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Investigation of the effects of hydrogen and deuterium on copper vapour laser performance
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1 September 1994
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EISEVIER
OPTICS
COMMUNICATIONS Optics Communications 110 (1994) 699-707
Full length article
Investigation of the effects of hydrogen and deuterium on copper vapour laser performance Michael J. Withford, Daniel J.W. Brown, James A. Piper Centrefor Lasers and Applications,Macquarie University,NorthRyde, NSW 2109, Australia
Received 11 January 1994;revised manuscript received 10 April 1994
We report a detailed study of the effects on copper vapour laser performance of the addition of small partial pressures of gaseous hydrogen, deuterium and other trace gases to the neon buffer gas. Increases in output power by typically 30% are observed for a small-scale (6 W) device and the spatial characteristics of the output beam are significantly altered. Increased power deposition due to improved impedance matching is shownto be a minor effect. Instead, the changesto output power and the
voltage/current characteristicsduringthe excitation pulseand the interpulseperiod are attributed primarilyto enhancedplasma coolingby elastic interactionsresultingin reduced prepulseelectron densities. 1. Introduction Improved performance of elemental copper vapour lasers (CVLs) with small admixtures of hydrogen to the neon buffer gas has been reported by a number of researchers in recent years. Huang et al. [ 1 ] first reported increases in output power of up to 50% and a 60% improvement in efficiency with a 0.40.7 torr (0.6-11) admixture of hydrogen to the neon buffer gas in a medium bore (32 mm) CVL. Changes in the spatial protile of the laser output intensity were also noted, with the typically annular profile becoming gaussian-like with increasing levels of hydrogen. Enhanced laser performance with hydrogen additive has also been reported for CuBr [ 2-5 ] lasers, CuCl lasers [ 5 1, Au vapour lasers [ 6 ] and Ba lasers [ 7 1. A number of mechanisms by which hydrogen improves output power and spatial profile have been suggested. Huang et al. [ 1 ] proposed that the relatively large elastic collision cross section of hydrogen effectively cools electrons in the interpulse period, increasing the rate of deexcitation of the copper
*D3,2,5,2metastable levels. In a study of hydrogen admixtures to a CuBr laser, Astadjov et al. [ 2-41 suggested that improved laser performance derived from a decreased pre-pulse electron density, possibly due to electron attachment to hydrogen. Further, Astadjov et al. [4] reported a reduced depopulation of the copper 2S,,2 ground state during the excitation pulse, along with more rapid repopulation of the copper ground state (20 x faster) in the interpulse period when hydrogen was present. Small increases in output power with helium admixtures to neon buffer gas have also been reported. Yamanaka et al. [ 8 ] noted small increases in output power in a large bore CVL for a 5Ohadmixture of helium, an increase they attributed to enhancement of 5 10 nm emission solely in the central portions of the output beam profile. Reduction of the axial gas temperature was suggested as a mechanism due to the high thermal conductivity of helium. Lesnoi [ 91 reported no change in the discharge characteristics of a CVL when argon, krypton and xenon were admixed with the neon buffer gas. In con-
0030-4018/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDiOO30-4018(94)00276-Z
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M.J. Withford et al. /Optics Communications 110 (1994) 699-707
trast, the discharge current increased in duration and decreased in amplitude with helium additive. Lesnoi attributed this to large electron energy losses to helium due to its low mass yet relatively high elastic collision cross section. Further, Ref. [ 9 ] notes decreases in CVL output power as the partial pressure of added argon, krypton and xenon increases, reportedly due to inelastic electron energy losses with those gases. Apart from their influence on the kinetic processes in the laser medium, trace additives may simply alter the impedance matching between the laser discharge and its driving circuit, resulting in changed power deposition into the laser discharge tube and consequently changes in laser output power. Clearly there remains considerable ambiguity as to the mechanism, or mechanisms, by which hydrogen additive influences CVL performance which it is necessary to resolve in order that the affects on particular devices can be predicted. In this paper we present the results of a detailed study regarding the comparative effects of hydrogen and deuterium additive on the voltage/current characteristics and output power of a CVL. The difference in elastic energy transfer rates for these two gases gives a strong indication as to the primary mechanism by which the addition of hydrogen and deuterium to the laser discharge influences laser performance. The significance of improved impedance matching relative to that of volume kinetic processes in their effects on laser performance has also been investigated for various trace gaseous additives (Ha, Dz, He,AandXe).
2. Experimental method The CVL used in this investigation was a smallscale device (plasma tube dimension 18 mm dia. X 7 50 mm long giving an active volume of 190 cm2 ) coupled to a ultra-high vacuum system (Fig. 1) incorporating a turbomolecular pump (Alcatel 5080) and mass quadrupole spectrometer (Inficon Quadrex 100). The low base pressure ( c lo-’ torr) available with this system minim&d the effects of residual gases trapped in the insulating substrate (Zircar AL-30 AAH) while the mass quadrupole spectrometer allowed real-time analysis of the composition of the buffer gas. The various partial pressures of hydro-
gen were mixed with neon in a chamber separated from the active volume, and the resulting admixtures were then introduced at the cathode end of the laser at low flow rates ( N 3 cm3/min) with a set buffer gas pressure of 25 torr (chosen because it lay in a region where laser performance was optimized). In all cases, ultra-high purity gases (99.999% pure) were used. The CVL discharge circuit involved a standard thyratron-switched (EG&G 3001) pulse charging circuit (Fig. 2). The storage capacitor C, (3 X 2 nF Condenser Products TSG 202 24M) was resonantly charged through the saturable inductor L and charging resistor R ( 100 R wirewound). The excitation voltage pulse was monitored using a high voltage probe (Tektronix P6015) across the plasma tube and peaking capacitor C, (2 x 1 nF Condenser Products TSG 102 24M). Current through the laser discharge tube was monitored during the excitation phase (the discharge current) using a loop current probe (Pearson 11OA) . Identical current probes were also used to monitor both the leakage current through the discharge tube during the interpulse period (at point A; Fig. 2) and the charging current through resistor R (point B; Fig. 2) as the storage capacitors recharge during the interpulse period. The oscilloscope traces (Tektronix 2245a 100 MHz) of the voltage and current pulses were digit&d using an oscilloscope mounted video camera (Tektronix C 100 1) and its associated software. A flat/flat laser resonator was used incorporating a high reflector and a 4% output coupler. The total laser output power and the spatially averaged 578 nm component of total output power (separated by dichroic beam-splitters) were measured using a calorimetric power meter (Scientech 36000 1). The temperature of the silica vacuum tube was monitored with a chromel-alumel thermocouple mounted on its external surface. The discharge tube wall temperature was also measured using an optical pyrometer (Minolta TR 630). The change in water coolant temperature through the thyratron was monitored by alcohol thermometers mounted on the water lines, offering qualitative information regarding the deposited power in the thyratron when the laser was operated with different gas mixtures. The CVL was operated at a pulse repetition frequency of 6.0 kHz and a supply voltage of 6.6 kV,
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M.J. Withford et al. /Optics Communications 110 (1994) 699-707 Laser
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slightly below the optimal supply voltage of 6.9 kV (Fig. 3). During previous investigations, the power deposition in the discharge tube was observed to increase (evident from observed increases in the discharge tube wall temperature) as the various trace gaseous additives produced small changes in the impedance matching of the laser. As a result, a near optimal supply voltage (6.6 kV) was selected because it lay in an operating regime where the laser output power was relatively insensitive to increases in the discharge tube wall temperature (Fig. 3). An investigation of the effects of H2 and D2 additive was also undertaken at a second lower supply voltage (6.2 kV). Care was taken to remove residual hydrogen and deuterium between experimental runs to allow accurate assessment of the various admixtures. It was found by mass spectroscopy that the fibrous alumina insulator used in conventional CVL design was capable of absorbing considerable amounts of hydrogen when cooling, this residual hydrogen outgassing
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Fig. 3. Laser output power and discharge tube wall temperature with respect to the supply voltage for the CVL operating in pure neon buffer gas. The optimum output power resulted from a sup ply voltage of _ 6.9 kV.
