Atmospheric pressure pulsed modulated arc discharge plasma

Ji LI (李寄),Jingfeng TANG (唐井峰) ,Haoran ZHANG (张浩然),Lu WANG (王璐),Tianyuan JI (纪添源),Daren YU (于达仁) and Ximing ZHU (朱悉铭)

School of Energy Science and Engineering,Harbin Institute of Technology,Harbin 150001,People’s Republic of China

Abstract Direct-current(DC)arc plasma has great application values in the field of the chemical industry,but it has the problem of low energy efficiency.Facing the requirement for improving the energy efficiency of the arc,this paper proposes a unique method of pulsed modulated arc(PMA).This method uses high-frequency pulses and reduces the arc current to improve the control of electron temperature.The electrical characteristics,optical characteristics and products are tested.The test results show that during the PMA process,all of the experimental results which include voltage,current and light will significantly increase.These results are analyzed from the perspective of functionality,repeatability and energy conversion.The analysis results show that although the PMA method does not show good parameter consistency,it has potential application prospects because it increases the energy conversion rate by 4.5% and 8% from the perspective of light and products,respectively.

Keywords: experimental research,atmospheric pressure plasma,DC arc,PMA

1.Introduction

Direct-current (DC) arc plasma technology,due to high temperature and chemical activity properties [1,2],has wide application value in high-temperature chemical fields such as nano-material preparation [2],CO2reforming [3],hazardous waste treatment [4],and device combustion [5].However,due to the poor energy selectivity of the plasma itself,it always has the fatal problem of low electron temperature and low efficiency in the chemical industry[6].This problem has seriously affected its extensive industrial application,and has become a problem that scholars have to consider solving.

In view of the low efficiency of the DC arc,the solutions from scholars around the world can be divided into two types:reform and improvement.Reform refers to completely abandoning thermal plasma and adopting non-thermal plasma with strong energy selectivity to solve the problem of low efficiency.In this solution,Starikovskaia et al [7] firstly proposed the concept of a fast nanosecond pulse ionization wave.Later,Qiu et al[8],Lacoste et al[9],Babaeva et al[10]and others carried out the coupling of nanosecond pulse and metal electrodes,and successfully realized the forms of pulse spark discharge and pulse diffuse discharge.Pai et al [11],Zhang et al [12],Liu et al [13],Boeuf et al [14] and others carried out the combination of nanosecond pulse and dielectric barrier discharge (DBD) device,and explored the optimization,mechanism and product characteristics of DBD nanosecond pulse.These methods are common in some lowtemperature reaction applications (such as water treatment with plasma),and their energy efficiency is about 2.240 g(kW·h)-1(energy efficiency is defined as the mass of organic matter that can be degraded per unit energy when the pollutant degradation rate reaches 50%).The improvement type refers to adding new techniques for existing thermal plasma,such as realizing a warm arc or adding a highfrequency source.In this solution,Czernichowski et al [15]promoted the concept of a sliding arc,and then Sun et al[16],Lu et al [17],Zhu et al [18],Feng et al [19] and others applied it to a variety of electrodes,such as blade electrodes,rotating electrodes,Raval electrodes and so on,and explored the combination of sliding arc and other devices.This type of method is usually applied to some endothermic reaction applications (such as fuel reforming),and its efficiency is usually about 50%(the efficiency is defined as the ratio of the reaction enthalpy and actual energy expenses).Kawajiri et al[20] proposed the concept of a DC-RF (direct-current-radiofrequency) hybrid plasma torch,and then Takana et al [21],Nishiyama et al [22],Seo et al [23] and Jiang et al [24]explored the effect of DC-RF on the preparation of nanosecond materials from the perspectives of experiment,theory and simulation.The researchers discussed the magnitude of the spheroidization rate (a comprehensive indicator of the degree of spheroidization of all particles within a specified field of view),but did not account for efficiency improvements.The development of these two solutions shows that non-thermal plasma has a strong ability to increase electron temperature and solve the problem of low efficiency.It is an important way for increasing the electron temperature of thermal plasma to add DC power supply of arc additional high-frequency source to modulate its electronic properties.

