Rai, S B

Singh, Shiv Kumar

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Indian J. Phys. 75B (4), 363-367 (2001)U P  man inlemational JournalStudy of plasma parameters of a dischargeShiv K um ar Singh and S B R ai'Department of Physics. Banaras Hindu University, Varanasi-221 005, Uttar Pradesh, India E-mail ’ sbrai@banaras erncl inReceived 18 September 1998, accepted 23 Mav 2001Abstract • Plasma parameters viz. electron temperature, electron-, ion-saturation current density, electron density etc have been measured (or the discharges of Ni, Op and Ar under various excitation conditions of discharge using a triple probe method It is found that electron temperature for monoatomic gas argon is much larger compared to diatomic gases oxygen and nitrogen in the same expcrimcnUil conditions A reverse thing is observed for electron density The electron temperature increases whereas electron density decreases with the increase of discharge voltage (discharge current).Keywords Discharge plasma, plasma parameters, triple probe methodlV\rSNo. 52 90.fz1. In tro d u c tio nA  discharge, plasm a consists o f  electrons, positive and negative ions and neutral species at low  pressure, the ratio  of which varies from  case to case. It strongly depends upon the discharge conditions such as applied voltage, pressure Lind nature o f  the gas in the discharge, shape, size and reparation o f  the electrodes etc. Several processes such as dissociation, association, ion ization , recom bination etc. are possible s im ultaneously in the discharge plasma. This makes a discharge a v ery  com plex system to study. I f  the discharge paiameters are kept constant, the plasm a can be considered stationary. Thus plasm a param eters can be measured in this condition. These param eters are very im portant to understand the physical processes in a discharge.In order to understand the interaction o f  laser beam w ith  discharge plasm a, w e  have used various kinds o f  probes (single probe, double probe, trip le probe) reported in literature N -6 J to m easure plasm a param eters in presence and absenceof laser.A p re lim inary  study o f  plasm a in various conditions o f  discharge o f  N 2, O 2 tm d A r  using trip le  probe in absence o f  htser is reported here. A  sym m etrica l trip le probe method is 't more d irect m ethod and do not require a voltage orCorresponding Authorfrequency sweep to get V -1  characteristic curve. It is basically a floating probe so that a little  disturbance o f  plasma results. It is useful not only fo r stationary plasma but also in the case o f  rapidly changing tim e dependent ones or in the cases where the probe device travels through plasma  such as sounding rockets or space ships. The other advantage w ith trip le  probe is that the plasma parameter can be determined instantaneously w ithout any tim e consuming tedious calculations. The results o f  these studies are reported here.2. E x p e rim e n ta l m ethodThe experim ental set up used in the present study is shown in Figure 1. It consists o f  a glass discharge tube o f  25 cm  in length and 2 cm in diameter. It is fitted w ith  tw o parallel cylindrical electrodes (1 cm in length and 0.5 cm in diam eter) 3 cm apart. One end o f  the discharge tube is connected w ith  a rotary pum p to create low pressure in the discharge tube. The other end o f  the tube is connected to the cylinder to introduce the gas to be studied. A  pirani gauge is also attached to this to measure the pressure in the tube. The triple  probes are three tungsten w ire  0.5  mm in diam eter and 2 cm  in length located centrally in the tube at a separation o f  2  m m .O 2001 I ACS3 6 4 Shiv Kumar Singh and S B RaiThe lube was first degassed (up to 10 ^^  m m ) and then filled  w ith  the desired gas (A r , O 2 or N 2) to an appropriate pressure (0 .4  m m ). A  suitable D .C . voltage (6 00  -650 volts)D C Power supply8 Discharge No voltagePressure Voltage Vji -  7 voltSeveral sets o f  measurements were taken by selecting different values o f  V jj and correspondingly vary ing  l l ie s e  measurements correspond to a particular value o f discharge voltage and pressure. Measurem ents were also made by vary ing the discharge voltage keeping discharge pressure the same and the current h  and ly fo r different values o f  Vdi and were noted