at lasing temperatures. Several hours of cycling the laser in pure neon were required to reduce the residual hydrogen to negligible partial pressures of less than 10-7 torr. The spatial characteristics of the beam profile were monitored using a telescope arrangement (200 mm and 60 mm AR coated convex lens) to image the laser output onto the CCD array of a laser beam analyser (Spiricon LBA- 100 ) .
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h4.J. Withford et al. /Optics Communications 110 (1994) 699-707
3. Results Fig. 4a shows the total laser output power (green & yellow) for different quantities of Hz, D1, He, A and Xe added to the neon buffer at a supply voltage of 6.6 kV. An increase in output power of over 30°h was noted for a 2% hydrogen/neon admixture (Fig. 4a) with an associated increase in the discharge tube wall temperature from _ 1550°C to 1620°C (Fig. 4b). For all admixtures investigated the proportion of yellow output remained 33% ( z!z1%) of the total laser 10,
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output. Significant increases in output power were also observed for added deuterium, and marginal increases for a 5Ohhelium/neon admixture. Addition of argon and xenon reduced the output of the CVL. Maximum increases in output power were observed for a 2% Hz and -4W DJNe admixtures. Above these levels of Hz and Dz the output power falls rapidly. At first glance, this decrease in output power may be attributed to overheating (i,e. beyond 1620°C) due to increased power deposition in the plasma tube, however, it is not immediately obvious why the laser can tolerate twice as much Dz as Hz additive. In fact, repeating the experiment at a lower supply voltage of 6.2 kV (Fig. 5: i.e. at lower wall temperatures consistent with low power deposition) laser power at first increases (by 38%) for 2% H2 additive but then decreases even though the wall temperature remains well below the optimum temperature. Moreover, the optimum concentrations for H2 and D2 additive remain approximately in the ratio 1:2. The deposited power in the thyratron was observed to decrease (by up to 14Ohfor a 5% H,/Ne admixture) with increasing levels of trace gaseous additive, evident from a drop in the temperature gradient of the coolant between the in-flowing and out-flowing ports of the thyratron. This observation is consistent
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Fig. 4. (a) Laser output power with respect to various partial pressures of Hx, D,, He, Ar and Xe gases added to the neon buffer gas of a CVL operating at 6.0 kHz pulse repetition frequency and 6.6 kV supply voltage. (b) Discharge tube wall temperature relative to partial pressures of Hr. Dx, He, Ar and Xe gases added to the neon buffer gas of a CVL operating at 6.0 kHz pulse repetition frequency and 6.6 kV supply voltage.
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Fig. 5. Laser output power at a supply voltage of 6.2 kV for various admixtures of H,/Ne and DJNe. A maximum output power of 2.8 W was observed for a 2% Hr/Ne admixture. In contrast, a maxima in the output power behaviour for deuterium additive was observed for a 5% Dr/Ne admixture.
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M.J. Withfordet al. /Optics Communications 110 (I 994) 699- 707
with improved impedance matching of the laser with its driving circuit, therefore increasing power coupling to the discharge tube. Increased power coupling (implied by an increased tube wall temperature) for Dz, He, A and Xe additives (Fig. 3b) are also attributed to improved impedance matching. The general effects of improved impedance matching by H2 additive were approximated for a pure neon buffer gas by supplementary heating of the discharge tube [lo] and alternatively increasing the supply voltage (thereby increasing the peak amplitude of the discharge pulse). The maximum in the output power occurred for a 2% hydrogen/neon admixture for which the wall temperature was 1620°C (compared to 1550°C for neon only). The same wall temperature of 1620°C (and therefore similar copper vapour pressure) was obtained for pure neon buffer gas by supplementary heating of the discharge tube while maintaining the same stored input power as for the H,/Ne admixture. An increase of only 0.3 W was observed, falling well below the increases of several watts observed for admixed hydrogen. Increasing the input power (supply voltage) to produce a wall temperature of 1620°C also resulted in a minimal output power increase of 0.3 W even though the peak amplitude of the excitation voltage pulse was similar to that observed for a 2W HJNe admixture. It is therefore clear that the improved CVL performance observed for hydrogen additives is due predominantly to mechanisms other than one where the overall copper density is increased as the wall temperature increases in response to increased input power. Increases in the peak amplitude of the discharge voltage (Fig. 6a), similar to those reported by Astadjov et al. [ 2,3 1, and decreases in the peak amplitude of the discharge current (Fig. 7) were observed for hydrogen added to the CVL. In light of the relatively minor decreases in the peak amplitude of the discharge current observed with 1Olband 2% HJNe admixtures, increasing the supply voltage in pure neon buffer gas (thereby increasing the peak amplitude of the discharge voltage pulse) is a reasonable approximation of the effects of small partial pressures of H2 on impedance matching (as described above). Increases in the peak amplitude of the discharge voltage were also observed for deuterium (Fig. 6b) and helium (Fig. 6c), though larger proportions of deuterium (about 10%) and helium (approx. 50%)
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Fig. 6. (a)-(d) Discharge voltage pulses for various Hz, Db He and Ar/Ne admixtures. Comparable increases in the peak amplitude of the voltage pulse were observed for 5% H,/Ne, 10% D2/ Ne and 50% He/Ne admixtures. No significant increases in the peak amplitude of the voltage pulse were observed for additives of argon (shown) or xenon.
were required to cause changes comparable to those induced by a 5Wmix of hydrogen. An associated decrease in the peak amplitude of the discharge tube current was also observed for deuterium and helium though again larger partial pressures were required to give similar effects to those observed for hydrogen. The changes to the voltage pulse and the discharge tube current, during the excitation phase, were negligible for argon (voltage pulse: Fig. 6d) and xenon admixtures.
