Auluck, S K H

Garg, A B

Khan, H A

Rao, P Meenakshi Raja

Rout, R K

Saraswathy, P

IACS Institutional Repository

Indian J. Phys. 7 3 B  (1 ), 65-71 (19 9 9 )I JP  Ban international )ournalS p e c tr o s c o p ic  s tu d ie s  o f  a  p la sm a  fo c u s  a fte r g lo wP Saraswathy, R K Rout*, A B Garg, H A Khan, P Meenakshi Raja Rao and S K H Auluck*Spec t ioscopy  Division, Bhahha Atomic Research Centre, Trombay,M um ba i-400  085, IndiaR e i e n e d  .? A p n l  199S.  a n e p t e d  M> N n v e m b e i  I 9 9 SA b s t r a c t  Carbon spectrum produced in the afterglow of a 2 U  Plasma focus operatedwith an admixture  o f  20 rA acetylene in hydrogen at a pressure of  S mb was studied in the 4000  -  bOOOA region using a lm  C /e rn y  -  Turner Spectrograph The spectrum consisted  mainly of lines due to Cl, CU, Clll  and CIV and hydrogen Balmet series From the obseived line width o f  H and H y lines the election density o f  the plasma was evaluated and found to be of the ordet of 6 x I0 '7cc  From the intensity of the carbon ion lines the lempeiature ot the afteiglow plasma was evaluated Band spectrum of C, molecule also was observed Vibrational temperature  calculated indicated the existence o f  a cooler  /one  Details o f  the exper imental findings are presented in this paperK e y w o r d s  Plasma spectra, af terglow, electron densit> temperatureP A C S  Nos .  M  50 Fa. 82 40 Ra 52 2S Rv1. IntroductionPlasma diagnostics by emission spectroscopic method is a well developed branch of plasma spectroscopy [ 1 ]. From the emitted wavelengths, intensities and shapes of the spectral lines, it is possible to gain valuable information about a plasma and understand the different processes of plasma formation. While in eonventional spectroscopy one deals with atoms and molecules m isolated environment, the radiating atoms and molecules in a plasma arc influenced by electric, and magnetic fields and also multiple collisions. These effects are reflected on the shape of the spectral lines. Two of the basic plasma parameters, the electron density and temperature can be evaluated from the Stark broadening and intensity of the emitted lines respectively.An interesting application of emission spectroscopy for plasma diagnostics is the analysis of the afterglow plasma in a low energy plasma focus device. Neutron Physics Division of B. A. R. C. has a 2kJ plasma focus source being used as a fusion neutron source [2]. A hot deuterium plasma lasting for about 100ns is produced in this device. After the hot phase,* Neutron  Physics Division©1999IACS66 P Saraswathx et alhaving a temperature of several million degree Kelvin, the plasma expands and cools and also interacts with the electrodes, leading to eiosion of electrodes. Most of the visible light emission in the device comes from this afterglow plasma. This afterglow plasma causes deposition of eroded electrode material on glass insulator which in turn has a deleterious effect on the plasma produced in the subsequent discharge. This contamination of the plasma during the initial phase of the d;.