22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Laser triggered discharge gap
S.G. Davydov, A.N. Dolgov, V.O. Revazov, V.P. Seleznev and R.Kh. Yakubov
Dukhov All_Russian Research Institute of Automatics, ul. Sushchevskaya 22, RU-127055 Moscow, Russia
Abstract: Based on the data obtained in the experiment, an assumption about discharge
development mechanism in laser trigged discharge gap was made. Initially, under the
action of the laser pulse, a glow discharge in electrode erosion products is ignited. Then as
the result of ionization-overheating instability the current channel undergoes contraction
and glow discharge transforms into an arc.
Keywords: vacuum gap, laser pulse, current delay, glow discharge, arc, instability
1. Introduction
The aim of the presented work is to study physical
processes, which occur during laser excitation of
electrical discharge in a vacuum. The range of laser pulse
intensity (q) used in experiments was between 3×106 and
3×109 W/cm2. In such a wide range of radiation intensity
values a wide range of physical phenomena and processes
is possible: from melting and evaporation to ionization of
the vapor and absorption of incident radiation by the
plasma.
2. Scheme of the experiment
Pulsed solid-state LGI-60 laser in Q-switched mode
generates radiation with a wavelength of 1.06 µm and
time duration at the base of 20 ns. The discharge gap was
placed in a glass vacuum chamber, which was evacuated
to a residual gas pressure not higher than 10-1 Pa. Flat
cathode target was at a zero potential with respect to a
ground bus. The aluminium anode shaped as a flat ring
before switching of a vacuum gap had a positive voltage
of 3 kV. Laser radiation was focused on the surface of
the cathode through the wall of the vacuum chamber so
that the radiation beam passed through an opening with
diameter of 2.5 mm in the anode center without losses.
A focusing lens with a focal length of F = 50 mm was
used in experiments. A focal spot with diameter of
0.8 mm was obtained on the cathode. The energy flux of
the laser radiation incident on the cathode was regulated
by a set of calibrated absorbing filters placed in front of
the converging lens. The distance between the cathode
and the anode was 1 mm. A circuit, switched by a
discharge gap, consisted of a capacitor and a set of
resistors. The current in the cathode-anode gap had an
amplitude up to 100 A. The duration of the front edge of
the current pulse was about 100 ns with a rise rate of the
current dI/dt = (1-2)×109 A/s. Measurements of a current
flowing through the gap during the discharge process
were conducted with a calibrated Rogowski coil. The
signal from the Rogowski coil was fed through a coaxial
cable to the input of a high-speed oscilloscope. The
signal from a high-speed photodetector was fed to the
second input of the same oscilloscope.
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3. Results
A wide range of laser radiation energy incident on the
cathode surface was used, E LP = 2×10-6 - 2×10-3 J. The
average intensity of the laser radiation on the surface of
the cathode within the focal spot was calculated as:
q=
E LP
,
τS
(1)
where τ = 10 ns - laser pulse duration at half-height,
S - area of the focal spot. The range of laser pulse
intensity (q) used in experiments was between 3×106 and
3×109 W/cm2. In such a wide range of radiation intensity
values a wide range of physical phenomena and processes
is possible: from melting and evaporation to ionization of
the vapor and absorption of incident radiation by the
plasma [1].
In the course of these experiments, a minimum energy
of the laser pulse radiation required to initiate a discharge
in the studied vacuum gap was determined, and
measurements of time parameters of the commutation
process of discharge gap at a different energy of laser
pulse radiation, constant pulse duration and constant area
of a laser beam focal spot were conducted. Oscillograms
of the discharge current were obtained and with a fixed
value of E LP a distance on a time scale between specific
points of discharge current oscillogram was determined
(Fig. 1): 1 - start of the laser pulse; 2 - the time when the
current with significant value occurs in a discharge
circuit; 3 - feature (splash or fracture) on the oscillogram;
4 - moment when the maximum current in the circuit is
achieved. Based on the data obtained, graphs of relation
of duration of time intervals τ 1-2 , τ 1-3 , τ 1-4 to the E LP value
were plotted for different cathode materials. Fig. 2 shows
graphics of obtained experimental data for cathode made
from Kovar (alloy 29NK).
Note, that dependences τ 1-2 (E LP ), τ 1-3 (E LP ), τ 1-4 (E LP )
for mentioned materials (as well as for a variety of other
materials that were tested, including aluminum, wolfram,
stainless steel, copper , tantalum, titanium, graphite) have
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almost identical shapes, which indicates the uniformity of
the processes.
2
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current pulse with increase of E LP , and the second branch,
which corresponds to rapid changes of mentioned
parameters with increase of E LP , and then the
approximate position of intersection point of these lines
was determined.
For small values of E LP the current in the discharge
circuit is not registered during 100 ÷ 200 ns after the end
of laser pulse. For large E LP current in the discharge
circuit is registered almost immediately after the start of
cathode surface irradiation.
Fig 1. Character oscillograms of vacuum gap
commutation.