M.J. Withfod et al. / Optics Communications I10 (1994)
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Fig. 7. The current through the discharge tube during the excitation phase with respect to pure neon and various Hs/Ne admixtures. Small decreases in the peak amplitude of the excitation current pulse were observed for 1% and 2% H2 additive. Relatively large decreases in the peak amplitude of the current pulse were noted for > 5% H2 additive.
Increases in the current through the charging resistor during the interpulse period (i.e. as the storage capacitors recharge) were observed with increasing levels of hydrogen (Fig. 8a), deuterium and helium admixed to the neon buffer. In contrast, the total leakage current through the discharge tube, during the same period, decreased with increasing levels of hydrogen (Fig. 8b), deuterium and helium admixtures. Note that although the leakage current eventually converges in Fig. 8b for all admixtures investigated, the voltage present across the discharge tube during this time is larger with added H2 (by some 30% for 10% Hz additive), representing an increased prepulse impedance (note the larger charging current for the final 10 ps; Fig. 8a). Negligible change was observed in the levels of leakage current through the discharge tube or current drawn through the charging resistor for argon and xenon admixtures. The observed increase in the charging resistor current, coupled with a decrease in the total leakage current through the discharge tube for the same period clearly indicates reduced tube conductivity during the interpulse period for Hz, D2 and He admixtures. In a further experiment a (2% hydrogen + 2% deuterium) /neon mixture was introduced into the CVL and the resulting buffer gas constituents sampled by the mass quadrupole. Significant quantities of D-H
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Fig. 8. (a) The current through the charging resistor during the interpulse period for 0, 1,2, 5 and 10% HJNe admixtures. The current through the charging resistor increased as the level of Hz additive was increased. (b) The leakage current through the discharge tube during the interpulse period for 0, 1, 2, 5 and 10% HJNe admixtures. The leakage current was observed to de crease (while the current through the charging resistor increased: Fig. 7a) with increasing H2 additive implying decreased tube conductivity.
molecules were detected relative to the amounts of H-H and D-D measured. It follows that a significant proportion of the molecular gases is dissociated in the active volume, thus atomic hydrogen and deuterium must be also be considered when determining the effects of Hz and Dz additives. The change in the spatial nature of laser output from an annular (Fig. 9a) to a gaussian-like profile (Fig. 9b) was easily observed in the far field for a 2%
hf. .I. Withford et al. / Optics Communications 110 (I 994) 699- 707
Fig. 9. (a) The far-field image of laser output with no hydrogen additive. (b) The far-field image of laser output with a 2% Hz/ Ne admixture. The heam profile is peaked on axis in contrast to that of the clean case (Fig. 8a) which is relatively annular.
hydrogen/neon admixture. Similar beam profile changes were observed for 1OWdeuterium and large helium admixtures but no such changes were apparent for argon or xenon.
4. Discussion of results Gas additives introduced into the buffer gas of CVLs may influence laser performance by improving the impedance matching of the discharge tube with its driving circuit or more fundamentally through
105
changes in the kinetic processes of the plasma. Improved impedance matching has been shown here to account for only minimal increases in laser output power (up to 5%). The bulk of the observed increases in output power ( > 30%) must therefore be attributed to changes to the plasma kinetics entailing modification of the electron behaviour during the excitation and interpulse phases of the laser duty cycle, and modification of the thermal conductivity of the plasma (and thus radial gas temperature [ 111 and density profiles). Molecular hydrogen and deuterium share a similar momentum transfer cross section [ 121 and electronic structure, only their masses differing, thus they have a similar influence on inelastic energy interactions in the laser active volume. However, up to twice the partial pressure of deuterium, relative to hydrogen, is required to induce comparable changes in the voltage/current characteristics (Fig. 6a, b) and output power behaviour (Figs. 4a, 5) of the laser. This observation implies elastic interactions involving the low mass gaseous additives are the primary mechanism contributing to enhanced laser performance. The rate of energy transfer from the electrons to a buffer gas particle (j) by elastic collisions is proportional to 2m, < Oj Y, > Nj/Mj where m, is the mass of an electron, Y, the electron velocity, Mj the mass, Oj the momentum transfer cross section and Nj the number of species j. In a pure neon buffer gas, neon atoms form the bulk of elastic energy exchanges with the electron gas due to the large neon population relative to copper (99% NNe compared to 1% No-: Fig. 10). Due to the large momentum transfer cross section [ 12,15 ] and low mass of molecular hydrogen and deuterium, relative to neon, only small admixtures of hydrogen (2%) and deuterium (4%) are required to dominate electron elastic energy transfer rates (Fig. 10). A proportion of dissociated hydrogen and deuterium is also expected, further increasing the rate of energy transfer due to the larger momentum transfer cross section [ 14 ] and lower mass relative to molecular hydrogen and deuterium. Larger admixtures of helium ( 1OI) are required to effect similar elastic energy transfer rates due to the larger mass and lower momentum transfer cross section of helium, relative to hydrogen (Fig. 10). An overall decrease in the discharge tube conduc-
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M.J. Withford et al. /Optics Communications 110 (1994) 699-707