~chargc is responsible for the failure of the neutron yield.In this paper, a preliminary study of the time integrated carbon spectra obtained using acetylene mixed with hydrogen as working gas and the results obtained are presented. Using the Stark broadened line width of the line, the density of the plasma at the iime of maximum emission of the line is determined. By the line intensity ratio method, the excitation temperatures of various species al carbon at the time of their maximum emission are determined. From this temperature and Saha's equation, the density is evaluated. This is found to agree with the density from the Stark broadening. An interesting result is the observation of the C., molecular band spectra, which indicate a vibrational temperature of 4900°K.2. Experimental setupThe source used to generate the carbon plasma is a pulsed Z-pinch plasma focus device. Figure 1 shows the schematic of the device. It has multiple cathodes arranged around a centralSpectroscopic studies o f a plasma focus afterglow 67anode enclosed in a metallic vacuum chamber maintained at a pressure of 1(H mbar. A 2 kJ capacitor bank having an internal inductance of 100 nil is used to drive the plasma locus source. For producing carbon plasma a graphite cap was fitted on to the central anode made of copper. The plasma gas used was hydrogen with -  20% acetylene. The pressure inside the chamber was 5 mbar. Cathodes were of copper. It was observed that the addition of acetylene, enhanced the intensity of carbon lines considerably and spectral lines from copper were suppressed. A one meter Czerny-Turner spectrograph was used for photographing the spectrum. A fibre optic wave guide was used to collect the emitted light and focus it on to the entrance slit of the spectrograph. The carbon spectrum was recorded m the region 4000-6800 A, using ORWO NP7 photographic film. The lower limit was imposed by the fibre optic cable and the higher limit by the photographic emulsion. The resolution obtained was of the order of 1 A. The intensity profiles were recorded using a recording micro densitometer. More details about the experimental setup, recording of the spectrum and calibration procedures are dealt with in ref. [3,3a]3. Results and discussionsA detailed analysis of the spectrum emitted from the 2 kJ plasma focus device was carried out. The emitted lines were mainly due to CII, CIII and CIV. Very few lines of neutral carbon were observed as most ol the intense lines are in the UV-VUV region. Since the plasma gas was hydrogen, H(x, and were also observed with good intensity. In addition to these, a lew bands were observed They were assigned toC, Swan System [4]. Table 1 contains wavelengths of the observed linesTabic  1. Observed lines from a carbon plasma sourceSpecies A(A) In tens i ty  T ra n s i t io n  j j(mini4771 7 5 3 3 3 s ' P ' - 4 p , P 2- 25 3 8 0  34 4 0 3 s 'P "  - I p ' P 1- 14 2 6 7  00 192 ^ r D - 4 f ; r " 3/2- s /24 2 6 7  26* 3 d : l> -4 f : F" 5 /2 —7/24 3 7 2  35 10 3dJ P " - 4 f ' P 3/2 3/2. 5/24 3 7 2  49* 3dJ Pl,-4 f*D I / 2 - 1 / 24 3 7 4  27 12 3dJ P" 4f*l) 5 1 2 - 1 / 24411  16 32 3d2l ) " - 4 f 2F 3 / 2 - 5  n4411  51* 3 d 2l ) " -4 t ‘2F 5 /2 - 7 /24 6 1 8  40 34 3d2F"- 4 f2G 5 / 2 - 1 / 24 6 1 9 .2 3 36 3d2F*'-4f2G 7 / 2 - 9 /25 1 3 2  94 59 3 s4P " - 3 p 4P 1 /2 -3 /25 1 3 3 .2 8 * 3 s 4P " - 3 p 4P 3 /2—5/25 1 4 3 .4 9 85 3 s 4P ° - 3 p 4P 3 / 2 - 1 /25 1 4 5 .1 6 123 3 s 4P" 3 p 4P 5 / 2 - 5 /25 1 5 1 .0 9 63 3 s 4P " - 3 p 4P 5 / 2 - 3 /268 P Saraswathy et alTable  1. Cant'd ---------------- ------——-------------------Species A< A) In tensi ty(mm)T ra n s i t io n J “ JC l l 5 6 4 0  55 31 3s4P ° -3 p 4S1 /2 -3 /25648  07 48 3s4P ° -3 p 4S 3 /2 —3/25662  47 62 3s4P " - 3 p 4S 5 / 2 - 3 /2588 9  27 83 3 d 2D - 4 p 2P" 3 /2 —3/2588 9  1 1 * 3 d 2D - 4 p 2P" 5 / 2 - 3 /25891 59 6 0 3 d 2D - 4 p 2P" 3 /2 - 1 /2CIII 4121 84 18 4 p lP " - 5 d ‘D 1 - 24 1 8 6  90 71 4 f 1F ° - 5 g lG 3 - 44 3 2 5  56 14 3 s , P0- 3 p ' D 1 - 24 6 4 7  42 65 3 s ' S - V P " 1 - 24 6 5 0  25 34 . V S  3 p 'P " 1-14651 47 30 V S  3 p 'P " 1 - 04 6 5 9  