03-01-2013, 01:23 PM
Triggering, guiding and deviation of long air spark discharges with femtosecond laser filament
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ABSTRACT
In the perspective of the laser lightning rod, the ability of femtosecond filaments
to trigger and to guide large scale discharges has been studied for several years.
The present paper reports recent experimental results showing for the first time
that filaments are able not only to trigger and guide but also to divert an electric
discharge from its normal path. Laser filaments are also able to divert the spark
without contact between laser and electrodes at large distance from the laser. A
comparison between negative and positive discharge polarities also reveals important
discrepancies in the guiding mechanism. Copyright 2012 Author(s). This article
is distributed under a Creative Commons Attribution 3.0 Unported License.
INTRODUCTION
If lightning is one of the most fascinating phenomena occurring in the atmosphere, it is also
one of the most dangerous. This spark discharge of several kilometers can cause severe damages
to ground infrastructures. During history, several techniques have been developed for lightning
protection, such as Benjamin Franklin’s famous lightning rod or the rocket triggering device.1 The
laser lightning rod would be a valuable alternative to lightning rockets. This concept relies on the
generation by powerful lasers of a long plasma column acting like an extension of the classical
rod toward thunder clouds and would be able to significantly empty electrically charged clouds
preventing lightning stroke to hit sensitive building or facilities.
Imagined in the early 60’s the concept of laser-triggered discharges was first investigated with
high energy CO2 and YAG lasers.2 Despite the first real scale demonstration of triggering in 1996,3
this path was progressively abandoned because of the discontinuous profile of the plasma generated
with such long pulses through avalanche breakdown.4 Following the development of femtosecond
CPA (chirped pulse amplification) laser systems, the study of ultrashort filaments in air5 and their
ability to generate a thin uniform plasma channel over very long distances opened new perspectives
in the field.
INFLUENCE OF THE VOLTAGE POLARITY
Experimental setup
Experiments were performed at the DGATA center in Toulouse in the high voltage facility
FOUDRE. The first experimental setup is presented in Figure 1. A large planar electrode connected
to a high voltage Marx generator was placed 2.5 m above a spherical one (15 cm of diameter)
connected to the ground. The high voltage generator could deliver up to 2 MV in both polarities.
The voltage applied consisted in a standard voltage waveform modeling a fast lightning process. To
produce the plasma filament, the laser ENSTAmobile built by Amplitude Technologies was used.
This laser is a mobile Ti:Sa CPA laser chain delivering pulses of up to 350 mJ energy with a duration
of 50 fs (7 TW) at a repetition rate of 10 Hz. To postpone the onset of filamentation and transport
the beam without damaging the optics, a linear chirp of about 15000 fs2 was impressed to the pulse.
The laser beam of 40 mm diameter was weakly focused in order to create the plasma filaments
tangentially to one side of the sphere electrode. The plasma column was composed of a bundle of 80
filaments starting at a distance of 7 m after the lens and continuing over a distance of 4 m. In the gap
separating the two electrodes, the multiple filaments formed a quasi homogeneous circular plasma
column several mm in diameter. The current I1 circulating through the electrode was measured with
a Rogowski coil, while the voltage V on the planar electrode was measured through a resistive probe.
Results
Aresult of guided and unguided discharge for a displacement of the second electrode d2 =20 cm
is shown in Figure 11. In the absence of filament the discharge follows a quasi direct trajectory given
by the field lines inside the gap. When the filament is formed in the gap it is able to deviate the
discharge path increasing the discharge length by ∼30%. Similar results were obtained with a
negative polarity.
The maximum separation allowing laser guiding was 20 cm for d1 and 13 cm for d2 (see
illustration on Figure 12(a)). When both electrodes were displaced the guiding was maintained up
to d1 = 5 cm, d2 = 5 cm (Figure 12(b)).
Again, we observed that this guiding was very robust as long as the delay between laser and
voltage pulse was maintained at an optimal value.
For all results presented in Figures 11 and 12 the plasma filament was placed in the plane defined
by the two electrodes which contains the lines of maximum electric field. We also tested a scheme
in which the filament was laterally displaced with respect to electrode gap (see Figure 13). In this
case the discharge deviation by the laser was not as systematic as in the previous case but several
shots were positive.
Influence of the voltage polarity
We now discuss the effect of voltage polarity observed in part II, following models described
in the literature for long air gap discharges (> 1 m).4, 19 Due to the electrodes geometry the field
induced between the electrodes is strongly non uniform and its amplitude is much larger near the
sphere than near the planar electrode as shown in Figure 5. For this reason the discharge is expected
to start always from the sphere electrode, as indeed observed in the experiments.
For a negative polarity applied to the planar electrode, a positive upward leader is developing
from the top of the spherical electrode. This leader develops as follows. When the high voltage
waveform is applied, the free electrons present in the air near the spherical electrode are accelerated
and produce other free electrons by avalanche effect. These electrons aremoving toward the direction
of an increase of the field and regroup near the electrode. A corona discharge thus appears with the
development of a streamer bundle.Within the risetime of the voltage pulse, these positive streamers
can attain a typical length of 1 m in 1 μs with an appropriate local external field. These streamers
regroup at the stem of the corona in a hot plasma (T ∼ 1500 K) column which progresses toward the
high voltage source: the leader. The positive leader propagation is sustained by the corona discharge
at its tip which provides the current necessary to heat the leader. When the leader propagates, it
increases locally the electric field at its extremity leading to a further enhancement of the corona
discharge.
CONCLUSION
We have demonstrated the ability of the filament to deviate long air gap spark discharge from
their natural point of attachment. When the delay between the laser and the voltage pulse are
cleverly optimized the deviation is 100% efficient for both voltage polarities. We also observed
a very important reduction in the threshold voltage (especially for a positive polarity) with laser
filament. Filament induced triggering and guiding has been observed at a distance of 50 m from the
laser, a distance limited by the available space. We have also observed filament guided discharges
even when a natural discharge had already started from a rival tip electrode. All these results are
encouraging for the realization of a laser lightning rod.