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tivity during the interpulse period (Figs. Sa, b) implies reduced pre-pulse electron densities which also lead to increases in the voltage pulse (Fig. 6a) and decreases in the current pulse (Fig. 7)) during the excitation phase. Electron attachment is unlikely to be a significant mechanism contributing to reduced interpulse electron density as helium was observed to reduce the plasma tube conductivity similarly to hydrogen and deuterium. In addition, equal quantities of hydrogen and deuterium would be expected to induce comparable effects if electron attachment predominated in the interpulse period, their electronic structure being similar. On the contrary, approximately twice as much deuterium as hydrogen was required to effect comparable changes in the output power and voltage/current characteristics of the CVL. An increased rate of electron cooling during the interpulse period due to enhanced elastic energy transfer rates increases the deexcitation rate of excited copper states, increasing the repopulation of the copper ground state, an observation reported by Astadjov [ 41. As a result, some increase in the rate of e-/ Cu+ recombination is expected resulting in a reduced pre-pulse electron density. It remains unclear by what intermediate mechanism (or mechanisms) elastic interactions with Hz and D2 additive result in increased total output power and modification to the spatial nature of laser output
(Figs. 9a, b). Improved laser performance may occur as enhanced electron cooling facilitates repopulation of the copper ground state [4] from the metastable and other excited states, during the interpulse period. Alternatively, improved laser operation may result from modification of the radial profile of the applied electric field (as a reduced skin effect is experienced with a lower pre-pulse electron density [ 13 ] ) or from an altered rate of power deposition due to a reduced pre-pulse conductivity. The positive influence of hydrogen and deuterium admixtures on laser performance (via elastic effects ) , is counterbalanced by ionisation/excitation energy losses (inelastic effects) during the excitation phase. Inelastic electron energy losses to neon buffer gas during the excitation pulse are small due to the small cross section of neon. However, hydrogen and deuterium have a high momentum transfer cross section, relative to neon [ 131, therefore an increase in inelastic energy losses with H2 and Dz additive is expected. Ionisation/excitation energy losses are significant even for the small partial pressures of hydrogen and deuterium investigated. The peak output power (Figs. 4a, 5) for hydrogen and deuterium admixtures occurs for approximately 2% and 4% additive respectively. However, the output power for the 2% HJNe admixture is up to 1W larger than that for the 4% D2/ Ne admixture; this difference could be attributed to the larger ionisation/excitation energy loss, during the excitation phase, in the deuterium case. Further increases in the proportion of hydrogen/deuterium additive increase energy losses via inelastic interactions, contributing to the overall reduction in the laser output power. If the detrimental effect of ionisation/excitation energy losses outweigh the positive effects of elastic interactions for larger than optimal partial pressures of Hz and D2 then laser output power would be expected to decrease concurrently for similar H2 and Dz admixtures ( > 2%) and the total output power for deuterium admixtures should never exceed that for a similar hydrogen admixture. However, the output power observed for a 4W DJNe admixture does exceed that observed for a 4OhHJNe admixture and it is apparent other factors are at play. Large admixtures of helium were able to effect comparable changes to the voltage/current charac-
M.J. Withfordet al. /Optics Communications 110 (1994) 699-707
teristics of the laser to those induced by smaller admixtures of hydrogen and deuterium. However, only small increases in output power were observed with helium admixes, most likely due to the large inelastic energy losses associated with a large helium admixture. Similarly, decreases in laser output power likely result from ionisation/excitation energy losses to small partial pressures of argon and xenon. Conflicting opinion exists as to the role of the high thermal conductivity of hydrogen on laser petformante. Ref. [ 111 suggested that hydrogen additive modifies the radial gas temperature profile thereby improving CVL output power. In contrast, Astadjov et al. (from thermal modelling results [ 31) determined that only minimal change to the gas temperature profile results with similar partial pressures ( -2%) of hydrogen in a CuBr laser. Our observations of the ratio of 578 nm/total output (remaining fixed at 33% with respect to increasing percentages of added H,) also imply that little change to the gas temperature profile occurs, however, only direct measurements of the laser gas temperature can accurately determine whether the high thermal conductivity of hydrogen contributes to improved CVL performance.
5. Conclusion Admixtures of hydrogen and deuterium to the neon buffer gas were observed to significantly increase the output power of a CVL. Marginal increases in output power (~5%) are accounted for by improved impedance matching between the discharge tube and driving circuit, increasing power deposition in the plasma tube. However, the majority of the observed increase in laser output power is attributed primarily to enhanced electron cooling due to an increased rate
101
of elastic energy transfer in the interpulse period. Further experimentation is required to determine the exact mechanism by which elastic interactions may increase total output power and alter the spatial characteristics of laser output.
Acknowledgements The authors are grateful to Dr. R.J. Carman for helpful discussion regarding the kinetic issues of copper vapour lasers.
References [ 11 Z.G. Huang, K. Namba and F. Shimizu, Japan J. Appl. Phys. 25 (1986) 1677. [2] D.N. Astadjov, N.V. Sabotinov and N.K. Vuchkov, Optics Comm. 56 (1985) 279. [ 31 D.N. Astadjov, N.V. Sabotinov and N.K. Vuchkov, IEEE J. Quantum Electron. 24 (1988) 1927. [4] D.N. Astadjov, A.A. Isaev, G.G. Pen-ash, I.V. Ponomarev, N.V. Sabotinov and N.K. Vuchkov, IEEE J. Quantum Electron. 28 ( 1992) 1966. [ 5] N.V. Sabotinov, N.K. Vuchkov and D.N. Astadjov, Optics Comm. 95 (1993) 55. [ 6 ] Z.G. Huang, H.Y. Shan and H.H. Wang, Appl. Phys. B 44 (1987) 57. [ 71 S.N. Halliwell and C.E. Little, CLEO (1993) CThN6. [ 8 ] T. Yamanaka, E. Fujiwara et al., CLEO ( 1987) MG4. [9] M.A. Lesnoi, Sov. J. Quantum Electron. 14 (1984) 142. [lo] M.D.Ainsworth,D.J.W.Brown,D.W.CouttsandJ.A.Piper, Opt. Quantum Electron. 23 ( 1991) S539. [ 111 K. Hayashi, Y. Iseki, S. Suziki, I. Watanabe, E. Noda and 0. Morimiya, Jpn. J. Appl. Phys. 3 1 ( 1992) L1689. [ 121 A.G. Engelhardt and A.V. Phelps, Phys. Rev. 131 (1963) 2115. [ 13 ] R.J. Carman, private communication. [ 141 I. Bray, private communication. [ 151 E.W. McDaniel, Atomic collisions (Wiley, New York, 1989).
