The electron distribution functions relative to the velocity component directed along the magnetic field are compared for PIG and arc type discharges. The identity of these functions for the plasma region pierced by the primary electron beam and their difference in the peripheral part of the discharge are shown. It is concluded that the electron distribution function in the PIG type discharge is formed during one transit of the primary electron through the discharge gap. The mechanisms of electron energy spectrum formation in both the axis region and the peripheral region of the discharge are discussed

Romanyuk, L. I.

Suavilnyy, N. Y.

English

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NASA TECHNICAL MEMORANDUM: _' NASA TM-75255
_COMPARATIVE_CHARACTERISTICS_0_P ELECTRON J|NERGY_
.^SPECTRUM''IN PIG AND A'RC TYPE DISCHARGE PLASMAS
1
 *'
L.I. Romanyuk. and N.Ye. Svavil'nyy
TfFASA-TM-75255) COMPARATIVE CHARACTERISTICS N78-18307
OF-'ELECTHOH ENERGY SPECTRUM IN PIG AND ABC
TYPE filSCHAEGE PLASMAS (National Aeronautics „,-,.,„
and Space Administration) 8 p CSCL 09C Unclas
Tr.arislaflon of "Sravnltel' nye kharakterlstlkl energetlcheskogo
spektra elektronov v plazme razryada pennlnga 1 pryamogo
razryada," Ukrainskly Fizicheskiy Zhurnal, Vol.221, No. 12,
Dec. 1976, p. 1978-1982.
Copyright Statement: "T-<—* . ;
"NOTICE. Because of Copyright Restriction this
translation has not been published. This copy ,
is for the internal use of NASA personnel and any
reference to this paper must be to the origlfa]
foreign s(
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WASHINGTON, D.C. 205^6 FEBRUARY 1978
https://ntrs.nasa.gov/search.jsp?R=19780010364 2020-03-22T05:56:45+00:00Z
STANDARD TITLE PAGE
1. Report No.
NASA TM-75255
2. Government Accession NO. 3. Recipient's Cotolog No.
COMPARATIVE CHARACTERISTICS OF ELEC-
TRON. 'ENERGY SPECTRUM IN PIG AND ARC"""
TYPE DISCHARGE PLASMAS
S. Report Data
•February 1978
6. Performing Organ!zotion Code
7. Author(s)
L.I. Romanyuk and N.Ye. Svavil'nyy
Institute of Nuclear Research, Academy
of Sciences/Ukrainian .SSR, Kiev
8. Performing Orgonizotion Report Mo.
10. Work Unit No.
9. Performing Orgonizotion Nome and Address
Leo Kanner Associates
Redwood City, California 9^063
11. Contract or Grant No.
NASW-27QQ
12. Sponsoring Agency Name and Address
National Aeronautics and Space Admin-
istration,, Washington, D.C 205*16
13. Type of Report and Period Covered
Translation
14. Sponsoring Agency Code
IS. Supplementary Notes
Translation of "Sravnitel 'nye kharakteristiki :" ,
energeticheskogo spektra elektronov v plazme raz~ryad"a~
penninga 1 pryamogo razryada," Ukrainskiy Fizicheskiy
Zhurnal, Vol. 21, No. 12, December 1976, p. J1978"-1982,.,
16. Ab.troet The electron.distribution functions relative .to. the
velocity component directed along the magnetic field are
compared for PIG and arc type discharges. The identity of
these functions for the plasma region pierced by the
primary electron beam and their difference in the peripheral
part of the discharge are shown. It. is concluded that the.o
electron distribution• function.iinct.he PIG type discharge is
formed during one transit of the- primary electron through
the discharge gap.. The mechanisms of electron energy
spectrum formation in both the axis region and the peripheral
region of the discharge are discussed.
