Measurement of the temporal evolution of electron density in a

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Measurement of the temporal evolution of electron density in a nanosecond pulsed argon microplasma: using both Stark broadening and an OES line-ratio method

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2012 J. Phys. D: Appl. Phys. 45 295201 (http://iopscience.iop.org/0022-3727/45/29/295201) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 45 (2012) 295201 (11pp)

doi:10.1088/0022-3727/45/29/295201

Measurement of the temporal evolution of electron density in a nanosecond pulsed argon microplasma: using both Stark broadening and an OES line-ratio method Xi-Ming Zhu1 , James L Walsh2 , Wen-Cong Chen1 and Yi-Kang Pu1 1 2

Department of Engineering Physics, Tsinghua University, Beijing 100084, People’s Republic of China Department of Electrical Engineering and Electronics, University of Liverpool, L69 3GJ, UK

Received 30 March 2012, in final form 30 May 2012 Published 2 July 2012 Online at stacks.iop.org/JPhysD/45/295201 Abstract The temporal evolution of electron density in a nanosecond pulsed argon microplasma is measured using a combination of Stark broadening and the optical emission line-ratio method. In the initial discharge period (0–100 ns), the electron density can reach as high as ∼1018 cm−3 . It decreases to ∼1017 –1016 cm−3 in the early afterglow period (100 ns–1 µs after the ignition) and ∼1016 –1013 cm−3 in the late afterglow period (1–20 µs). It is demonstrated that the optical emission spectroscopy (OES) line-ratio method can obtain the electron density in the range 1013 –1016 cm−3 , while in the range 1016 –1018 cm−3 , the Stark broadening technique with argon 2p–1s lines (in Paschen’s notation) is a better choice. These results are in good agreement with those from the Stark broadening technique with hydrogen Balmer lines. Finally, a possible mechanism for such a density evolution is briefly discussed. (Some figures may appear in colour only in the online journal)

the hydrogen Balmer α and β lines (Hα , Hβ ) are used, the Stark broadening method may be used when ne is as low as ∼1014 cm−3 [1, 12]. However, these lines are observed with the addition of H2 , which may affect the characteristics of microplasmas significantly [1], or otherwise, they are observed due to the impurity of H2 O (e.g. from a stainless steel surface) and thus suffer from low signal-to-noise ratio [12]. In addition to the Stark broadening method, a line-ratio method to obtain ne in atmospheric-pressure microplasmas has been proposed recently [16]. It uses the population ratios of Ar(2p) atoms (from line ratios of Ar 2p–1s lines), which is sensitive to the variation of ne in the range 1013 –1016 cm−3 , as predicted by a collisional–radiative (CR) model [17]. Therefore, by a combination of the Stark broadening and line-ratio methods of using Ar 2p–1s lines, a wide range of ne (say, ∼1013 –1018 cm−3 ) in high-pressure discharges can be determined. In this work, we make the measurement of ne evolution in a nanosecond pulsed microplasma (described in figure 1) to illustrate this point. In particular, we obtain ne ∼ 1018 –1017 cm−3 during the initial discharge period (0– 100 ns after the ignition), ne ∼ 1017 –1016 cm−3 in the early

1. Introduction Recently, many kinds of microplasmas at high and atmospheric pressures (∼100–760 Torr), such as the dc- [1, 2], rf- [3, 4] and microwave-coupled microplasmas [5, 6], dielectric barrier discharges (DBDs) [7, 8] and nanosecond pulsed microplasmas [9, 10], have been reported for potential applications in many fields [11]. An important parameter to characterize these microplasmas is the electron density, ne , which can vary over a wide range, e.g., from ∼1013 cm−3 for argon DBDs [7], ∼1014 cm−3 for argon microwave microplasmas [12] to ∼1016 cm−3 for argon dc microplasmas [2] at atmospheric pressure. The Stark broadening technique is widely used to obtain ne in microplasmas (for example, [13–15]). However, this method has a fundamental limitation due to the presence of van der Waals, resonance and Doppler broadenings, the sum of which may dominate the line broadening when ne is low. For example, the Stark broadening method of using Ar 2p–1s lines (in Paschen’s notation) is only valid when ne > 1016 cm−3 in an argon microplasma at atmospheric pressure [13]. When 0022-3727/12/295201+11$33.00

