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THE HIGH GAIN CO2 LASER BY EFFECTIVE
MIXING OF N2 AND CO2 GAS
H. Hara, A. Fujisawa
To cite this version:
JOURNAL DE PHYSIQUE CoZZoque C9, suppl6ment au nO1l, Tome 41, novembre 2980, page C9-203
THE HIGH GAIN C02 LASER
BY
EFFECTIVE MIXING OF N2 AND C02 GAS
H. Hara and A. Fujisawa.
First Research Center Japan Defense Agency, Nakameguro, 2-2-1, Meguro, Tokyo, Japan.
Abstract.- A high-gain C02 laser is described in which vibrationally excited N2 gas and cold Con
gas are mixed effectively by means of the diffusion of C02 gas into Ng gas. By using different ty-
pes of mixing techniques, a maximum gain of 1 1 ml' was obtained when Cop gas was injected parallel
to the expanding N p gas flow. An output power of 4 W was obtained from an 1.2 cm active length. In
addition, He gas addition to the N2 gas flow was found to decrease the small-signal gain with increa- sing He gas flow rate.
INTRODUCTION
Numerous papers have been published in the literature on C 0 2 l a s e r
' 4
which used a discharse in N2-He -CO mixture g a s , however 2 there a r e remarkably little data on the one f o r which N2 i s vibrationally excited in an electric discharge and mixed with cold C 0 2 g a s . In this type of the l a s e r , b o t h the dissociation of C 0 2 molecule and the excitation of lower l a s e r level do not take place, and the effective mixing of N2 and CO g a s i s v e r y important to obtain high-2 gain
.
We have investigated the gain distribution of C 0 2 l a s e r by injecting C 0 2 g a s parallel o r perpendicular to the supersonic o r subsonic N2 g a s flow, and have obtained the maximum gain of 11 m-I when C 0 2 g a s was injected parallel to the expanding N2 gas flow by using a conical nozzle a t a static p r e s s u r e of
18.3
T o r r and ag a s composition of N2:He:C02=9.0: 6.5:
14.5
(tfmin). The small-signal gain was found to decrease gradually when increasing the He g a s flow rate;in contrast to the previous measure- ments. 8 , 9EXPERIMENTAL SETUP
Figure 1 shows the schematic diagram of the experimental setup. The discharge tube i s
having 6-mm-diam b r a s s -ring electrodes placed separately i n the discharge tube with a distance of 17 cm. Dl- TUBE
n
-
I VACUUM 0 0 0-
He,
PUMPFigure 1 Experimental setup of N2-CO mixing 2 l a s e r .
TYPE A-1 TYPE A-2
TYPE B-I TYPE 8-2
Figure 2 Four different types of C 0 2 gas injector.
N2 g a s was introduced through an anode ring into the discharge tube. He g a s was premixed with N g a s s o a s to stabilize a glow
2
discharge. Premixed N2 and He gas flowed out composed of a water-cooled 15-mm-diam g l a s s through the discharge tube into mixing region
JOURNAL DE PHYSIQUE
where CO g a s was Injected from various g a s 2
injectors shown i n Flg. 2. N He-C02 mixture 2 -
gas introduced into the measuring region was exculded by a vacuum pump of 3000 j/min capa- bility. In the present experiments, four differ- ent types of C 0 2 g a s injectors were employed in o r d e r to clarlfy the influence of mixing techniques on the small-signal gain. The used C 0 2 gas
injectors a r e a s follows; A-1 C 0 2 gas injector
C 0 2 gas was injected perpendicular to N 2 g a s flow from four 0.5-mm-dim holes placed in side walls of 12-mm-dim mixing tube. A -2 C 0 2 gas injector
C 0 2 gas was injected parallel to N 2 gas flow from an 1 -mm-diam pipe positioned in the c e n t e r of 12-mm diam mixing tube
B-1 C 0 2 gas injector
C 0 2 g a s was injected perpendlcular to N2 g a s flow from four 0.5-mm-diam holes placed in a conical nozzle throat.
B-2 C 0 2 gas injector
C 0 2 g a s was injected parallel to N 2 gas flow after N expansion from an 2 1 -mm-diam pipe positioned a t
5
mm downstream from the conical nozzle throat.Figure
3
shows the small-signal gain (determined wlth a P(20) stabillzed TEMOO mode2 and l a s e r power density of 500 mW /cm ) a s a function of the distance from C 0 2 gas injection point. Although the distribution of the small - signal galn in each case showed somewhat differ- ent dependence on the distance from C 0 2 g a s lnjectlon point, the small -signal gain decreased smoothly with increasing the distance along the manstream consisting of N2, He and C 0 2 g a s .