dow n. These values for the argon are given in Tab le  1. S im ila r measurements were made for oxygen and nitrogen also and the corresponding values are given in Tables 2 and 3 respectively.Table 2. Electron temperature, electron density, ions saturation currcnl density and electrons saturation curircni density of02 at dilTerent discharge voltage.S Discharge No voltagePressure Voltage Vj2 -  7 volt in voltFigure I. Experimental set-up used for the study of discharge plasma using triple probe methodwas applied to the electrodes to get a suitable discharge. O nce a stable plasma condition was achieved, the trip le  probes w ere biased w ith  tw o low  voltage stable D C  pow er supplies Vdi and w ith  central probe com m on as shown in Figure 1. The voltage o f one pow er supply Vd2 was kept constant fo r one set o f  measurements w h ile  that o f  the other Vd-s was varied. The  currents in the tw o circuits viz. I2 and h  w ere noted w ith  m icro am m eter for d ifferen t values o f  Vds.Table 1. Electron temperature, electron density, ions saturation current density and electrons saturation currrcnl density of Ar at different discharge voltage7 5 10 15 20 25600 volts 0 4 mm Temp (x H H )“K 2 0 0.60 0.31 0 26 0,17current (3 mA) 10 " ) c m ‘^ 47 2 182 114 3 7 2 10./,(> 10^)amp'^ 95 5 20 2 9 14 2 72 1 2110'‘)a m p ^ 169 9 35 9 162 4 80 2 15620 volts 0 4 mm Temp (x 10^)°K 2 5 0 65 0 35 0 28 0 19current (5 2 mA) T]^ {> 10“’')cm~^ .37 5 16 1 9 20 3 4 1 90,/,(a 10^) amp ^ 84 9 186 7 83 2 57 1 18I0^)am p^ 151 1 33 0 13 9 4 5 2 11650 volts 0 4 mm Temp (x I0^)''K 3 0 0 75 0 40 0 30 0 20current (8 mA) 77r(x 10 " )  cm'^ 30.2 130 8 3 2 3 1 30J,(x I0^)amp"^ 74 9 16 1 7 56 1 84 0.88yf(x 10*) amp ^ 133.2 28 7 134 3 2 1 50250 volts 0 1 mm 1^ 1R. F power I0“‘’)cm 1 0in volt-7 c in 1 < -in Table 3. Electron temperature, electron density, ions saturation current! J lU density and electrons saturation cunrent density ofNj at different discharge1 600 volt 0,4 mm Temp (> I0^)“K 8 1 5,1 26 2 5 23 voltage.current (3 mA) 7;^ .(y 10 ")cm 52.0 6 5 4 1 30 I 5 S Discharge Pressure Voltagey(x l()2)amp'J 21.2 2 12 0 95 0.64 0 32 No voltage = 7 volt10*) amp-' 37.8 3.7 1 7 0 78 0.48 in volts75 10 15 20 252 620 volt 0 4 mm Temp (> lO YK 92 62 32 27 24 1 600 volt 0 4 mm Temp (x 1(H)'»K 4,0 1.8 0.32 0 25 0 15currcnl (5 2 mA) 7 .^(> 10 ")cm ' 195 8 2 36 25 14 current (3 mA) i7p(x 10 ")cm~' 25 1 55 7.8 14 077My I0^)amp-' 8,5 2 9 0 93 0 65 031 .//(x 10^ ) amp ' M65 167 103 1 7 069Jg{)i 10*)amp"' 15 1 5 1 1 7 1 0 0 46 y,(x 10*)amp'' 128 1 18.3 113 1 9 073 650 volt 0 4 mm femp (x 10-^ )‘’K 116 7.2 42 3 1 2.5 2. 620 volt 0.4 mm Tcmp.(x 10-*)'*K 50 23 0 38 0.28 018current (8 mA) /7,.(x 10 ' ‘)cm ' 130 99 25 19 1 3 current (5.2 mA) i;c(x 10") cm ' 98 4.2 6 1 1.2 067y(x 10^)amp"' 63 3 8 0 74 0 49 0.29 y,(x 10') amp-' 50.9 15.0 8.8 1 4 0 68y^ (x 10*) amp' 113 68 1.3 0 87 0 52 J,,(x 10*) am p' 55 9 16 5 96 1 5 0 7.14 100 volt 4-31 mm Tcmp,(x 10^ )®1C 1.0-2.5 [7] 3 650 volt 0.4 mm Tcmp.(x 1(H)*K 5.8 2.9 041 0.31 020current (2-8 A) r/e(x 10'")cm"' 10-100 current (8 mA) ;;.(x 10") cm-' 73 4.4 24 1.0 0.595 60-300 volts 10 mm Temp (x I0-*)“K 2.0 181 y,(x 10')amp'' 41.2174 67 1.3 061current (4 0 A) y,,(x 10*) amp' 45.2 19.1 73 1.4 0,68Study o f plasma parameters o f a discharge 3653. Results and discussionI f  / i ,  h  and h  are the currents flowing in the probe P ,^ P2 and respectively, then according to Chen and Sekiguchi [1]h =  h -  (1)Now assuming that( i )  l l i e  electron energy d istribution in the plasma is M a x w e llia n .( i i )  The  thickness o f  the ion sheath form ed in the discharge near probe electrode is sm aller than the probe separation and( i i i )  The  m ean free path o f  the electrons is much larger than the thickness o f  the ion sheath around each probe and the probe radius.In general, in equ lib rium  situation, the current heating the probe is- ‘S ' ( 2)  where a  is the species o f  the partic le  (e lectrons/ions), qa is diarge o f  the partic le  o f  species a, /„  is current dinsity o f  the species a, V is the potential at probe for biasing and •S' IS the surface area o f  the probe. In  the case o f  electronsJ,. = /27IW , , ( 3)where r/„ -  - e  and"  kT.. (4 )IS the electron saturation current density, 7/  ^ is the electron density, k is the B o ltzm ann  constant and c and are charge