SELF-SUSTAINED ELECTROPHOTOIONIZED DISCHARGE IN COMPRESSED GASES

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Submitted on 1 Jan 1979

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SELF-SUSTAINED ELECTROPHOTOIONIZED DISCHARGE IN COMPRESSED GASES

N. Basov, E. Glotov, V. Danilychev, A. Milanich, A. Soroka

To cite this version:

N. Basov, E. Glotov, V. Danilychev, A. Milanich, A. Soroka. SELF-SUSTAINED ELECTROPHO- TOIONIZED DISCHARGE IN COMPRESSED GASES. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-393-C7-394. �10.1051/jphyscol:19797193�. �jpa-00219172�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppZ6ment au n07, Tome 40, JuiZZet 1979, page C7' 393

SELF-SUSTAINED ELECTROPHOTOIOhllZED DISCHARGE lN COMPRESSED GASES

N.G. Basov, E.P. Glotov, V.A. Danilychev, A.I. Milanich and A.M. Soroka.

P:N. Lebedev PhysicaZ I n s t i t u t e , U.S.S.R. Academy of Sciences Leninsky prospect 53, 117924 Moscow, U . S. S. R.

Compressed rare gases (or mixtures of

rare gases and haloids) are the most fitting

[K~ ($

⁽¹⁾

active media for powerful lasers. Presently the radiation power of 2 GW / I / and 20%

efficiency can be obtained in such lasers

/r/.

Theory predicts the efficiency of 50%

/3/. But the realization of laser potentia- lities is impeded by strong absorption /3/

at radiation wavelengths of the existing lasers and at possible wavelengths of new laser~systems,viz.Ax2Cl*, Xe2C1*, Kr2F*

etc. The absorption is connected with 1) IJe2+, Ar2+, Bt2, xe2+ absorption by molecular ions (632 5). 10-I7cm2/4/),

2) electron photodetachment from negative ions F-, ~ 1 -

(6'4

0-~~-10-~~crn*

I ,

3) photoionization of low metastable levels of rare gases Ar*, Ne*, Kr*, Xe*

(6i,~~-17cm2/5/) ,etc. Note that process 3) results not only in vanishing of in- duced radiation photon (unfavourable process for laser generation) but also in the production of a new ion and an electron

(additional ionization),which may produce additional photons (favourable process for laser generation). The laser exciting discharge may burn without the sources of external ionization and without indepen- dent ionization of the active medium by electrons under the following threshold conditions:

Here

Y-1

is the number of particles X*

Ye

which excite the electron during its life- t i , the probability of producing the laser quantum by excited particle;

Kg ,

the probability of the photon occurence at electron destruction;

c ~ X * ,

the frequency of X particles photoionization ( C is the speed of light;

6 ,

photoionization cross section);

Vj- ,

the speed of quantum loss in all channels except photoionization (if

. .

photoabsorption by all particles is insig- nificant (excluding X* particles), then

$$= c R ~ 4 1 6 5 @ ~

,

^where.I,! is the length of the act* region;l;

,h

are the reflection coef f icients of mirrors). value(i

is the photoionization efficiency. The value in square brackets is equal to the number of quanta excited by electrons during lifetime. The intensity Ye of the excita- tion of the active particles X* by electrons is a strong function of E/P parameter

( E

is the field intensiw of the discharge,

p

is the pressure). As follows from Eq.

(1) there exists a threshold value of E/P starting from which there occurs burning of EPI-discharge. It should be noted that there exists the possibilty of burning of the self-sustained discharge in all mixtures where the laser quantum energy is

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797193

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sufficient for photoionization of the excited X* states. Technical feasibility of such a discharge requires that E/P thre- ehold value should not exceed the field intensLty which corresponds to the break- down of the discharge gap.

The most favourable conditions for the EPI-discharge burning are realized in

three-component mixtures containing rare gases and haloid molecules, where the laser quantum energy is sufficient to photo- ionize the low metastable levels of the rare gas R, and is not enough to photoio- nize the buffer gas A.

The present paper demonstrates the technical feasibility of an EPI laser pumping in Ar:Xe:CCl -1500:50:1 mixture and

4-

P = 2 atm. Threshold value of E/P for the EPI-$ipcharge burning does not exceed (in this mixture) E/P values for the designed laser based on electron transitions in excimer XeCL* molecule /6/. The laser radiation quanta of this molecule

(3

⁼³⁰⁸

tailed in / 6 / .

If

u/@

⁼1.5 kV/cm.atm

(U

is the ap- plied voltage) and the ballast resistance

f$

= 0. I Ohm, then the electrons and the laser radiation disappear after cessation of the external ionization (see curves I

~ / 4

⁼³kV/cm. atm and

4 */Q%-)

1 1

= 0.5 Ohm then the discharge current and the light flux density insidesthe resonator are available after cessation of ionization (curves I1 in Fig. I ). PIIoreover

,

they have the stationary values,which re-

rim) may ionize the low metastable energy main unchanged over the calculated time.

levels of Xe*. The following discharge ee- Here the specific power injected into the ometry has been calculated:L=50 ~m,&~/~q= BPI-discharge is 300 W/cm , a d the power 3 1, the space between electrodes l=2cm; dis- fluxes in the resonator are& =3.6 W/cm 2

.

charge Volume ,~=200cm'. The Kirchgof f eq- uation has been taken into account. The power of external ionization source

u-@

was equivalent to an electron beam with energy fe =I 50 keV

,

and m ~ i m u m current density

k

.%6 ~/crn~ (the beam has a tri- angular form; the rise time is 30 ns, the decay time, 22D ns). The main proces- ses that affect kinetics of the laaer mixture generation,and which have been taken into account in our calculations, are de-

Note Chat the discharge chain impedance determines the steady-state regime. The EPI discharge may burn rather uniformly due to %ixingU of plasma parameters in- side the resonator by the radiation. Be- sides laser pumping,the EPI-discharge may find applications,viz.,for modulation of high currents by affecting the Q-factor of the resonator.

References

7. J.Hoffman et al.Appl.Phys.Lett.28,1976.

2. R.Bradf ord, Opt. Corn.%, 21 0, lT6.