HIGH POWER AND SINGLE LINE OPERATIONS OF AN HBr CHEMICAL LASER

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HIGH POWER AND SINGLE LINE OPERATIONS

OF AN HBr CHEMICAL LASER

K. Toyoda, H. Osada, S. Namba

To cite this version:

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' H I G H POWER AND S I N G L E L I N E OPERATIONS OF AN - H k - C H E M I C A L LASER K. Toyoda, H. ~sada* and S. Namba

Riken, the I n s t i t u t e of Physical and Chemical Research, 2-1 Hirosawa, Wako-Shi, Saitama 351, Japan.

R6sumd.- Un laser chimique 1 HBr (Bromure dlhydrogPne) de grande puissance a dtd consu et construit pour assurer le maximum d'dnergie de sortie et de puissance de pointe de 1,4J et 1MW respectivement, dans les op6rations B plusieurs lignes. Le tuyau du laser a 6 mOtres de longueur et emploie une ddcharge en gerbe B excitation transversalle de type B pointe entre les anotes h barrette et les cathodes h 7980 pointes. Le volume de ddcharge effectif est estimd 1 2 litres. Par le fait d'ajouter de llargon gazeux tampon, 1'Energie du laser devient h peu prOs deux fois plus que celle obtenue en Br2 + H2 seulement.

Pour les op6rations B une seule ligne avec des rdseaux de diffraction de type Littrow, 29 lignes de PlO(l)

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PlO(lO), PZ1(1)

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PZ1(5), P21(7)

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P21(11) et P32(4) - P32(ll) ont 6t6 observses dans les longueurs d'onde allant de 3,934 2 4,586 pm. Les sept d'entre elles, ~ ~ ~ ( l ) , Plof2), P10(3), Ppl(l), P21(2), P21(3) et P32(ll) ont St6 observges pour la premi2re fois.

Ce laser peut servir d'instrument trSs efficace pour Gtudier sur la laser photochimie des mol6cules contenant -OD, -CD et -CN.

Abstract.- A high power HBr chemical laser was designed and constructed to give the maximum output energy and peak power of 1.45 and IMW, respectively, in multiline operation. The laser tube is 6m long employing a pin-type TE shower discharge between bar anodes and 8000 pin cathodes. The effective discharge volume is estimated to be two liters. With addition of Ar buffer gas, the laser energy became almost twice as much as that in Br2 + H2 mixture.

For single line operation with a Littrow mounted grating, twenty-nine lines of P l O ( l ) - PdlO), P21(1)

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PZ1(5), ~ ~ ~ ( 7 )

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~ ~ ~ ( 1 1 ) and P32(3)

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P32(Il) were observed in the wavelengths ranging from 3.934~1~ 4.586~111. Seven lines of them, PI0(1), P10(2), P10(3), P21(1). P21!2), P21(3) and P32(11) were observed for the first time. This laser can be powerful tool for studylng laser photo-chemistry of mole- cules containing -OD, -CD and -CN bonds.

1. Introduction

The laser action and the spectral data large4) that the laser can be properly

of HBr chemical laser using an electrically used for laser isotope separation of hydro-

pulsed discharge were reported in 1967 by gens.

T. F. ~eutschl)

.

The transverse discharge In addition, it is well known that

co2

excitation of HBr laser was demonstrated by and N 2 0 have strong absorption in 4pm re-

O.R. Wood and T.Y. Chang in 1972~). The gion. An HBr chemical laser has been used

operation of transverse excitation at high as an effective source for exciting the CO,

pressure made it possible to obtain high

.

and N 2 0 lasers5r6r7r8r9).

output power3) of chemical laser. In considering these applications in

This laser is of very interests because mind, we have made paremetric study of TE

its oscillating wavelengths are very close discharge HBr laser to obtain high output

to those of the absorption bands of 0-D, C- energy. And also, we have obtained spec-

D and C-N stretching modes of molecular tral distribution in both of multiline and

vibrations. Therefore, this laser can be single line operations. The laser tube had

a powerful source for studying laser photo- six pin-bar electrodes pair of each 90cm

chemistry of moldhules containing these long with transverse discharge excitation.

bands. Especially, the isotope shifts of The detailed parametric study of the TE

the absorption bands of 0-H and 0-D is so discharge HBX laser has led to reliable

.