N,-He-C02 =90-65-1&61~5-10-10) A TYPE A-l X TYPE A-2 TYPE 8-1 =f= 0 TYPE B-2 $= z 2 3 - 2
-
I - 10 15 2 0DISTANCE FROM CO? GAS INJECTION POLNT l c r n l
Figure 3 Small-signal gain a s a function of distance from C 0 2 gas injection point
A
: A- 1 injectorN2:He:C02=9.0:6.5:14.6 (Plmin) Discharge voltage Vd=4. 0 kV, Discharge current: Id=40 mA
Static p r e s s u r e , P , i n measuring region a t
4.5
cm downstream from C 0 2 g a s injec- tion polnt =22 T o r r .)( : A-2 injector
N2:He:C02=9.0: 6.5:14.6 ([/min) Vd=2.
8
kV,
ld=40 mAP s a t 4 . 0 cm downstream from C 0 2 gas injection point=20 T o r r . : B-1 injector N2:He:C02=4.5:10:10 (B/min) Vd=3. 2 kV
,
Id=40 mA P at 4 . 5 cm downstream from C 0 2 g a s injection p o i n t = l l T o r r0
: B-2 injector N2:He:C02 =9.0:6.5:14.6 ( j / m i n ) Vd=4. 0 'kV,Id=40 mAP a t
3.5
cm downstream from C 0 2 gasS
This reduction of the gain along the N2-He -C02 mixture gas i s thought to be connected with the relaxation of upper l a s e r level due to the colli- sion between C 0 2 and other gas o r wall. As can be seen from the figures shown in F i g . 3 , a few cm downstream the gain f o r B-2 injector i s l a r g e r than those obtained f o r the A - I , A-2 and B-1 injectors. While ,the magnitude of gain f o r all types of injectors a r e nearly equal a t points away from C 0 2 gas injection point. These results indicate that the gain depends strongly on the mixing efficiency of N2 and C 0 2 g a s at a few cm downstream from C 0 2 gas injec- t o r . In contrast to this r e s u l t , the gain does not change s o much at 1 6 cm downstream from C 0 2 gas injection point, since the N2 and C 0 2 gas can be mixed homogeneously until N2 and C 0 2 g a s
reached i t . The increase in the injected C 0 2 gas flow r a t e f o r the case of B-1 injector r a i s e s the static p r e s s u r e of the discharge tube and hence the glow discharge becomes instable.
F o r the c a s e of B-1 and B-2 injectors ,the small- signal gain revealed the peak in the vicinity of
4
cm downstream from C 0 2 gas injection point,while no peak was observed f o r the c a s e of A-1 and A-2 mjectors. By comparing the magnitude of the gain f o r B-1 and B-2 injectors, the parallel injection technique of C 0 2 gas into N2 gas was found to be superior to the normal injection technique of C 0 2 g a s , because the parallel injection did not disturb the flow condi- tion m contrast to the normal injection.
Moreover, the magnitude of the gain f o r B -2 injector was found to be g r e a t e r than the value obtained f o r A-2 injector from the small-signal gain measured f o r the parallel injection tech- niques with and without u s e of the nozzle.
From these results,the conical nozzle and the
parallel injection technique was found to be hlghly effective to produce the high-gain in o u r setup.
175cm 0
0 5 10 15 20
Cot GAS FLOW RATE ( I l r n ~ n )
Figure
4
Small-signal gain a s a function of C 0 2 gas flow r a t e f o r the c a s e of B-2 injector. The experimental conditions a r e of the same values a s in Fig. 3Figure
4
shows the gain a s a function of C 0 2 g a s flow r a t e f o r the case of B-2 injector. The maximum small-signal galn of 11 m-I was obtained at a p r e s s u r e of18.3
T o r r with a g a s composition of N2:He:C02 =9.0:6.5:
1 4 . 6 (1
/min). This high-gain i s resulted from the effective mixing of N2 and C 0 2 g a s , which was thought to be made by shock waves produced in the N2-He mixture gas by means of the introduc- tion of C 0 2 gas with one Mach number. TheC9-206 JOURNAL DE PHYSIQUE
1.36 at
3.5
cm downstream when C02 gas flow rate was adjusted to 14.6[/min. . This Mach number i s smaller than the value observed in a smooth expansion without C 0 2 gas injection, and indicates the generation of shock waves in N2-He gas flow.
