OPTICAL INVESTIGATIONS OF THE SPONTANEOUS EXTINCTION OF THE HIGH-CURRENT KNUDSEN ARC DISCHARGE IN Cs-Ba GAS MIXTURE

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Submitted on 1 Jan 1979

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OPTICAL INVESTIGATIONS OF THE SPONTANEOUS EXTINCTION OF THE

HIGH-CURRENT KNUDSEN ARC DISCHARGE IN Cs-Ba GAS MIXTURE

V. Babanin, V. Kaplan, I. Kolyshkin, V. Kuznetsov, A. Martsinovskii, A.

Mustafaev, V. Sitnov, A. Ender

To cite this version:

V. Babanin, V. Kaplan, I. Kolyshkin, V. Kuznetsov, A. Martsinovskii, et al.. OPTICAL INVESTI- GATIONS OF THE SPONTANEOUS EXTINCTION OF THE HIGH-CURRENT KNUDSEN ARC DISCHARGE IN Cs-Ba GAS MIXTURE. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-201- C7-202. �10.1051/jphyscol:1979799�. �jpa-00219505�

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JOURNAL BE PHYSIQUE CoZZoque ~ 7 , s u p p t 6 m e n t a= n 0 7 , TO, 40, J u i Z t e t 1979, awe ~ 7 - 201

OPTICAL INVESTIGATIONS OF ME SWNTANEWS EXTINCTION OF THE HIGH-CURRENT KNUDSEN ARC 'DISCHARGE IN Cs-Ba GAS MIXTURE

V.I. Babanin,V.B. Kaplan, I.N. Kolyshkin, V.I. Kuznetsov, A.M. Martsinovskii, A.S. Mustafaev, V.I. Sitnov and A.Ya. Ender.

loffe PhysieaZ-TeehnicaZ Institute of t h e U.S.S.R. Academy of S c i e n c e s , Leningrad, U.S.S.R.

The investigations of the spontaneous arc extinction in a switching Cs-Ba triode with a fine-mesh grid, operating in the

collisionless mode, have been carried out.

The use of Cs-Ba mixture, where cesium is a plasma-forming component, allowed to obtain emission currents from the cathode up go 100 A / C ~ in Cs-pressure range 10'

-

^10-2^{Torr and,}thus, easily attain the limits of gas current-conduction capacity.

It is shown, that the critical current jcrit, preceding the spontaneous extinction of the arc ( the upper current limit), doesn't depend on Ba pressure and is proportional in a wide range to Cs pressure

( Fig.1 ). It is ascertained, that jcrjt is the density of electron current, whlch can be compensated at the complete ionization of the plasma-forming atoms,

here

7.-

average ion velocity in the plasma,m.(h )

-

ion (electron) mass, P -gas

preshurg, f

-

temperature.

Time of existence of the arc discharge under conditions, when the critical value of the discharge current is exceeded, is inversely proportional to

A

j= j

-

^{jc it}

( Pig.2 ) and is almost by an order

OF

magnitude longer, than the similar period in the case of the inert gas discharges.

Such a prolonged existence of the current conduction state may be axplained by desorption of the plasma-forming atoms from the electrode surfaces. It is found, that the spontaneous arc extinction is followed by a relaxation period- the time period, in which the ignition of the arc discharge doesn't occur. This time proves to be commensurable with the time of existence of the plasma conduction state.

Further, it is found, that the arc extinction is preceded by the apperience of plasma instability and the development of plasma oscillations.

In order to study the mechanism of the spontaneous arc extinction the time dependencies of the luminosity of a series of Cs1,BaII and Bar lines were obtained; the luminosities of all lines investi ated were measured in the subcritical

7

^points

A

-

^C^), ^critical⁽^point^D⁾ and super- critical ( point E ) discharge modes, as well as at seven points across the interelectrode spacing ( Fig.3 ).

Recording and processing of the expe- rimental data were performed with the use of the multichannel measuring and calcu- lational system, based on the computer

BESM

-

4 and intended for the investiga- tion of non-stationary periodic processes*

the resolution of the order of Ipsec being provided. All the analogue signals were being strobed simultaneously. The recording with the time resolution of 0.2 Psec was being performed by the single-

-channel analogue strobe-integrator. For increasing the ratio llsignal-noisell the numerical averaging in the computer was employed ( N=128 ). The synchronous recording of the electrical and optical relationships using the multi-channel system made possible to reveal the time shift between these relationships, as dell as between optical processes at different points of the interelectrode spacing.

To determine the value of the time shift the correlation analysis was employed. The correlation function

for the luminosity relationships, obtained for the BaTT line (h -7060%) in the cathode ( point 2 ) and anode ( point 6 ) regi- ons of the discharge, is presented in Pig.4. The examination of the correlation method,being applied in our case, showed, that the measurement error of the time shift values didn't exceed 20% of the time step, i.e. was equal to 0.2pec.

The results of data processing demon- strated, within the accuracy indicated above, the absfience of time shifts during the oscillation development in the discharge modes A

-

^D. It means, that the oscillations in the subcritical mode are

I

not connected with the propagation of potential waves, potential jumps and other disturbances across the interelectrode spacing and are accompanied by the synchronous change of the potential at all points in the spacing. The results, obtained in the mode E, in which the spontaneous arc extinction occurs, are different. As is seen from F i a 5 and 6, under these conditions the process$s non-synchronous throughout the spacing: before the arc extinction flashes of the highly excited BaI and BaII lines radiation occur in the anode region by 3psec earlier, than in the cathode region, which is due to the abrupt change of the plasma potential. The spon- ,taneous arc extinction is preceded by pre- ,paratory processes involving atom concen-

tration depletion, manifested by the de- crease of luminosity with time.

Prom the optical measurements it may be concluded, that the oscillatory pro-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979799

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cesses in the subcritical mode occur syn- chronously at all points in the spacing;

these measurements confirm also the assump- tion about the spontaneous extinction of the high-current low-pressure discharge being due to the atom depletion in the spacing.

Thus, the arc extinction in the triode, having the fine-mesh, highly-transparent grid, is due to the high degree of atom ionization and to the escape of atoms from the spacing, while the large duration of the current pulse is determined by atom desorption from the electrode surfaces.

+

2

Pig.4 a

-

Time depende ce of the BaI

I line (.A = 7060

ft

⁾ luminosity

at the points2 and 6 in the

2 spacing ( mode B ) ,

, t o r r b

-

Correlation function.

Pig.1 Critical current density as a function of cesium pressure 1

-

experiment

2

-

^'theory

Time of existence of

8,

_\ the arc discharge as

a function of the ¹⁰^'

%! subcritical current I luminosity of Ba and

\% density

1 1 I 11 ¹

^{Cs lines}⁽^mode^E^1:

a

^PCB=^8*l0-~^Torr ^{I 1}

1

^lo

' 4 0 -

^'^B^;⁼ ^2.1^0-4^Torr ^.^-^(point

^k

^j^;

^,

o jA/cm 1 ,I a-CSI, _A=7279&:

2,2"-~a1, A =5535A;.

3, ~"-B~II,A =4934 A.

dT,rsec\

' 8

Fig.2

Time of existence of

! the arc discharge as Time dependencies of

81 \ a function of the the current and

%! subcritical current luminosity of Ba and

\% density

ya PCB= 8 * 1 0 - ~ Torr

\D\

%k0-

.

^'^;^B^' 2.1

o - ~

Torr

0 10 20 a 4A/cm

Volt-ampere characteristic of' the discharge and the schematic view of the interelectrode spacing