17. Key Words (selected by Author(s)) 18. Distribution Stotement
U. S. Government Agencies and
Contractors only
19. Security Clossif. (of this report)
Unclassified
20. Security Clossif. (of this page)
Unclassified
21. No. of Poges 22. Price
NASA-HQ
COMPARATIVE CHARACTERISTICS OP ELECTRON ENERGY
SPECTRUM IN PIG AND ARC TYPE DISCHARGE PLASMA
L.I. Romanyuk and N.Ye. Svavil'nyy
Institute of Nuclear Research, Academy of Sciences
Ukranian SSR, Kiev
In studies of the electron distribution functions by veloc^ /I §7.8*
ity component parallel to.the magnetic field ( v i , ) in a PIG ^
discharge from an Incandescent; cathode [1], it has been shown
that it differs significantly from Maxwellian_in the axis region
of the-'plasma, at electron energies E=mvAy2^23' eV. This dif-
ference is that, in the high electron velocity region, their
number considerably exceeds the value corresponding to the
Maxwellddistribution. By analyzing the experimental results,
we concluded that this excess of highly energetic particles,
the spectrum of which reaches all the way to E%1.5 eVa (Va is
the discharge voltage), forms by means of effective scattering
of the primary electron beam, resulting from collective processes.
to a considerable extent. The extent to which electron oscil-
lations .in a PIG discharge /affects the form of their energy
spectrum should have been determined. This could have been done
by comparing the electron distribution functionbby velocity in
a PIG discharge with the measurements of the functions for an
arc type discharge under the same discharge conditions. The l-.t
latter is understood to be a discharge,' in which the deflecting
electron is the anode, and the primary electrons complete a
total of only one transit through the. discharge system. The
meo.hanism- of formation of the nonMaxwellian part of the electron
spectrum in the. plasma /outside' the primary electron beam could
also have been determined by such a comparison.[1].
The experiments were carried out in a discharge chamber,
consisting of a 5^ mm diameter cylindrical anode, bounded on
one side by a flat indirect heating cathode,. with a d,i_ap_hr_agfe
.c oil ima ting the electron beam to a 10 mm diameter and, on the
other side, by a flat, 5^ mm diameter deflecting electrode. The
distance between the cathode and. the deflecting electrode was
1^5 -mm. All the results described below were obtained with a
steady state discharge in a H=135 oersted magnetic field (if
not separately}- stipulated), in helium at a pressure p=2. 6!;-10-22
mm Hg. The volta.gelmn the discharge was Va=150 V and the dis-
charge current jEa»-0. 37 A. To change the PIG discharge to an
arc type discharge, the anode potential was fedC the deflecting
electrode, and the electrode previously serving as the anode
*Numbers in the margin indicate pagination in the foreign text.
remained under a floating potential. To investigate the electron"
distribution by velocity component parallel to the magnetic field,
a five electrode electrostatic analyzer was used, located in the
plane of the deflecting electrode. The electron velocity dis-
tribution function was determined by automatic differentiation
of the electron blocking current reaching the collector of the
analyzer. The layout and methods of measurement of the electron /1979
velocity distribution functions was previously described in
greater detail [1].
The electron velocity distribution functions in anp.arc type
discharge, obtained at various distances r from the system axis,
are presented irT'Eig. 1. It can be seen that "the electron veloc=:t
ity distribution function in the region of the primary electron
beam (curve 3), both by the form of the highly energetic part
of the spectrum (E>20 eV), and in general structure, hardly dif-
fers from that obtained in a PIG discharge [1]. Actually, from
the results presented in Fig. 2, of a direct comparison of the
two functions, taken for this region of the plasma in an arc
type (curve 2) and PIG (curve 1) discharge , under the same dis-
charge conditions, it follows that, in both types of discharge,,
the Maxwellian part of jthe^  spectrum extends all the way to an
energy E%20 eV, and a'^lateaA^jregion and more rapid decrease of
JP_ (_Yjj_)_ _tnen follow. As is seen, in both cases, there is a
'Sj^ s_tantlalnfr.aGtion of anomalously fast electrons (EseVa), and
no noticeable maximum,.indicating its beam nature, is observed
in the distribution functions.
