CW CARBON MONOXIDE LASER WITH MICROWAVE EXCITATION IN THE SUPERSONIC FLOW

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CW CARBON MONOXIDE LASER WITH

MICROWAVE EXCITATION IN THE SUPERSONIC

FLOW

W. Schall, P. Hoffmann, W. Schock, H. Hügel

To cite this version:

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JOURNAL DE PHYSIQUE CoZZoque C9, suppZJrnent au n O 1 l , Tome 41, novembre 1980, page ~ 9 - 2 1 7

CW CARBON MONOXIDE LASER W I T H MICROWAVE E X C L T A T I Q N 1 N T H E SUPERSONIC FLOW

W. Schall, P.Hoffmann,W. Schock and H. Hiigel.

I n s t i t u t fUr Teehnisehe Physik, DFVLR, D-7000 S t u t t g a r t 80, Fed. Rep. Germany.

Abstract.- A new technique of efficiently coupling microwave energy into a supersonic gas flow is described. The stable self-sustained microwave discharge is applied to excite a cw CO gasdynamic laser. Electron densities well above the cut-off value have been achieved together with a reduced field strength within the discharge that is appropriate for efficient vibrational excitation. With an energy loading of 0.15 eV/CO a laser power of 340 Watts is obtained at an overall efficiency of 7 %. The effect of some additives on stability and laser performance is studied.

1

.

Introduction

Several requirements have to be met by a discharge for the efficient vibrational excitation of CO in the supersonic flow of a gasdynamic laser: an appropriate value of the reduced field strength E/n, a high electron density, and a stable and homoge- neous discharge without concentration in the boundary layers. This can be accom- plished with the e-beam sustained dis- charge1,' 2, the pulser-sustainer discharge 3 and by an rf discharge perpendicular to the flow. 4

The use of microwaves with a wavelength comparable to the dimension of the flow channel allows the control of the field distribution by suitable geometric design of the discharge system.' The coupling of microwave power into a gas at rest or in subsonic motion was investigated in many different devices (Fig. 1 ) .

One-group of such devices are the different

.varieties of microwave cavities. 6'7 Here, use is made of the fact that particularly high field strengths occur when geometry and coupling is suitably selected.

LATERAL COUPLING

SLOW WAVE STRUCTURE _HORN

Fig. 1

Different devices for coupling of microwave radiation into a subsonically or supersonically flowing gas. At the bottom of the figure the device presented by the authors. Broad arrows indicate the direc-

tion of the gas flow and small arrows the

direction of microwave propagation.

One is often interested in allowing as large quantities of gas as possible to be excited by the microwave discharge. How- ever, the size of the cavities with well defined modes is limited by the wavelength of the radiation. To s o l v ~ this problem, different approaches are applied. Accord

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C9-218 JOURNAL DE PHYSIQUE

ing to the process of Ref. 8 a slow wave structure acting as an appropriately shaped antenna is used for the coupling of radiation into the discharge, whereas in the process according to Ref. 9 ionizing surface waves are produced on a long plasma column. Common to both processes is, however, the problem that it is not possible to deposit all of the available microwave power in the plasma. 10

According to Ref. 1 1 , microwave energy was successfully coupled into a discharge channel having a laser-active supersonic gas flow therein. The coupling is carried out with the aid of the slow wave struc- ture8 and serves to preionize an alectri- cally excited gas-dynamic C02-laser with excitation in the supersonic flow region. However, this lateral coupling into the flow channel has some disadvantages. The microwave field strength is highest imme- diately adjacent to the antenna, i.e., at the edge of the channel where the boundary layer is located, and decreases gradually towards the center of the channel. Conse- quently, the discharge develops preferen- tially in the boundary layer rather than in the flowing volume. In the boundary layer the gas temperature is high and the number density of heavy particles is low. Hence, ionization takes place preferen- tially and creates a layer of high electron density. Therefore further pene,tra- tion of the.radiation into the core of the supersonic gas flow is obstructed, thus causing a further decrease of the field strength in the actual flow region. Al-

though the ensuing field asymmetry in the channel can be eliminated by a similar configuration.on the other side of the channel, a field strength minimum still remains at the center of the channel. This problem can be solved by using the walls of the flow channel as a waveguide, where both the directions of gas flow and of microwave propagation are identical. The cross-section of the waveguide was selected to allow the propagation of the TEol mode only, in order to obtain a defined field distribution with the maximum in the center of the flow channel. 1 9

2. Experimental design

The investigated device is shown schemati- cally in Fig. 2.

GAS MICROWAVE DISCHARGE INLET

WAVEGUtOE MICROWAVE DIELECTRIC NOZZLE LASER

TRANSPARENT CAVITY

PRESSURE WINDOW

Fig. 2 Schematic of laser device.

Amplitude stabilized cw microwave radiation at 2.45 GHz with power up to 5 kW is propagated through a microwave-transparent pressure window along the rectangular waveguide with a cross-section of 2.5 x

2

1 1 cm

.

