The role of Ne, Ar, and Kr as a buffer gas was investigated in a KrF laser pumped by a coaxiale-beam. The coaxial diode has a length of 20 cm and the anode tube diameter is 1 cm. The tube can be pressurized with the laser gas mixture up to 12 bar while a current density of 250 A/cm2can be achieved. It is found that both the maximum energy extraction and the optimized gas pressure with each buffer gas increase in the sequence Kr-Ar-Ne. From the experimental results, the quenching parameters of Kr, Ar, and Ne are deduced

Kleikamp, B.M.H.H.

Peters, P.J.M.

Witteman, W.J.

English

NARCIS 

An investigation of the role of Ne, Ar, and Kr as buffer gas in a coaxial e-beam pumped KrF laser

University of Twente Research Information

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-22, NO. 2,  FEBRUARY 1986 219 Quantum  Electronics  Letters An Investigation of the Role of Ne, Ar, and Kr as Buffer  Gas in a Coaxial e-Beam Pumped KrF Laser B. M. H. H. KLEIKAMP, P. J. M. PETERS, AND W. J. WITTEMAN Abstrcf-The role of Ne, Ar, and Kr as a buffer gas was investigated in a KrF laser pumped by a coaxial e-beam. The coaxial diode has a length of 20 cm  and the anode tube diameter is 1 cm. The tube can he pressurized with the laser gas mixture up to 12 bar while a current density of 250 A/cmZ can be achieved. It is found  that both the maxi- mum energy extraction and the optimized gas pressure with each buffer gas  increase in the sequence Kr-Ar-Ne. From the experimental results, the quenching parameters of Kr, Ar, and Ne are deduced. F I. INTRODUCTION OR e-beam pumped excimer systems, the stopping power  is  only  a  fraction of the  electron  beam  energy. Since  the  stopping  power is proportional to the gas den- sity, it is  important  to  optimize  the  energy  extraction  den- sity with  respect  to  the  gas  density. We have treated  this problem  in  the  past  for  the  conventional  mixture of Ar,  Kr, and  F2  [l].  It was argued  that for practical  situations,  the analysis can be simplified by following the complicated chain of many  kinetic  reactions  as  a  quasi-stationary  pro- cess.  The  predictions  obtained  in  this way are  suitable  for comparison  to  results of different  experiments. Although the energy transfer from the e-beam is pro- portional to the gas density, this density must be limited because of its  quenching effect and  the  production of ab- sorbing  species.  The  density of KrF*  is  determined by the optical  quality  of  the  resonator  and  the  absorption of the medium.  The  latter  is  mainly  dependent on the  partial  gas densities  and  current  density.  This  means  that  the  density of KrF*, and consequently the quenching losses, can be decreased by decreasing  the  outcoupling.  The  stimulated emission  process  will  then  overcome  the  quenching losses by a  stronger  intracavity  laser  flux.  The  more  intense  laser flux on its turn causes increased absorption. Therefore, the  system  can  be  optimized.  It was found that the opti- mized  densities of all  individual  gas  components  increase with the current density, and that in a large range, the outcoupling is not a sensitive parameter. These results were  found  in  good  agreement  with  our  experiments  [2], [3] where we investigated laser cavities of different vol- umes  and  current  densities. Manuscript  received  June 26, 1985; revised October 8, 1985. This work was supported by the Dutch Research Foundation Stichting Fundamenteel Onderzoek  der  Materie. The  authors  are  with  the  Department of Applied  Physics,  Twente  Uni- versity of Technology.  Enschede,  The  Netherlands. IEEE Log Number 8406663. Since  for  high  performance we  have to optimize  the  bal- ancing process between the excitation of the  gas  and  its quenching  and  radiation  losses,  it  is  interesting  to inves- tigate mixtures with different buffer gases that are ex- pected to show large differences in their quenching and absorption  properties.  Recently,  some  calculations  and  ex- periments have been reported on this subject. Measure- ments  with  high-pressure  Ne  mixtures have been  de- scribed [4], whereas  a  growing  interest  can  be  seen  in low- pressure  Kr  buffered  gas  mixtures 1.51-[7]. In  the  present contribution, we study  the  radiation  energy  extraction density for, respectively, Ar, Kr, and Ne as a buffer gas. The experiments allow us to estimate the quenching pa- rameters of these  gases. 