/W.c/f"
.,, • e so no BO wo ts0f.jaa'
Fig. 1. Electron veloc-
ity distribution functions
in arc type discharge: 1.
r=10 mm; 2. r=6 mm; 3. r=0.
Key: a. E, eV
S .-' "• ••••-•V.t.-v :V- . .
Fig. 2. Electron velocity
distribution function in
PIG discharge (1) an arc
type discharge (2) on system
axis (r=0).
Key: a. E, eV
It follows from the results presented above, that formation
of the electron velocity distribution functions in both cases
takes place in a single transit of the primary electrons through
the discharge gap. Since the length of the latter is less than
or on the order of the. free-:path length of the primary electrons
for pair collisions, such effective energetic scattering of them
in both discharges can only be caused by collective interactions
of the primary electron beam with the plasma. The energy spectrum
of the anomalously fast electrons indicates the existence of a
nonlinear mechanism--;of this interaction.
The studies showed that transformation of the electron veloc-
ity distribution function with radial distance from the system
axis in an arc type discharge is significantly different than in
a PIG discharge. The difference is that, in the arc type dis-
charge, a sharper decrease in the limiting energy of the electrons
recorded and fraction of the nonMaxwellian particles in the
spectrum occurs, with distance from the primary electron beam,
than in a PIG discharge. Thus, at a distance of 1 mm from it
(curve 2, Fig. 1), the maximum energy decreases by approximately
50 eV, and the number of electrons with E=100 eV decreases ap-
proximately ' sixfold. With further increase in the distance, the
fraction of fast electrons continues to decrease, the electron /1980
velocity distribution function approaches single temperature
Maxwellian, and the maximum energy becomes much less than eVa
(see Fig. 1 curve 1). The latter never has been observed [1],
in a PIG discharge, where the maximum energy always was at least
eVa. The results of a direct comparison of the electron velocity
distribution functions, recorded under the same discharge con-
ditions, at distance r=ll mm, for the arc type (curve 1) and PIG
(curve 2) discharges, are presented in Fig. 3. They show that,,
besides the significant differencesiintthe maximum electron ene •
energies in the spectrum indicated above, the relative number
of fast.particles is considerably less in the arc type discharge/ --
than in the PIG discharge. . ; •
It was proposed earlier [1] that the nonMaxwellian section
of the electron energy spectrum, extending all the way to EfyeVa,
in a PIG discharge plasma outside the primary electron column,
can be due to either angular scattering or radial export of the
particles from the axial region of the discharge, or by secondary
ion-electron emissions from the end electrodes ('diaphr4gm7Q[e^ b•-'-
fleeter) . The conditions for secondary ion-electron emission, ^ .from
the diaphragm in an arc type discharge are the same as in a PIG
discharge. Therefore, the absence of particles with energies
E^eVa in the peripheral plasma of an arc type discharge unambigu-
ously indicates that the contribution oftthis effect to the elec-
tron energy spectrum in an arc type discharge and, consequently,
in a PIG discharge, is negligibly small.
Only one single act of angular scattering of electrons inside
the beam, which ejected them outside it, also cannot explain the
type of energy spectrum in the peripheral region of the plasma in
Fig. 3. Electron veloc-
ity distribution functions'
in arc type (1) and PIG
(2) discharges outside the
primary electron beam (r=
11 mm).
Key: a. E, eV
a PIG discharge, obtained at distances
from the beam greater than the Larmor
diameter of the particles. Evidently,
the main factors which determine the
type of spectrum in this region, at
least of its high energy section,
should be considered the radial flux
of particles entering it from the
plasma column penetrated by the pri-
mary electron beam and the accumu-
lation of fast particles in the space,
as a consequence of their oscillations.
