A. Desparois

Filamentation of ultrashort pulse laser beams resulting from their propagation over long distances in air

The propagation of high-power short-pulse laser beams over considerable distances in air is studied both experimentally and via numerical simulations. Filaments are formed after 5–10 m and their propagation over distances in excess of 200 m is reported for the first time. The lateral dimensions of the filaments are found to range from about 100 μm to a few millimeters in diameter. The early values of plasma electron density have been inferred to be a few times 1016 cm−3 using longitudinal spectral interferometry. For 500 fs pulses and a wavelength of 1053 nm, the energy in the filament can be quite high initially (∼8 mJ) and is found to stabilize at about 1.5–2 mJ, after about 35 m. A simple model based on the nonlinear Schrodinger equation coupled to a multiphoton ionization law appears to describe several experimental results quite well.

Triggering and guiding high-voltage large-scale leader discharges with sub-joule ultrashort laser pulses*

The triggering and guiding of leader discharges using a plasma channel created by a sub-joule ultrashort laser pulse have been studied in a megavolt large-scale electrode configuration (3–7 m rod-plane air gap). By focusing the laser close to the positive rod electrode it has been possible, with a 400 mJ pulse, to trigger and guide leaders over distances of 3 m, to lower the leader inception voltage by 50%, and to increase the leader velocity by a factor of 10. The dynamics of the breakdown discharges with and without the laser pulse have been analyzed by means of a streak camera and of electric field and current probes. Numerical simulations have successfully reproduced many of the experimental results obtained with and without the presence of the laser plasma channel.

Modeling the triggering of streamers in air by ultrashort laser pulses

The physical processes involved in the triggering of ionization waves (streamers) by ultrashort laser pulses, focused in air at 350 Torr and in a uniform electric field, are investigated by means of a one-dimensional (1-D) numerical model. The model describes the interaction of the laser pulse with air and takes into account many of the reactions in the laser-created plasma as well as the radial expansion of the plasma. Consequences of the model are that the threshold electric field for the appearance of streamers is an increasing function of the delay between the laser pulse and the electric field pulse and a decreasing function of the laser energy. Also, it appears that the electron temperature, the plasma density and radius, and the conduction of heat across the plasma boundaries play major roles in the capacity of the laser-created plasma to trigger streamers. The results of the model are compared with the available experimental data.

Guiding large-scale spark discharges with ultrashort pulse laser filaments

Using the nonlinear propagation properties of ultrashort laser pulses in air, we were able to produce long ionized filaments that served to guide spark discharges. With a laser pulse energy of 20 mJ, one or two ionized filaments were created and could guide streamer discharges over 2 m air gaps, where the electric field was fairly uniform and had an average value of 0.6 MV/m. Such a guiding effect was observed for times of 1–3 μs after the laser pulse created the ionized filaments. Longer delays (10–15 μs) were recorded at higher laser pulse energy, with a larger number of filaments. Images of the early stages of the discharge of a uniform air gap show that the laser-produced ionized filaments do not initiate the discharge process but act rather as preferred channels where the leader growth is accelerated. In the end, these straight conductive channels carry the arc current as the voltage in the gap breaks down.

Triggering and guiding leader discharges using a plasma channel created by an ultrashort laser pulse

In a 2.8 m positive rod–plane air gap, we have studied how a plasma channel produced by focusing a 200 mJ ultrashort laser beam is able to trigger and guide a leader discharge. We have observed that the plasma channel allowed the lowering of the leader inception voltage by 50% and the guiding of the leader propagation on a distance of up to 2.3 m, with a tenfold increase of its speed. This led to an effective 40% reduction of the breakdown voltage. For the conditions studied here, the laser energy per unit length required to guide a leader is between 60 and 100 mJ/m.

