Computational Study of Emitted Spectra from the Neon Plasma Focus

Transcription

1 J Fusion Energ (2013) 32: DOI /s ORIGINAL RESEARCH Computational Study of Emitted Spectra from the Neon Plasma Focus M. Akel S. Alsheikh Salo C. S. Wong Published online: 16 March 2013 Ó Springer Science+Business Media New York 2013 Abstract The expected emission spectra (full, Bremsstrahlung, recombination, and line) of neon focussed plasma have been studied for different conditions. Expected neon plasma spectra at certain electron temperature range have been plotted. The suitable electron temperatures ranges for neon plasma soft X-ray emission and extreme ultraviolet emission have been investigated. The X-ray ratio curves for various electron temperatures with probable electron and ion densities of the neon plasma produced have been computed with the assumption of non-local thermodynamic equilibrium model for the distribution of the ionic species. These ratio curves could be used for electron temperatures deduction of neon plasma focus. Keywords Neon plasma Soft X-ray EUV emission X-rays ratio method Introduction The plasma focus device has been studied intensively as a plasma source capable of producing high density high temperature plasma that emits intense radiation ranging from hard and soft X-rays, UV and extreme ultraviolet (EUV) [1 4]. The radiation output and the emission spectra that may be obtained depend on the operating parameters of the plasma focus discharge, which include the discharge M. Akel (&) S. Alsheikh Salo Department of Physics, Atomic Energy Commission, P. O. Box 6091, Damascus, Syria pscientific@aec.org.sy C. S. Wong Physics Department, Plasma Technology Research Center, University of Malaya, Kuala Lumpur, Malaysia energy, operating pressure and electrode geometry [5, 6]. The dense plasma focus pinch discharge has recently been considered as a possible light source for extreme ultraviolet lithography (EUVL) [6]. This is of interest to the semiconductor manufacturing industry due to the expectation that the Next Generation Lithography (NGL) will be using the wavelength of 135 Å [7, 8]. Various types of EUV radiation sources, including the laser produced plasma and pulsed discharge sources such as the capillary discharge [9, 10], vacuum spark [11, 12] and plasma focus [13, 14] are being considered. These radiation sources, especially the pulsed discharge sources are favorable as X-ray and EUV radiation sources because of their lower cost and simplicity in operation when compared to other radiation sources. The X-ray emission from dense plasma focus is characterized by high intensity and a wide spectral range, the emission times ranging from a few to a few tens of nanoseconds for a small focus. The predominant spectral range that is actually radiated can be controlled by using a specific gas at a specific temperature. Good soft X-ray yield can be achieved by neon as a filling gas with characteristic spectral energies around 1 kev. Based on corona model incorporated in Lee model code, it is shown that for operation in neon, a focus pinch compression temperature of ev is suitable for generating H-like and He-like ions in neon plasma (therefore neon soft X-ray emissions) [15 22]. Liu [15] has shown that for the soft X-rays from neon operated 3.3 kj UNU-ICTP plasma focus device it was found that 64 % of soft X-ray emission can be attributed to line radiations at 922 ev ( Å) (He-like alpha line) and 1,022 ev ( Å) (H-like alpha line) and the remaining 36 % by the rest, mainly recombination radiation, for optimized operations [23]. Generally it is hardly possible to get the detailed accurate knowledge of the states of the plasma. Approximate estimations, by calculations based on simplified plasma models,