Fig 2. Time parameters τ 1-2 , τ 1-3 , τ 1-4 as a function of
laser pulse energy E LP for cathode of Kovar alloy.
With relatively small E LP (below the threshold) in the
range 2 ÷ 20 mJ (q = 3×106 ÷ 3×107 W/cm2) timeslots
τ 1-2 , τ 1-3 , τ 1-4 vary slightly. For relatively large E LP
(above the threshold) in the range of 200 ÷ 2000 mJ
(q = 3×108 - 3×109 W/cm2) with increase of E LP the
duration of the intervals τ1-2 and τ 1-3 decreases rapidly,
and the duration of the interval τ 1-4 apparently reaches a
plateau, which is defined by the parameters of the
switching circuit. The threshold value of the E LP for each
of mentioned materials was determined by finding the
inflection point on the corresponding plot. For this
purpose, an approximation of two branches of the graph
was done, i.e., approximation of the first branch, which
corresponds to slight changes of time parameters of the
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4. Discussion
For relatively small values of E LP , i.e., less than the
threshold, based on the intensity value of laser radiation
incident on the cathode, proceeding from the fact that the
current in the circuit, which is commutated by vacuum
gap, is not registered during considerable period of time
after the end of laser pulse, one can assume, that the
weakly ionized vapor clot expands into the interelectrode
space [2, 3]. Probably, a low-current glow discharge is
ignited in the mentioned area and an external border of
vapor clot, which is pointed at anode, plays the role of
electrons emitter. The closure of the interelectrode gap by
conducting medium and neutralization of space charge
significantly increases the current registered on the
oscillograms. Further, apparently, ionization-overheating
instability of the glow discharge leads to the contraction
of the current channel [4-6] and, respectively, leads to
further heating of local region of cathode surface. This is
followed by an increase in flux density of electrons
produced by thermionic emission, in particular, due to the
electric field intensity at the cathode surface and increase
in flux density of ions from plasma to the cathode. The
increase in density of evaporable or sublimated substance
of electrode at its surface will lead to the decrease of
electron's mean free path and, accordingly, to the decrease
of the layers width of cathode potential drop, i.e., again
electric field intensity and density of electron emission
current from cathode will increase. The action continues
till formation of the cathode spot, i.e., initiation of selfreproducing centers of explosive electron emission – the
so called ectons [1]. Discharge transforms into an arc.
The presence of bursts on the oscillogram (on the current
pulse front) indicates, in our opinion, development of
instability in a glow discharge, when it undergoes
transformation from normal to abnormal form [7], and
then transforms into an arc.
For relatively large values of E LP , larger than the
threshold, influence of laser radiation on the process of
development of ionization-overheating instability in
plasma of glow discharge, apparently, becomes a
distinctive feature of the discharge. When E LP reaches
the threshold value, an effective ionization of the cathode
material vapors begins due to the breakdown of neutral
gas in the field of electromagnetic wave with the further
active absorption of laser radiation in produced plasma
[8].
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In the presence of laser radiation flux development of
instability happens faster, as local increase in temperature
and degree of vapor ionization leads to increase of
absorption of laser radiation and to further heating and,
accordingly, to increase of ionization degree and
conductivity of plasma, i.e., current contraction.
Moreover, probably, the process of filling of
interelectrode space by plasma develops faster. The result
is acceleration of process of condition formation for arc
ignition with the increase of E LP . Indeed, this assumption
is in agreement with experimental data obtained in
research [9] of scattering of laser-plasma torch by shadow
photographing methods with pulsed laser as illuminator
and with high-speed photographing.
If our hypothesis is correct (we have glow discharge in
the cathode material vapor on the first stage of a process),
what parameters will turn out to be defining for the ELP
value in the first approximation? Probability of molecule
vapor ionization by electron impact will be determined by
an average value of free electrons energy and the latter in
turn will depend on the thermal energy of electrons, i.e.,
on the electrons temperature T and on the possibility of
gaining the energy in electric field applied to the gap.
The latter circumstance is determined by expansion rate
of vapor formed during heating of the cathode by laser
pulse. Energy is proportional to (Т∕μ)1/2. Further,
influence of the initial density of vapor should be
considered, which will be determined by the amount of
vaporized substance, i.e., cathode heating depth by laser
radiation, which, in turn, can be estimated as (τχ)1/2 ,
where τ – laser pulse duration, χ – coefficient of thermal
diffusivity. By temperature we assume the melting
temperature of cathode material as we are talking about
the minimum value of required radiation energy. The
diagram which demonstrates interconnection of (E LP ) min
and designed parameter (Т3∕χμ)1/2 is presented in the
Fig. 3.
Fig. 3. Distribution diagram of cathode material for
minimum value of radiation energy (E LP ) min
dependence of parameter (Т3 ∕χμ)1/2.
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The diagram shows the notable trend and can be used
for preliminary search of the optimal cathode material for
energy minimization of the laser pulse.
The threshold value (E LP ) thre is determined by plasma
expansion rate, which does not depend on the ion mass
[10], according to experimental data, in condition of
continuing laser pulse and is determined by electrons rate.
