVD in SF6-C2H6mixtures has the characteristics of spatial uniformity in the discharge gap and pulse to pulse repeatability.The holding scope of SSVD mode is also confirmed and can be used to guide future discharge parameter optimization.Quantitatively analysis of the dependence of SSVD peak current on discharge voltage and gas pressure shows that the SSVD of SF6-C2H6mixtures is essentially an α ionization avalanche,which provides the basis for the correction of SSVD numerical simulation in the future.

Figure 1.Schematic of the setup for pulsed discharges in SF6-C2H6 mixtures.HV:DC high voltage;SG1,SG2:rotary spark gaps;Cs:energy storage capacitor;Cp:peaking capacitor;Cpre:pre-ionization capacitor.

2.Experimental setup

The schematic for SF6-C2H6mixtures discharge device is shown in figure 1.Two electrodes are made of alloy and parallel to each other,whose edges conform to the Rogowski shape to reduce the electric field edge effect.The discharge volume is about 160 ml,wherein the interelectrode gap d is 20 mm.For discharge ignition,the cathode(100×100 mm2)is equipped with 4×4 surface sliding array spark pins structure.The detail of one sliding is shown in the inset of figure 1.The sliding electrodes are electrically parallel to the anode through pre-ionization capacitors.The anode is 100×60 mm2rectangle and the inner surface is polished to avoid flashover.The reason for using a larger cathode is to reduce the discharge current density over the cathode surface and restrain the cathode arc spot.

As discharge gases,SF6and C2H6are filled into the discharge chamber with a total flow rate of 16 l min-1but different mixing ratio,which refreshes the discharge chamber within 0.1 s.By adjusting the exhausting rate,the chamber gas pressure P can change in the range of 20–70 Torr.The molar ratio of SF6to C2H6can vary from 15:1 to 30:1 by adjusting their flow rate ratio,and the resultant concentration of SF6was 94%–97%.

Shown also in figure 1,the DC high voltage input is pulsed to drive the discharge with two spark gaps (SG1and SG2) and capacitors (Csand Cp).The SG2and Cpaim to reduce the rise and fall time of the voltage pulsed,which compose the so-called pulse peaking circuit.The capacitor bank Csplays a role of discharge energy storage,which can be 11.2 nF,14.2 nF and 16.8 nF in capacitance and is charged to 17–30 kV respectively.The pulsed discharge is controlled by two rotary spark gaps (SG) with frequency up to 10 Hz.The other capacitor bank Cp,the so-called peaking capacitor,which is fixed as 5.6 nF.The reason why the pulse peaking circuit is installed is that it reduced the wire length to reduce the circuit inductance,which is beneficial to suppress the formation of arc.The pre-ionization capacitor is 4.08 nF,which is a partial-pressure element in pre-ionization circuit to produce the voltage of the pre-ionization gap.

Discharge current and voltage waveforms were measured with a TCP303 current probe at B and a P6015A probe at A,as shown in figure 1,and were recorded by a TDS1012B oscilloscope.The overall luminescence of the discharge gap was pictured by a Canon G5 digital camera.

3.Results and discussion

3.1.Mode transformation of the pulsed discharge in SF6-C2H6 mixtures

Some typical photos of the discharge gap luminescence are presented in figure 2 to display the discharge spatial forms and transformation with discharge conditions.In the photos,the anode is on the left and the cathode is on the right.All the photos are taken at discharge conditions of total gas pressure P=40 Torr,the molar ratio n(SF6):n(C2H6)=20:1,SG frequency f=10 Hz and charging voltage Uc=17–30 kV.According to different storage capacitance,the photos are divided into three rows(the top row Cs=11.2 nF,the middle row Cs=14.2 nF,the bottom row Cs=16.8 nF.)and in each row five photos with different charging voltages are shown.

Figure 3.The typical photographs and corresponding discharge waveforms of SSVD and arc discharge.(a),(b) SSVD:Cs=11.2 nF,Uc=23 kV,P=70 Torr.(c),(d) Arc discharge:Cs=16.8 nF Uc=29 kV,P=30 Torr.

