22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Small-size controlled vacuum spark-gap in an external magnetic field
A.A. Pshenichnyi, S.G. Davydov, A.N. Dolgov and R.Kh. Yakubov
Dukhov All_Russian Research Institute of Automatics, ul. Sushchevskaya 22, RU-127055 Moscow, Russia
Abstract: It is demonstrated that the operation of a small-size controlled spark-gap can be
controlled by applying a uniform external magnetic field. It is shown that the magnetic
field of such a simple configuration efficiently suppresses the effect of localization of the
discharge current after multiple actuations of the spark-gap.
Keywords: spark-gap, cathode spot, electron emission centers, shift in a magnetic field
1. Introduction
Contributed In order to provide long lifetime and high
stability of a vacuum spark-gap, it is necessary to resolve
the problem of localization of the discharge region in it.
Due to this effect, in the course of multiple switchings,
the discharge occurs in a limited region of the spark-gap.
As a result, the discharge affects only definite regions of
the electrode system, thereby enhancing their surface
erosion and decreasing the lifetime and stability of the
spark-gap as a whole.
One of the possible means to prevent discharge
localization is to place the discharge unit in an external
magnetic field. It is well-known that the presence of a
magnetic field tangential to the cathode surface results in
the shift of the cathode spot. In a spark discharge, this
leads to the appearance of new electron emission centers,
due to which the cathode spot moves in the direction of
the Ampère force [1].
2. Scheme of the experiment and its results
The vacuum spark-gap used in our experiments is a
three-electrode coaxial system (the cathode, anode, and
igniter electrode) placed in a dielectric vacuum case
(Fig. 1). The ignition system consists of a cathode, an
igniter electrode, and a 100 μm thick dielectric (mica)
washer tightly pressed between them. The use of
breakdown along the dielectric surface allows one to
decrease the breakdown voltage of the igniter and,
accordingly, the amplitude of the start pulse. Such a
spark-gap is capable operating in a broad voltage range,
from 100 V to 10 kV [2]. The length of the cathodeanode gap is 1 mm. The diameter of the aluminium
cathode is 5 mm. The cathode is grounded, while the
anode before switching is at a constant positive voltage of
2 - 3 kV. The spark-gap is triggered by applying a 3.5 kV
positive (with respect to the cathode) voltage pulse with a
rise rate of (1-2)×1010 V/s to the igniter electrode. The
switched current pulse in the cathode-anode gap has a
duration of 3 ×105 s and an amplitude of 200 - 300 A.
When a voltage pulse is applied to the igniter electrode,
the electric field near the cathode edge adjacent to the
dielectric washer increases and reaches a value at which
field emission currents become sufficient to evaporate
micropoints on the cathode surface and ionize the
P-II-4-12
Fig. 1. Scheme of the controlled spark-gap: (1)
igniter electrode, (2) covar spacers, (3) dielectric
washer, (4) titanium spacer, (5) anode, (6) cathode,
(7) permanent magnets.
resulting vapor of the cathode material and the gas
molecules sorbed on the cathode and the dielectric
washer. As a result, a cathode spot and plasma cloud
form. The cloud expands into the ambient space under
the action of the voltage applied to the cathode-anode gap
and propagates toward the cathode due to the high
electron mobility. This leads to the formation of a
cathode plume that closes the cathode-anode gap, and the
spark discharge transforms into an arc [3]. In order to
control the discharge operation and increase the symmetry
of the discharge action on the electrodes and dielectric
washer, two 12 mm diameter 15 mm long permanent
magnets were placed coaxially outside the vacuum case,
their opposite poles facing one another (Fig. 1). The
magnets created an almost uniform magnetic field of
about 10-1 T inside the spark-gap with field lines parallel
to the symmetry axis of the system.
Two series of 1000 switchings each were carried out by
using two identical spark-gaps, one in the presence and
the other in the absence of the above magnetic field.
After the experiments, the spark-gaps were visually
inspected by using an MBS-9 optical microscope with a
14- to 100-fold magnification and a JSM-35F electron
1
microscope with a 100- to 700-fold magnification. In the
absence of a magnetic field, 20 - 30% of the perimeter of
the cathode side surface, as well as of the side surface of
the dielectric washer, underwent appreciable erosion. The
most intensely eroded regions of the cathode surface
coincided with those of the washer surface. No erosion
was observed on the end surface of the cathode. In the
presence of a magnetic field, cathode erosion was visually
uniform along the entire perimeter of the cathode side
surface. Erosion of the dielectric washer was also
uniform along its perimeter. No erosion of the cathode
end surface was also observed in this case. Thus,
applying a uniform magnetic field with an easily
implemented magnitude and configuration to the
small-size controlled vacuum spark-gap allowed us to
increase the homogeneity of the action of the vacuum
discharge on the electrodes and dielectric washer of the
ignition system. A more uniform erosion results in the
slower degradation of the dielectric washer and a decrease
in the flux of the conducting electrode material onto the
washer surface. This, in turn, leads to an increase in the
lifetime of the spark-gap and improves stability of its
operation. A decrease in the pulsed voltage required for
breakdown along the surface of the dielectric washer with
increasing number of switchings also slows down
appreciably (Fig. 2).
switchings. The vertical bars show the scatter in the
pulsed breakdown voltage.
