Diagnosis of gas temperature, electron temperature, and electron density in helium atmospheric pressure plasma jet Zheng-Shi Chang, Guan-Jun Zhang, Xian-Jun Shao, and Zeng-Hui Zhang Citation: Phys. Plasmas 19, 073513 (2012); doi: 10.1063/1.4739060 View online: http://dx.doi.org/10.1063/1.4739060 View Table of Contents: http://pop.aip.org/resource/1/PHPAEN/v19/i7 Published by the American Institute of Physics.
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PHYSICS OF PLASMAS 19, 073513 (2012)
Diagnosis of gas temperature, electron temperature, and electron density in helium atmospheric pressure plasma jet Zheng-Shi Chang (常正实),a) Guan-Jun Zhang (张冠军),b) Xian-Jun Shao (邵先军), and Zeng-Hui Zhang (张增辉) State Key Laboratory of Electrical Insulation and Power Equipment, School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, Shaanxi 710049, China
(Received 27 February 2012; accepted 21 June 2012; published online 20 July 2012) The optical emission spectra of helium atmospheric pressure plasma jet (APPJ) are captured with a three grating spectrometer. The grating primary spectrum covers the whole wavelength range from 200 nm to 900 nm, with the overlapped grating secondary spectrum appearing from 500 nm to 900 nm, which has a higher resolution than that of the grating primary spectrum. So the grating P secondary spectrum of OH (A2 þ(t0 ¼ 0) ! X2G(t00 ¼ 0)) is employed to calculate the gas temperature (Tg) of helium APPJ. Moreover, the electron temperature (Te) is deduced from the Maxwellian electron energy distribution combining with Tg, and the electron density (ne) is extracted from the plasma absorbed power. The results are helpful for understanding the physical C 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4739060] property of APPJs. V I. INTRODUCTION
The spectra of plasma contain lots of information which reflects the basic physical characteristics and many parameters of plasmas, such as gas temperature (Tg),1 electron temperature (Te),2 electron density (ne),3 electric field,4 thermodynamics parameters, and transport parameters.5 As well known, Tg, Te, and ne are usually important parameters of atmospheric pressure plasma jet (APPJ). Tg significantly influences the application of APPJs and needs to be well diagnosed. Among many diagnostic methods, optical emission spectroscopy (OES) was usually used to diagnose plasmas Tg because it is a measurement method of no interference and non-contact.6 From spectroscopy analysis, the grating secondary spectrum is a better choice than the grating primary spectrum for its higher resolution. However, it is difficult to be identified because the lines intensity decreases sharply with increase of the series of grating spectrum, and spectrum overlapping usually happens. The electron temperature was approximated by electron excitation temperature (Texc) under the condition of local thermodynamic equilibrium (LTE) in published works.7,8 However, the APPJs are usually worked in non-LTE where Te and Texc are different. Therefore, other way is necessary to be introduced for diagnosing the electron temperature. Assuming that the electrons obey a Maxwellian electron energy distribution, Te can be deduced from the ions number balance and the electron energy distribution function (EEDF).9,10 Further, ne can be calculated from the energy balance and plasma absorbed power.9,10 In this paper, the spectral identification of helium APPJ is conducted in wavelength range from 200 nm to 900P nm. Clearly visible grating secondary spectrum of OH (A2 þ(t0 ¼ 0) ! X2G(t00 ¼ 0)) and nitrogen molecules (the a)
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b)
1070-664X/2012/19(7)/073513/5/$30.00
first positive system (FPS) and the second negative system) are found from 500 nm to 900 nm. Then, Tg (represented by molecular rotational temperature11,12) is diagnosed P by using the grating secondary spectrum of OH (A2 þ(t0 ¼ 0) ! X2G(t00 ¼ 0)). Finally, the electron temperature is deduced from EEDF, and the electron density is further obtained by using the plasma absorbed power. II. GAS TEMPERATURE OF PLASMA A. Experiment setup
In our experiment, helium APPJ is generated by a linear-field electrode configuration (the flow field and the electric field are parallel), as shown in Fig. 1. The powered electrode 2 and the ground electrode 1 are 2 cm wide copper strips, which are wrapped around a hollow quartz tube with inner and outer diameters of 0.2 cm and 0.4 cm, respectively. The distance between the two electrodes is 2 cm. A high voltage source is used to generate high voltage of 8.9 kV in magnitude and a frequency of 17 kHz continuous sinusoidal wave. The distance from electrode 2 to the quartz tube nozzle is 1 cm. The flow rate of helium is fixed at 7.5 slm (standard litres per minutes). A three grating spectrometer (Acton spectrapro 2300i, Princeton Instruments) is used to measure the plasma spectra. The grating of 1200 groove/mm is selected and blazed at 500 nm in this work. B. Spectral identification
The helium APPJ spectra are captured by using the grating spectrometer aforementioned and the identified lines are shown in Fig. 2. In Fig. 2, the grating primary spectrum covers the whole P range from 200 nm to 900 nm, which includes OH P (A2 þ (t0 ¼ 0) ! X2G(t00 ¼ 0)) radical, the FPS B3Pg!A3 þ u and 3 3 P ! B P of N the second positive system (SPS) C u g 2, the P 2Pþ þ ! X of N and three first negative system (FNS) B2 þ 2 u g lines of helium atoms as well as one line of oxygen atom. The
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FIG. 1. Schematic of the experimental setup.