In most of these photos(besides figure 2(bottom row,part e)),the interelectrode gap is full of blue or purple luminescence,and many bright spots distribute on the cathode surface.The bright spots are generated from the pre-ionization sliding spark and bring out fading and expanding discharge channels(called diffusion-like discharge channels).These discharges consisting of diffusion-like discharge channels ignited synchronously by the sliding spark pin array are called SSVD,as described in [2].However,the case in figure 2 (bottom row,part e) is quite different from the others,in which many discharge channels transform into bright arc channels,due to the highest charging voltage and the biggest storage capacitance among them.Thus,figure 2 (bottom row,part e) illustrates another discharge mode in which the discharge gap is mostly filled with arc channels,which is called pulsed arc mode.

Comparison of the pictures in each row illustrates that the incremental charging voltage leads to evident increment in the volume and brightness of the cathode spots and therefore the resultant brighter discharge channels (diffusion-like channels or arc channels).However,despite the same voltage applied,the inhomogeneity of the cathode spots brightness in each picture is observed.Since the diffusion-like channels are closely related to the electron production of their birth sparks,the randomness of the pre-ionization sliding sparks results in the inhomogeneous luminescence of the discharge gap.On the other hand,the diffusion-like channels are homogeneous in discharge kinetics and spatially overlap with each other,which leads to discharge uniformity.As reported in [16],the more homogeneous discharge mode shows a more uniform glow-like state except for the regime of cathode spots,which is termed SSVD.

By comparing the rows in figure 2,the brightness and volume of the spark spots and their resultant diffusion-like discharge channels increase also with the increasing storage capacitance.High energy storage capacitance can increase the duration of the discharge pulse,which makes the spark discharge development more mature.As the charging voltage and the energy storage capacitance increase,the brightness of the SSVD increases significantly,which indicates that the ionization intensity of SSVD region is determined by the charging voltage and the energy storage capacitance.

To sum up,the pulsed discharge in SF6-C2H6mixtures presents two different discharge morphologies,one is called SSVD,the other is the pulsed arc discharge,where the inhomogeneity and randomness are more significant.To explain the different mechanisms of the two modes more clearly,their discharge pulsed waveforms are compared as follows.

Figures 3(b) and (d) are plotted the typical discharge waveforms of SSVD and arc discharge for n(SF6):n(C2H6)=20:1,in order to visually display the differences between the two discharge modes,the corresponding typical photographs are presented in figures 3(a) and (c).Comparing the discharge waveforms of two discharge modes,the voltage waveforms are the similar attenuated oscillation waveform except for different residual voltages (shown in the horizontal red dotted line in figures 3(b)and(d)).The residual voltage indicates that when an SSVD pulse extinguishes and all electrons generated in the process reach the anode,those electrons cannot neutralize all charges transferred from the storage capacitor.The maxima reduced electric field strength of SSVD is 89.84 kV cm-1bar-1in figure 3(b),which is close to known critical reduced electric field strength [11].The maxima reduced electric field strength of arc discharge is 227.5 kV cm-1bar-1in figure 3(d),which is much larger than the critical reduced electric field strength.

However,the difference in current waveforms is significant.The current waveform of SSVD consists of only one pulse with peak value of about 0.54 kA and pulse width of about 130 ns in figure 3(b).Except for the first voltage peak,the amplitude of other voltage peak is not enough to trigger the avalanche ionization gas again in a discharge cycle.Accordingly,as shown in figure 3(a),multiple simultaneous avalanche ionization channels are overlapped with each other within the discharge gap to form the active media.Different from the current waveform of SSVD,the current waveform of arc discharge has obvious multiple current peaks following the first current peak.Compared with figure 3(b),the peak value of the first current pulse is much larger,close to 6 kA,and the pulse width is about 190 ns in figure 3(d).On the other hand,as shown in figure 3(c),there are not only blue or purple luminescence,but also breakdown pulsed arc channels in the discharge gap.These arc pulses consume most of the electrical energy in the form of arc heat,which reduces the conversion rate of the laser pumping process.Therefore,SSVD should be kept to avoid arc discharge during laser operation to improve the energy deposition and the electro-optical conversion rate of the laser.

Figure 4.(a) The current waveforms of SSVD at different charging voltages with Cs=11.2 nF.(b) Dependence of SSVD current peak on storage capacitance and charging voltage.

Figure 5.(a) Discharge current waveforms with different storage capacitances.(b)Dependence of peak current on storage capacitance at various voltages.