3. Discussion
Let us consider a possible mechanism through which
the magnetic field affects the discharge conditions and
leads to the observed results. When a voltage pulse is
applied to the igniter electrode, the magnetic field of the
above configuration does not affect the process of
breakdown along the surface of the dielectric washer,
because the magnetic field is parallel to the direction in
which charged particles leave the cathode spot. Estimates
show that the propagation velocity of aluminium plasma
of the cathode plume is about 4×104 m/s [4]. Taking into
account that the thickness of the dielectric washer is
10-4 m, we find that the duration of the spark phase of the
discharge along the washer surface is about 2×10–9 s. In
contrast, the propagation of the cathode plume toward the
anode in the presence of a magnetic field is hampered,
because, in this case, the magnetic field is perpendicular
to the propagation direction of the charged particle flux.
Indeed, for a plasma temperature in the cathode spot of
3 - 5 eV [4], the electron velocity in the vicinity of the
spot is:
1
 2W  2
6
v e ≈  e  ≈ 10 m/s,
 me 
(1)
where W e is the mean thermal electron energy and m e is
the electron mass. For a magnetic field in the electrode
gap of B ≈ 10-1 T, the electron Larmor radius is:
R Le ≈
mev e
−5
≈ 5 × 10 m,
eB
(2)
where е is the electron charge. This is much less than the
cathode−anode gap length. The plasma density in the
center of explosive electron emission is close to the solid
density (about 1028 m−3) [4]. If we assume that the plasma
expands isotropically, i.e., the charged particle density
decreases inversely proportionally to the third power of
the distance form the center of explosive electron
emission, whose size is about 10−6 m [4-6], then we find
that, at a distance of 10−3 m (the distance between the
cathode and anode), the plasma density is n≈1019 m−3 and
the Debye length is:
( )
εW
R D ≈ 02 ε
εn
Fig. 2. Mean pulsed voltage required for breakdown
along the surface of the dielectric washer (V) as a
function of the number of spark-gap switchings (N) in
the (a) absence and (b) presence of a magnetic field.
Each experimental point is averaged over ten
2
1
2
−5
≈ 2 × 10 m,
(3)
where ε 0 is the permittivity of vacuum. This is much less
than the dimensions of the plasma plume. The free path
length of plasma particles is:
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λ≈ 1 ,
nσ
(4)
where σ ≈ 10 - 10 m [6] is the cross-section for
elastic collisions. In the case of isotropic plasma
expansion, λ is longer than the electron Larmor radius
already at a distance of 10−4 m form the cathode spot.
The size of the cathode spot is about 10−5 m; hence,
plasma electrons are unmagnetized only in the close
vicinity of the cathode spot. Therefore, the propagation
velocity of the cathode plume toward the anode is
determined by the diffusion velocity of the plume plasma
in the magnetic field, which is certainly lower than the ion
thermal velocity. Thus, for a plasma temperature of 3 –
5 eV (i.e., for an ion thermal velocity of about
5×103 m/s), the time during which the cathode plume
closes the cathode−anode gap is no less than 2×10–7 s.
Under the action of the magnetic field, the cathode spot in
the spark phase of the discharge moves in the direction of
the Ampère force with a velocity of 104 m/s [4]. During
2×10–7 s, it shifts over a distance of about several
millimeters, which is comparable with the linear
dimensions of the cathode. After the transition into the
arc phase, the propagation velocity of the cathode spot
decreases by two orders of magnitude and its propagation
direction in the magnetic field reverses. However, the
duration of the arc phase (about 3×10–5 s) is such that the
reverse shift of the cathode spot is of the same order of
magnitude as that in the spark phase. It is also worth
noting that, according to estimates, the lifetime of an
explosive emission center is about 10−8 s [2], whereas the
time during which a spark discharge develops in the
cathode−anode gap in the presence of a magnetic field is
much longer. While the spark discharge develops, a high
voltage across the cathode−anode gap is preserved.
Therefore, new cathode spots can form in the spark phase
due to, e.g., radiation from the cathode plume.
–19
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–20
2
4. Conclusions
Thus, our experiments have shown that the operation of
a small-size vacuum spark-gap can be controlled by
applying a uniform magnetic field created by using simple
technical means. It is shown that the magnetic field of
such a simple configuration efficiently suppresses the
effect of localization of the discharge current after
multiple actuations of the spark-gap. As a result, the
impact of repeated discharges per unit area of the surface
of the igniter elements decreases substantially, which
leads to an increase in the lifetime of the spark-gap and
improves stability of its operation.
5. References
[1] M.M.
Tsventukh,
G.A.
Mesyats
and
S.A. Barengol'ts. in: XL International Zvenigorod
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[2] G.A. Mesyats. Pulsed Energetics and Electronics
(Moscow: Nauka) [in Russian] (2004)
[3] A.A. Pshenichnyi and R.Kh. Yakubov. in: Proc.
XVIII Conference on Vacuum Science and
Technology. (D.V. Bykov; Ed.) (Moscow: MIEM)
153 (2011)
[4] G.A. Mesyats. Ectons in Vacuum Discharges:
Breakdown, Spark, and Arc. (Moscow: Nauka) [in
Russian] (2000)
[5] V.I. Rakhovskii.
Physics of Electric Current
Commutation in Vacuum. (Moscow: Nauka) [in
Russian] (1970)
[6] J. Cobain, G. Ecker, G. Farrall, A. Greenwood and
L. Kharris.
in: Vacuum Arcs: Theory and
Application. (J.M. Lafferty; Ed.) (New York:
Wiley) (1980)
(Moscow: Mir) (1982)
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