The influence of electron density on the formation of streamers in electrical discharges triggered with ultrashort laser pulses

In an ongoing program using ultrashort laser pulses to provoke discharges in air over considerable distances at electric fields below breakdown threshold, we have studied the conditions for the onset of streamers in such laser-produced plasmas, both experimentally and through numerical simulations. The results demonstrate the importance of the electron density and of its gradient on the generation of streamers. Also, a significant reduction of the breakdown voltage for a 30 cm plane-plane gap in air was observed with a laser pulse energy of 15 mJ. Finally, a direct comparison of laser-induced breakdown in air and in nitrogen shows the influence of electron attachment to oxygen on the discharge process.

Triggering and guiding large scale leader-discharges with ultrashort laser pulses

Summary form only given, as follows. Ultrashort pulse lasers, which can produce long continuous plasma channel with a relatively small amount of energy, are interesting candidates to achieve laser triggering and guiding of lightning. In order to investigate this possibility, we have used picosecond pulses of 400 mJ delivered by a Ti:sapphire laser to trigger and guide leader discharges in gaps of 3 to 7 m. We used two types of discharge configuration: (i) gap between a positive rod and a grounded plane and (ii) gap between a negative plane and a rod on a grounded plane. The discharges were studied by means of optical and electrical diagnostics, such as a streak camera and a current probe. First, we have observed that the laser pulse can initiate a corona at the tip of the positive rod and instantaneously trigger leader propagation, at a voltage that can be 30% lower than the normal minimum inception voltage of the first corona. Also, we have seen that the leader propagation can be guided by the laser-created plasma channel on a distance of up to 4 m, with a tenfold increase of its velocity. The leader current records revealed that this latter effect is accompanied by a significant reduction (by 4 to 5 times), of the amount of charge, fed to the leader tip by its corona, needed to generate by Joule heating a given length of leader channel. Our results show that the lower leader inception voltage combined to the higher leader speed can lead to a 40% smaller gap breakdown voltage. The length and the quality of the leader guiding has been studied as a function of the focal length of the lens used to produce the plasma channel and of the laser pulse characteristics (energy, duration).

Modeling large scale leader discharges in presence of a laser-created plasma channel

Summary form only given, as follows. In a laboratory study of the feasibility of the triggering and guiding of lightning by an ultrashort pulse laser, we have shown that a plasma channel produced by such a laser can guide the propagation of a positive leader on a distance of up to 4 m, with a tenfold increase of its velocity. We have made a theoretical study of the results of these experiments with a numerical model of the positive leader propagation. Previously developed by ONERA and the University of Padova for discharges occurring in ambient air, we have adapted this model to take into account the presence of a laser-created plasma channel. We found that this could be done simply by modifying a key parameter for the leader propagation, called qL, which is the amount of charge, flowing through the leader tip from its corona, required to heat up by Joule effect a unit length of leader channel. By correctly modifying qL, the results of the numerical simulations of the laser-guided discharges were in good agreement with the experiments. The nearly correct increased leader velocity and current, as well as reduced qL and breakdown voltage have been computed. We also propose a model linking the value of qL with the negative oxygen ions density in front of the leader tip, which leads to a direct relation between the negative ions left in the laser-created plasma channel and the accelerated leader velocity.

Single-shot chirped-pulse spectral interferometry used to measure ionization dynamics of air on a femtosecond time scale

Summary form only given.We have developed a new type of spectral interferometry with ultrafast temporal resolution based on the use of linearly chirped pulses. We have demonstrated that this single-shot technique can be used for measurements of the ionization dynamics of air produced by an intense short laser pulse. In principle, chirped pulse spectral interferometry could be used as a phase oscilloscope over a significant time span.

Laser-triggered spark discharges using ultrashort-pulse lasers

This paper presents result form a systematic study of the feasibility of triggering lightning in a controlled fashion, using ultrashort pulse lasers. We show the importance of producing a plasma with local gradients of electron density by focusing the laser beam in order to trigger streamers, which are the first condition required for the initiation of large-scale spark discharges. We present evidence of the ability of laser filaments to guide streamer discharges, which are akin to the final jump phase of a lightning discharge. We also demonstrate that the leader propagation can be considerably modified by the presence of a laser- produced plasma channel, in a rod-plane electrode geometry. Finally, we have developed numerical models for the ultrashort pulse laser beam propagation through air, plasma production and streamer inception.