11. DESCRIPTION OF THE LASER SYSTEM The  laser  system,  consisting of a  coaxial  vacuum  diode driven directly by a low-inductance ten-stage Marx gen- erator,  has  been  described  in  detail  previously 131. In  the present  situation,  the  laser  chamber  is  made of Ti-foil of 50 pm  instead of 25  Fm, so that  it  can  be  used  at  pressures up to 12  atm  under  discharge  conditions.  The  thicker foil causes  a lower current  density  and  electron  energy  in  the gas. The diameter of the chamber is 10 mm and the pumped  length  is  20 cm,  resulting in an  active  volume of 16 cm3. The chamber is ended with two plane parallel MgF, flats. At one side, a quartz prism is attached as a total  reflector. All  experiments  described below have been  performed at  a  diode  voltage of 280 kV and  a  current of 7.5 kA  in 30 ns pulses. From this, we estimate a maximum current density of approximately 375 Aicm’ and  an  average  cur- rent of about  250  A/cm2.  The  maximum  electron  energy inside  the  laser  tube is approximately 170 keV. 111. EXPERIMENTS The  classical  mixture of an  e-beam  pumped  KrF  laser consists of Ar, Kr, and F2. This mixture has been well established,  and  the  results  are  all  based  on  the  thorough investigations by many researchers of fundamental pro- cesses  such  as  formation  rate  constants  and  radiation  in- teractions. It was found that for maximum extracted en- ergy, depending on the current density, the Ar pressure should be between 3 and 5 atm and the Kr pressure be- tween 0.2 and 0.5 atm [ l]-[3]. The  output  dependence on the  total  pressure of the  clas- 0018-919718610200-0219$01.00 0 1986 IEEE 220 IEEE JOURNAL OF QUANTUM ELECTRONICS. VOL. QE-22, NO. 2. FEBRUARY 1986 Fig. 1. The  dependence  of  laser  output  in  artitrary  units  on  Ar  pressure. Each  curve  has  constant  Kr  pressure  as  indicated  in  the  figure. Fig. 2. The output energy in arbitrary units is plotted versus the Kr per- centage for the three-component mixtures. Each curve follows the data for  total  pressures as indicated. sical mixture is shown in Fig. 1. For each curve, the Kr and F2 partial  pressures  are  kept  constant  as  parameters. All mixtures have shown  that  the  output  is not  sensitive  to F2 concentrations around its value of about 10 mbar, so that all experiments have been performed for this partial pressure of F2.  The experiments  show  clearly  that  the  op- timized values of the three-component system for a cur- rent density of approximately 250 A/cm2  are 4.5 atm  Ar and 0.5 atm Kr. Increasing  the  Kr  concentration  shows a lower extraction energy, but also a shift to lower total pressure for better energy extraction. Further, it is seen that  the  lower  the  total  pressure,  the  less  sensitive  the ex- traction  energy is on  the  Kr  pressure,  and below 2 atm  the maximum  is even  obtained  for  a  mixture  without  Ar, i .e. ,  a  two-component  system. This  is  also  shown  in  Fig. 2 where  the  output is plotted as a  function of the  Kr  percentage in F,-Ar-Kr mixtures with  total pressures of 1, 2, 3,  4, and 5 atm, respectively. The results of the two-component system Kr-F2 are I it O U T P U T   E N E R G Y  (arbitrary units) I Kr = but fergas F2 :10mbar J. Kr. Pressure [At  m.] - I 1 2 3 4 5  Fig. 3. The  output  energy in arbitrary  units of the two-component system Kr-F, as a function of the  Kr  pressure. F, pressure is 10 mbar. I t r  I Kr= 2 atm 5 pressure [ mbar ] 10 20 30 Fig. 4. The  output  energy in arbitrary  units of the F,-Kr mixture as  a  func- tion of F2.  The Kr  pressure is 2  atrn. shown in Fig. 3.  The F2 concentration of 10 mbar  is kept constant.  The  maximum  extraction  energy  is  more  sensi- tive to  the  gas  pressure,  is  much  smaller,  and  reaches  its value at a much lower pressure than in the case of the three-component  system.  The F2 dependence of this sys- tem is shown in Fig. 4. The optimized value of the F2 concentration  has  a relatively broad  range  around 10 mbar, similar to  the  three-component  system.  Apparently,  at 10 mbar, sufficient F ions are produced by the interaction of F2 and secondary electrons. Higher F, concentrations lead to increased absorption of radiation by photodisso- ciation. Since in the  absence of Ar  the  optimized  gas  pressure and  maximum  output  are much smaller  than in the  pres- ence of Ar, one may wonder  whether  this  difference  is  re- lated to stronger  quenching  and  absorption  phenomena in IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. QE-22, NO. 2,  FEBRUARY 1986 22 1 I OUTPUT ENERGY (arbitrary units) / Ar+Ne=BuItergas Kr1500mbar F2: lombar 8atrnAr 2 4 6 8 10 ' 12 Fig. 5. The  output  energy of the  four-component  mixture Ar-Kr-Ne-F, a: a  function of the Ne pressure  with  the  Ar  pressure as a  parameter. The F, pressure  is 10 mbar  and  the Kr pressure 0.5 atm. a  Kr  dominating  mixture.  