2 504 J Fusion Energ (2013) 32: may be carried out. The most tractable plasma models are the local thermodynamic equilibrium (LTE), the non-local thermodynamic equilibrium (NLTE) or the corona equilibrium (CE), and the collisional-radiative equilibrium (CRE) [24 27]. One of the best known of these is the suite of three codes (POPULATE, SPECTRA, and RATIO) called RATION [28 31]. The POPULATE code uses the principal of detailed balance to calculate the rate of inverse processes. The SPECTRA code computes the expected emission spectrum of the plasma while the RATIO code allows the user to view the results of POPULATE in graphical forms. The graphical outputs include the population, ratio of the intensities of selected transitions and the optical depths of transitions as a function of temperature or density. Since the code is to provide details of the populations in Lithium-like through fully stripped ions, an X-ray filter analysis code, XRAYFIL, [32] has been developed to allow more accurate non-dispersive X-ray plasma diagnosis with absorption filters by using a series of trial spectra of the emitting species. The code calculates a set of emission spectra for a given plasma using RATION, and convolves it with the transmission characteristics of the filter set used, as well as the response function of the detector chosen. Comparison of the ratio of the signal through the different filters from these calculated values to that recorded in the experiment allows us to obtain a measurement of the plasma temperature and density. The code incorporates a number of options for various emission scenario and detector. The main purpose of XRAYFIL code to calculate the emission spectra of neon plasma and then to work out the number of photons passing through the chosen composite filter (BPX65 PIN diode detector, in our case). So, the emission spectra (full, Bremsstrahlung, recombination, and line) are computed using XRAYFIL code (in unit of number of photons/cm 3 / Angstrom/Sec./Sterad.) [32]. In this work, the XRAYFIL code coupled with the POPULATE code is used to study the EUV and X-ray emissions of neon focussed plasma. The spectra of radiation emissions (full, Bremsstrahlung, recombination, and line) from the neon plasma focus have been simulated for different plasma conditions. The calibrated X-ray ratio curves for electron temperature measurements of neon plasma focus have been deduced. The neon plasma focus has been also presented as X-ray and EUV source. expected to consist of ionic species that can be considered as emitters of line radiations at wavelength around 135 Å. At a temperature of around ev, the Ne?4 is prominent, whereas at a temperature of around ev, the neon plasma is expected to consist of predominantly the Ne?8 ionic species. These ionic species are known to be able to emit intense line radiations at or near 135 Å as listed in Table 1 [33]. Figure 1 shows the population distribution of these two groups of ionic species as predicted by the coronal equilibrium model. XRAYFIL code has been also used to synthesize the EUV emission spectrum from a plasma which is characterized by the plasma parameters available in a POPU- LATE file. The expected EUV neon plasma spectrum generated in plasma focus devices at different operational conditions has been calculated. Based on populate output file, the neon plasma spectrum has been deduced at different temperatures ev for NLTE. Figure 