Figure1.Block diagram of a pulsed system for studying the effects of modulation on the arc discharge plasma chemistry.

The method of modulating the electronic properties of plasma by a high-frequency source has not yet been discussed in DC,but it has been widely studied in the RF field.In the field of RF etching,Goto et al [25] and Tsuyoshi et al [26]introduced high-frequency RF into RF etching silicon-based chips to realize electronic control,and also discussed its internal heating mechanism.In the field of RF chemical vapor deposition,Xu et al [27] independently controlled the electron density and energy of plasma by using high radio frequency,and prepared graphene film at low temperature without a metal catalyst substrate.In the field of material preparation,Liu et al[28]introduced high-frequency RF into the preparation of thin film silicon,improving the efficiency of material preparation.In the field of RF mechanism,Krishnakum et al[29],Nienhius et al[30],Rehman et al[31],Yan et al [32],Kushner [33],Bruneau et al [34],Radouane et al [35],Jia et al [36] and others also deeply explored the electrical and product characteristics of high and lowfrequency coupling,showing the feasibility of the sources modulating plasma electron temperature.

Referring to the idea of high-frequency excitation and modulation of electrons in the RF field,this paper proposes a PMA method.The difference from the DC-RF works described earlier is that this work not only uses the pulses with higher frequency and high transient power attributes to modulate DC,but also evaluates the electron temperature and products of plasma to verify the PMA effect.

2.Method of modulation

The basic idea of PMA is to construct a rapidly varying electric field,and use the characteristics of electronic response to the variation of electric field to realize the improvement of electronic energy.As the electronic energy is improved,the situation that only a small amount of energy is used for chemical energy will be expected to improve.

In order to realize this assumption,this study will modify the DC source.Firstly,the current is decreased as much as possible to reduce the plasma density of the arc.Then the pulse source is connected in series in the DC circuit to provide a high-frequency and high-current oscillation signal with a MHz cycle(figure 1).At this time,although the plasma density of the arc is still around 1021cm-3(the minimum plasma density of a thermal arc [37]),it is much lower than 1024cm-3,which is the plasma density of a typical arc of high current [37].Meanwhile,although the frequency of the oscillation signal is slightly lower than the value of the ion characteristic frequency (6.9 × 109s-1is calculated according to the literature[38]),it can also play a certain role in improving the electron temperature.

Figure2.Voltage (a) and current (b) characteristics of arc (red line) and of PMA (violet line).

In the specific experiment,a DC source and a pulse source are connected with pin-pin electrodes (figure 1).At the gap of the electrodes,slow flow is produced to collect plasma-treated gas.The flow velocity is as low as possible so that the plasma is not affected by the flow.To get electrical characteristics,a high-voltage probe,a sampling resistance,a passive probe and an oscilloscope are applied (green pane).To get optical characteristics,a camera,a photodiode and a spectrometer are used (pink pane).To indicate the ozone concentration,the transmittance of indigotin disulfonate sodium (IDS) solution treated by the plasma feedstock in the spectrophotometer is tested(yellow pane).Then according to the test results of electrical characteristics,optical characteristics and ozone concentration,the promotion effect of impulse-modulated arc on plasma chemistry is demonstrated from the path with three nodes of the ‘source-plasmaproduct’.

On the pin-pin electrodes,there is a point to be explained.The electrodes gap can be selected between 0.5 and 2 mm,because too large or too small electrodes gap will bring the risk of difficult discharge or short circuit.When the electrode spacing is between 0.5 and 2 mm,the pulse modulation power will change slightly with the variation of the electrode gap.The choice of spacing has deviated from the theme of this paper,so we only explain here: this work finally chooses the electrode gap of 1 mm,which is the common gap for arc ignition [5].