Further, if this  is  the  case,  it is interesting  to  investigate  the  effect of  a lighter  rare  gas  as a buffer that  is  expected  to have a smaller  quenching  ef- fect.  This  has  been  done by considering again the curve of the  optimized  three-component  system with 500  mbar Kr  and 10 mbar F,. At the  positions of, respectively, 2, 4, 6, and 8 bar Ar of this curve, the gas has been further mixed  with  Ne.  This  is  shown  in  Fig. 5. At  points  where we start  to  add Ne instead of Ar,  we find indeed  that Ne has  a  favorable  effect  on  the  energy  extraction.  The  lower the  Ar  concentration,  the  higher  the  maximum  energy  ex- traction.  The  result  also  shows  that  for  Ar below  4 atm,  a further  decrease of Ar  results  in  an  increase of total  pres- sure for which maximum energy is obtained, indicating that absorption and quenching of Ne are much less than those  for  the  Ar  gas.  The next step by substituting  Ne  for Ar is to investigate  the  Kr  dependence  in  this  mixture  be- cause  Kr  is  essential  for  producing  the  lasing  molecules. It  is  most  likely  that,  similar  to  the  case with  Ar, the Kr' ions are formed in collisions with Ne;.  We studied the relation  between  the  partial  pressures of Kr  and  Ne. In  Fig. 6, we  plotted  the  extraction  energy  as  a  function of the  total  pressure  with  Ne  as  a buffer gas. In all  exper- iments, we used  a  constant  F2  pressure of 10 mbar which was observed to be the optimized concentration of F2. Each  curve  describes  the  measurements  with  constant  Kr pressure and a constant Ar pressure of 490 mbar. This presence of Ar  is  due to the  fact  that  the  available  F2 was diluted  in  Ar  gas.  The  curves of Fig. 6 show  that  the  max- imum of each  curve  increases  with  decreasing  Kr  concen- tration  and  reaches  a  maximum  for  500  mbar,  similar  to the  three-component  system.  The  higher  maximum  is again  obtained  for  higher  total  pressure. We also  noticed that  if  one  chooses  a  maximum  pressure below  4.5 atm,  a higher Kr pressure is more favorable. In this pressure range below  4.5 atm,  the  use of Ar  is  even  more  favorable. The  maximum  admissible  pressure of our  system  is 12 atm. We may extrapolate the optimized Ne pressure at about 15 atm. 2 4 6 8 10 12 Fig. 6. The  output  energy of the  four-component  mixture Ar-Kr-Ne-F, as a  function of the Ne pressure  with  the  Kr  pressure as  a  parameter.  The F, pressure  is 10 mbar  and  the  Ar  pressure  is 490 mbar. IV. DISCUSSION OF THE RESULTS The  above-described  experiments  were  performed  with Kr,  Ar, and Ne as  a buffer gas.  It was  found  that  the  lighter the buffer gas,  the  higher  the  maximum  extracted  energy density is from the e-beam, but also that this higher en- ergy is extracted at a higher pressure of the buffer gas. The  heavier  gas, however, has a large  stopping  power so that  the  excitation  density  per unit pressure  is  higher.  Since there  is  a  balancing  process  between  the  excitation of the gas  and  the  losses  (quenching  and  absorption), it is clear that  the  losses of a  lighter  gas  are much smaller  than  those of a  heavier  gas.  The  experimental  results  can  be used to estimate  the  quenching  parameters of the buffer gases. We proceed  as  follows.  The  experiments  with  the  high  e-beam current  density  show  that  the  output  power is very  insen- sitive  to  the  reflectivity of the  outcoupling  mirror.  For  the steady state, which is reached after a few nanoseconds, we  have per  unit  length  the  condition  that  the  stimulated emission  is  equal  to  the  outcoupling  and  absorption  losses: o,,Z[KrF] = l ( y  + a) where Z is the intracavity photon intensity, [KrF] is the density of the lasing molecules, y is the outcoupling per unit  length, a is  the  absorption  per  unit  length,  and oI is the  cross  section  for  stimulated  emission.  