While the conical nozzle i s known to be effective for mixing N2 and C 0 2 gas due to the 2-dimensional diffusion of C02 gas into the expanding N2 gaslO~this scheme i s believed to be more effective for the diffusion of C 0 2 into N2 gas when N2 and C02 gas had different exit pressures and different Mach numbers. In ad- dition, since the translational temperature of N2- He-CO mixture gas in the measuring region was 2 found to be only 266 K at
3.5
cm downstream,this low translational temperature of the mixture gas i s highly effective to produce the high-gain in the present l a s e r system.TYPE
-k
u'w
12 [-r -
A A
&TYPE A-I1
X'TYPE A-2 o TYPE 8-1 0:TYPE 0-2 x- X-x . x -~ -h-C02=9.0-11 6 I L5-7 3 ) Id = LOmA L c m DOWNSTREAMv
I I I I 1 I 0 5 10 15 20 25 He FLOW RATE ( I l r n ~ n )Figure
5
Small-signal gain a s a function of He gas flow rate. The experimental conditions a r e of the same values as in Fig.3.Figure
5
shows the relation between the small-signal gain and the He gas flow rate.He gas was purposely added to N2 gas in order to stabilize the discharge, however, i t gave much influence on the small-signal gain. This figure shods the profile of the gain for the B-2 injector i s different from those observed for the A-1, A- 2 and B-1 injectors. For instance,the gains f o r the A-1, A-2 and B-1 injectors exhibit peaks at different He gas flow rates. While the gain for the B-2 injector decreased gradually with increasing the He gas flow rate. In all cases, with CO gas injector the electric input power
2
decreased with increase in the He gas flow rate until-5 Amin (STP), and became constant with
the increase of the He gas flow rate. Since
the energy in the lower l a s e r level of C02 mole- cule doesnot change because the lower l a s e r level i s empty due to the low translational tem- perature of C02 gas, the influence of He gas addition on the small-signal gain for the B-2 injector has no relation to the relaxation of l a s e r level. The distribution of the small-signal gain for the B-2 injector a s a function of the He gas flow rate can be explained by assuming that the electric input energy was carried away by the
added He gas in proportion to the He gas flow rate, and the vibrational energy stored in N2 molecules decreased, although i t i s not clear at
present time.
We have observed an output power of
4
W from an 1.2 cm long active length and a static pressure of 18.3 T o r r by using a gold coated total reflective mirror and a 85 % reflectivity dielectric-coated ouput mirror. The saturation2 parameter was determined to be. -500 W/cm by
In summary,we have reported the attainment T o r r , t h e maximum gain of 11 m-I was obtained. of a high-gain C 0 2 l a s e r by using the conical Zn addition, the influence of He gas addition to nozzle to achieve the effective mixing of N2 and N g a s was measured. An output power of
2
C 0 2 gas to decrease the translational temperature
4
W was obtained. of l a s e r gas. At a static pressure of 18.3REFERENCES
1) C .N.Patel,Phys. Rev. ,136 A(1964)1187. 2) W. J.Witteman
,
Appl. Phys .Lett., ll(1967)337.
3) T.G.Roberts,G.T.Hutcheson, J.J.Ehrlich, W.L.Hales and T.A.Barr, J r , ,IEEE J. Quan-
tum Electron.
,
QE-3(1967)605.4)
W. B. Tiffany, R. Targ andJ.
D.
Foster, Appl.
Phys.Lett., 15(1969)91.5)
C.O.Broun,Appl.Phys.Lett.,17(1970)388.6)
A.C.
Eckbreth, J. W .Davis, and E.
A.Pinsley,Appl. Phys. Lett. ,18(1971)73.
7) A.C.Eckreth and J.W.Davis,Appl.Phys. Lett., 19(1971)101.
8) P .K.CHeo and H.G.Cooper,IEEE J.Quantum Electron.
,
QE -3(1967)79.9) P .K.Cheo,
J.
Appl.Phys. ,38(1967)3563. 10) H.Wirels and W. R.Warren, Jr., AIAA J .15(19771602.
