Magnetic field influence on the shape of eroding surface of graphite cathodes
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Національний науковий центр «Харківський фізико-технічний інститут» НАН України
2002
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| Cite this: | Magnetic field influence on the shape of eroding surface of graphite cathodes / I.I. Aksenov, V.V. Vasil’ev, A.O. Omarov, V.E. Strel’nitskij // Вопросы атомной науки и техники. — 2002. — № 5. — С. 142-144. — Бібліогр.: 5 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1860131441941151744 |
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| author | Aksenov, I.I. Vasil’ev, V.V. Omarov, A.O. Strel’nitskij, V.E. |
| author_facet | Aksenov, I.I. Vasil’ev, V.V. Omarov, A.O. Strel’nitskij, V.E. |
| citation_txt | Magnetic field influence on the shape of eroding surface of graphite cathodes / I.I. Aksenov, V.V. Vasil’ev, A.O. Omarov, V.E. Strel’nitskij // Вопросы атомной науки и техники. — 2002. — № 5. — С. 142-144. — Бібліогр.: 5 назв. — англ. |
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MAGNETIC FIELD INFLUENCE ON THE SHAPE OF ERODING SURFACE
OF GRAPHITE CATHODES
I. I. Aksenov, V. V. Vasil’ev, A. O. Omarov, V. E. Strel’nitskij
National Science Centre " Kharkov Institute of Physics and Technology",
Academicheskaya str. 1, Kharkov 61108, Ukraine,
Phone: (0572) 356561; Fax: (0572) 350755; E-mail: strelnitskij@kipt.kharkov.ua
PACS: 52.77.-j; 52.40.-w
1. INTRODUCTION
The vacuum - arc method of synthesis of amorphous
carbon а-С allows to shape films of this unique material
with the highest physical-mechanical performances [1].
As fields of practical application of this method widen
more and more actual become questions of productivity
of the process equipment, clearing of macroparticles from
plasma, and, in the first instance, also problems of
reliability and lifetime of a plasma source as basic
instrument for realization of the method. The success in
the solution of the problems, in turn, will be defined, as
far as effectively will be eliminated difficulties in
discharged an ignition and stability in the vacuum - arc
carbon plasma source stipulated by specificity of a
cathode spot (CS) behavior on the graphite cathode. The
velocity of the CS motion on graphite is essentially (on 2-
3 orders of magnitude) less, than on metal; the averaged
lifetime of CS strongly depends on the sort of graphite. In
practice it results in strongly nonuniform erosion of the
working surface of the cathode, that in turn is the reason
of unpredictable changes of plasma stream parameters,
raise of an arc exiting frequency and loss of serviceability
of the device.
In the present work the influence of a magnetic field
on the shape of the graphite cathode eroding surface, at
different pressures and arc currents was investigated.
2. EXPERIMENTAL
The schema of the experimental plasma source is
submitted in Fig. 1. The replaceable water-cooled cathode
(1) had the shape of the cylinder of 60 mm in diameter
and of 20 to 60 mm in height. The cathode end (2)
oriented to the anode (3) served as a working surface. The
anode was made from stainless steel as a cylinder with
water-cooled walls. Inner diameter of the anode was 210
mm, its length was 300 mm. The housing of the cathode
unit (4) was embraced by the cathode coil (5) (2300
turns). The additional coil (6) (100 turns) there was
disposed. At the anode disposed is the anode coil (7) (4
sections, 600 turns per section). The igniter (8) here was
located at the working end of the cathode and forms a
spark plasma injector in a couple with the auxiliary anode
(9) [2]. On this electrode the demountable ring from steel
(10) was disposed. Being ferromagnetic, this ring served
as the concentrator of a magnetic field. The cathode coil
(5) was enclosed with demountable magnetic screen (11).
At presence of the concentrator the system ensured
ignition with probability not less than 90 %.
The exit end of the source was overlapped by a flat
collector (12) for imitation of a substrate under negative
potential (-70 V) and for measuring ion component of a
plasma stream. The cathode was made of graphite of
several marks. Argon was used as a working gas.