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J. Phys. D: Appl. Phys. 45 (2012) 295201

X-M Zhu et al

Figure 1. Schematic diagram of the experimental setup. The spatially resolved optical measurement system is shown on the left bottom. On the right bottom is a zoom-in figure showing the stainless steel needle tips and the discharge gap.

Figure 2. (a) Current and voltage measurement of a pulse of Ar discharge in ambient air. (b) Instantaneous power (product of current and voltage in (a)). (c) Comparison of currents of Ar discharges in ambient air and in the vacuum chamber.

afterglow3 period (100 ns–1 µs) and ne ∼ 1016 –1013 cm−3 in the late afterglow period (1–20 µs). The results are in good agreement with those from the Stark broadening method of using Hα and Hβ lines. In the following, section 2 describes the experimental setup. Section 3 illustrates how to obtain ne by the Stark broadening and line-ratio methods. The measured temporal evolution of ne in the microplasma is presented and analysed in section 4. Section 5 discusses the uncertainties and limitations in the ne measurement.

gives a negative pulsed high voltage (figure 2(a)) across two stainless steel needle tips with a gap of about 100 µm, where the microplasma is formed. Firstly, we investigate the nanosecond pulsed argon discharge in ambient air. An argon gas outlet is placed 1 mm away from the discharge gap with a gas flow of 2.5 slm. In this case, we measure the I –V curve of the discharge using a highvoltage probe (Tektronix, P5100) and a current sensor (Pearson 2877) with an oscilloscope (Tektronix, TDS540). The results are shown in figure 2(a). Figure 2(b) shows the instantaneous power (product of current and voltage). The discharge current increases to a peak value of ∼10 A and the power increases to ∼8 kW in about 10 ns. After 80 ns, the current decreases to ∼20% of its peak value while the power is less than 5% of its peak value. Secondly, the discharge is ignited in a stainless steel vacuum chamber (discharge gas is a mixture of argon (partial pressure 700 Torr) and neon (30 Torr)). Figure 2(c) compares the obtained discharge currents in ambient air and in the vacuum chamber. The discharge current in the vacuum chamber increases to a peak value of ∼9 A in 10 ns and decreases to ∼10% after 80 ns, while ∼10 A and ∼20% in

2. Experiment As shown in figure 1, the microplasma power source consists of a nanosecond switch (home-made), a pulse generator (SRS, DG645) and a dc high-voltage power supply (Spellman, PTV350). The pulse generator provides a square voltage (trigger signal) with frequency 1 kHz and width 600 ns. The dc power supply provides a high voltage of 1 kV. The switch 3 In this work, ‘afterglow’ refers to the plasma temporally removed from the initial discharge (initial period 0–100ns and afterglow period 100 ns–20µs), but not that spatially removed.

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Table 1. Selection of Ar 2p–1s lines for the Stark broadening and line-ratio methodsa .

a

This table lists the wavelengths of Ar 2p–1s lines in units of nm. The number in parentheses is the total angle momentum quantum number. Lines in bold are recorded with a grating of 600 g mm−1 and those in shades are selected to obtain the relative population of Ar(2p1 )–Ar(2p8 ) levels for the line-ratio method. In addition, the two lines underlined are also recorded using a grating of 1200 g mm−1 for the Stark broadening method. The other lines are too weak to be recoded due to their small Einstein coefficients or being out of the wavelength range of the optical system.