06 2 0 V P ' - l p ' p 1-1466 3  64 32 , V P " - 3 p ’P 1 - 0466 5  86 39 3 s , P " - 3 p 1P 2 - 25695  92 51 3 p lP " - 3 d ' l ) 1 - 2civ 5 8 0 1 .3 3 43 3,s2S - 3 p : P" 1 /2 -3 /25811 98 57 3 s2S 3p2P" 1 / 2 - I / 24 6 5 8  30 3 1 5f: F " - 6 g : G -6 5 6 2  79 2 p 2P " - 3 d 2D»» 4861 332 p 2P‘' - 4 d 2n" y 4 3 4 0  47 2 p 2P" s d 2n* Blended lines3.1. Temperature e\alnation :Atomic emission lines arc used to evaluate the temperature of the plasma, using the intensity ratio oflinc pairs. The expression [9];T =5040 (Vu -  Vh)>°g ^  J" ~ log -  log Ia(8A)h / ,( I )where V is the excitation potential (cV),g statistical weight, A is transition probability, A is the wavelength in A, / is the intensity the subscript a and b correspond to the two lines used. The intensity of the line is taken as proportional to the peak height fin mm) of the spectral profile. The temperature was evaluated using different line pairs, for Cli, CIII and CIV species, and the temperatures so obtained are listed in Table 2. It can be seen that for higher order ionised states the temperature is high which indicates an expanding plasma.Spectroscopic studies o f a plasma focus afterglow 69T ab le  2. Calcula tion  o f  excitat ion  temperatureSpecies line pair (A) i,/'. Tem peratu re  (°K)C II 5151 08 / 5648 08 6 3 /48 6 1 9 35151 08 /  5662 47 6 3 /62 9 8 2 45151 .08  /  5640  55 63/31 3 6 8 7cm 4647  40  / 5695 .92 65/51 5 9 8 0 74647  40  /  4121 84 6 5 /1 8 6 6 8 1 6CIV 5801 5 / 4658  30 43 /3  1 5 2 5 9 3 03.2. Electron density evaluation :It is a well known fact that m plasma the spectral lines are broadened by Stark effect (pressure effect), Doppler effect, and instrumental effect. Details of the line broadening mechanism are dealt with in great details by Griem [5, 6] and Sobelman [7]. The broadening of the hydrogen lines is mainly from linear Stark effect and is made use of in determining the electron density using the expression [8] ;A l,,, =2.5xl()~q a U2 (2)where AXin is FWHM (full width at half maximum) in A, nr is the electron density in c irr \ a W2 is the shape factor. a ]p is weakly dependent on temperature, but remains constant over a wide temperature range. The Hp and Hy profiles were recorded, and the half widths were evaluated [3J and the n was calculated using the above expression. a {p was taken to be 0.086 for Hp and 0.090 for Hy. The criteria for selecting this value are discussed by Griem |6]. Figure 2 shows the profile of Hp line. In Table 3 the average values of AAIP from differentMA)-F ig u re  2 Profi le o f  Hh linespectra and ne calculated using Hp and Hy lines arc listed. Ha line was too intense in our spectra and H§ was too weak, hence they were not used in the present experiments for electron density evaluation. Electron density of the plasma focus device evaluated is 5.9 x 10,6/cc.70 P Saraswathy et alTable  3. Evaluation of election densityLine M i/2 (A) ii (x10 1 r> / n n ht» v34 0 0 8 6 5 21(4861 33 A)11, 36f) 099 5 56(4 3 4 0  47 A )Electron density and temperature of a plasma are interdependent. It is possible to evaluate, temperature of the excitation source if the electron density is known and vice versa. We have evaluated the value of nv using the Saha’s expression [9| ;rlog n = -log —  + log - 1—  '  / R.. As '  A* v +V<W <1Pi v + v + -  V )j  '/ n if+ 3 /2  log 7 '+ 15.684, (3)where ne is the electron density per cc , / is the intensity of neutral line # is the statistical weight, 4 is the transition probability, is the wave number in cm 1 of the spectral line, Vfi is the ionisation potential. V the excitation potential