The  production of the  inversion  density P depends  on  the  gas  pressure  and e-beam  current  density. For steady  state, we  have (1) P = [KrF] Q + Z(y + a) (2) where Q described  the  quenching of a  lasing  molecule  per second. The output power per unit length, given by y l ,  can  be  calculated  from (1) and (2): In  the  case  where  the  output  is  insensitive  to  variations of the outcoupling, we have Nay (Zy) = 0. Using (3), we 222 IEEE  JOURNAL OF QUANTUM  ELECTRONICS,  VOL. QE-22, NO. 2, FEBRUARY 1986 obtain Substituting (1) and (2) into (4), we obtain TABLE I QUENCHIXG CONSTAYTS OF Kr, Ar. A N D  Ne FOR KrF where p < 1. The last expression shows that the intracavity absorp- tion  is  smaller  than  the  quenching. In order to make an estimate of the quenching para- meters of the  various buffer gases, we again  consider (2) for the optimized high-pressure gas densities. At these pressures,  the  quenching mainly occurs by three-body collisions which allow the  formation of a new species  with a large  probability of transferring  the relatively small  ex- cess  energy  into  the  translational  motion of the  colliding particles. The quantity Q for  high  pressure  can  then  be x- pressed as QR = kR IR1 (6) where kR and [R] are, respectively, the quenching para- meter  and  the  density of the  rare  gas. The production term P is proportional  to  both  gas  den- sity and current density i. This can be expressed as fol- lows: P R  = BRI‘[R]. (7) Substituting ( 5 ) ,  (6), and (7) into (2), we obtain = BRi [R] - (1 i- 0) [KrF]  kR[RI2. (8) For  the  maximum  energy  extraction  as a  function of [R] ,  we have or  from (8) BRi = 2( 1 + p) [KrF] kR[R] (9) where  the  dependence of /3 on [R] is expected  to  be  neg- ligible. The quenching  parameter  is now  given by k -  asBRi - 2(2y + a)[R]  where  we have used (1). The pumping  rate BR can  be  deduced  from  the  stopping Kr 111 5.2 x I O ”  4.6 x IO-”  Ar  55.5 1.35 x 10” 9 x Ne 28 3.9 x IO”’ 1.5 x IO-’’ power as  given by Berger  and  Seltzer [SI. The values are B,, = 111, BAr = 55.5, and BNe = 28 cm2 . A - l  . s . The outcoupling y for an outcoupling  reflectivity of about 10 percent  and  a  length of 20 cm for the  active  me- dium is 5.75 X cm-’.  The  cross section  for  stimu- lated  emission  is a, = 2.4 X cm2  [9]. The absorption  losses a near  the  maximum  output  can be calculated with (4). The optimized values of [R] can be deduced from ,the experimental data. The quenching parameter kR for the three gases can now be estimated with (10) where  the  used  value of [KrF]  is  obtained  from (1). The results  are  presented in Table I. To our knowledge,  the  quenching  constants of Kr and Ne are not known in the  literature.  The  three-body quenching of Ar  is quoted as 8 X cm6* s-I [ lo],  which is in good agreement with our  results. - 1  REFERENCES W. J. Witteman and B. M. H. H. Kleikamp, “On the electron-beam pumped KrF laser,” J. Appl.  Phys.. vol. 55, pp. 1299-1307, 1984. P. J. M. Peters, H. M. J. Bastiaens, and W. J. Witteman, “Compact coaxially  excited  high  energy  density  KrF  laser,” J. Appl. Phys., vol. 55, pp. 1410-1412, 1984. B. M. H.  H. Kleikamp  and W. J .  Witteman,  “High  energy  extraction of electron beam pumped KrF lasers at multi atmospheres,” Opt. Commun., vol. 49, pp. 345-348, 1984. T. Gerber, P. J. M. Peters, H. M. J.  Bastiaens, and W. J.  Witteman, “Enhancement of‘ the specific output energy of an electron beam pumped  KrF  laser by using  Ne as  the  main buffer gas,” Appl. Phys. Lett., vol. 45, pp. 356-357, 1984. F. Kannari, A. Suda, M. Obara, and T. Fujioka, “Theoretical evalu- ation of electron-beam excited KrF lasers using argon-free mixtures of one  atmosphere,” Appl. Phys. Lett., vol. 45, pp. 305-307, 1984. A.  Suda, M. Obara,  and T. Fujioka,  “Atmospheric  pressure  operation of an  electron  beam  excited  KrF  laser  using  KrlF,  mixtures,” Appl. E. T. Salesky  and W. D.  Kimura,  “Gain  and  absorption  measurements of electron  beam  pumped,  high  Kr  concentration  KrF  gas  mixtures.” Appl.  Phys. Lett., vol. 46, pp. 927-929, 1985. M. J. Berger and S. M .  Seltzer, Nat. Aeronaut. and Space Admin., Washington, DC, Rep. NASA sp-3012, 1964. T. H .  Johnson and A. M. Hunter, “Physics of the krypton fluoride laser.” J. ADDL Phvs., vol. 51, OD. 2406-2420, 1980. Phys. Lett., VOI.  45, pp. 1165-1167. 1984. & ‘  I ,  [ lo]  J. A. Mangano, J .  H. Jacob, M. Rokni, and A .  Hawryluk. “Three- body  quenching of KrF* by Ar  and  broad-band  emission  at 415 nm,” Appl. Phys. Lett., vol. 31. pp. 26-28, 1977. .. 

An investigation of the role of Ne, Ar, and Kr as buffer gas in a coaxial e-beam pumped KrF laser

Abstract

Similar works

Full text

Available Versions

NARCIS

University of Twente Research Information