Figure 6.(a) Discharge current waveforms of SF6-C2H6 mixture ratio with Cs=11.2 nF,Uc=23 kV.(b) Peak current variation with the mixture ratio for different capacitances with Uc=23 kV.

Figure 7.The holding scopes of SSVD with n(SF6):n(C2H6)=20:1.

3.2.Holding conditions for the SSVD mode in SF6-C2H6 mixtures

3.2.1.Effects of charging voltage and storage capacitance on the SSVD current.The SSVD current waveforms versus the charging voltage of storage capacitor are shown in figure 4(a)and the dependence of SSVD current peak on storage capacitance and charging voltage in figure 4(b),while the total gas pressure P=40 Torr and molar ratio n(SF6):n(C2H6)=20:1.It is evident that the SSVD current peak increases rapidly with the charging voltage.As shown in figure 4(a),when the voltage increases from 17 kV to 26 kV only by 80%,the current peak increases from 1 kA to 3 kA by 200%while the current pulse shape and duration keep almost constant about 200 ns.The area of current pulse can reflect the discharge energy deposition in the electrode gap in SSVD mode.Thus,increasing the charging voltage is quite efficient to elevate discharge energy.However,it is noteworthy that the charging voltage of 29 kV leads to a slightly changed shape of the current pulse,as arc channels begin to appear in the discharge gap with the voltage.The charging voltage of 29 kV is regarded as the upper limit for holding the SSVD mode.Arc channels will continue to set off in the gap if the charging voltage surpasses the limit.The tendencies in figure 4(b) reveal that the peak current of SSVD almost linearly rises with the charging voltage,and the slope is almost the same for different storage capacitances,which implies that SSVD mode keeps.According to Townsend’s theory,Townsend’s first ionization coefficientα=APe(-BPd/Vs),whereα,P,d andVsare Townsend’s first ionization coefficient,gas pressure,interelectrode gap,voltage,A and B are Paschen constants of gas.If the ionization events occur mostly in the discharge gap,the discharge current densityi=i0e(αd),where i andi0are the discharge current density and seed current density at cathode surface.When the discharge area is constant,the current intensity is proportional to the current density.If gas composition,P and d remain constants,the current density i(therefore the current intensity) is an incremental function of Vs.

Figure 5 shows the variation of the SSVD current waveform and its peak value with storage capacitance.The pulse shape of SSVD current varies slightly with increasing storage capacitance.As shown in figure 5(a),the capacitance increased from 11.2 nF to 16.8 nF by 50%,accordingly,the current peak increased from 2.4 kA to 2.8 kA only by 17%and the current duration increased from 190 ns to 210 ns by 10%.Similarly,all the current peak tendencies versus storage capacitance in figure 5(b) went upward slightly for different charging voltages.Therefore,increasing the storage capacitance is not effective to raise the amplitude and duration of SSVD current pulse,i.e.,energy deposition in discharge gap cannot be effectively increased by higher capacitance.

3.2.2.Effects of gas composition and pressure on SSVD current.As shown in figure 6 for gas pressure P=40 Torr and charging voltage Uc=23 kV,though the C2H6ratio in mixture gas increased twice (from 3% to 6%),the current waveform kept almost the same shape.So,it can be said that C2H6ratio had hardly effect on SSVD current,which means that the development of SSVD current was dominated by SF6,and slight C2H6did not affect the discharge energy deposition in the SSVD mode.For more detail,figure 6(b) illuminated different storage capacitances resulted in similar slight dependence of the SSVD current on C2H6doping.Instead of the mixture ratio,the charging voltage is the key factor for controlling SSVD energy deposition.

Figure 8.The discharge waveforms of SSVD with gas pressures of 25 Torr (a),40 Torr (b),60 Torr (c),and (d) the dependence of peak current and peak voltage of SSVD on gas pressure at n(SF6):n(C2H6)=20:1,Cs=11.2 nF,Uc=23 kV.