~ * ~ e p r t ~ ~ - : ~ Hoperation with a maximum output of 1.4 @ . V ~ j J. ,

Kyoto 601, Japan.

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C9-252 JOURNAL DE PHYSIQUE

2. Experimental setup and procedure

The experimental setup is schematically shown in Fig.1, which is consisted of a laser tube with pin-bar electrodes of

transverse discharge, laser optical cavity,

gas handling system, H.V. power supply and

pulse generator and measuring equipment for laser output energy.

a) The laser tube Fig.2 shows the cross- sectional view of laser tube. It was made of licite in frame and was constructed in connecting six discharge tubes of each one meter in length.

The effective discharge length was ap- prox. 6m. Each descharge tube has 1300 pin as cathodes so that the small sparks dis- tribute uniformly in the wide discharge tubs. The cathodes and anodes were made of brass, which were Cr-Plated so as to avoid corrosion in contacting with reactive gases used. The cathode pins were arranged 14 rows in radial and 90 columns along the optical axis. The separation of cathode and anode was 3.5cm and width, 3.9cm. b) The laser cavity For multiline operations, the cavity consisted of a total reflector of 30m radius of curvature and an output coupler of Ge flat mirror. The re- flectivity of output mirror was chosen from 50, 75 and 90%. For single line operation,

UqW N, Trap

Fig. 1 Experimental setup. I23

the total reflector was replaced by Littrow mounted gratings, which are PTR standard 402 (blazed at 5.4pm, 97% reflectance at 3.5pm-5.5pm, 50rnm in diameter) and special

order PL 402 (blazed at 5 . 4 ~ ~ ~ 20m radius

of curvature, 97% reflectance at 3.5~~-5.5 pm, 50x75mm2 in size). The output coupler used for single line operation was a Ge concave mirror of 20m radius curvature with

80% reflectance.

c) Gas handling system In gas mixing system, where the corrosive compounds like Br2 and HBr flowed, all tubes and valves were made of teflon and glass. Bromine gas evaporated from reservoir was mixed with

flowing H2, I!e and Ar gases. After drift-

ing 4m teflon pipe, the mixed gas was led into the one end of the iaser tube. From the other end of the laser tube was evacu-

ated by a rotary pump. The exhausted gas

was passed through the molecular sheaves and liquid N2 trap to remove the remaining Br2.

d) Output measurements The output energy

was measured by a power meter (Scientec 3 6 -

0001). The'pulsed waveform and the spec- troscopic data were taken by the use of a

Ge:Au IR detector (SBRC, rise time < loons)

and a 25cm monochromator (Jasco CT25N)

.

I T 1 T I

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\ ANODE / LUClTE LUCITE

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a) Effect of Gas mixture Fig.3 shows the changes of the output energy in the multi line operations as the functions of the Hz pressure for various Br2 gas pressures. The reflectance of the output mirror was 75% and the charging voltage was 30kV. The

electrical energy in the capacitor of 0.24

pF was 108J. The output energy varied with the flowing H2 pressure for constant Br2 pressure, and had a maximum for each Br2 pressure. For low Br2 pressure, the output varied rather insensitively with the vari- ation of the H2 pressure and vice versa.

The output energy is optimized around 40

Torr of H2 for the Br2 pressure of 7 Torr.

Ar, He PRESSURE (Torr )

-

7

-

.

i2

09-

9

U I- 07- I) 0

Fig.4 Dependency of output energy upon the buffer gas pressure.

A Ar

0 He

Reflectance 75%

b) Foreign gas additions EIe or Ar gas addition to Br2+H2 mixture enhanced the out-

put energy as are shown in Fig.4. In the

case of He addition, the maximum output ob-

tained was about. 680mJ at Br2+H2+He=5+27+58

Torr, which is 1.15 times as high as that

O.30 40 50 60 70 80

The maximum energy obtained for the gas of no buffer gas. Yhen Ar gas was added at

mixture of Br2+H2=7+40 Torr was 585mJ. a pressure of 57 Torr, the output energy of

However, in the variations of Br2 pressure 980mJ was obtained, that is 1.68 times as

from 2.5 Torr to 8.4 Torr, the output compared to the case of no guffer gas.

energy can exceed 500mJ for each optimum C) Mirror reflectance Fig.5 illustrates

condition. the dependency of the output energy on the

reflectance of the output coupler. Among three mirrors, the 75% reflecting mirror gave the maximum output.