The diagram which demonstrates interconnection of
(E LP ) thre and designed parameter (Т∕χ) is presented in the
figure 4, where T – boiling temperature of cathode
material.
Fig.4 Distribution diagram of cathode material for
threshold value of radiation energy (E LP ) thre
dependence of parameter (Т vap ∕χ).
The major portion of researched materials fits into the
scope of trend, the minor portion, aluminum and tungsten,
do not fit in the scope. It can be assumed that this
circumstance is tied with features of change of phase state
dynamic of different materials under the action of highenergy laser radiation.
Initially, under the influence of high-energy laser
radiation on the surface of condensed matter, a thin
surface layer of material heats up. This layer, in turn,
through heat transfer mechanism becomes a source of
heat wave, which spreads inside of the body. The heat
wave rate in case of the constant heat conduction, density
and specific heat of material decreases within time as t-1/2.
On the other hand, at some moment of time the
temperature on the material surface increases so much
that the considerable evaporation begins. From this
moment an evaporation wave will spread inside the matter
and the wave rate will increase jointly with the
temperature increase and will reach the stationary value of
v evap ≈ q/ρr, where ρ – the density of condensed matter ,
r – specific heat of evaporation. At the moment of time
t crit the evaporation wave rate will be equal of the heat
wave rate. Provided that t crit < τ, where τ – laser pulse
duration, the influence of laser radiation on the condensed
matter practically comes down to the evaporation regime
in which the heat conduction does not play a considerable
role. The radiation intensity value should exceed q crit
value, which is determined by expression [2]:
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χ 1
(2)
τ
The obtained data demonstrates that the regime of
influence of laser radiation at intensity threshold value
q thre for aluminium and tungsten radically differs from the
regime for other researched materials. If the major part of
researched materials is in conditions close to boundary
conditions, then aluminum is in the conditions of heatconducting regime and tungsten – in evaporation regime.
qχρiτ ≈ ρρ ( ) 2 ,
Table 1.
material.
Distribution q crit /q thre parameter of cathode
Material
W
Al
Kov
Cu
Mo
q crit /q thre
1.8
0.017
0.2
0.11
0.2
Material
Fe
Ta
Ti
C
q crit /q thre
0.07
0.2
0.3
0.5
[6]
E.P. Velihov, A.S. Kovalev and A.T. Rahmanov.
Physical phenomenon in the gas-discharge plasma.
(Moscow: Nauka) 160 [in Russian] (1987)
[7] S.K. Gdanov, V.A. Kurnaev, M.K. Romanovskii
and I.V. Tsvetkov. Fundamentals of physical
processes in plasma and plasma plans. (Moscow:
MEPHI) 368 [in Russian] (2207)
[8] Yu.P. Rayzer.
Laser spark and diffusion of
breakdowns. (Moscow: Nauka) 308 [in Russian]
(1974)
[9] N.B. Delone. Interaction laser radiation with
matter. (Moscow: Nauka) 280 [in Russian] (1989)
[10] V.S. Vorob’ev.
“Plasma arising during the
interaction of laser radiation with solids”. PhysicsUspekhi (Adv. Phys. Sci.) 36, 1129-1157 (1993)
5. Conclusions
In the present paper it is demonstrated that the character
of dependence of registered time parameters from
radiation energy is the same for different cathode
materials.
For relatively small E LP (less than the threshold value
(E LP ) thre ) in the range of 2 - 20 μJ (q = 3×106 - 3×107
W/cm2), registered time intervals change slightly or even
remain constant within the random error interval. For
relatively large E LP (larger than the threshold value
E LP ) thre ) in the range of 200 – 2000 μJ (q = 3×108 - 3×109
W/cm2), the duration of time intervals, which characterize
the speed of discharge gap commutation process by wellconducting medium, reduces with increase of E LP value.
Value of the minimal laser pulse energy required for
ignition of a vacuum gap (E LP ) min and threshold energy
(E LP ) thre , from which the dependence of the registered
time parameters of commutation process from laser pulse
energy is observed, is determined by thermodynamic
parameters of cathode material.
6. References
[1] G.A. Mesyats. Ectons in Vacuum Discharges:
Breakdown, Spark, and Arc. (Moscow: Nauka) [in
Russian] 424 (2000)
[2] O.B. Anan`in, Yu.V. Afanas`ev and O.N. Krohin.
Laser plasma. Physics and application. (Moscow:
MEPHI), 400 [in Russian] (2003)
[3] D.A. Cremers and L.J Radziemski. Handbook of
Laser Induced Breakdown Spectroscopy. (New
York: Wiley) (2006)
[4] L.M. Biberman, V.S. Vorob`ev and I.T. Yakubov.
Kinetics of nonequilibrium low-temperature plasma.
(Moscow: Nauka) 375 [in Russian] (1982)
[5] Yu.P. Rayzer. Physics of gas discharge. (Moscow:
Nauka, Moscow) 536 [in Russian] (1992)
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