ambient air. This difference may be due to the ne variation with gas composition [1, 2]. Note that the impurity in the discharge is mainly H2 O from the stainless steel vacuum chamber wall. With an H2 O concentration of 0.1%, the emission lines of H atoms, coming from the dissociation of H2 O, are strong enough to use the Stark broadening technique. In addition, the Ar line-ratio method can also be applied in this case, since the concentration of impurity species is 1016 cm−3 ) and that using the line-ratio method.

simplifications (i) and (ii) for Ar 2p–1s lines. We also consider another source of uncertainty (∼10%) from the experimentally determined coefficient a [24]. For the Hα line, λS also depends on Te . The averaged relationship, λS = a · nbe , which neglects the Te dependence, may cause an uncertainty of ∼30% in the ne measurement [20]. The Hβ line has a large broadening and is possibly influenced by nearby Ar, Cr and Ar+ lines. Therefore, we only use this line when it is narrow enough at ne 1016 cm−3 . In this case, we estimate an uncertainty of ∼10% for using the simplified relationship between λS and ne and another uncertainty of ∼10% in the fitting procedure due to its low signal-to-noise ratio. On the other hand, the uncertainty in species temperature, as mentioned in section 4.1, may also affect the ne determination due to the temperature dependence of λV and λD , say, λV ∼ T −0.7 (at a fixed pressure) and λD ∼ T 0.5 [13]. We also include an uncertainty in λI , ∼1%, due to the determination of instrument broadening width. Therefore, the uncertainty in ne determination by the Stark broadening method in this work is λV λD λI δne ∼ δS + δV + δD + δI λS λS λS 0.7λV + 0.5λD λI ∼ δS + δT + δI . (12) λS λS

at the electrode surface (important in the afterglow period when ne is low) [17]. On the other hand, some collisional ionization processes in the microplasma slow the decay of ne . For example, Ar(1s) atoms are produced by the electron-impact excitation in the initial discharge period. In the early afterglow period (100 ns–1 µs), the Penning ionization of Ar(1s) atoms is a very strong ionization process, which slows the decay of ne [33]. Ar(1s) are mainly depopulated by the collisions with Ar(gs), due to which the lifetime of Ar(1s) is ∼100–300 ns [17]. As a result, Ar(1s) density significantly decreases in the late afterglow period (1–20 µs). Excimers are produced by the collisions between Ar(1s) and Ar(gs) and mainly depopulated by the electron quenching [17]. In the late afterglow period, with a very low ne , the excimer density roughly stays constant and the Penning ionization of excimers plays the dominant role to slow the decay of ne [33].

5. Discussion In this work, we use two simplifications for the Stark broadening method: (i) we assume a symmetric Lorentzian profile for the Stark broadening, (ii) we use approximate forms, λS = a · ne , for Ar 2p–1s lines (constant a from the measurement in [24]) and λS = a · nbe , for Hα and Hβ lines (constants a and b from the simulation in [20]). In fact, the iondependent Stark broadening may lead to an asymmetric profile for Ar lines as well as an electron temperature (Te ) dependence of λS [13]. For the Ar 2p–1s lines, the contribution of ion Stark broadening is 10%, in comparison with the electron Stark broadening, as measured in [34]. Therefore, we estimate an uncertainty of ∼10% in the ne determination when using

Here δne , δS , δV , δD , δI and δT refer to the uncertainty in ne and those in λS , λV , λD , λI and species temperature, respectively, as given above. With λS , λV , λD and λI in figure 9, we give δne for Ar 696.5 nm, Hα and Hβ lines, as shown in figure 12. We find that, in the Stark broadening method, these three lines can be used when ne 1016 cm−3 , ∼1015 cm−3 and ∼3×1013 cm−3 , respectively, with an uncertainty of 50%. We rewrite the line-ratio equation (10) as R ∼1+ 9

C , 1+X

(13)