Actual_ly_,_radial density and poten- /198l
tial gradients in a PIG discharge
plasma, directed alike [2], essenti-
ally ensures the radial transport
of electrons to the anode, if ad-
ditional transport mechanisms con-
nected with plasma instability [3]
do not arise. These gradients are
directed opposite each otheriin an
arc type discharge [4], as a con- '
sequence of which, the radial trans-
port of electrons is hampered and,
moreover, there are no conditions
for the accumulation of particles.
This evidently explains the significant difference in the appear-
ance o.f the high energy part oft the electron spectrum of the arc
typerarid PIG discharges noted above.
'Data on the transformations of
the electron distribution function
in the-'peripheral region of a. PIG
discharge plasma with increase in
magnetic field strength,1, presented
in Fig. 4, are evidence in favor of
this point of view. It can be seen
that, withiincrease in the magnetic -
field, both the temperature of the
low energy, -Maxwellian part of the
spectrum, and the fraction of the
high energy particles in the total
number of electrons decrease. This
is natural, since, with increase in
the magneticefield, the radial trans-
port-velocity of the.-particles de-
Fig. 4. Electron veloc-
ity distribution function
in peripheral plasma (r=
16 mm) with different :.:;.;•;•
magnetic field strengths:
1. H=l65 oersted; 2. 105;
3. 75.
Key: a. E, eV
creases, and they successfully (Lose
their energy in the axial region of
the discharge, as a consequence of
which, it becomes Ucolder" on the
periphery of the plasma. The absence
of an appreciable number of anoma- i
;lousty.fast electrons in this region
o
of the-plasma is connected-with the fact that they form primarily
in the primary electron beam, and they do not accumulate in space,
since they leave by the end electrodes.
In summarizing what has been reported above, the following
conclusions can be drawn.
The electron distribution functions by velocity component
parallel to the magnetic field, in the plasma column region
penetrated by the primary electron flux, are approximately the
same in a PIG and arc type discharge under the same discharge
conditions, and they are characterized by an excess (compared
with Maxwellian distribution) of highly energetic, including
anomalously fast particles. The results of work [5] become
understandable in this connection. In it, considerable (on the
order of Va) potential drops were obtained in the plasma pene-
trating from the arc type discharge .into pt.he vacuum.
In both arc type and PIG discharges ,tt.he nonMaxwellian part
of the electron velocity distribution function is formed by one
transit of the electrons through the discharge gap, and the
specific nature of the PIG discharge is reduced to the accumulation
of particles with energies E^eVa in the space, due to their oscil-
lation. The general structure of the electron distribution func- .
tion in the PIG discharge evidently does not have to depend on
the electron free path length for pair collisions, if the latter
exceeds the discharge gap length.
The nonMaxwellian, highly energetic part of the electron
velocity distribution function is caused mainly by collective
interactions of the primary electrons with the plasma in both
types of discharge. This interactioniisllocalized primarily
within the plasma column penetrated by the primary, electron
beam. The nature of.the nonMaxwellian part of the electron
energy spectrum outside the column is determined basically by
the radial transport of particles and their accumulation in the
space.
In conclusion^, the authors express thanks to V.A. Sayenko
and A.A. Gurin for valuable discussions.
••* ' :
References
1. Romanyuk, L.I. and N.Ye. Svavil'nyy, UFZh? 21, .981 (1976). /1982
2. Chen, F.F., Phys. Rev. Lett. 8, 23^ (1962).
3. Romanyuk, L.I. and V.M. Slobodyan, UFZH 1^, 455 (-1973).
4. Reykhrudel1, E.M. and G.V. Spivak, ZKETF 1_0, 1408 (19^ 0).
5. Gabovlch, M.D., LII. Romanchuk and Ye.A. Lozovaya, AhTF 35,
94 (19^5).


Comparative characteristics of electron energy spectrum in PIG and arc type discharge plasmas

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