in eV and T the temperature in degree Kelvin. The superscript (+) corresponds to values belonging to ionic line. In the present case, taking the temperature evaluated from the line intensity ratio methods from different line pairs, the electron density was evaluated and the value of the n( = ~ 6 x 1010 cm \  which agrees well with the electron density evaluated from line width.3.3 Vibrational temperature evaluation .In the spectrum of Carbon plasma, we have observed some bands identified as the C, Swan System. We have evaluated the vibrational temperature using the expression [ 1 ]l m l v \  v") = K V V  v"  |FCF ( v \  u") /?; ] e ~G(l/)hllkI (4)where / uV is the band intensity, K is a constant, Vv.v» is the band head wave number in cm *, FCF is the Frank-Condon factor, is the electronic transition moment, G(l/) is vibrational energy and Fis the temperature. FCFs were calculated for this band system using the constants reported in literature [ 1 ()]. Assuming electronic transition moment/?^ as constant and simplifying above equation reduces toIn / u.()„ / v V u "  FCF = -  C(u") / 0.6925 T + constant (5)A plot of the LHS against G( v r) is a straight line whose slope gives the temperature. The Table 4 gives the details of calculation. The temperature obtained was 4900°K.4. ConclusionThe observations of Cl- CIV lines and C2 band spectrum at different excitation temperatures indicates that as the afterglow plasma cools during its expansion, various charge states as well as molecular species are formed during recombinations.T ab le  4. Evaluation o f  vibrational temperature from band head intensit iesSpectroscopic studies o f a plasma focus afterglow 71l / l / ' L v - 1 FCF G(r')In FCFV , „ o u0 - 0 516 5  2 19378  44 27 0 735 817 76 - 3 5  881 -  0 4737.1 2 1 1 3 2  13 45 0 2 3 9 6 2435 77 34 602 - 0 4 7 1 5  1 2 1 2 3 0  38 93 0 3714 4 0 1 0  44 14 1?3 - 2 4 6 9 7  6 21311 48 89 0 4 1 8 9 5601 77 - 3a 504 - 6 5958 7 16777 54 38 0 171 7 1 4 9  76 >33 520 -  1 5 6 3 5  5 17760  43 18 0 213 8 17 76 - 34 701 - 2 5 5 8 5  5 17919  45 2 9 0 2 8 7 2 243 5  77 - 34 552 3 5540 7 18064  38 5 3 0 2677 4 0 3 0  44 33 91R e f e r e n c e sI l |  (i  V Marr P la sm a  S p e e t i o u  o p \  (A msterdam Elsevier) (1968)(2 j  Anuiag Shyatn Ph D  Thesis (Mumbai Umv Mumbai, India) (1981)(3) P M eenakshi Raja Rao, P Saiaswalhy, G Krishnamurty, R K Rout, S K 11 Auluik. A S h \a m .  1 VKulkarni and Dipak Oja Pi annum -J  Phys  32 627 (1989)H a |  P Meenakshi Raja Rao, S Padmanabhan. G Krishnamurty and B N Rajasekhar  J Quant S p e i t i o u  R a d ia l  h a n s f a  47 113 (1992)[ 4 1 R W B Pearse and A G Gaydon Indentifnat ion  of M o l n u l a i  S p e i t t a  (London Chapman and Hall) ( 1 9 5 0 )[51 H R Griem S p e i t t a !  Line R to a d e n m g  m P l a \m a \  (New York Academic.) (1978) f6 J H R G ncm  Plasm a  S p e i t n m o p s  (New Yoik Mcgraw Hill) (1964)17 1 11 Sobol man Atom u S p e i t t a  a n d  R a d m t n e  h a n s i tu m s  (Berlin Spnngei Yerlag) (1979)j8]  A Seheeline. G J Kamala and M J Zoellner S p a  inn him A i ta  B39 677 (1984)[91 P W J M Boumans I h c o i s  of Spin tun h e m n a l  / u  nation  (London Hilgcr and Walts) (1966)|1 0 j  R P Huber and G H c i /bc rg  M o l e m l a r  S p e i t t a  a n d  M olet  ulai S m a  tu n  C on s tan ts  of D ia to n m  Mulct ule\  Yol. IV (New Yoik Van Nostrand Reinhold) (1978)

Spectroscopic Studies of a Plasma Focus Afterglow

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Spectroscopic Studies of a Plasma Focus Afterglow

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