J. Phys. D: Appl. Phys. 45 (2012) 295201

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with R=

nAr(2p2 ) , nAr(2p3 )

line-ratio method is valid. Using the combination method, we experimentally obtain the temporal evolution of ne in a nanosecond pulsed microplasma. In the initial discharge period, ne is as high as ∼1018 –1017 cm−3 with a non-uniform distribution, indicated by the spatially resolved emission intensity measurement. A double-Voigt fitting method is developed to obtain ne,centre and ne,edge , corresponding to the ne in the centre and edge regions in the microplasma, respectively. The temporal scalings of ne,centre and ne,edge are different in the initial discharge period, which indicates that a dense ‘plasma core’ with high ne expands to the edge region with low ne . In the afterglow period, ne exponentially decreases from ∼1017 to ∼1013 cm−3 in a time of 20 µs, which indicates that the electron kinetics is dominated by two competing processes—the recombination and the Penning ionization of energetic Ar(1s) atoms and excimers, by considering the previous modelling investigations.

que

X=

ne Qe,Ar(2p2 ) que

nAr(gs) · QAr(gs),Ar(2p2 )

,

que

C=

QAr(gs),Ar(2p3 ) que

QAr(gs),Ar(2p2 )

,

C 1.

(14)

We have ∂R X X2 /C + X + 1 ∼ δX ∼ δR / δR . ∂X R X

(15)

Here δX and δR are the uncertainties in parameters X and R, respectively. We estimate δR ∼ 3% for the OES line-ratio measurement in this work. From equations (14) and (15), we have X2 /C + X + 1 δR + δN + δQ δn e ∼ X X2 /C + X + 1 δR + δT + δQ . ∼ (16) X Here δne is the uncertainty in ne determination by the lineratio method. δN and δQ are the uncertainty in Ar(gs) density −1 , δN ∼ δT ) and that in the rate coefficient data in (nAr ∼ TAr the CR model, which is at least ∼10% [35, 36]. Considering its uncertainty shown in figure 12, the line-ratio method is valid in the ne range 1013 –1016 cm−3 . Another issue to be addressed is the variation in Hα line intensity. It is weak in the first 100 ns and we cannot use the double-Voigt fitting due to the large noise in the measured line profiles. However, the signal-to-noise ratio of Hα line increases after 100 ns. This is because (i) we use a relatively long integration time in the OES measurement after 100 ns and (ii) the H atom density may increase due to the Penning dissociation of H2 O with Ar(1s) (considering that Ar(1s) density keeps increasing in the first 100–300 ns due to its long lifetime). Therefore, we can use both single-Voigt and doubleVoigt fitting after 100 ns. The last issue to be discussed is the Te (or mean electron energy) variation in the afterglow period. According to the simulation in [33], the mean electron energy is ∼0.6– 1.4 eV in 200 ns–20 µs after the ignition. This is mainly due to the reheating mechanisms, including the superelastic collisions and the Penning ionizations of Ar excited atoms and excimers. In the CR model, we find that the line ratio of 2p levels is insensitive to Te , in the range 0.5–1.5 eV, when the electron-impact excitation process dominates the recombination process. However, when Te decreases to ∼0.1– 0.3 eV, the recombination process is important and the lineratio method may be invalid, e.g., in 40–50 µs after the plasma ignition [33].

Acknowledgments The authors thank Professors N Sadeghi and U Czarnetzki for the enlightening discussions, Professor R Boswell and Dr H Boettner for providing their original experimental data and Dr J Li, Z W Cheng, Dr X Meng and Prof W X Pan for the help with the experiment. This work is supported in part by the National Natural Science Foundation of China (Nos 11075093 and 10935006) and the China Postdoctoral Science Foundation (No 20100480327).

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6. Conclusion In this work, we investigate the possibility of measuring the electron density, ne , by a combination of Stark broadening and line-ratio methods in microplasmas at high to atmospheric pressure. The Stark broadening method (using Ar 2p–1s lines) is valid when ne 1016 cm−3 ; when ne 1016 cm−3 the Ar 10

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