The arc stability was defined as 1/n, where n is a
number of the arc excitations per hour. The ignition
probability was defined as the relation of the arc ignition
number to the number of all triggering impulses which
have arrived per unit of time on the electrode 8.
At the starting stage of our experiments the cathodes
from titanium were used. It was done for simplification of
procedure of a choice of optimum condition ensuring
maintenance of the flat shape of the cathode working
surface during its "burnup". At those velocities of CS
motion, which is characteristic for Ti (hundreds m/sec),
the area of the most probable existence of CS is easily
identified visually as a part of the working surface, filled
with brightly flashing twisting lines – the CS trajectories.
Selecting an appropriate arcing mode (arc current,
magnetic field, gas pressure, etc.) so that the area of the
CS existence takes up all working surface of the cathode,
one could expect that under these condition the uniform
erosion of this surface will be supplied, and hence a plane
form of it will be maintained.
3. RESULTS AND DISCUSSION
3.1. EXPERIMENTS WITH Тi CATHODES
The experiments have shown, that the most effective
passage of plasma along the anode, which takes place in
case of adding magnetic fields of the anode coil sections
(Fig. 1 (b)), can’t be realized at presence of the negative
collector due to a strong arc instability. Therefore all
experiments were carried out at the opposing fields of
those sections, when the rather free sink of magnetized
electrons along magnetic lines crossing the anode (Fig. 1
(с)) was ensured. Though such operation mode of the
source yielded to variant with adding fields, nevertheless,
it ensured rather high ion current on exit (Fig. 2).
In presence of the concentrator 10 ignition remained
stable at lowering pressure up to 4·10-3 Pa. In its absence
the pressure could be reduced without disadvantage for
ignition reliability only up to 1,3·10-2 Pa. The positive
influence of the concentrator on ignition is stipulated by
that at its presence the course of magnetic lines is more
favorable for CS transition from a lateral surface of the
cathode to its working end: the acute angle between the
lines and this surface enlarged (Fig. 3). The similar effect,
basically, could be reached with the help of the screen 11.
However in our condition the expected effect has
appeared inappreciable owing to a small thickness and
large distance of the screen from the cathode.
142 Problems of Atomic Science and Technology. 2002. № 5. Series: Plasma Physics (8). P. 142-144
Fig. 1. Schematic of the experimental plasma source (a);
lines of adding (b) and opposing (c) magnetic fields
(arrows)
Fig. 2. Output ion current dependence on the arc current
Fig. 3. Magnetic field lines in the source without
concentrator (thin lines) and with concentrator (bold
lines)
The character of "burnup" of the cathode owing to
erosion under action of the CS is defined by low of the
spot motion on the cathode surface. In turn, the CS
behavior on the cathode of source of the explored type is
defined by balance of magnetic fields: external fields
created by the coils, and so-called internal fields
generated by currents through the CS. In absence of
external fields the zone of CS random travel is restricted
to a central part of the working surface of the cathode. In
this place a concave [3] is formed. With occurrence of an
exterior axisymmetric magnetic field, which lines are
canted from centre to periphery of the working end of the
cathode, the CS rotation around the cathode centre
("retrograde" motion under action of the magnetic field
radial component Нr) is superimposed on its random
motion. According to "the rule of an acute angle" the CS
shifts on such distance r, at which the action of an
external field on the spot is balanced by action of internal
fields. At some magnetic field strength the equilibrium
circular trajectory of CS shifts to the periphery of the
working surface. Its edges erode most intensively. CS
frequently runs on the lateral surface of the cathode and
dies away. The device, practically, is disabled. At some
intermediate value of a magnetic field intensity the CS,
making motions on a circle with r ≈ R/2 and being
deviated from it chaotically on distance ≈ ± R/2, "visits"
all points of the effective area with equal probability and
equal frequency. (R is radius of the cathode). Thus the
erosion of the surface is uniform, and its shape remains
flat up to complete "burnup" of the cathode (see Fig. 4a).