Br2 PRESWRE ( Torr )

0 , L "20 " . '&O 60 80 100 120 J

HI PRESSURE (Torr)

Fig.3 Output energy of multiline HBr laser as func- tions of partial pressure of Hz with various

B2 pressure.

REFLECTANCE ( % )

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C9-254 JOURNAL DE PHYSIQUE 0 Br2 5.0 Twr H2 27 Torr Ar 58 Torr

*

Br250 Hz 27 He 57.5 o Br25.0 H2 27

'"-

_{REFLECTANCE 75%}

-

B

- W

5

1.0

-

0.6

YO

20 30 40 50 VOLTAGE (KV)

Fig.6 Output energy vs. charging voltage. d) Voltage dependency With increasing the applied voltage to the discharge tube, the output energy monotonously increased within the experimental condition as shown in Fig. 6. The Ar addition has given the highest output energy which amounted to l.4J at the applied voltage of 46kV. In this case, the conversion efficiency was calculated to

Table 1. Laser line pio (4) p l o (5) P1o (6) Pl0 (7) P1o (8) P21(4) p2 1 (5) P,, (5) P32 (6) ~ 3(7)' 2 P4 3 (5) P43 (6) P43 (7)

output energy of each line of multiline

HBr laser. Gas mixture: Br2+H2=5+27 Torr.

He pressure: 6 0 Torr. Ar pressure: 57 Torr. Charging voltage: 30kV.

NO buffer He buffer Ar buffer

(mJ) (mJ) (mJ) 32 3 4 18 8 3 70 9 8 29 17 39 4 5 17 0 0 7 120 118 127 118 119 170 24 138 87 3 5 101 222 2 3 2 0 146 11 8 0 3 1 6 0 39 7 7 19

e) Oscillating lines in multiline operation The oscillating lines and their output energy are listed in Table 1. The assign- ments of lasing lines referred to reference LO). The output energy of individual line was estimated from the area od the waveform detected. by Ge:Au detector displayed on CRT. The number of simultaneously.oscillating lines was 12 and did not change significantly when the buffer gas was added. However, the energy of a line increased by the additions of buffer gases. The highest energy in a single line is 222

mJ of P 3 2 (6).

£1 Single line operation The PBr laser was

operated in the line-selected lasing in which the gratings were used as the total reflector. The output energy of individual line is listed in Table 2.

Table 2 Output energy of single line HBr laser.

Gas mixture: Br2+H2+Ar=5+27+57 Torr. Charging voltage: 30kV. Grating: 50mm

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a) The output energy

The HBr laser emitted the output energy of 0.58J per pulse for the optimum gas mixture of Br2+H2=7+40 Toor. The output energy increases with additions of He or Ar. With Ar addition, the output amounts to 1.

1.4J at the applied voltage of 48kV. The

output is the monotonously increasing functions of the applied voltage within our experimental condition. However in the case of He addition and no buffer gas added, the output energy does not increase

remarkably at the voltage over 3 5 k V . Then,

the Ar addition is found to be most effective to enhance the laser output.

b) The optimum gas mixture

As mentioned above, the maximum output energy is obtained for the Hz-rich gas mixture in which Br2 content is 14.9%. The optimum Br2 content is an increasing function of Br2 partial pressure as shown in Fig. 7.

The typical waveforms of laser output pulse are shown in Fig.8 for three cases of different reflectance of output mirror. In the case of low reflectance of 50% or 75%, the output pulse has bumps or splits into multiple pesks. However, in the case

of 99% mirror, the pulse becomes smooth and

is composed of a single spike.

Fig.9 show the waveforms of typical lines of v=1+0, v=2+1 and v=3+2 transi-

tions, respectively, for 75% output mirror.

The v=1+0 lasing line has no bumps, however, the lasing lines from higher vibra- tional levels have two or three peaks. The splitting of pulse would be caused by the. temporal change of the population inversion followed by the cascade lasing.

Br, PRESSURE ( Torr)

Fig.7 Optimum ratio of Br2/Br2+H2 vs. Brp pressure.