2 presents EUV (0 150 Å) neon plasma focus spectra at T e = 15, 25, 55, 95 ev for NLTE model, N e = cm -3. Neon Plasma Focus as Soft X-Ray Source Based on corona model, a focus pinch compression temperature of ev is suitable for generating H-like ( Å) and He-like ( Å) ions in neon plasma (see Fig. 3). In our calculation, the non-lte model has been used to obtain an estimate of the ionic distribution in the neon plasma. This is believed to be able to give sufficiently accurate results. XRAYFIL code has been used to synthesize the emission spectrum from a plasma which is characterized by the plasma parameters available in a POPULATE file, where the computed spectral lines emitted from a plasma focus are broadened by Doppler broadening [28, 34]. Table 1 Expected lines near 135 Å from neon ions Ne?4 and Ne?8 Ion Wavelength (Å) Transition Upper level Lower level Ne? s 2p ((2p 2 P)4s) 1 P 1 (2p 2 ) 3 P 0 Ne? s 2p (2p4s) 1 P 1 (2p 2 ) 1 S 0 Ne? p 2s ((2s2p 24 P)3p) 3 S 1 (2p 2 ) 3 P 0 Ne? p 2s ((2s2p 24 P)3p) 3 S 1 (2p 2 ) 3 P 1 Numerical Experiments: Results and Discussion Neon Plasma Focus as Extreme Ultraviolet (EUV) Source According to the coronal equilibrium model, there are two possible ranges of temperature at which neon plasma is Ne? p 2s ((2s2p 24 P)3p) 3 S 1 (2p 2 ) 3 P 2 Ne? f 3d (1s6f) 3 F 4 (1s3d) 3 D 3 Ne? f 3d (1s6f) 1 F 3 (1s3d) 1 D 2 Ne? p 3d (1s6p) 3 P 2 (1s3d) 3 D 2 Ne? p 3d (1s6p) 3 P 2 (1s3d) 3 D 1 Ne? p 3d (1s6p) 3 P 2 (1s3d) 3 D 3 Ne? d 3p (1s6d) 1 D 2 (1s3p) 1 P 1

3 J Fusion Energ (2013) 32: Ion population Te = ev Ne+4 (C-like) ,000 Temperature (ev) Te = ev Ne+8 (He-like) Fig. 1 The temperature ranges at which neon ions Ne?4 and Ne?8 (EUV * 135 Å) are prominent predicted by NLTE Model EUV neon plasma spectrum at Te = 15 ev Te = 25 ev Te = 55 ev Te = 95 ev Fig. 2 Computed EUV neon plasma focus spectra at different temperatures for NLTE model The neon plasma spectrum has been deduced at different temperatures (T e in the range of ev), electron density (N e in the range of cm -3 ) and ion density (N i in the range of cm -3 ). Figure 4 presents the expected radiative emissions (full, Bremsstrahlung, recombination, and line) of neon plasma focus at T e = 400 ev for non-lte model, N e = cm -3,N i = cm -3. The electron temperature and density effects on the plasma emissions have been studied. The electron plasma temperature influence on the radiative emission has been found to be more dominant than electron density. Figure 5 shows the variations of the expected full emission spectra of neon plasma focus at various temperatures, while the variations versus densities are illustrated in the Fig. 6. As expected from theoretical consideration of plasma emission [35, 36], the continuum of the X-ray emission spectrum is observed to shift towards shorter wavelength (higher photon energy) with increasing electron temperature, with the wavelength at the peak of the continuum given approximately by 12.4/T e (according to equations used for computing Bremsstrahlung, recombination, and line radiations in the XRAYFIL code), where T e is in kev (in our case, the peak of the continuum is at about 31 Å). The relative population of the ionic species present in also affected by the temperature. The prominent species present in neon plasma at electron temperature of 200 ev are Ne?8 and Ne?9, while at 400 ev, Ne?8, Ne?9 are present with small fraction of Ne?10. Finally at electron temperature of 500 ev, Ne?10 becomes prominent, together with Ne?8 and Ne?9.This will affect the recombination and line radiations. At electron temperature much higher than 2 kev, when the plasma becomes fully ionized, Bremsstrahlung is expected to dominate. As have been shown in Fig. 6, the shape of spectra for different electron densities ( cm -3 ) Ion population Ne+8 (He-like) Te = ev Ne+9 (H-like) 1E19 Full neon spectra at Te = 400 ev, Ne = E19, NLTE Brem. emission Recomb. emission A o A o A o A o ,000 Temperature (ev) Fig. 3 The temperature range at which neon ions Ne?8 and Ne?9 are prominent predicted by NLTE Model Fig. 4 Computed full, Brem. and Recomb. spectra for neon plasma with T e = 400 ev