As the cathode was expended owing to erosion it
gradually was moved in the source so that it’s working
end was in the auxiliary anode vicinity.
Fig. 4. Titanium cathode exposed during 360 min., Id =
110A, I5 = 0.7A, I6 = 7A (a); graphite cathode exposed
during 510 min., Id = 110A, I5 = 0.65A, I6 = 9A (b)
The data on influence of the magnetic field of the coils
5 and 6 on the arc stability and shape of the eroding
surface of the cathode are given in the Table. Here I6 is
the coil 6 current, Нr and Hz are the radial and axial
components of the magnetic field measured at the surface
of the working end of the cathode at radius r when the
current in the coil 5 was I5 = 0.7 A.
Table. Influence of magnetic fields on the arc stability
and the shape of the cathode eroding surface
I6, A
Hr, Oe / Hz, Oe
r =0 r =15
mm
r =30
mm
n,
hour -1
The
cathode
end
shape
0 0/57 5/54 8/40 ∼1 concave
6 0/67 6/64 12/49 6 ÷ 7 flat
9 0/73 8/70 12/59 ∼30 convex
3. 2. EXPERIMENTS WITH GRAPHITE
CATHODES
The experiments have shown, that in case of a vacuum
arc with graphite as well as with metal (Ti) cathode the
CS reaction on an exterior magnetic field is qualitatively
the same. In both cases the spot rotates around the centre
of the electrode under actions of radial component of the
magnetic field Нr, being simultaneously displaced from
centre to the periphery against the gradient of axial
component ∇Нz. Last circumstance is in accordance to the
conclusion of ref. [4], which could be interpreted as
follows: in an oblique nonuniform magnetic field the CS
143
moves athwart tangential component of the magnetic field
in retrograde direction and also drifts along this
component against the gradient of the normal component.
A special case of this rule is the known rule of an acute
angle.
In fig. 5 the dependences n(∇Hz) and Ud(∇Hz) are
shown. The character of dependences is explained as
follows. The higher ∇Hz value corresponds to greater Hz
absolute values. Hz amplification, in turn, leads, on the
one hand, to diminution of electron-type conduction of
plasma in direction to the anode and therefore Ud raises.
On the other hand, it lead’s to greater CS shift toward the
working end edges, and also to increasing probability of
it’s moving off from the end surface and, hence, to it’s
extinction (Fig. 6). The strong correlation between n and
Ud can be used for self-acting correction of an arc
stability, and, consequently, of the cathode erosion
character by means of a magnetic field control device
with a back coupling on Ud.
Fig. 5. Extinction frequency (n) and arc voltage (Ud) as
functions of ∇Hz. (Hz – axial component of the magnetic
field)
Fig. 6. Extinction frequency (n) as a function of the
coil 6 current
Fig. 7. Extinction frequency as a function of pressure:
I6 = 0; I5 = 0.65A (1), I5 = 0.2A (2)
Fig. 7 illustrates the influence of pressure of argon on
the arc stability. It follows from the figure, that in
presence of Ar in moderate magnetic fields, close to
optimum, the arc stability rises with growth of pressure,
and at р ≥ 1,5 mTorr the arc burns almost without
extinctions (curve 1). But in feeble magnetic fields at the
same character of dependence n (p) the n quantity remains
different from zero in all explored range of pressure and
does not fall below ~ 6 hour - 1.
Dependence of the shape of the graphite cathode
eroding end on the magnetic field, as a whole, is the same,
as for the metal cathode. The photo of the cathode from
MG graphite, saturated with pyrocarbon, is show in Fig.
4(b). The cathode was subject to undergone an arc action
during 510 minutes at Id = 110А, I5 = 0,65А and I6 =
0,9А. One can see, that in operation mode, close to
optimum for the Ti cathode, the end of the graphite
cathode till the test finishing has maintained the flat
shape. It must be noted, that the uniform burnup of the
graphite cathode was achieved only at high arc stability,
when the CS average lifetime essentially exceeded the CS
circular motion period on the equilibrium trajectory.