i!3
1 September 1994
.. mm _ __ 1 4.-
EISEVIER
OPTICS
COMMUNICATIONS Optics Communications 110 (1994) 699-707
Full length article
Investigation of the effects of hydrogen and deuterium on copper vapour laser performance Michael J. Withford, Daniel J.W. Brown, James A. Piper Centrefor Lasers and Applications,Macquarie University,NorthRyde, NSW 2109, Australia
Received 11 January 1994;revised manuscript received 10 April 1994
We report a detailed study of the effects on copper vapour laser performance of the addition of small partial pressures of gaseous hydrogen, deuterium and other trace gases to the neon buffer gas. Increases in output power by typically 30% are observed for a small-scale (6 W) device and the spatial characteristics of the output beam are significantly altered. Increased power deposition due to improved impedance matching is shownto be a minor effect. Instead, the changesto output power and the
voltage/current characteristicsduringthe excitation pulseand the interpulseperiod are attributed primarilyto enhancedplasma coolingby elastic interactionsresultingin reduced prepulseelectron densities. 1. Introduction Improved performance of elemental copper vapour lasers (CVLs) with small admixtures of hydrogen to the neon buffer gas has been reported by a number of researchers in recent years. Huang et al. [ 1 ] first reported increases in output power of up to 50% and a 60% improvement in efficiency with a 0.40.7 torr (0.6-11) admixture of hydrogen to the neon buffer gas in a medium bore (32 mm) CVL. Changes in the spatial protile of the laser output intensity were also noted, with the typically annular profile becoming gaussian-like with increasing levels of hydrogen. Enhanced laser performance with hydrogen additive has also been reported for CuBr [ 2-5 ] lasers, CuCl lasers [ 5 1, Au vapour lasers [ 6 ] and Ba lasers [ 7 1. A number of mechanisms by which hydrogen improves output power and spatial profile have been suggested. Huang et al. [ 1 ] proposed that the relatively large elastic collision cross section of hydrogen effectively cools electrons in the interpulse period, increasing the rate of deexcitation of the copper
*D3,2,5,2metastable levels. In a study of hydrogen admixtures to a CuBr laser, Astadjov et al. [ 2-41 suggested that improved laser performance derived from a decreased pre-pulse electron density, possibly due to electron attachment to hydrogen. Further, Astadjov et al. [4] reported a reduced depopulation of the copper 2S,,2 ground state during the excitation pulse, along with more rapid repopulation of the copper ground state (20 x faster) in the interpulse period when hydrogen was present. Small increases in output power with helium admixtures to neon buffer gas have also been reported. Yamanaka et al. [ 8 ] noted small increases in output power in a large bore CVL for a 5Ohadmixture of helium, an increase they attributed to enhancement of 5 10 nm emission solely in the central portions of the output beam profile. Reduction of the axial gas temperature was suggested as a mechanism due to the high thermal conductivity of helium. Lesnoi [ 91 reported no change in the discharge characteristics of a CVL when argon, krypton and xenon were admixed with the neon buffer gas. In con-
0030-4018/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDiOO30-4018(94)00276-Z
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M.J. Withford et al. /Optics Communications 110 (1994) 699-707
trast, the discharge current increased in duration and decreased in amplitude with helium additive. Lesnoi attributed this to large electron energy losses to helium due to its low mass yet relatively high elastic collision cross section. Further, Ref. [ 9 ] notes decreases in CVL output power as the partial pressure of added argon, krypton and xenon increases, reportedly due to inelastic electron energy losses with those gases. Apart from their influence on the kinetic processes in the laser medium, trace additives may simply alter the impedance matching between the laser discharge and its driving circuit, resulting in changed power deposition into the laser discharge tube and consequently changes in laser output power. Clearly there remains considerable ambiguity as to the mechanism, or mechanisms, by which hydrogen additive influences CVL performance which it is necessary to resolve in order that the affects on particular devices can be predicted. In this paper we present the results of a detailed study regarding the comparative effects of hydrogen and deuterium additive on the voltage/current characteristics and output power of a CVL. The difference in elastic energy transfer rates for these two gases gives a strong indication as to the primary mechanism by which the addition of hydrogen and deuterium to the laser discharge influences laser performance. The significance of improved impedance matching relative to that of volume kinetic processes in their effects on laser performance has also been investigated for various trace gaseous additives (Ha, Dz, He,AandXe).
2. Experimental method The CVL used in this investigation was a smallscale device (plasma tube dimension 18 mm dia. X 7 50 mm long giving an active volume of 190 cm2 ) coupled to a ultra-high vacuum system (Fig. 1) incorporating a turbomolecular pump (Alcatel 5080) and mass quadrupole spectrometer (Inficon Quadrex 100). The low base pressure ( c lo-’ torr) available with this system minim&d the effects of residual gases trapped in the insulating substrate (Zircar AL-30 AAH) while the mass quadrupole spectrometer allowed real-time analysis of the composition of the buffer gas. The various partial pressures of hydro-
gen were mixed with neon in a chamber separated from the active volume, and the resulting admixtures were then introduced at the cathode end of the laser at low flow rates ( N 3 cm3/min) with a set buffer gas pressure of 25 torr (chosen because it lay in a region where laser performance was optimized). In all cases, ultra-high purity gases (99.999% pure) were used. The CVL discharge circuit involved a standard thyratron-switched (EG&G 3001) pulse charging circuit (Fig. 2). The storage capacitor C, (3 X 2 nF Condenser Products TSG 202 24M) was resonantly charged through the saturable inductor L and charging resistor R ( 100 R wirewound). The excitation voltage pulse was monitored using a high voltage probe (Tektronix P6015) across the plasma tube and peaking capacitor C, (2 x 1 nF Condenser Products TSG 102 24M). Current through the laser discharge tube was monitored during the excitation phase (the discharge current) using a loop current probe (Pearson 11OA) . Identical current probes were also used to monitor both the leakage current through the discharge tube during the interpulse period (at point A; Fig. 2) and the charging current through resistor R (point B; Fig. 2) as the storage capacitors recharge during the interpulse period. The oscilloscope traces (Tektronix 2245a 100 MHz) of the voltage and current pulses were digit&d using an oscilloscope mounted video camera (Tektronix C 100 1) and its associated software. A flat/flat laser resonator was used incorporating a high reflector and a 4% output coupler. The total laser output power and the spatially averaged 578 nm component of total output power (separated by dichroic beam-splitters) were measured using a calorimetric power meter (Scientech 36000 1). The temperature of the silica vacuum tube was monitored with a chromel-alumel thermocouple mounted on its external surface. The discharge tube wall temperature was also measured using an optical pyrometer (Minolta TR 630). The change in water coolant temperature through the thyratron was monitored by alcohol thermometers mounted on the water lines, offering qualitative information regarding the deposited power in the thyratron when the laser was operated with different gas mixtures. The CVL was operated at a pulse repetition frequency of 6.0 kHz and a supply voltage of 6.6 kV,
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M.J. Withford et al. /Optics Communications 110 (1994) 699-707 Laser
Tube
q
UHV
q
Leak
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Valves
Fig. 1. Schematic of the laser assembly incorporating an ultra-high vacuum system with on-line mass spectrometric and laser beam analyser diagnostics.