4 506 J Fusion Energ (2013) 32: E19 Full neon spectra, Ne = E19, NLTE Te = 200eV Te = 300eV Te = 500eV He α ( A o ) Te = 200 ev Te = 300 ev Te = 400 ev Te = 500 ev Te = 600 ev Te = 700 ev Wavelength, A o Fig. 5 Computed spectra of neon plasma at three different temperatures for N e = cm -3 1E30 1E29 1E19 1E18 1E17 Full neon spectra, Te = 500 ev, NLTE Ne = E18 cm -3 Ne = E19 cm -3 Ne = E20 cm Fig. 6 Computed spectra of neon plasma at three different densities for T e = 500 ev are similar, but different in amplitude. This is because of the N 2 e dependence. Figures 7 and 8 show variations of the soft X-ray intensity (He-like and H-like ions) versus electron temperature. From Fig. 7 it can be seen that the most suitable temperature for He-like is about 300 ev, while for H-like is found to be about 500 ev (Fig. 8). So, based on our obtained results, the suitable T e ranges for soft X-ray emitted from neon plasma focus may be determined. From the above mentioned results, it can be said that the POP- ULATE and XRAYFIL codes are a good tools for plasma diagnostic, especially for X-ray plasma focus study and electron temperature measurements. Calibration Curves for Electron Temperature Measurements of Neon Plasma Focus The electron temperature of the plasma can be deduced from the measurement of the X-ray continuum emitted by the plasma. It is well known that the temperature of the focused plasma is in the range of kev. Most of the radiation from the neon plasma is expected to be due to Bremsstrahlung produced by electron retardation in the Coulomb field of the ions. However, if impurities are present, recombination and line radiations obscure the free free radiation, and the interpretation of the experimental results becomes very complicated [37]. The electron temperature can be determined from the analysis of radiation in the X-ray region [38, 39]. The five channels BPX65 PIN diodes with different filters have been widely employed to record the X-ray pulses generated by a plasma focus devices [40 44]. So, the attenuated radiative emissions of plasma through different channels of BPX65 PIN diodes with varying absorption filters have been calculated using the Ratio- BPX65 code [45]. Briefly, the code has been written in FORTRAN 77 for studying the effect of the response of BPX65 photodiode, with Mylar and Aluminum foils filtering on the emitted spectra from plasma focus. The input data for this code are: XRAYFIL output file, Mylar, BPX65 and aluminum mass attenuation coefficient data versus wavelength (k). Where the attenuated plasma spectrum through Mylar foils and BPX65 photodiode has been determined by using the following formula: Z I 0 ¼ Pðk; T e ÞSðkÞexp l mylarðkþx mylar dk ð1þ Fig. 7 Variation of He-like ion emission line intensity of neon plasma with T e for NLTE model Then, emitted spectrum through additional different aluminum foil thicknesses will be:

5 J Fusion Energ (2013) 32: Z I ¼ Pðk; T e ÞSðkÞexp ½ l mylarðkþx mylar þl Al ðkþx Al Š dk ð2þ Finally, the ratio of the generated spectrum by these combinations due to the same X-ray pulse can then be calculated as R = I/I 0 : R ¼ I I 0 ¼ R Pðk; T e ÞSðkÞexp ½ l mylarðkþx mylar þl Al ðkþx Al Š dk R Pðk; T e ÞSðkÞexp l mylarðkþx mylar dk Hα ( A o ) Te = 200 ev Te = 500 ev Te = 700 ev Fig. 8 Variation of H-like ion emission line intensity of neon plasma with T e for NLTE model ð3þ Where S (k) is the BPX65 sensitivity, l is mass absorption coefficient of material, x is the absorption foil thickness As an example, the radiative emission from the neon plasma focus actually detected by the BPX65 PIN diode with 12 lm aluminized Mylar and Aluminum foils with varying thicknesses (10 90 lm) have been calculated. Figure 9 shows the emitted neon plasma spectrum, which shows the attenuated X-ray intensities recorded after passing through different filters at T e = 500 ev. The signals recorded by the BPX65 detector provide information on the time evolution of the X-rays produced by the plasma focus and they are used to determine the electron temperature of the plasma focus by the X-ray foil absorption technique. For this purpose, the sets of neon plasma spectrum (continuum) for different temperatures (T e = 200 5,000 ev) have been calculated to get the X-ray signal ratio R = I/I 0. As an example, for our calculations, the set of X-ray signal ratio graphs for T e = 200, 500, 1,000, 1,500, 2,000, 3,000, 4,000, and 5,000 ev at the above mentioned conditions are shown in Fig. 10. These ratio curves can be used as the calibration curves for the measurement of electron temperatures for neon plasma. 1E18 1E16 1E14 1E Conclusions Neon specta Te = 500 ev NLTE, Ne = E19, Full spectra Spectra + BPX μm Mylar Spectra + BPX μm Mylar + 10 μm Al Spectra + BPX μm Mylar + 30 μm Al Fig. 9 Computed spectra of neon plasma through different sets of filters (BPX65 PIN diode with 12 lm aluminized Mylar (D.1), BPX65 PIN diode with 12 lm aluminized Mylar coupled to Aluminum foil thickness of 10 lm (D.2), and BPX65 PIN diode with 12 lm aluminized Mylar coupled to Aluminum foil thickness of 30 lm (D.3) at electron temperature T e = 500 ev Ratio E-3 X-ray ratio curves of neon continuum spectra at different electron temperatures ( ev) T e = 200 ev 1000 ev 500 ev Al foil, μm 5000 ev 4000 ev 3000 ev 2000 ev 1500 ev Fig. 10 Calculated X-ray ratio (R = I/I 0 ) curves of BPX65 PIN diode coupled to Mylar (12 lm) and sets of BPX65 PIN diode coupled to Mylar (12 lm) with different aluminum foil thicknesses (10, 20, 30, 40, and 90 lm) for X-rays of neon plasma (NLTE, N e = cm -3 ) at various temperatures The radiation emission spectra (full, Bremsstrahlung, recombination, and line) of neon plasma focus at various plasma parameters have been computed using the XRAYFIL code by assuming a non-lte model for the plasma. Neon plasma focus spectra have been calculated for plasma focus operation as soft X-ray and EUV sources. The suitable electron temperatures ranges for neon plasma soft X-ray emission were found to be ev, while for EUV

6 508 J Fusion Energ (2013) 32: emission were estimated to be ev. The calibration X-ray ratio curves for electron temperature deduction of neon plasma have been computed. These ratio curves could be used as the calibration curves for the measurement of electron temperatures for neon plasma focus. Acknowledgments The authors would like to thank general director of AECS for support, guidance and encouragement. References 1. S.H. Lee, S.L. Yap, C.S. Wong, in AIP conference proceedings volume 1250, progress of physics research in Malaysia: PERFIK (2009) 2. S.P. Moo, C.S. Wong, Laser Part. Beams 13(1), 129 (1995) 