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The n o z z l e i s made of d i e l e c t r i c microwave- t r a n s p a r e n t m a t e r i a l which t o a f i r s t o r - d e r approximation d o e s n o t d i s t u r b t h e e l e c t r i c f i e l d d i s t r i b u t i o n . Due t o t h e p r e s s u r e d e c r e a s e d u r i n g t h e expansion t h e Townsend breakdown f i e l d s t r e n g t h i s de- c r e a s e d l i k e w i s e . The d i s c h a r g e i s i n i t i - . a t e d when t h i s p a r a m e t e r f a l l s below t h e

peak f i e l d s t r e n g t h o f t h e microwave r a d i a - t i o n . F o r t h e TEol mode, t h e l a t t e r depends o n l y on i n p u t power, impedance matching, and t h e h e i g h t o f t h e waveguide ( F i g . 3 ) . The breakdown f i e l d i s r e a c h e d approximate- l y a t a d i s t a n c e o f t e n t h r o a t h e i g h t s downstream o f t h e n o z z l e t h r o a t where t h e reduced f i e l d s t r e n g t h E/n i s c a l c u l a t e d t o 2 be 2 . 3 - 1 0 - ~ ~ V cm

.

FLOW MRECTION

F i g . 3 Q u a l i t a t i v e development of p r e s s u r e p , m i - crowave peak f i e l d s t r e n g t h and breakdown t h r e s h o l d Ed i n a g a s , f l o w i n g t h r o u g h a d i e l e c t r i c s u p e r s o n i c n o z z l e w i t h s u p e r - imposed microwave r a d i a t i o n . The f i l l e d c i r c l e i n d i c a t e s t h e o c c u r e n c e o f breakdown some t h r o a t h e i g h t s downstream o f t h e noz- z l e t h r o a t , i n d i c a t e d by h*. (The dashed c u r v e shows t h e development o f peak f i e l d s t r e n g t h w i t h i n a m e t a l l i c n o z z l e and re- s u l t i n g breakdown i n t h e s u b s o n i c p a r t o f t h e flow, a s i n d i c a t e d by t h e open c i r c l e . ) wave f i e l d needs n o t p e n e t r a t e t h r o u g h a boundary l a y e r and i s t h e r e f o r e a b s o r b e d e x c l u s i v e l y i n t h e s u p e r s o n i c c o r e o f t h e g a s flow. There t h e c o l l i s i o n f r e q u e n c y i s

h i g h and hence e f f i c i e n t c o u p l i n g o f gene- r a t o r power t o t h e g a s f l o w i s p o s s i b l e . With a minor t r a n s f o r m a t i o n o f t h e plasma impedance, u s i n g a d o u b l e screw t u n e r , a 100 p e r c e n t power d e p o s i t i o n i s o b t a i n e d .

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JOURNAL DE PHYSIQUE

3. Laser characteristics

The spectrum of' laser output depends main- ly on gas temperature and energy loading. For typical parameters, described below, vibrational-rotational transitions between V = 3-15 and j = 4-13 occur with several strong laser lines below 5.1

pm.

Electron densities have been measured in 5

a similar device

,

investigated earlier at comparable parameters with a microwave in- terferometric method. They show a decrease from 10" cm-3 at a position 10 cm downstream of the nozzle throat to some 10 1 0 cm-3 at 20 cm downstream of the throat. From the high electron density gradient near the 10 cm position it can be concluded, that the density is much higher near the front of the discharge.

Typical experimental conditions are: total mass flow up to 50 g/s of different laser gas mixtures, static pressure at the ca-

vity position p = 5-25 mbar, depending on

C

mass flow, and gas temperature Tc = 50-60K. The Mach numbers, depending on gas composition, lie between M = 3.5 and 4.5.

Fig. 4 shows the dependence of laser power and efficiency on the CO mass flow rate. At constant microwave power increasing total mass flow corresponds to an increasing plenum pressure and a decreasing energy loading. The laser power shows a pronounced maximum.

At low mass flow rates the decrease of number density 'results in a decrease of gain. At high mass flow rates the energy loading decreases until laser threshold is reached at 0. I eV/molecule CO.

ENERGY LOADING PER CO MOLECULE. eV/CO

1 0.4 0 2 01

2 4 6 8 1 0 20 CO MASS FLOW, g/s

0 . ~ 0 . 6 1 2 4 6

PLENUM PRESSURE. bar

Fig. 4

Measured laser power versus CO mass flow resp. plenum pressure. Microwave input power was held constant at 4750 W, He/CO =

95/5 by volume. Also shown is laser efficiency and specific energy loading. The 2"

coupling mirror had a reflectivity of 95 %. The dependance of laser output from energy loading of the CO molecule at different ca-

ENERGY LOAD1 NG, eV/molecule

CO

Fig. 5

Laser power versus energy loading per CO molecule at different cavity pressures.

