ON THE PECULIARITIES OF PASSAGE OF A LOW-CURRENT ELECTRON BEAM THROUGH GASES

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ON THE PECULIARITIES OF PASSAGE OF A LOW-CURRENT ELECTRON BEAM THROUGH

GASES

A. Pal, I. Persiantsev, Yu. Petrushevith, A. Starostin

To cite this version:

A. Pal, I. Persiantsev, Yu. Petrushevith, A. Starostin. ON THE PECULIARITIES OF PASSAGE

OF A LOW-CURRENT ELECTRON BEAM THROUGH GASES. Journal de Physique Colloques,

1979, 40 (C7), pp.C7-345-C7-346. �10.1051/jphyscol:19797170�. �jpa-00219147�

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JOURNAL DE PHYSIQUE CoZZoque C7, supplgment au n07, Tome 40, JuiZZet 1979, page C7- 345

ON THE PECULIARITIES OF PASSAGE OF A LOW-CURRENT ELECTRON BEAM THROUGH GASES

A.F. Pal, I.G. Persiantsev, Yu. V. Petrushevith and A.N. Starostin.

I n s t i t u t e of Nuclear Physics, Moscow State University, 11 7234, Moscow U.S. S.R.

In studying the non-self-sustained\ discharge controlled by an electron beam the current of pulsed beam is measured by me- ans of a Rogovsky coil or an electron collector,which collects the beam passed thro ugh the evacuated discharge chamber ( e

.

^g.,

in

[I]

both methods yielded identical res- ulfs). Steady-state beams are measured on- ly with the use of the collector.

In our experiments carried out on an installation similar to that described in

[ 2 ] , the 1x1 cm anode of the discharge 2

chamber,which was grounded through a resis tance R,,,semed as the electron collector.

The beam was injected into the chamber through a graunded grid that served as the discharge-chamber cathode.The distance bet ween the electrodes was L-1 cm.!Che electron gun wag feeded by variable voltage (fig

I

,osc,

I

) .Oscillogram 2 of fig.

I

represents the collector potential with respect to the grid, which is proportional to the current through the discharge gap filled

with the I-atm. air. The same oscillograms have been obtained

in

Cop or a vacuum-lo-' Torr. As the beam passed through the chamber filled with Ar at the same current and voltage of the gun, oscillogram 3 of fig.1 has been obtained. It is seen that at a low energy of the beam electrons the collector potential becomes positive, which may be associated with the passage, through the discharge gap, of a current whose direction is opposite to that of the electron beam. By decreasing the gun voltage one can obtain a positive collector potential at a

maximum

beam energy, i.e.,a ful- ly "reversedn current, Analogous results have been obtained when the beam passed through the nitrogen. The potential which appears on the collector grounted through the R H and which is positive relative to the grounded grid potential may be expla- ined by the presence of electric gields near the electrodes.These fields and the

corresponding near-electrode potential jumps arise due to the redistribution of charges produced by the beam in accordance with the boundary conditions necessary for the passage of current (see fig.2 showing schematic E-field distributions,concentra- tion of electrons

ne

and ions n i (dashed curve) and also indicating the directions ofthe ion current and those of the elect-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19797170

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ron-current components in the various re-

Zn

considering the anode region we in+-

We now turn to the qualitative conside- ration of the observed phenomena. The ini- tial set of equations has the form:

.i7\Jini ()-/ne"i

T - 4

⁽¹⁾

3 E 4fi/ni-ndj,.ne/o)=ne/r/=o

T -

Here %,=-keE is the electron drift velocity, Wi=kiE-the 'ion drift velocity, Dl- the longitudial electron-diffusion coeffi- cient, q-the rate of generation of secon- dary electrons by the fast electrons of the beam. From the set (1 ) :

?Be

⁽²⁾

d=eh/ini -eh&e+egL,

where j is the current density in the discharge chamber, Let us divide the discha*

ge-chamber volume into three regions (fig.

2) : 1 .The interval

8 ~ - 1

o - ~ c ~ near the grid surface-the "cathode regionw. 2.The "positive column", which occupies the main por- tion of the discharge-chamber volume.

3

.The "anode regionw-the interval#*

-

¹^0-2

cm near collector.

Let us consider the experimentally observed case of the "reverse" current pas- eing from "cathode"

to

"anode", Let us ex- amine the "positive columnn reaon. From (2) it follows:

E(%)= a/e

_Kene

- 8-n ₊

_Ki

*

_ffi

Beglect g kini, we have:

'% a~ ne(;6p), ,

u ^c-- ^- $ ; b x = - ( j / e ~ e ) ~ ' G ) + ~

^r

^n, ⁴

roduce the approximate boundary conditions (cf

. ,

cathode-laxer theory

1)

ne=~)i(l-6)=~~(.--6)zo.

⁾

4 'i(~)=j ⁽⁴⁾

From th$ set

(I 1,

taking account of

(4)

we

From the conditions at the boundary of the

"positive columnr1 region,

u,=

^u(~K))=

^P, -$$

^{,we get}( 6 ) Substituting the obtained voltage-drop into the set for external electrical cir- c ~ i t . ~ e obtain U=U~+UK+UA=

r h ; a=f/S-jt(7)

Here S is the electrode area,

#

is the fast electron current density. As a result

where

electron near c~llector,(~~//?e)~ in the

"positive columnn and

(8f/k'e)k

in the "cathode regionw.

For Ar under p=l atm,f-70 keV andd;/=

16

.

50

mkA we have using [I]

p6.10

_?/cdeec,

Dl/ke=0,2 eV and

I=17

mkA is opposite to beam current .For ,air ne-I ~ ' O c m - ~ ,ke O,uo3 cm3/v.sec and I=d/S< 0 is equal to the beam current.

References

Phys.

,B,

4991, (1977). [41~.~.~.Huxle~ and R.W.Crompton "The diffusion and drift of electrons in gasesu(In russian),Mir Pub- lishers, Koscow