other lines are of N2 (SPS), which is not yet marked in Fig. 2. The lines of red marked (like N2 (SPS) [HeI706.52 nm]) mean the intensity and shape of line 706.5 nm are contributed by the grating primary spectrum of HeI (706.52 nm) and the grating secondary spectrum of N2 (SPS, 353.6 nm). Besides, the lines overlapping happens between the grating primary spectrum and secondary spectrum range from 500 nm to 900 nm in Fig. 2, which can also be confirmed by the grating equation dsinðh0 þ hÞ ¼ jk ðj ¼ 0; 61; P 62; …Þ. The spectra of OH (A2 þ(t0 ¼ 0) ! X2G (t00 ¼ 0) radical are further discussed here. The grating primary spectrum and the grating secondary spectrum of OH radical are shown in Figs. 3(a) and 3(b), respectively. Obviously, the line intensity of grating secondary spectrum is larger than that of the grating primary spectrum because the grating is blazing at 500 nm in our experiment. Moreover, its resolution is so high that up to 12 rotational transition lines can be identified in Fig. 3(b). Then, the lines are used to diagnose Tg of APPJ, marked as P(J) for DJ ¼ 1, Q(J) for DJ ¼ 0, and R(J) for DJ ¼ þ1, and J is rotation quantum number. Corresponding parameters of these lines such as wavelength (k), upper energy level (EJ0 ), and Ho¨nl-London factor (SJ0 J00 ), are listed in Table I. The intensities of helium atom lines are weaker than that of molecule radicals as shown in Fig. 2. It is because the line intensity is positive correlation with the population of
FIG.P3. Grating primary spectrum (a) and its secondary spectrum (b) of OH (A2 þ(t0 ¼ 0) ! X2G (t00 ¼ 0)). The letters marked denote the lines were used to diagnose the gas temperature of plasmas.
particles in energy state, which is negative exponential correlation with the excited energy of particles, and the excited energy of helium atoms exceeds 20 eV, while that of most molecule radicals lines is under 13 eV. C. Gas temperature calculation
The gas temperature Tg of plasma is usually approximated by the rotational temperature of OH radical.11,12 This approximation is acceptable P under the condition of low rotational levels of OH (A2 þ(t0 ¼ 0) ! X2G(t00 ¼ 0)) and small rotational quantum numbers J. The corresponding transition wavelength can be expressed in the terminologies defined by Herzberg13 and Bai et al.14 as ( 0 0 J kAt Xt00 J 00
¼
1 p 0 0 A Ypq t0 þ ½J ðJ þ 1Þq 2 p¼0 q¼0 )1 1 p 00 00 q X 00 Ypq t þ ½J ðJ þ 1Þ ; 2 na
5 X 1 X
(1)
where na is the refractive index of air and Ypq are spectroscopic constants related to the rovibrational transition in TABLE I. Wavelength k, energy EJ0 , and Ho¨nl-London factor SJ0 J00 of OH P radical (A2 þ(t0 ¼ 0) ! X2G(t00 ¼ 0)).
FIG. 2. Identification of the helium APPJ spectra. The grating primary spectrum (ranging from 200 nm to 900 nm) and its grating secondary spectrum (500 nm–900 nm).