3.Effect of modulation

3.1.Discharge characteristics

When the arc is modulated by the pulse,its voltage waveform has not only an overdamped oscillation signal with a period of 1.2 μs,but also an oscillation signal with a superimposed period of 40 ns due to impedance matching (as shown in figure 2).If the oscillation wave with the period of 1.2 μs is ignored,and only the oscillation with the period of 40 ns is focused on,it can be found that there are many significant peaks in the voltage signal.Compared with the arc voltage of 25 V,these peaks always exist during the impulse current duration,and the maximum values can reach 280 V (at 1 μs)and 800 V (at 2 μs).

The instantaneous voltage peak means the rapid variation of the electric field,and it means the instantaneous displacement current [39].When the instantaneous displacement current is generated,the plasma will generate a conduction current equivalent to the displacement current.At this time,the charged particles in the plasma,especially electrons,will obtain more energy and begin to make a directional motion.The charged particles with increased energy collide with neutral particles,forming more charged particles as a result.Since the number density of charged particles increases instantaneously,the conduction current and total current also increase instantaneously.The result is that in addition to voltage,the current will also reach a certain peak in an instant,and the peak value (52 A/110 A) is significantly higher than the 4 A of the DC arc.

3.2.Optical characteristics

The increase of instantaneous total current comes from the increase and movement of charged particles [40],while the increase and movement of charged particles come from the improvement of electron temperature and the acceleration of collision reaction.According to the preliminary estimation of the collision-radiation model(which will be reported later),the electron temperature of the PMA is improved (marked in figure 3).High-energy electrons collide with other particles high frequently,leading to the increase of instantaneous total current and the acceleration of the easily observed collision process of emitting photons.This acceleration will show the difference of light between the arc and PMA in space,time and frequency spectrum.

The difference of light between the arc and PMA in space is shown in figure 4.It can be seen in the figure that when the arc is modulated by the pulse,the anodic area of the arc plasma and the positive column near the anodic area will be significantly brighter.The reason is that the negative particles(such as e-)near the sheath of the anode move away from the anode under the effect of the high-frequency electric field,and the movement of negative particles leads to more collisions between particles.As a result,more luminescence processes occur and the plasma becomes brighter [38].

The difference of light between the arc and PMA in time is shown in figure 4.Although the photon energy of each light-emitting collision process is different and the number is limited,the total number of photons produced by all processes is significantly increased.From the perspective of time evolution,the instantaneous optical signal of PMA has significant variations compared with that of unmodulated ones,and its optical signal shows an oscillation state with a period of 1.2 μs (as shown in figure 4) just like the voltage signal.This oscillation state continuously exists throughout the duration of the impulse current and does not decay until the end of the pulse.

Figure3.Electron temperature Te near anode of arc (above) and PMA (below).

Figure4.Optical signal difference between arc (red line) and PMA(violet line).

The emission intensity spectrum for arc and PMA in time is shown in figure 5.It can be seen from figure 5 that the intensity of the light from the de-excited radiation of O is enhanced when the arc is modulated by pulse.At this time,the plasma obtains more energy under the modulation from pulse,and increases the ground state O atom by increasing the electron temperature.Thus,more excited oxygen(O(5 P)and O(5 S)) is formed,resulting in more de-excited radiation processes (such as O(5 P) →O(5 S)+hv and O(3 P) →O(3 S)+hv) than unmodulated ones.

Table 1.Difference between arc and PMA in treatment of IDS solution.

Figure5.Emission intensity spectrum for arc (red line) and PMA(violet line).

3.3.Product

Like other plasmas [41],high-energy excited particles produced by PMA cannot exist for a long time.These particles will collide with neutral gas molecules to form metastable particles with higher stability.O3is one of these metastable particles.

As for whether the PMA can generate additional O3,the variation of transmittance of IDS solution can provide qualitative analysis.The basic principle of the transmittance of IDS to describe the concentration of O3is that the ozone generated by the plasma can transform the blue IDS into another colorless substance [42].The more O3produced by the plasma,the more IDS is transformed,and the more obvious the fading of the corresponding blue solution [43].According to the variation of transmittance before and after fading of solution,the output of O3can be qualitatively evaluated.

