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Submitted on 1 Jan 1979
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ON THE FORMATION OF PLASMA CARBON COMPONENT IN THE HOLLOW CATHODE
ANOMALOUS GLOW DISCHARGE
S. Ibadov
To cite this version:
S. Ibadov. ON THE FORMATION OF PLASMA CARBON COMPONENT IN THE HOLLOW
CATHODE ANOMALOUS GLOW DISCHARGE. Journal de Physique Colloques, 1979, 40 (C7),
pp.C7-173-C7-174. �10.1051/jphyscol:1979785�. �jpa-00219491�
JOURNAL DE PHYSIQUE CoZZoque C7, suppzdment au n07, Tome 40, JuiZZet 2979, page C7- 173
ON THE FORMATION OF' PLASMA CARBON COMWNNT IN THE HM_LOW CATHODE ANOMALOUS GLOW DISCHARGE
S. Ibadov
AstrophysicaZ I n s t i t u t e of the Academy of Sciences of Taszhikistan, U. S. S . R.
Plasma, containing a component of atoms and molecules of carbon, occured in a series of important cases, for example it may appear in the laboratory high-temperature plasma, contacting with graphite /1/. At this point the situations with simultaneously bombard- ment of a surface by different ions in the range 100 eV remains poorly studied / 2 / . Cometary molecules C2 and C3 determine the visual diameter of cometary heads /3/ and may create the cometary atmospheres dust component directly in the environment of the cometary nucleus by condensation /4/.
Laboratory investigation of the formation of solid phase in the heads of comets at the cost of Action of condensation mechanism required the creation in a plasma medium an over-saturated carbon vapor phase with the concentration n c a 10" rnolecula/cm 3
.
The use of the hollow cathode glow discharge /5-10/ is of interest for the studying of possibilities of over-saturated carbon vapor generation and also for the studying of graphite behaviour in the flow of different ions with energies 100 eV
.
All base possibilities of formation of car- bon component in plasma (evaporation and sputtering of cathode by ion bombardment;
dissociation of organic molecules in plasma of discharge) are laid in such a discharge with a graphite cathode
.
In the present work the results of investigations on determinating of carbon component concentration in the plasma of anomalous glow discharge in the graphite hollow cathode in helium and in propan
( C H ) at the discharge currents , 3 8
Ia = 50
-
500 mA (the densities of ion ir- radiation of graphite-
1017 ion/cm s) 2 the energies of ions irradiating graphite450
-
500 eV ( ~ e + ) and 600-
650 eV (ions from plasma of discharge in propan), the pres- sure of filling gase p N lmm Hg (He) andg
p 4 0.1 mm Hg (C3H8) are presented
.
The discharge system used was a modifi- cation of the discharge part of a plasma ion source /9/
.
It consisted of an almostclosed cylindrical hollow cathode (the radius R = 1.2 mm) and the length L=10 mm) with plane ends and pivotal anode (a tungsten wire with the diameter d =1.5 mm) introduced directly into a the cathode cavity along its axis ( on the length la=7-8 m m ) . The whole system was in- serted into homogeneous magnetic field of a coaxial coile.
The determination of the carbon vapor concentration in a plasma was carried out by measuring of the graphite layer mass AM, condensed on the anode pivot during certain irradiation time t
0
Hereo( is the accomodation coefficient of carbon a t o ~ on the anode surface; mc is the
-
mass of a carbon atom; Sa is the anode area corresponding to the condensate mass A M ; TC and VT =
1 -
are the tempe- rature ans the mean thermal velocity of car- bon atoms in the plasma.
The relation (1) was applied under the conditions j( 1 A/cm 2
,
p R n 1 0 . l-
1 mm Hg.cmq
when the motion of atoms-in a plasma has the diffusion character and the heavy component of a plasma may be supposed isotexmic :
T =T =T /11/. Here R is the linear dimension c g w
of the cathode cavity; T is the temperature g
of the filling gase; Tw is the temperature of the cathode discharge chamber wall which was measured with a thermocouple and also pirometrically
.
The anode temperature TaArticle published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979785
was also measured pirometrically
.
form :In helium discharge the measured densi- ty of the condensation flow of carbon atoms on the anode was Jc= A M/SatOm 5 (1 0-'-1
o - ~ )
g/cm s 2 ; for the discharge in propan Jc was approximatelly three times as large
.
Thetemperature of the cathode wall (of the graphite target) was in the range T W = 500
-
1400 K at discharge power IaVa = 20-
300 W.
In Fig. 1 the results of the calcula- tions carried out by (1) for one of regimes of the discharge are given
.
Fig. 1.- The concentration of carbon atoms in the plasma versus discharge current
.
1-discharge in helium at pressure in the cathode cavity p ~ , ~ l mm Hg, discharge vol- tage Va@500 V; 2-discharge in propan :
.v 0.3 mm Hg, Va&650 V; B0=50
-
400 Gs,::g~nst; T, rnax@ 1500 K,H 1
.
As far as the temperature T w ~ 2 6 0 0 K is required for the generation of carbon vapors with the concentration n C d l ~ ~ ~ c m - ~ , i
.
e.with the partial pressure pc.v10 -4 mrn Hg by heating of a graphite /12/, the carbon vapor in the plasma is in the state of strong over-saturation and has a non-evaporation origin
.
The discharge in propan with the hollow cathode in which the surface of a graphite is protected against ion irradiation by the tantalum envelope gives very small flow of carbon atoms Jc
.
At the same time the ob- served rise of the inner diameter of the cylindrical cathode surface indicates an intensive cathode sputtering process.
In the conditions of the dominating role of an ion sputtering the relation for the stationary concentration of carbon atoms
Here s = s (Val M+ etc.) is the coefficient of cathode sputtering atom/ion ; e is the charge of ion with the mass M+ irradiating graphite; S is the area of the plasma boun-
PI
dary at the cathode which owing to small thickness of near-cathode ion layer practi- cally equals to the area of the cathode S;
d =
b / ( ~ ~ , Va ) is the generalized coeffi- cient of the secondary electron emission from cathode which may be determined expe- rimentally /lo/.
The relation (2) satisfactorily descri- bes the curves of Fig. 1. For example in the caseoftheheliumdischargeatrealvalues s-0.2.
(for EHee500 eV
+ -
data of /1/ are extrapo- lated), Ia= 200 mA, 8-2, S = 16 cm 2 , VTca 1.1 x lo5 cm/s we get by (2)n c ~ 2 x 10 l1 ~ m - ~ . In the case of the propan discharge an accordance between nc from Fig.1 with that calculated by (2) takes place at
The author is indebted to E.M. Dubinina and O.V. Dobrovolsky for helpful discussions.
References
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M . I . Guseva and J . V . Martinenko, Phys.
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.
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44
(1978) 469
.
/3/ O.V. Dobrovolsky, Comets, Moscow, 1966
.
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(1967) 434
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/5/ P.F. Little and A. Engel, Proc. Roy.
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(1954) 209.
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/10/S. Ibadov, Proc. XIth Intern. Conf. on Phenomena in Ionized Gases. Prague, 1973.
/11/V.L. Granovsky, Electrical Current in Gase, M., 1971
.
/12/A.N. Nesmejanov, Pressure of Vapor of Chemical Elements, Moscow, 1961
.
in the considered plasma in the first appro- ximation may be presented in the following