Otherwise cathode burned off nonuniformly: an electrode
eroded, in basic, near to the igniter. As a result the
ignition rather soon became impossible. The alignment of
erosion in such case is achieved by means of application
of several triggering electrodes disposed around the
cathode on perimeter of its working end. The igniting
impulses are supplied to the electrodes alternately. Such
method of ignition is especially expedient in case of use
of graphite characterized by lowered arc stability [5].
4. CONCLUSION
It is established experimentally, that in an end-type
vacuum-arc plasma source with magnetic steering of the
cathode spot the uniform erosion of the graphite cathode
end can be ensured by selection of an appropriate
intensity of an axisymmetric magnetic field with gradient
of axial component.
At the presence of argon the arc stability essentially
rises; in an optimum mode at р ≅ 1,5 mTorr the arc burns
practically without extinctions.
At use of graphites characterized by low arc stability,
the using of the multielectrode triggering device is
recommended.
REFERENCES
1. I. I. Aksenov, V. E. Strel’nitskij. Kharkov Science
Assembly ISDF-5, Kharkov, 2002. Proc. p. 39-64.
2. I. I. Aksenov, V. A. Belous. Teplophisika vysokih
temperatur. 17 (1) (1979) 1-4 (Rus).
3. I. I. Aksenov et al. VIIIth Int. Conf. “Vacuum Sci.
and Technol.” Krimea, 2001. Proc., p. 252-256.
4. I. I. Aksenov, A. A. Andreev. Pis’ma v ZhTF, 3 (23)
(1977) 1272. (Rus).
5. V. V. Vasil’ev et al. 5th Int. Symp. “DRMTF”, 2002,
Kharkov, Ukraine, Proc. , p. 111-115.
144
I. I. Aksenov, V. V. Vasil’ev, A. O. Omarov, V. E. Strel’nitskij
Academicheskaya str. 1, Kharkov 61108, Ukraine,
Phone: (0572) 356561; Fax: (0572) 350755; E-mail: strelnitskij@kipt.kharkov.ua
References
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| id | nasplib_isofts_kiev_ua-123456789-79282 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 1562-6016 |
| language | English |
| last_indexed | 2025-12-07T17:44:44Z |
| publishDate | 2002 |
| publisher | Національний науковий центр «Харківський фізико-технічний інститут» НАН України |
| record_format | dspace |
| spelling | Aksenov, I.I. Vasil’ev, V.V. Omarov, A.O. Strel’nitskij, V.E. 2015-03-30T09:25:43Z 2015-03-30T09:25:43Z 2002 Magnetic field influence on the shape of eroding surface of graphite cathodes / I.I. Aksenov, V.V. Vasil’ev, A.O. Omarov, V.E. Strel’nitskij // Вопросы атомной науки и техники. — 2002. — № 5. — С. 142-144. — Бібліогр.: 5 назв. — англ. 1562-6016 PACS: 52.77.-j; 52.40.-w https://nasplib.isofts.kiev.ua/handle/123456789/79282 en Національний науковий центр «Харківський фізико-технічний інститут» НАН України Вопросы атомной науки и техники Low temperature plasma and plasma technologies Magnetic field influence on the shape of eroding surface of graphite cathodes Article published earlier |
| spellingShingle | Magnetic field influence on the shape of eroding surface of graphite cathodes Aksenov, I.I. Vasil’ev, V.V. Omarov, A.O. Strel’nitskij, V.E. Low temperature plasma and plasma technologies |
| title | Magnetic field influence on the shape of eroding surface of graphite cathodes |
| title_full | Magnetic field influence on the shape of eroding surface of graphite cathodes |
| title_fullStr | Magnetic field influence on the shape of eroding surface of graphite cathodes |
| title_full_unstemmed | Magnetic field influence on the shape of eroding surface of graphite cathodes |
| title_short | Magnetic field influence on the shape of eroding surface of graphite cathodes |
| title_sort | magnetic field influence on the shape of eroding surface of graphite cathodes |
| topic | Low temperature plasma and plasma technologies |
| topic_facet | Low temperature plasma and plasma technologies |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/79282 |
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