Measurements were performed with He/CO =

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vity pressures, resp. total mass flows is shown in Fig. 5. The upper energy loading limits represent the limit set by the available generator power. Since no thermal or ionization instabilities have been en-

countered in this gas mixture, higher energy loadings and higher pressures are,possible. The same behaviour is shown in Fig. 6, where the dependance of laser power versus input power is shown at different CO mass flow rates.

, 4. Variation of gas composition

Nearly since the invention of the CO laser it is well known, that the addition of small amounts of oxygen will increase laser output considerably.

'

This behaviour is confirmed also in the microwave excited laser system.

Fig. 7 shows the steep increase of laser

power.at O2 mole fractions below 0.3 %, followed by a flat maximum, corresponding

1 2 3 4 5

MICROWAVE INPUT POWER, kW

O2 MOLE FRACTION, %

Fig. 7

Dependance of relative laser power on 02 mole fraction. The experimental parameters are: CO mass flow

h0

= 8 g/s, corresponding to a CO mole fraction of 5 %. For this and the following figures the microwave

power input was held constant at 4750 W,

and plenum pressure at 2 bar; 2" coupling mirror 95 %.

to the ionic dominance of 0; in the pres-

ence of traces of oxygen. The adverse effect of deactivation of the CO vibrational levels by 0 at higher oxygen mole fractions,proposed by Morgan, Fisher and Light- man, 14'15 can not appear in our case, be- cause the residence time of particles between discharge and resonator is much

smaller than the vibration-translation, re- laxation time for deactivation of excited C0. l 5 The laser power decrease beyond an O2 mole fraction'of 3 % may be due tq the energy loss of the discharge into excitation of O2 and/or the formation of COi. This behaviour is also recognized in Fig.8, where the dependance of laser power on CO mole fraction for various laser gas mix-

tures is plotted. The admixture of little

>

argon shows no effect on laser power, but at mole fractions between 10-20 % the dis-

Fig. 6 charge becomes unstable and laser output

Laser power versus microwave input power at

different CO mass flows. Gas composition is disappears. In contrast to this behaviour,

He/CO = 95/5. The upper limits of the

curves are set by available generator pow- the admixture of nitrogen causes an almost

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JOURNAL DE PHYSIQUE V He/CO 0 3 % 02 75%N2 I I I 5 10 15 CO MOLE FRACTION, % F i g . 8 L a s e r power v e r s u s CO mole f r a c t i o n a t d i f - f e r e n t g a s c o m p o s i t i o n s . l i n e a r d e c r e a s e o f o u t p u t power and an i n - c r e a s i n g tendency t o d i s c h a r g e i n s t a b i l i t y w i t h i n c r e a s i n g N 2 mole f r a c t i o n . A t a g i v e n r e l a t i v e o u t p u t power l e v e l t h e p e r - m i s s i b l e amount o f N 2 s t r o n g l y depends on t h e O 2 mole f r a c t i o n ( F i g . 9 ) , w i t h an optimum n e a r l y a t t h e molar c o m p o s i t i o n of a i r . 0 -100 PARAMETER: LASER POWER, I I V V v u ₅₀ 0 10 20 3 0 LO 50 - - Nt MOLE FRACTION, % F i g . 9 Curves' o f e q u a l l a s e r power i n f r a c t i o n s of naximum power v e r s u s g a s c o m p o s i t i o n . CO mo&e f r a c t i o n w a s h e l d c o n s t a n t n e a r t h e optimum a t 6.5 8 . ( T h i s optimum was n o t s e n s i t i v e t o g a s c o m p o s i t i o n , a s i s demon- s t r a t e d i n Fig. 8.) The dashed l i n e c o r - r e s p o n d s t o t h e c o m p o s i t i o n of a i r .

0

o

oa

02 a3

CO, MOLE FRACTION, %

P i g . 10

Dependance o f r e l a t i v e l a s e r power v e r s u s C02 mole f r a c t i o n . The moLe f r a c t i o n s o f CO

and 02 were h e l d c o n s t a n t a t 5 % and 0 . 3 %

r e s p .

One can deduce t h a t N2 s t o r e s v i b r a t i o n a l e n e r g y which c a n n o t be t r a n s f e r r e d t o CO w i t h i n t h e s h o r t flow t i m e o f 100 u s from d i s c h a r g e t o r e s o n a t o r . 16

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of the CO fraction, we must conclude, that in our case C02 has a detrimental effect on discharge parameters. This is confirmed by the appearance of an increasing mis- match of plasma impedance with a resulting decrease of absorbed microwave power. So we conclude, that in spite of comparable high CO concentrations C02 acts as a strong

,electron attaching agensl thus reducing

considerably the electron density.

5. Conclusions

The microwave excitation of supersonically flowing, gasdynamically cooled CO-laser gas mixtures has proven to be an efficient technique. Even at unoptimized conditions and restricted input power levels a re- markable laser performance is achieved. The influences of various gas compositions on output power are shown and interpreted.

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