STUDIES OF DISCHARGE PHENOMENA DURING TRANSITION FROM BREAKDOWN IN VACUUM TO BREAKDOWN IN GAS

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STUDIES OF DISCHARGE PHENOMENA DURING TRANSITION FROM BREAKDOWN IN VACUUM

TO BREAKDOWN IN GAS

A. Pokrovskaya-Soboleva, T. Borisova, G. Grishutin, J. Nastitch

To cite this version:

A. Pokrovskaya-Soboleva, T. Borisova, G. Grishutin, J. Nastitch. STUDIES OF DISCHARGE PHE- NOMENA DURING TRANSITION FROM BREAKDOWN IN VACUUM TO BREAKDOWN IN GAS. Journal de Physique Colloques, 1979, 40 (C7), pp.C7-405-C7-406. �10.1051/jphyscol:19797198�.

�jpa-00219177�

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JOURNAL DE PHYSIQUE CoZZoque C7, suppliment au n07, Tome 40, JuiZZet 1979, page C7- 405

STUDIES OF DISCHARGE PHENOMENA DURING TRANSmClN FROM BREAKDOWN IN VACUUM TO BREAKDOWN !N GAS

A.S. Pokrovskaya-Soboleva, T.S. Borisova,

G.S.

Grishutin and

J.N.

Nastitch.

AZZ-Union EZectrotechnicaZ I n s t i t u t e , Moscow U.S.S.R.

Electric strength of

a

vacuum gap may be described by a generalized curve of breakdown voltage as pressure Ubr= f (P) as shown in

Fig.1

(curve a,b,c). It is known that the horizontal part of this curve relates to breakdown in a vacuum.

The slope of the left branch of the Pas- chents curve for the ignition voltage of glow discharge is connected with secon- dary electron emission processes initia- ted by positive ions excited atoms, fast neutral atoms and photons / r -processes/

/?I.

Of specific interest are studies of discharge phenomena in a narrow dange of pressures when a transition from vacuum breakdown to glow discharge ignition takes place.

Some investigators /2/ observed maxL- mum, in some cases, doubled electric strength values at these pressures.

Studies were performed with experi- rpental discharge tubes to which square voltage, pulses were applied. A bipolar palse-forming line with high-voltage hy- drogen thyratron as a ltswitchN was used as a voltage source. Crest voltages up to 100 kV with Z * I O - ~ S pulse duration were applied to the gap. The phenomena studies could be registered by an oscil- lograph with high voltage plates, so that the test voltage was applied directly with- out using a voltage diviser. Pulses of fraction of a microsecond duration could be recorded.

When square voltage pulses were ap- plied to a discharge gap of d=0,5 + ^{6 mm}

at pressures within + lom2 mm Hg diffuse luminescence resembling a glow discharge was observed visually. Along

with this diffuse luminescence spontaneou- sly rupted spark discharges accompanied by X-ray. Oscillographic records of dis- charge voltage and current wareforms show that the diffuse luminescence relates to a high voltage glow discharge with a ris- ing current voltage characteristic /3/.

The discharge voltage was several tens of kilovolts at currents up to 200 mA. In a high voltage glow discharge the field strength on microprotrusions is sufficient for the formation of spark discharges.

They are accompanied by a local release of gases from the electrodes which in turn support the glow discharge. Oscillo- grams of voltage presented in Fig.2 (a, b,c) confirm that simultaneous existence of two types of discharges is possible.

The authors of /4/ also observedM concurrent types of discharges at

d = 2 0

mm, One of the discharges was constricted spark accompanied by an abrupt increase of current; the second was a diffused glow typ2 discharge. The quantity of re- leased gases is proportional to the elec- tric charge that had passe& through the gap. Thus the passage of a single elemen- tary charge liberates two to three gas atoms. In this work a diffused discharge was observed after several constricted

spark discharges appearing upon the ap- plication of a sequence of pulses.

Ori

the basis of their investigations the authors of /4/ indicated the possibility of an exchange mechanism described by Van-Atta et al. which could explain the phenomena observeda It appears to us ha- wever that regenerative sputtering of ions of opposite signs from the cathode and anode would be unlikely.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797198

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The comparison of results obtainedin sustained discharge modes in thus sugges- this paper and in previous works /1,3,5/ ted

as well as the phenomena.observed by oth- er workers

/ 2 , 4 /

allow to represent a breakdown of a gap by a family of curves

"b

r= f (p) for different gaps, as given in Fig.?.

Formation of spark discharges is a statistical phenomenon. It corresponds to the horizontal part of curve ubr= f (P) (portion a-b) where the electric field strength is the dominant parameter. The right part is the Paschenls curve (cl, **c*...)** for the ignition of glow discharge due to r -processes.

It would be natural to assume the existence of a gradual transition from E to -processes in the "transitional regionu (part b-c) where two types of discharges may occur simultaneously. Ex- periments shown that in the "transitio- nal regioni1 of pressures where deviations from the Paschen's curve take place, the passage of current within the discharge gap is influented by field strength, ove- rall voltage and pressure,.

With the increase of interelectrode spacing the deviations from Paschen's law occur at higher voltages and at lo- wer pressures. In this case a maximim of breakdown voltage appears in the Ittran- sitional regiont1 of pressures (Fig.?, dotted line).

Thus the following pattern in the

transitional

regiont1 of pressures may be assumed. To form a high voltage glow dis- charge a sufficiently high gas pressure in the discharge gap is necessary. A certain amount of gas or metallic vapour may be released from the electrode due to local heating of microprotrusions by electron current during spark discharges.

Subsequent pressure reduction results in the growth of voltage drop across the glow discharge. A positive space charge near the cathode increases accordingly leading to a growth of the field strength.

The latter contributes to a spark initia- tion. A mechanism of two competitive self-

a - high voltage glow discharge (curve c in Fig.1)

b

- spark discharges (part a-b of the curve in Fig.

1)

c - glow discharge simultaneous with spark breakdown (part b-c of the Fig.

1 )

References

I. 5.E.

b @ ~ i b ~ ,

J.T. Yycesa, A.C.

I I O K ~ O B C I C ~ R - C O ~ O ~ I ~ B E ~ , XTa, 1966,

BbIII.

4,

TOM

36, 704 CTP.

2. Chathan M. Cooke. Second Inter.

Symp. on Insulation of high voltages in Vacuum. 1966, 181 p.

3 .