Fig.8 Pulsed waveforms of laser output for

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JOURNAL DE PHYSIQUE

enough. Then, the two lines happen to mix in some cases. The detailes are now under examination.

Fig.10 Energy distribution of single line HBr laser.

Fiy.9 Typical laser pulse waveforms of various V-R e) The scalability transitions, output mirror reflectance: 75%.

In course of making 6m laser, three

d) The spectral lines lasers of different length have been con-

In the case of multiline oscillations, structed. And the output energy in the

the outpui consists of 12 lines ranging multiline operations is plotted in Fig.11.

from 4.017um P 1 O (4) to 4.608pm P 4 3 (7). It is found that the output energy in-

The number of oscillating lines does not creases almost exponentially. The extra-

change with the additions of buffer gases. polation (dotted line) differs only by the

The increase of the output energy comes factor of two to the experiment of 6m

from the increase of the energy of individ- laser. This means that the more energy

ual line. could be obtained by increasing the plasma

In the case of line-selectable oscilla- length.

tion, the number of oscillating lines amounts to 29. The wavelengths cover from

3.935um to 4.585um. In this case, the

lasing of v=4+3 is not obtained.

The P21(6) line does not oscillate in any case. The reason would be the reduc- tion of net gain at the wavelength, because it is strongly absorbed by CO2 in the air in the cavity.

The spectral lines obtained by large grating are graphically shown in Fig.10. In the case of large grating, the discrimi- nation of neighboring line 'is sometimes not

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I I I I I I 0 1 2 3 4 5 6 7 PLASMA LENGTH (rn)

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The high power HBr laser in the multiline operation and the line-selective os- cillation by the use of grating as the total reflector of the cavity mirror have been demonstrated. The results are summa- rized and concluded as follow:

a) In the case of Br2+H2 mixture, the output energy in multiline operation is optimized at the mixing ratio of Br2:H2= 1:5.7 and the total pressure of 47 Torr. b) The additions of He and Ar enhance the

output energy by the factor of 15% and 68% respectively, as compared with the case of Br2+H2 mixture.

C) Thy maximum energy obtained is 1.4J at the gas mixture of Br2+H2+Ar=5+27i-5E

Torr at the charging voltage of 46kV.

The conversion efficiency is 0.55%. d) The number of oscillating lines is 13,

ranging from 4.017um to 4.607um for multiline operation.

e) In the case of single-line operations, the number of lasing line is 29 ranging from 3.93pm to 4.58ym.

f) Seven lasing lines, that is P (1)

,

P,,(2) 1 P10(3) 1 P2l (I), P21(2)r P21(3)

and Pg2(ll) are newly observed. g) The output energy would increase to a

certain extent by increasing the plasma length.

discharge hydrogen halide lasers",

~ p p l . Phys. Lett.

20

77 (1972).

3) E.N. Rutt: "A high-energy hydrogen bromide laser", J. Phys. D: Appl. Phys. 12 345 (1979).

-

4) H. Osada, T. Oyama, K. Toyoda, R.

Nakane, S. Namba and H. Gamo: "Kulti-

photon dissociation of CF3COOD by HBr chemical laser radiation", Technical digest of Conference on Laser and

Electro-Optical Systems, WCC21, Sari

Diego, California, February 26-28, 1980.

5) T.Y. Chang and O.R. Wood 11: "Optically

pumped atmosheric-pressure CO2 laser",

Appl. Phys. Lett.

21

19 (1972).

6 ) T.Y. Chang and 0.2. T*lood: "Optically pumped N20 laser", Appl. Phys. Lett. 22 93 (1973).

-

7) T.Y. Chang and 0.R. Wood: "Optically pumped 33-atm C02 laser", Appl. Phys.

Lett.

23

370 (1973).

3 ) T.Y. Chang and O.R. Wood: "Optically

transfer 42-atm N2 0 laser", Appl.

Phys. Lett.

24

182 (1974).

9) R.M. Osgood: "Optically pumped 16-pm

C 0 2 laser", Appl. Phys. Lett.

28

342 (1976).

10) C.S. Willett: "Introduction to Gas lasers: Population Inversion Mecha- nisms" P478 Pergamon Press, 1974.

References

1) T.F. Deutsch: "New infraxed laser transitions in HC1, HBr, DC1 and DBr",

IEEE J. Quantum Electron. QE-3 419