and mass o f  the electron respectively. Since the cross sectional area o f  the three probes are the same, S\ S2 = = S\ as a special case, it m ay be assumed that-  SJfiyi)  ~  =  S/,(V). In this case, w e canderive(/1  +  / 3 ) 1f l - g -[ l - t -<PK,<pr, (5)where / | ,  /2  and A  are the currents in the first, second and third probes and f  J2 and are battery potential between 1, 2  and 1, 3 electrodes. From  this equation, the values o f  <^and thereby the electron tem perature Te can be determ ined  provided 7], / 2 , J3,/ ¥^2  and are know n. The values o f  electron tem perature thus obtained in d ifferen t conditions fo r the three gasses are given in Tables ( 1 - 3 ) .  O ne can also derive the ion saturation current density using the relationIn this expression since a ll the terms are know n, therefore  ion saturation current density could be determ ined.The value o f  electron density can not be determ ined  directly using trip le  probe m ethod. H o w e ve r, it can becalculated from  T^ , and Jt since the ion saturation current density is related to the electron density as(7)where m, is the ion mass. The electron density thus obtained  fo r the three gases are tabulated in Tables 1, 2 and 3 respectively. From  the table, it is clear that the electron  temperature for the m onoatom ic gas argon is much larger compared to diatom ic O 2 and N 2 . A  reverse th ing is observed for electron density. V aria tion  o f  electron tem perature w ith  f  for a constant value o f  k (~  7 V )  and discharge pressure 0.4  m m ) for the three gases at three d ifferent voltages are shown in Figures 2 a, 3a and 4a. S im ila rly , Figures 2b, 3b and 4b represent the variation o f  electron density w ithFigure 2. (a) Varialion of electron temperature with in Ar discharge at different discharge voltages and (h) Variation of electron density with Fj 3 in Ar discharge at differenl discharge voltages.3 6 6 Shiv Kumar Singh and S  B RaiFrom  the figures one can see that for low er value o f  the electron tem peratures are higher fo r all the three gases. TheIt is interesting to note that the electron temperature in crea se s  w ith  th e  in c re a s e  o f  d is c h a rg e  vo ltag e(a)Figure 3. (a) Variation of electron temperature with in N2 discharge at different discharge voltages and (b) Variation of electron density with Vji in N2 discharge at different discharge voltageselectron tem perature decreases w ith  the increase o f  and shows a saturation fo r h ig h e r values o f  A  sim ilar behav iour is m arked  fo r d ifferen t discharge voltages.(a)Figure 4. (a) Variation of electron temperature with K/3 in O2 discharge at different discharge voltages and (b) Variation of electron density with F,/3 in O2 discharge at different discharge voltages.(current); how ever, the electron density decreases with discharge vo ltage  (c u rren t). T h e  varia tio n  o f  electron tem perature and density w ith  discharge voltage fo r a fixedStudy o f plasma parameters o f a discharge 367value o f  discharge pressure, V j2 and is shown in Figure 5.riKiirc 5. Variation of electron temperature and electron density with discharge voltageW e have also com pared our results fo r 7'e and 7]c fo r argon and oxygen w ith  the earlier measurements reported inliterature [7 -9 ]. Our values are slightly different from these reported values due to different experimental conditions o f  our discharge.Rcrcrcncci[1] S L Chen and T Sekiguchi J, AppI Phys. 36 2363 (1965)[2] F: 0  Johason and L Maltcr Phys Rev. 76 1141 (1949); 80 58 (1950)[3] S Kojima and K Takayama J. Phys Soc Jpn. 4 (1949); S 357 (1950)[4] T Okuda and K Yamamoto J. Appl Phys. x i  158 (1960)[5] A D Shukla, T M Desai. S V Oogawak and N K Joshi Recent Advance in Plasma Science and Technology, Plasma 95 (cds) R P Singh et al (New Delhi : Allied) p 270 (1995)[6] K Yamamoto and T J Okuda Phys Soc. Jpn  II 57 (1956)[7] K Kano, M Su/uki and H Akatsuka Plasma Sources Set and Tech 9 314 (2000)[8] A Bzoubtehenko, N Ohtsuki and II Akatsuka Plasma Sources Sci and Tech 7 136 (1999)[9] H M Katsch, T Sturm, F Quandt and H F Dobele Plasma Sources Sci and Tech 9 323 (2000)

Study of Plasma Parameters of a Discharge

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Study of Plasma Parameters of a Discharge

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