LONG-TIME CHANGES OF AN IONIZATION WAVE IN THE CO2 + N2 + He MIXTURE

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LONG-TIME CHANGES OF AN IONIZATION WAVE IN THE CO2 + N2 + He MIXTURE

H. Urbánkova, L. Pekárek

To cite this version:

H. Urbánkova, L. Pekárek. LONG-TIME CHANGES OF AN IONIZATION WAVE IN THE CO2 + N2 + He MIXTURE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-375-C7-376.

�10.1051/jphyscol:19797184�. �jpa-00219162�

JOURNAL DE PHYSIQUE CoZZoque C7, supple'ment au n07, Tome 40, JuiZZet 3979, page C7- 375

LONG-TIME CHANGES OF AN IONIZATION WAVE IN THE C o p + N2 + H e MIXTURE

H. Urbankova, L. ~ e k a r e k .

I n s t i t u t e o f Physics, Czech. Acad. S c i . , Na SZovance 2, 180 40 Prague, C.S.S.R.

For the investigation of slow changes in the composition of gas mixtures in c w C02-lasers, several methods have been used: gas sample

bottles for mass analysis /1,2/; side- -light emission of the 4835 A (0-1) Angstrom band of CO / 3 / ; freezing out of the mixture in li uid nitrogen and gas chromatography /47; mass spectrometer joined to the system /5,6/ etc. We used the changes in parameters of self-excited ionization waves

/7/

for continuous registration of temporal changes of the state of the mixture of C02, CO, N2 and He. The dispersion curves of the

ionization waves were taken by the method given in / 8 / . At the e d of the

experiment, i. e

.

^{when %e} discharge ceased to operate due to the expansion of the cathode fall almost over the whole tube, the discharge tube was connected to the manometer and to the mass spectrometer to measure the total pressure and composition

of the remaining gas mixture.

The tube 2.2 cm in diameter was water cooled, made of Pyrex glass, with

cylindrical nickel electrodes and the electrode distance was 70 cm. Two wire probes placed

7

and 13.6 cm from the cathode were used to measure the longitudinal electric field in the discharge positive column. The

measurements were accomplished in the mixture 1 Torr C02

+

^{1 lorr N}

+

^{1 Torr He}

at a constant current of 40 m%. The tube was sealed off before the experiment. To achieve the s h o ~ t e s t possible lifetime of the gas fill the tube was designed with minimum ballast volume.

Pig. 1 is one example of the obtained time dependencies. The ionization wave, which is of the forward anode directed type, changes its parameters slowly until 2 4 hours. At this moment a point is reached where the wave converts in a relatively s h ~ r t time into a backward wave with cathode directed phase velocity. The bending of the frequency dependence upwards followed by change of its sign

(and, hence, of the direction of phase velocity) can be attributed to growing influence of helium, which itself has only backward ionization waves /9/. In another experiment where only 1 Torr C02

+

¹ Torr He mixture was used, the result was

qualitatively identical, i.e. with helium- -like backward waves observed in the end of the experiment. The abrupt fall in wave number as well as the falling

electric field corroborate the latter

conclusion, i.e. the diminishing abundance of molecular gases.

Surprisingly, under the initial partial pressures given in Fig. 1 , the total pressure in the end of the

experiment was found equal to 0.26 Torr which was significantly lower than the initial partial pressure of each of the three gases including helium. As expected according to the observed wave pattern, the mass spectrometer revealed that the main remaining gas was He, with CO and/or N and 0 not exceeding their (rather

s$rong) gackground measured prior to the connection of the experimental tube to the spectrometer chamber. There was no peak corresponding to C02.

Comparing these results with mass spectrometer measurements of Carbone /I/, we conclude that the slow changes of the electric field, the wave number and the frequency of the forward ionization wave correspond to the known slow decrease of C02 during the discharge operation (CO*, CO and O2 being adsorbed and chemisorbed on the tube walls and on the electrodes).

The rather abrupt change around 2 4 hours accompanied by a drastic fall of pressure must, however, correspond to another phase of the changes in the mixture of which only the very beginning was observed

/I/, Indeed, the abrupt fall of pressure begins apparently when all CO molecules are dissociated and correspon$ing oxygen part is absorbed. Dissociation of CO molecules with further loss of oxygen then leads to the observable thin layer of carbon deposit on the tube walls near cathode. This carbone deposit in turn should cause further adsorption of helium and nitrogen ending in an appreciable lowering of the total pressure. Chemical analysis proved at the same time a

considerable amount of nickel nitride formed at the cathode region.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797184

Fig.1. Time dependencies of the electric field, the wave number and the frequency of the ionization wave. The minus sign of the latter parameter designates the cathode+anode directed phase velocity v_ = U /k. Bear the = 0 transition

1C02

+

^1N2

+

^1He

SNIT. TOT.PRESSURE 3 Torrs I = 40 mA

~;Eint (at ~ 2 4 hours) the frequency measurement was inaccurate due to very flat maximum of the increment curve.

-

E 60-

s _-

I 3 40--

_I

e

LL 20- I!

Cf

t;

9

w

REFERENCES

.

^{0 .}

. ..

t , . . , I , .

2 ' 4' 6' 8'10' ' ' ' ' 2 0 ' '

/1/ R.J. Carbone, IEEE J.Quant.El.

QE-3,

P. 373-375, Sept

.

¹⁹⁶⁷

TIME OPERAT ION(hours1

/2/ R.J. Carbone, IEEE J.Quant.El. QE-4, p. 102-103

/3/ W.J. Wiegand et al, Appl.Phys.Lett., 16, 1970, p.237-239

-

/4/

E.B. Lotkova et al, Khim.vys.en.,z, (19681, p.278-283; E. S. Gasilevitch et al, Zh.T.F.

2,

(19691, p.126-132

/5/ K.M. ~ ' ~ m i c o et al, J.Phys.D: Appl.

Phys., l0, (19771, p.261

/6/ N. Karube et al, J. Appl.Phys.,

41,

(1970), p. 2031-2042

/8/ J. Skgla, Czech.J.Phys. B (1973), p.284

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m,

^(19761,

P. 764