HIGH PRESSURE OPTICAL DISCHARGES

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Submitted on 1 Jan 1979

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HIGH PRESSURE OPTICAL DISCHARGES

C. Carlhoff, J. Schäfer, K. Schildbach, J. Uhlenbusch

To cite this version:

C. Carlhoff, J. Schäfer, K. Schildbach, J. Uhlenbusch. HIGH PRESSURE OPTICAL DISCHARGES.

Journal de Physique Colloques, 1979, 40 (C7), pp.C7-757-C7-758. �10.1051/jphyscol:19797366�. �jpa-

00219362�

JOURNAL DE PHYSIQUE CoZZoque C7, suppze'ment a u n 0 7 , Tome 40, J u i Z Z e t 1979, page C7- 757

HIGH PRESSURE OPTICAL DISCHARGES

C. Carlhoff, J.H. Schafer, K. Schildbach and J. Uhlenbusch.

P h y s i c s I n s t i t u t e I I , U n i v e r s i t y o f DiisseZdorf, F.R. G.

Continuous optical discharges (COD) were investi- be seen from fig. 2. As the temperature measure- gated in the past by many authors, see

-

71

.

As a result of these experiments it was shown that in horizontal configurations beyond a cri- tical pressure of say 30 bar COD cannot exist, whereas in vertical configurations pressures of 45 bar were regched. In our experiment dis- charges were sustained up to 145 bar and there is no doubt that even higher pressures are possible.

As a light source a cw

-

C02-laser with a maximum power of 1500 W is used. The laser assembly is built up as oscillator- amplifier system. The C02-monomode beam is vertically focussed inside of a high pressure chamber by a gold coated mir- ror of focal length f = 2,s an (see fig. 1). The plasma can be ignited by means of a thin tungsten wire which is moved for a short time through a region near the beam-focus. The high pressure chamber provides a conical KC1- or ZnSe-window of 20 nun thickness for entering the laser beam, which allows operation up to 1000 bar. Sapphire windows of good quality are mounted for spectro- scopic observation of the plasma, for schlieren pictures and interferometric studies.

The shape of the COD is conical, if the focussed laser beam is directed as shown in fig. 1. The spatial dimension of the plasma can be altered by changing the laser power. Higher power in the laser focus blows up the plasma dimensions as can

ments under high pressure conditions in fig. 3 show, a second temperature maximum can occur, so that the plasma has a shape like a "sandglass".

Local temperature and electron density measure- rnents in argon were performed by monitoring con- tinuum radiation at

A=

⁴⁴⁷⁰

8,

assuming that Saha's equation is valid. Figs, 2,3 show typical temperature plots and electron density profiles achieved by side on observation of the plasma using Abelfs inversion. It is interesting to no- te, that there is only a small increase of the electron density from 5x10' cm-3 to 7x10'~

if the chamber pressure changes over two orders of magnitude.

The temperature profile of the plasma can be deri- ved from the balance equations in the fluid dyna- mics approximation. A general two-dimensional so- lution of these equations is not known up to now, approximations are available as a cylindrical so- lution with one-dimensional flow and averaged transport processes [I]

,

cylindrical solution with one -dimensional flow but two-dimensional heat losses

181

all assuming parallel and homo- geneous laser beams. Converging beams without flow but two-dimensional heat flux distribution are considered in [3] and 191

.

Introducing the

rF

heat flwc potential S =

/ K.

^{dT (with )( ther-}

-%

ma1 conductivity and T, temperature far away

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797366

from the plasma) the energy balance (with one-di- mensional flow) reads for a Gaussian intensity distribution across the beam

.

^P

Here

A

^r:

moo

^{with Q,,} mass density, Cp Keo

specific heat capacity, &thermal heat conducti- vity, v, velocity far away from the plasma.

w z =

w:(3+

xz)with wo = beam waist and zR

%i

= Rayleigh length. A solution in the outer region without absorption is given by

e ^+(z - 4 7 7 3 1

S . w r ( f 2 )

v'm

This solution can be experimentally verified by interferometric measurements, as was done by Wroblewski, see fig. 4. For pressures in the 5 bar regime the assumption of an inner spherical absorption region (with X y wS) is a good approxi- mation. Then the energy equation can be solved

[6] C.D. Moody, J. Appl. Phys. 46,6 1975 171 Z.Mucha Bull.Ac.Po1. SST 25 1977 [8] J.H. Batteh IEEE PS 2 1974

[9] J. Uhlenbusch,LuPP 2 Physics Inst. 11, Univ. of Diisseldorf, 1973

Fig.1. High pressure chamber

Argon. Sbar.3 Mu-& laserpowers

inside the plasma with

The calculated laser power to sustain a given maximum temperature as well as the temperature profile from these simple formulas are in good agreement with the experiments, A more general solution of the energy balance is in preparation.

Fig.2. Ternperat~lrc profiles at constant pressure The authors thank Dr. Byszewski, Dr. Plochocki

and Mr. Wroblewski from the Polish Academy of Sciences for their support and helpful discus- sions.

References

b]

Yu.P.Raizer,Sov.Phys.JETP 31,6 1970 [2] N.A. Generalov, Sov.Phys

.

JETP 34,36 1972 [3] G.I. Kozlov, Opt. Spectr. 37,6 1974

[4] D.C.Smith,M.C.Fowler,Appl.Phys.L.22,10 1973 151 D.R. Keefer, J. Appl, Phys. 46,3 1975

Fig.3.T and ne at 100bar Fig.4.Interferogram 30bar