3. C.M. Ng, S.P. Moo, C.S. Wong, IEEE Trans. Plasma Sci. 26(4), 1146 (1998) 4. V. Raspa, C. Moreno, L. Sigaut, A. Clausse, J. Appl. Phys. 102, 303 (2007) 5. I.V. Fomenkov, N.R. Böwering, C.L. Retting, S.T. Melnychuk, I.R. Oliver, J.R. Hoffman, O.V. Khodykin, R.M. Ness, W.N. Partlo, J. Phys. D Appl. Phys. 37, 3266 (2004) 6. I.V. Fomenkov, R.M. Ness, I.R. Oliver, S.T. Melnychuk, O.V. Khodykin, N.R. Böwering, C.L. Retting, J.R. Hoffman, Proc. of SPIE 5374, 168 (2004) 7. R. Mongkolnavin, P. Tangitsomboon, C.S. Wong, J. Sci. Technol. Trop. 6, 43 (2010) 8. V. Banine, R. Moors, J. Phys. D Appl. Phys. 37, 3207 (2004) 9. S.R. Mohanty et al., Microelectron. Eng. 65, 47 (2003) 10. D. Hong et al., Rev. Sci. Instrum. 71, 15 (2000) 11. G. Xiaoming et al., Proc. SPIE 4343, 491 (2001) 12. S. Saboohi, S.L. Yap, L.S. Chan, C.S. Wong, IEEE Trans. Plasma Sci. 40(12), part 2, 3390 (2012) 13. I.V. Fomenkov et al., Proc. SPIE 5037, 807 (2003) 14. R.S. Rawat et al., Plasma Sour. Sci. Technol. 13, 569 (2004) 15. M.H. Liu, Soft X-ray from compact plasma focus. PhD Thesis, school of science. (Nanyang Technological University, 1996) 16. S. Bing, Plasma dynamics and X-ray emission of the plasma focus. PhD Thesis NIE ICTP open access archive: (2000) 17. S. Lee, S.H. Saw et al., Plasma Phys. Control. Fusion 51, (2009) 18. S.H. Saw et al., IEEE Trans. Plasma Sci. 37(7), 1276 (2009) 19. S.H. Saw, S. Lee, Energ. Power Eng. 2, 65 (2010) 20. M. Akel, S. Al-Hawat, S. Lee, J. Fusion Energ. 30, 39 (2011) 21. Sh Al-Hawat, M. Akel, S. Lee, J. Fusion Energ. 30(6), 494 (2011) 22. M. Akel, S. Lee, S.H. Saw, IEEE Trans. Plasma Sci. 40, 3290 (2012) 23. M.H. Liu et al., IEEE Trans. Plasma Sci. 26, 135 (1998) 24. H.K. Chunga, W.L. Morgan, R.W. Lee, J. Quant. Spectrosc. Radiat. Transf. 81, 107 (2003) 25. G.J. Phillips, J.S. Wark, F.M. Kerr, S.J. Rose, R.W. Lee, High Energ. Density Phys. 4, 18 (2008) 26. R.W. Lee, Manual the how to for fly, (1995) 27. H.K. Chung, R.W. Lee, M.H. Chen, Y. Ralchenko, Manual the how to for NIST, (2008) 28. R.W. Lee, User manual for RATION (Lawrence Liver more National Laboratory, California, 1990) 29. C.J. Keane, R.W. Lee, J.P. Grandy, DSP: a detailed spectroscopy postprocessor for H-, He-, and Li-like Ions UCRL-JC , DE Lawrence Livermore National Laboratory Liveimore, CA. Proceedings of the international workshop on radiative properties of hot dense matter Sarasota, Florida, February 22, (1991) 30. R.W. Lee, B.L. Whitten, R.E. Strout, J. Quant. Spectrosc. Radiat. Transf. 32, 91 (1984) 31. S.H. Kim, D.E. Kim, T.N. Lee, IEEE Trans. Plasma Sci. 26(4), 1108 (1998) 32. C. Dumitrescu-Zoita, Ph.D. Thesis, Université de Paris Sud. (1996) (2012) 34. M. Akel, S. Alsheikh Salo, C.S. Wong, J. Fusion Energ. (2012). doi: /s R.P. McWhirter, in Plasma diagnostics techniques, ed. by R.H. Huddlestone, S.L. Leonard (Academic Press, New York, 1965) 36. T.F. Stratton, in Plasma diagnostics techniques, ed. by R.H. Huddlestone, S.L. Leonard (Academic Press, New York, 1965) 37. C.S. Wong, Jurnal Fizik Malaysia 23, 4 (2002) 38. F.C. Jahoda et al., Phys. Rev. 119, 843 (1960) 39. R.C. Elton, in Determination of electron temperatures between 50 ev and 100 kev from X-ray continuum radiation in plasmas, NRL Report, 6738 (1968) 40. C.S. Wong et al., Malays. J. Sci. 17B, 109 (1996) 41. R. Mongkolnavin et al., Jurnal Fizik Malaysia 25((3&4)), 87 (2004) 42. C.M. Ng et al., IEEE Trans. Plasma Sci. 26, 4 (1998) 43. S.P. Moo, C.S. Wong, Jurnal Fizik Malaysia 15, 37 (1994) 44. Sh Al-Hawat, M. Akel, C.S. Wong, J. Fusion Energ. 30(6), 503 (2011) 45. M. Akel, S. Alsheikh Salo, C.S. Wong, J. Fusion Energ. (2012). doi: /s