1650
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Fig. 2. Schematic of the CVL pulse charging circuit. Current was monitored at points A and B during the interpulse period (direction of current flow shown).
1500
:
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slightly below the optimal supply voltage of 6.9 kV (Fig. 3). During previous investigations, the power deposition in the discharge tube was observed to increase (evident from observed increases in the discharge tube wall temperature) as the various trace gaseous additives produced small changes in the impedance matching of the laser. As a result, a near optimal supply voltage (6.6 kV) was selected because it lay in an operating regime where the laser output power was relatively insensitive to increases in the discharge tube wall temperature (Fig. 3). An investigation of the effects of H2 and D2 additive was also undertaken at a second lower supply voltage (6.2 kV). Care was taken to remove residual hydrogen and deuterium between experimental runs to allow accurate assessment of the various admixtures. It was found by mass spectroscopy that the fibrous alumina insulator used in conventional CVL design was capable of absorbing considerable amounts of hydrogen when cooling, this residual hydrogen outgassing
1400 6.0
6.5 Supply
Voltage
7.0 (kV)
Fig. 3. Laser output power and discharge tube wall temperature with respect to the supply voltage for the CVL operating in pure neon buffer gas. The optimum output power resulted from a sup ply voltage of _ 6.9 kV.
at lasing temperatures. Several hours of cycling the laser in pure neon were required to reduce the residual hydrogen to negligible partial pressures of less than 10-7 torr. The spatial characteristics of the beam profile were monitored using a telescope arrangement (200 mm and 60 mm AR coated convex lens) to image the laser output onto the CCD array of a laser beam analyser (Spiricon LBA- 100 ) .
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h4.J. Withford et al. /Optics Communications 110 (1994) 699-707
3. Results Fig. 4a shows the total laser output power (green & yellow) for different quantities of Hz, D1, He, A and Xe added to the neon buffer at a supply voltage of 6.6 kV. An increase in output power of over 30°h was noted for a 2% hydrogen/neon admixture (Fig. 4a) with an associated increase in the discharge tube wall temperature from _ 1550°C to 1620°C (Fig. 4b). For all admixtures investigated the proportion of yellow output remained 33% ( z!z1%) of the total laser 10,
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output. Significant increases in output power were also observed for added deuterium, and marginal increases for a 5Ohhelium/neon admixture. Addition of argon and xenon reduced the output of the CVL. Maximum increases in output power were observed for a 2% Hz and -4W DJNe admixtures. Above these levels of Hz and Dz the output power falls rapidly. At first glance, this decrease in output power may be attributed to overheating (i,e. beyond 1620°C) due to increased power deposition in the plasma tube, however, it is not immediately obvious why the laser can tolerate twice as much Dz as Hz additive. In fact, repeating the experiment at a lower supply voltage of 6.2 kV (Fig. 5: i.e. at lower wall temperatures consistent with low power deposition) laser power at first increases (by 38%) for 2% H2 additive but then decreases even though the wall temperature remains well below the optimum temperature. Moreover, the optimum concentrations for H2 and D2 additive remain approximately in the ratio 1:2. The deposited power in the thyratron was observed to decrease (by up to 14Ohfor a 5% H,/Ne admixture) with increasing levels of trace gaseous additive, evident from a drop in the temperature gradient of the coolant between the in-flowing and out-flowing ports of the thyratron. This observation is consistent
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Fig. 4. (a) Laser output power with respect to various partial pressures of Hx, D,, He, Ar and Xe gases added to the neon buffer gas of a CVL operating at 6.0 kHz pulse repetition frequency and 6.6 kV supply voltage. (b) Discharge tube wall temperature relative to partial pressures of Hr. Dx, He, Ar and Xe gases added to the neon buffer gas of a CVL operating at 6.0 kHz pulse repetition frequency and 6.6 kV supply voltage.
I
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/
2
4
6
a
10
% Additive/Neon
Fig. 5. Laser output power at a supply voltage of 6.2 kV for various admixtures of H,/Ne and DJNe. A maximum output power of 2.8 W was observed for a 2% Hr/Ne admixture. In contrast, a maxima in the output power behaviour for deuterium additive was observed for a 5% Dr/Ne admixture.
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M.J. Withfordet al. /Optics Communications 110 (I 994) 699- 707
with improved impedance matching of the laser with its driving circuit, therefore increasing power coupling to the discharge tube. Increased power coupling (implied by an increased tube wall temperature) for Dz, He, A and Xe additives (Fig. 3b) are also attributed to improved impedance matching. The general effects of improved impedance matching by H2 additive were approximated for a pure neon buffer gas by supplementary heating of the discharge tube [lo] and alternatively increasing the supply voltage (thereby increasing the peak amplitude of the discharge pulse). The maximum in the output power occurred for a 2% hydrogen/neon admixture for which the wall temperature was 1620°C (compared to 1550°C for neon only). The same wall temperature of 1620°C (and therefore similar copper vapour pressure) was obtained for pure neon buffer gas by supplementary heating of the discharge tube while maintaining the same stored input power as for the H,/Ne admixture. An increase of only 0.3 W was observed, falling well below the increases of several watts observed for admixed hydrogen. Increasing the input power (supply voltage) to produce a wall temperature of 1620°C also resulted in a minimal output power increase of 0.3 W even though the peak amplitude of the excitation voltage pulse was similar to that observed for a 2W HJNe admixture. It is therefore clear that the improved CVL performance observed for hydrogen additives is due predominantly to mechanisms other than one where the overall copper density is increased as the wall temperature increases in response to increased input power. Increases in the peak amplitude of the discharge voltage (Fig. 6a), similar to those reported by Astadjov et al. [ 2,3 1, and decreases in the peak amplitude of the discharge current (Fig. 7) were observed for hydrogen added to the CVL. In light of the relatively minor decreases in the peak amplitude of the discharge current observed with 1Olband 2% HJNe admixtures, increasing the supply voltage in pure neon buffer gas (thereby increasing the peak amplitude of the discharge voltage pulse) is a reasonable approximation of the effects of small partial pressures of H2 on impedance matching (as described above). Increases in the peak amplitude of the discharge voltage were also observed for deuterium (Fig. 6b) and helium (Fig. 6c), though larger proportions of deuterium (about 10%) and helium (approx. 50%)
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Fig. 6. (a)-(d) Discharge voltage pulses for various Hz, Db He and Ar/Ne admixtures. Comparable increases in the peak amplitude of the voltage pulse were observed for 5% H,/Ne, 10% D2/ Ne and 50% He/Ne admixtures. No significant increases in the peak amplitude of the voltage pulse were observed for additives of argon (shown) or xenon.