Symbol
k (nm)
EJ0 (eV)
SJ0 J00
R1(2) R1(1) Q1(1) Q1(2) P1(1) P1(2) P1(4) P2(3) P1(5) P2(4) P2(5) P2(6)
307.032 307.201 307.844 307.995 308.155 308.639 309.612 309.960 310.123 310.335 310.756 311.218
4.047 4.035 4.026 4.035 4.022 4.026 4.047 4.035 4.064 4.047 4.064 4.085
0.768 0.331 1.124 2.127 1.177 1.592 2.558 1.075 3.063 1.619 2.169 2.719
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P wavenumber. The values of Ypq for A2 þ and X2G electronic states of OH radical are spectroscopic constants from Ref. 15. When the vibrational transition (t0 ¼ 0 ! t00 ¼ 0) is given, the rotational transition (J0 ! J00 ) can be expressed as12,16,17 K EJ0 IJ0 J00 ¼ 4 SJ0 J00 exp ; (2) kB Trot k and K is also constant including all terms independent of rotational quantum number. IJ0 J00 can be measured in experiment. EJ0 is energy of upper level with rotational quantum number J0 corresponding to F(K) in Ref. 18. k is the wavelength of transition and Trot is the rotational temperature. kB is the Boltzmann constant. SJ0 J00 is called the line strengths or Ho¨nl-London factor.19,20 On the one hand, combining the data in Table I with Eq. (2) as well as IJ0 J00 measured in experimental spectra, the Boltzmann plot for ln(IJ0 J00 k4/SJ0 J00 ) versus EJ0 is shown as Fig. 4. Then the rotational temperature is extracted by slope of the straight line, which is about 312 K. On the other hand, the rotational temperature is also obtained by using software Lifbase.21 It is shown from Fig. 5 that the experimental and simulation spectra are satisfactorily fitted except the peak between 311.5 nm and 312 nm. The obtained rotational temperature is 320 K. The deviation at the peak between 311.5 nm and 312 nm may be due to the influence of band spectrum of N2 SPS in 316 nm. It can also be verified from Fig. 2. This is a relatively low temperature when compare to these obtained by the other results.6 III. ELECTRON TEMPERATURE AND ELECTRON DENSITY
The electron temperature Te is a vital parameter to describe plasmas properties. It is higher than the neutral temperature Tg in cold plasma so that the LTE is no longer valid. So diagnosis of Te by using LTE approximation method is not credible. However, the electron’s behaviour can still be described by using EEDF. So Te is further obtained combin-
FIG. 5. The best-fit between experimental and simulation spectra by using software lifbase.
ing the ions number balance with the obtained gas temperature, Tg. The Maxwellian electron energy distribution is assumed in this work without considering the effects of strong electric field: rffiffiffi 2 3=2 e exp ; (3) T f ðeÞ ¼ p e Te where e is the electron energy. The ion velocity Ð 1isf ðeÞobtained 10 i ¼ A from the sheath formation criterion m1i hv2 i 0 2e de. sheath containing an excess of positive ions develops on the boundary between plasma and a quartz tube inner wall when helium gas flows velocity is stable. According to the particle number balance equation, Te can be determined by equating the total volume ionization to the surface particle loss to obtain9,10 Kiz 1 ¼ ; vi ng deff
(4)
where ng is the neutral gas density can be obtained from the ideal gas law. Kiz is the rate constant for electron-neutral ionization, and it is calculated by integrating the electron-neutral impact ionization cross section given by Rejoub et al.22 over the electron energy distribution given by Eq. (3) or its simply expression can be found from Ref. 23. deff is the effective length of plasma about the ions lost9,10 and there is no sheath formed in axial direction, so it can be write as deff ¼
1R ; 2 hR
(5)
where hR is the normalized radial sheath edge densities.10 Considering the negative ions density far lower than the electrons density, it is expressed as 1=2 R 0:80RuB : hR ffi 0:80 4þ þ v01 J1 ðv01 ÞDa ki P FIG. 4. Boltzmann plot fitting of OH (A2 þ(t0 ¼ 0) ! X2G(t00 ¼ 0)) in Helium APPJ.
(6)
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi uB is Bohm velocity as uB ¼ eTe =mi (where e ¼ 1.6 1019 C, mi is mass of helium ion), J1(v) is the
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the electron density of 3 1017 m3 in non-thermal RF plasma jet at atmospheric pressure with helium as feed gas,25 the electron density of the magnitude of 1017 m3 in the atmospheric helium plasma needle,28 as well as other results given by Sakiyama et al.29 IV. CONCLUSIONS
FIG. 6. the digital photograph of plasma (R ¼ 1 mm and L ¼ 20 mm).