Although the mixture ratio has a slight effect on the generation process of SSVD current,it has been reported that C2H6can effectively inhibit the occurrence of arc discharge in SF6-C2H6mixtures [4].The reason is that C2H6has different ionization performance from SF6,which has low ionization energy and small electronegativity.It is easier to generate many free electrons evenly distributed in space,which can reduce the energy deposition of partial discharge channel and inhibit arc discharge.Besides,Ke et al [23]analyzed the dependence of laser output radiation energy on the mixture ratio of working media,and the optimal mixture ratio is SF6/C2H6=20:1.The holding scope of SSVD is investigated in mixture ratio n(SF6):n(C2H6)=20:1,as shown in figure 7.The solid and dashed lines in the figure form close polygons indicating the holding scopes of SSVD,where the solid lines represent the real boundary for SSVD.The closed polygon takes on the features:(1)the left and right solid lines connecting to the upper dotted line tend to closer together.(2)The upper and lower solid lines connecting the right dotted line also get closer.The features indicate that the holding scopes of SSVD exist the maximum allowable gas pressure and maximum allowable charging voltage.

The mixture gas pressure is another key factor of SSVD mode except charging voltage.Figure 8 shows the dependences of the SSVD waveforms on gas pressure while Cs=11.2 nF and Uc=23 kV.With the increase of pressure from 20 Torr to 70 Torr,the current peak decreased almost linearly from 3.6 kA to 0.54 kA,while the maxima discharge voltage peak,which is different from the charging voltage of the storage capacitor,increased from 13.5 kV to 16.8 kV only by about 25% for 25–40 Torr.When the pressure is greater than 40 Torr,the maxima of the discharge voltage tends to saturate,as shown in figure 8(d).By the way,the voltage waveforms gradually approach a residual voltage through attenuated oscillation after an SSVD pulse (shown in the horizontal red dotted line in figures 8(a)–(c)).

Together with figures 4(b),8(d) tells the peak current of SSVD linearly increases with charging voltage and decreases with gas pressure,which indicates that SSVD current is not coincident with the characteristics of normal low-pressure glow discharge.Therefore,it is inferred that SSVD formation is essentially dominated byαionization avalanche[3]instead of glow discharge.Although the maxima reduced electric field strength E/P=89–230 kV cm-1bar-1,the reduced electric fields are less than 100 kV cm-1bar-1for most of the current duration,for the SF6concentration of 95%,Uc=23 kV,d=2 cm.According to the experimental research results of Y.Qiu [24],can be formatted to a linear increasing function ofwhere M and N are constant and M＞0,N＜0,if the reduced electric field strength ranges in 15–94 kV cm-1bar-1.Thus,the effective first Townsend coefficient ¯α=+ME NP.When the electric field is uniform and steady,the discharge currentI=I0exp (MU+NPd),which increases exponentially with voltage and decreases exponentially with gas pressure.According to the pulsed discharge voltage,the electric field in the discharge gap is temporally dependent,()=E E t,and therefore the effective ionization coefficientwhere=α-η,αis electron collision ionization coefficient andηis electron attachment coefficient.Thus,(t)=ME(t)+NP,the discharge peak currentwheretpis the time of peak current,tdis the total time of electron avalanche from the cathode to the anode,which can be obtained byis the electron drift velocity.Ipshould increase non-exponentially with voltage and decrease non-exponentially with gas pressure.Nevertheless,our results show that the peak current increases almost linearly with charging voltage and decreases almost linearly with gas pressure.Although the peak current does not obey the expression of discharge peak current quantitatively,it presents coincident tendencies qualitatively.Therefore,we considered that SSVD formation is essentially dominated byαionization avalanche.

4.Conclusions

In this paper,the pulsed discharge and discharge mode transformation in SF6-C2H6mixtures are investigated using a parallel plate configuration with pre-ionization sliding arrayed cathode.As expected,two discharge modes are achieved under different conditions.One is SSVD mode which is formed by spatially overlapping multiple synchronous avalanche ionization channels.As a result,comparatively homogeneous discharge occupies the interelectrode gap.Regarding the SSVD,the peak current increases with charging voltage and decreases with gas pressure,while the influence of mixture ratio and storage capacitance on the current is insignificant.Therefore,the holding scopes of charging voltage and gas pressure for SSVD are depicted.Because the dependence of the SSVD peak current in SF6-C2H6mixtures on charging voltage and gas pressure do not conform to normal low-pressure glow discharges,the achieved SSVD mode can be attributed to theαavalanche ionization.

Acknowledgments

This work is supported by National Natural Science Foundation of China (No.11375041).