were required to cause changes comparable to those induced by a 5Wmix of hydrogen. An associated decrease in the peak amplitude of the discharge tube current was also observed for deuterium and helium though again larger partial pressures were required to give similar effects to those observed for hydrogen. The changes to the voltage pulse and the discharge tube current, during the excitation phase, were negligible for argon (voltage pulse: Fig. 6d) and xenon admixtures.
M.J. Withfod et al. / Optics Communications I10 (1994)
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Fig. 7. The current through the discharge tube during the excitation phase with respect to pure neon and various Hs/Ne admixtures. Small decreases in the peak amplitude of the excitation current pulse were observed for 1% and 2% H2 additive. Relatively large decreases in the peak amplitude of the current pulse were noted for > 5% H2 additive.
Increases in the current through the charging resistor during the interpulse period (i.e. as the storage capacitors recharge) were observed with increasing levels of hydrogen (Fig. 8a), deuterium and helium admixed to the neon buffer. In contrast, the total leakage current through the discharge tube, during the same period, decreased with increasing levels of hydrogen (Fig. 8b), deuterium and helium admixtures. Note that although the leakage current eventually converges in Fig. 8b for all admixtures investigated, the voltage present across the discharge tube during this time is larger with added H2 (by some 30% for 10% Hz additive), representing an increased prepulse impedance (note the larger charging current for the final 10 ps; Fig. 8a). Negligible change was observed in the levels of leakage current through the discharge tube or current drawn through the charging resistor for argon and xenon admixtures. The observed increase in the charging resistor current, coupled with a decrease in the total leakage current through the discharge tube for the same period clearly indicates reduced tube conductivity during the interpulse period for Hz, D2 and He admixtures. In a further experiment a (2% hydrogen + 2% deuterium) /neon mixture was introduced into the CVL and the resulting buffer gas constituents sampled by the mass quadrupole. Significant quantities of D-H
1
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b
c
I
I
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20
30
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Fig. 8. (a) The current through the charging resistor during the interpulse period for 0, 1,2, 5 and 10% HJNe admixtures. The current through the charging resistor increased as the level of Hz additive was increased. (b) The leakage current through the discharge tube during the interpulse period for 0, 1, 2, 5 and 10% HJNe admixtures. The leakage current was observed to de crease (while the current through the charging resistor increased: Fig. 7a) with increasing H2 additive implying decreased tube conductivity.
molecules were detected relative to the amounts of H-H and D-D measured. It follows that a significant proportion of the molecular gases is dissociated in the active volume, thus atomic hydrogen and deuterium must be also be considered when determining the effects of Hz and Dz additives. The change in the spatial nature of laser output from an annular (Fig. 9a) to a gaussian-like profile (Fig. 9b) was easily observed in the far field for a 2%
hf. .I. Withford et al. / Optics Communications 110 (I 994) 699- 707
Fig. 9. (a) The far-field image of laser output with no hydrogen additive. (b) The far-field image of laser output with a 2% Hz/ Ne admixture. The heam profile is peaked on axis in contrast to that of the clean case (Fig. 8a) which is relatively annular.
hydrogen/neon admixture. Similar beam profile changes were observed for 1OWdeuterium and large helium admixtures but no such changes were apparent for argon or xenon.
4. Discussion of results Gas additives introduced into the buffer gas of CVLs may influence laser performance by improving the impedance matching of the discharge tube with its driving circuit or more fundamentally through
105
changes in the kinetic processes of the plasma. Improved impedance matching has been shown here to account for only minimal increases in laser output power (up to 5%). The bulk of the observed increases in output power ( > 30%) must therefore be attributed to changes to the plasma kinetics entailing modification of the electron behaviour during the excitation and interpulse phases of the laser duty cycle, and modification of the thermal conductivity of the plasma (and thus radial gas temperature [ 111 and density profiles). Molecular hydrogen and deuterium share a similar momentum transfer cross section [ 121 and electronic structure, only their masses differing, thus they have a similar influence on inelastic energy interactions in the laser active volume. However, up to twice the partial pressure of deuterium, relative to hydrogen, is required to induce comparable changes in the voltage/current characteristics (Fig. 6a, b) and output power behaviour (Figs. 4a, 5) of the laser. This observation implies elastic interactions involving the low mass gaseous additives are the primary mechanism contributing to enhanced laser performance. The rate of energy transfer from the electrons to a buffer gas particle (j) by elastic collisions is proportional to 2m, < Oj Y, > Nj/Mj where m, is the mass of an electron, Y, the electron velocity, Mj the mass, Oj the momentum transfer cross section and Nj the number of species j. In a pure neon buffer gas, neon atoms form the bulk of elastic energy exchanges with the electron gas due to the large neon population relative to copper (99% NNe compared to 1% No-: Fig. 10). Due to the large momentum transfer cross section [ 12,15 ] and low mass of molecular hydrogen and deuterium, relative to neon, only small admixtures of hydrogen (2%) and deuterium (4%) are required to dominate electron elastic energy transfer rates (Fig. 10). A proportion of dissociated hydrogen and deuterium is also expected, further increasing the rate of energy transfer due to the larger momentum transfer cross section [ 14 ] and lower mass relative to molecular hydrogen and deuterium. Larger admixtures of helium ( 1OI) are required to effect similar elastic energy transfer rates due to the larger mass and lower momentum transfer cross section of helium, relative to hydrogen (Fig. 10). An overall decrease in the discharge tube conduc-
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M.J. Withford et al. /Optics Communications 110 (1994) 699-707
-
NE