first order Bessel function, v01 2.405 is the first zero of the zero order Bessel function J0. R is the radius of plasma column within the tube. ki is the ion-neutral mean free path depending on both the neutral densities and the ionization cross sections about 1.32 1019 m2 (Ref. 22) for single helium ions. Da is the ambipolar diffusion coefficient.24 In our experiment, under one atmospheric pressure condition, the plasma within the tube between the two electrodes is investigated, and the digital photograph is shown in Fig. 6. Based on the obtained gas temperature Tg ¼ 312 K and Eq. (4), Te has been calculated and Te ¼ 1.87 eV. It is higher than previous results of the electronic excitation temperature, Texc, about 1 eV.6 However, it has a good agreement with the electron temperature of 2 eV in non-thermal radio frequency (RF) plasma of helium APPJs.25 It is a fact that Texc is equal to Te only when LTE is valid. However, in non-LTE, Tg, Te, and Texc of APPJs present a large difference and normally satisfy the relations of Tg < Texc < Te.2 The above analysis indicates that the result of Te by using the method in this work is more reasonable than the approximation with Texc. The electron density, ne as an important parameter, can be deduced from the plasma absorbed power9,10: Pabs ¼ ne vi Aeff eT ;
(7)
where Aeff is the effective area as Aeff ¼2pR L hR . R and L are radius and length of plasma shown in Fig. 6, respectively. eT includes the collisional energy lost per electron-ion pair created ec , the mean kinetic energy lost per electron lost ee ¼ 2Te and the mean kinetic energy lost per ion lost ei 5:2Te . ec is defined as10,26 Kiz ec ¼Kiz eiz þ
X i
Kex;i eex;i þ Kel
3me Te ; mi
(8)
where eiz ¼24:6 eV is the ionization energy for helium, eex;i is the threshold energy for the ith excitation process, Kiz is the ionization rate constant in above section, Kex,i is the rate constant for ith excited state,23 Kel is the elastic rate constant and can be calculated by integrating the elastic scattering cross section given by Brunger et al.27 The plasma absorbed power is assumed equal to the discharge power. The discharge power is obtained from lissajous figure measured in the experiment, i.e., Pabs ¼ 0.74 W. The electron density ne is calculated by using Eq. (7) about ne ¼ 1.70 1017 m3. The result seems reasonable if we compare it with the electron density of other works such as
In this work, the plasma spectra of helium APPJ are identified from 200 nm to 900 nm. It is found that most of the P lines are due to molecular radicals such as OH radical 0)), N2 P the SPS (C3Gu (A2 þ (t0 ¼ 0) ! X2G(t00 ¼P þ 3 2 2 þ ! B Gg), and N2 the FNS (B u ! X þ g ). The grating secondary spectrum is overlapped with the primary spectrum between 500 nm and 900 nm. Moreover, the grating secondary spectrum of OH radical is used to diagnose Tg of plasmas, which is slightly higher than room temperature. Finally, because our APPJs is in non-LTE conditions, the electron temperature and the electron density are calculated according to the ions number balance and ions energy balance, and their values are Te ¼ 1.87 eV and ne ¼ 1.70 1017 m3, respectively. The overall results have a good agreement with formerly published results and are helpful for understanding real physical property of APPJ. ACKNOWLEDGMENTS
We wish to thank China National Funds for Distinguished Young Scientists (Grant No. 51125029), China Foundation for the Author of National Excellent Doctoral Dissertation (Grant No. 200338), and Intersection Subject Program of Xi’an Jiaotong University (Grant No. xjj20100160) for financial support. ´ . Gonz´alez et al., Plasma Sources Sci. P. Bruggeman, D. Schram, M. A Technol. 18, 025017 (2009). 2 A. Yanguas-Gil, J. Cotrino, and A. R. Gonza´lez-Elipe, J. Appl. Phys. 99, 033104 (2006). 3 L. F. Dong, W. Y. Liu, Y. J. Yang et al., Acta Phys. Sin. 60, 045202 (2011). 4 G. B. Sretenovi, I. B. Krsti, V. V. Kovaevi et al., Appl. Phys. Lett. 99, 161502 (2011). 5 V. Colombo, E. Ghedini, and P. Sanibondi, Prog. Nucl. Energy 50, 921 (2008). 6 J. L. Walsh and M. G. Kong, Appl. Phys. Lett. 93, 111501 (2008). 7 A. A. Garamoon, A. Samir, F. F. Elakshar et al., IEEE Trans. Plasma Sci. 35, 1 (2007). 8 A. Sarani, A. Y. Nikiforov, and C. Leys, Phys. Plasmas 17, 063504 (2010). 9 M. A. Lieberman and J. J. Licgtenberg, Principles of Plasma Discharge and Materials Processing (Wiley, New York, 1994), p. 333. 10 J. T. Gudmundsson, Plasma Sources Sci. Technol. 10, 76 (2001). 11 J. P. Pichamuthu, J. C. Hassler, and P. D. Coleman, J. Appl. Phys. 43, 4562 (1972). 12 O. Motret, C. Hibert, S. Pellerin, and J. M. Pouvesle, J. Phys. D: Appl. Phys. 33, 1493 (2000). 13 G. Herzberg, Molecular Spectra and Molecular Structure: I. Spectra of Diatomic Molecules, 2nd ed. (D. Van Nostrand Company, Princeton, NJ, 1964), p. 109. 14 B. Bai, H. H. Sawin, and B. A. Cruden, J. Appl. Phys. 99, 013308 (2006). 15 J. Luque and D. R. Crosley, J. Chem. Phys. 109, 439 (1998). 16 S. Pelleriny, J. M. Cormiery, F. Richardy et al., J. Phys. D: Appl. Phys. 29, 726 (1996). 17 F. Iza and J. A. Hopwood, IEEE Trans. Plasma Sci. 32, 498 (2004). 18 G. H. Dieke and H. M. Crosswhite, J. Quant. Spectrosc. Radiat. Transf. 2, 97 (1962). 1
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