The results of the transmittance variation of the IDS solution are shown in table 1.It can be seen from the table that when air is added to the solution,the variation of IDS solution is only 0.003,which is negligible,indicating that oxygen in pure air cannot make a significant variation in IDS solution.However,the transmittance of IDS solution varied by 0.264 and 0.313 respectively after the gases treated by the arc and PMA were introduced,which indicates that the plasma can produce observable ozone.Meanwhile,the transmittance of the IDS solution increased by 18.6%compared with the unmodulated one,indicating that the PMA can form more active O3.

Table 2.Comparison between parameters of PMA with an unmodulated arc.

4.Discussion: effect analysis

The results of the above three aspects of electrical characteristics,optical characteristics and products show that during the PMA process,the values of voltage and current of the arc,the electron temperature,the light near the anodic area of the arc,and the de-excited radiation process and metastable products produced by the arc plasma will significantly increase.These results show the functionality of the PMA method,and preliminarily describe this method as a way to generate additional active substances.

After discussing the basic mechanism,now let us discuss some of the available values of this method.The content includes the energy conversion,reliability and accuracy of PMA.

As for the energy conversion characteristics,there is no doubt that the PMA will inevitably cause an increase in energy input and material output.Compared with unmodulated arc,the energy input of arc plasma increases by 10%from 0.1 kW to about 0.11 kW (table 2).However,the average value of photodiode results from multiple pulses is increased from 37.5 to 39.2 V kW-1,and the luminous intensity per unit energy input is increased by about 4.5%(the value is increased by 15.2%).The difference in transmittance of IDS solution is increased from 2.64 to 2.85 kW-1,and the product formation per unit energy input is increased by about 8.0%(the value is increased by 18.5%).From the perspective of total energy conversion,the energy conversion and energy utilization benefits of PMA are significantly improved compared with those of unmodulated arc.

Reliability is proposed for the power supply.That is,the power supply should be able to provide sufficient output power to ensure that there is no lack of pulse.In contrast,if the power supply power is lower than the demand power(such as the result of using a small power switching power supply/linear power supply to supply the pulse source),the pulse will disappear where there should be a pulse.For this requirement,as long as the pulse itself has sufficient power,for example,when using mains power or a high-power DC source,the problem of pulse loss will not occur.

Figure6.Scatter plot of additional power and additional emission intensity of pulses.

The accuracy is proposed for the coupling between pulse and arc.In our actual test work,standard deviations of the voltage and current for both the arc and pulse are relatively stable.However,when the pulse and arc are coupled,that is when the pulse modulates the arc,the standard deviations of output electrical characteristic parameters increase significantly (see table 2).For example,the additional average power and additional emission intensity of PMA pulses(figure 6) can fluctuate over a large range of variations,and from the results shown in figure 6,it seems that power and emission intensity fluctuate independently,but they also exhibit a certain positive correlation.Further understanding of the coupling mechanism between the arc and pulse is needed to improve this behavior.

5.Conclusion

(1) Facing the requirement of improving the chemical efficiency of the arc,in order to improve the electronic energy in the arc as much as possible,this paper introduces high-frequency and high-current pulse while reducing the arc current,and forms a unique method of PMA.

(2) Electrical signals,optical signals and products confirm the effect of pulse on modulating arc.The electrical signal results show that there are many peaks significantly higher than the DC voltage in the pulse voltage/current.The optical signal results show that the PMA method enhances arc plasma significantly.The results of the product show that the gas treated by impulse modulated arc has stronger oxidation activity than that treated by ordinary arc.

(3) The engineering issues of the impulse-modulated arc are discussed from the perspective of energy conversion efficiency,reliability and accuracy.In the discussion of energy conversion efficiency,it is found that the energy conversion rate is increased by 4.5% and 8% from the perspective of light and product,so it has potential application prospects.In terms of the discussion of reliability and accuracy,we present some experiences with reliability and point out the shortcomings of the temporally method of PMA that do not show good parameter consistency.

Acknowledgments

This work was supported by National Natural Science Foundation of China (No.52177135) and the National Science and Technology Major Project of China (Nos.2017-III-0007-0032,2019-III-0013-0056).