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tivity during the interpulse period (Figs. Sa, b) implies reduced pre-pulse electron densities which also lead to increases in the voltage pulse (Fig. 6a) and decreases in the current pulse (Fig. 7)) during the excitation phase. Electron attachment is unlikely to be a significant mechanism contributing to reduced interpulse electron density as helium was observed to reduce the plasma tube conductivity similarly to hydrogen and deuterium. In addition, equal quantities of hydrogen and deuterium would be expected to induce comparable effects if electron attachment predominated in the interpulse period, their electronic structure being similar. On the contrary, approximately twice as much deuterium as hydrogen was required to effect comparable changes in the output power and voltage/current characteristics of the CVL. An increased rate of electron cooling during the interpulse period due to enhanced elastic energy transfer rates increases the deexcitation rate of excited copper states, increasing the repopulation of the copper ground state, an observation reported by Astadjov [ 41. As a result, some increase in the rate of e-/ Cu+ recombination is expected resulting in a reduced pre-pulse electron density. It remains unclear by what intermediate mechanism (or mechanisms) elastic interactions with Hz and D2 additive result in increased total output power and modification to the spatial nature of laser output
(Figs. 9a, b). Improved laser performance may occur as enhanced electron cooling facilitates repopulation of the copper ground state [4] from the metastable and other excited states, during the interpulse period. Alternatively, improved laser operation may result from modification of the radial profile of the applied electric field (as a reduced skin effect is experienced with a lower pre-pulse electron density [ 13 ] ) or from an altered rate of power deposition due to a reduced pre-pulse conductivity. The positive influence of hydrogen and deuterium admixtures on laser performance (via elastic effects ) , is counterbalanced by ionisation/excitation energy losses (inelastic effects) during the excitation phase. Inelastic electron energy losses to neon buffer gas during the excitation pulse are small due to the small cross section of neon. However, hydrogen and deuterium have a high momentum transfer cross section, relative to neon [ 131, therefore an increase in inelastic energy losses with H2 and Dz additive is expected. Ionisation/excitation energy losses are significant even for the small partial pressures of hydrogen and deuterium investigated. The peak output power (Figs. 4a, 5) for hydrogen and deuterium admixtures occurs for approximately 2% and 4% additive respectively. However, the output power for the 2% HJNe admixture is up to 1W larger than that for the 4% D2/ Ne admixture; this difference could be attributed to the larger ionisation/excitation energy loss, during the excitation phase, in the deuterium case. Further increases in the proportion of hydrogen/deuterium additive increase energy losses via inelastic interactions, contributing to the overall reduction in the laser output power. If the detrimental effect of ionisation/excitation energy losses outweigh the positive effects of elastic interactions for larger than optimal partial pressures of Hz and D2 then laser output power would be expected to decrease concurrently for similar H2 and Dz admixtures ( > 2%) and the total output power for deuterium admixtures should never exceed that for a similar hydrogen admixture. However, the output power observed for a 4W DJNe admixture does exceed that observed for a 4OhHJNe admixture and it is apparent other factors are at play. Large admixtures of helium were able to effect comparable changes to the voltage/current charac-
M.J. Withfordet al. /Optics Communications 110 (1994) 699-707
teristics of the laser to those induced by smaller admixtures of hydrogen and deuterium. However, only small increases in output power were observed with helium admixes, most likely due to the large inelastic energy losses associated with a large helium admixture. Similarly, decreases in laser output power likely result from ionisation/excitation energy losses to small partial pressures of argon and xenon. Conflicting opinion exists as to the role of the high thermal conductivity of hydrogen on laser petformante. Ref. [ 111 suggested that hydrogen additive modifies the radial gas temperature profile thereby improving CVL output power. In contrast, Astadjov et al. (from thermal modelling results [ 31) determined that only minimal change to the gas temperature profile results with similar partial pressures ( -2%) of hydrogen in a CuBr laser. Our observations of the ratio of 578 nm/total output (remaining fixed at 33% with respect to increasing percentages of added H,) also imply that little change to the gas temperature profile occurs, however, only direct measurements of the laser gas temperature can accurately determine whether the high thermal conductivity of hydrogen contributes to improved CVL performance.
5. Conclusion Admixtures of hydrogen and deuterium to the neon buffer gas were observed to significantly increase the output power of a CVL. Marginal increases in output power (~5%) are accounted for by improved impedance matching between the discharge tube and driving circuit, increasing power deposition in the plasma tube. However, the majority of the observed increase in laser output power is attributed primarily to enhanced electron cooling due to an increased rate
101
of elastic energy transfer in the interpulse period. Further experimentation is required to determine the exact mechanism by which elastic interactions may increase total output power and alter the spatial characteristics of laser output.
Acknowledgements The authors are grateful to Dr. R.J. Carman for helpful discussion regarding the kinetic issues of copper vapour lasers.
References [ 11 Z.G. Huang, K. Namba and F. Shimizu, Japan J. Appl. Phys. 25 (1986) 1677. [2] D.N. Astadjov, N.V. Sabotinov and N.K. Vuchkov, Optics Comm. 56 (1985) 279. [ 31 D.N. Astadjov, N.V. Sabotinov and N.K. Vuchkov, IEEE J. Quantum Electron. 24 (1988) 1927. [4] D.N. Astadjov, A.A. Isaev, G.G. Pen-ash, I.V. Ponomarev, N.V. Sabotinov and N.K. Vuchkov, IEEE J. Quantum Electron. 28 ( 1992) 1966. [ 5] N.V. Sabotinov, N.K. Vuchkov and D.N. Astadjov, Optics Comm. 95 (1993) 55. [ 6 ] Z.G. Huang, H.Y. Shan and H.H. Wang, Appl. Phys. B 44 (1987) 57. [ 71 S.N. Halliwell and C.E. Little, CLEO (1993) CThN6. [ 8 ] T. Yamanaka, E. Fujiwara et al., CLEO ( 1987) MG4. [9] M.A. Lesnoi, Sov. J. Quantum Electron. 14 (1984) 142. [lo] M.D.Ainsworth,D.J.W.Brown,D.W.CouttsandJ.A.Piper, Opt. Quantum Electron. 23 ( 1991) S539. [ 111 K. Hayashi, Y. Iseki, S. Suziki, I. Watanabe, E. Noda and 0. Morimiya, Jpn. J. Appl. Phys. 3 1 ( 1992) L1689. [ 121 A.G. Engelhardt and A.V. Phelps, Phys. Rev. 131 (1963) 2115. [ 13 ] R.J. Carman, private communication. [ 141 I. Bray, private communication. [ 151 E.W. McDaniel, Atomic collisions (Wiley, New York, 1989).