Laser-plasma accelerators emerge as ultra-compact and versatile sources for numerous applications. Although the acceleration length is short (a few millimeters), they typically require large-scale infrastructures including ultra-high-power lasers, vacuum chambers and strict stability for temperature and humidity. As a result, most experiments are conducted in laboratories in large areas with controlled environments. Here, we present a highly compact (footprint of ~ 9 m²) and transportable system capable of generating electrons and photons in the MeV range at high repetition rates (up to 10 Hz) with average charge levels of 0.5 nC and up to 1 nC. This achievement shows the feasibility of performing laser-plasma acceleration outside of laboratory environments with a transportable system, significantly expanding the potential for practical applications.

We study the influence of the gravitational force on the generation of low-density channels of air left in the path of femtosecond laser filaments at high repetition rate. We observe a more important density variation along the filament longitudinal axis in the case of a vertically created filament as compared to a horizontal one. This leads to a more important reduction of the electrical breakdown field using vertical filament. This geometry induced difference is only observed at high repetition rate because it is directly related to the cumulative effect appearing above 100 Hz.

We study the creation and evolution of meter-scale long-lived laser-guided electric discharges and the interaction between consecutive guided discharges. The lifetime of guided discharges from a Tesla high voltage generator is first increased up to several milliseconds by the injection of additional current. The subsequent discharge evolution is measured by recording the electric current and by Schlieren and fluorescence imaging. A thermodynamic model of the gas evolution is developed to explain the discharge evolution. Finally, we analyze the succession of laser-guided discharges generated at 10 Hz.

In the framework of the Laser Lightning Rod project, whose aim is to show that laser-induced filaments can guide lightning discharges over considerable distances, we study over a distance of 140 m the filaments created by a laser system with J-range pulses of 1 ps duration at 1 kHz repetition rate. We investigate the spatial evolution of the multiple filamentation regime using the fundamental beam at 1030 nm or using combination with the second and third harmonics. The measurements were made using both a collimated beam and a loosely focused beam.

Controlling lightning is a long-time dream of mankind. Along with the rapid evolution of laser technologies, the idea to develop lightning protection based on filamentation of high-power ultrashort-pulse lasers emerged in the 1990s [1]. The goal of the Laser Lightning Rod (LLR) project that started in 2017 was to investigate a new type of lightning protection based on the use of upward lightning discharges initiated through a high-repetition-rate terawatt laser [2]. To that end, a high-power laser was developed by Trumpf scientific with pulse energy in the Joule-range, 1 ps pulse duration, and a repetition rate of 1 kHz [3]. This CPA laser system is based on Yb-YAG thin-disk technology allowing amplification at very high average power at a wavelength of 1030 nm.

Electric currents circulating between charged clouds and the earth surface during lightning discharges are responsible for considerable damages and casualties. It is therefore important to develop better protection methods in addition to the traditional Franklin rod. Here we present the first demonstration that filaments formed by short and intense laser pulses can guide lightning discharges over considerable distances. We believe that this experimental breakthrough will lead to progress in lightning protection and lightning physics. An experimental campaign was carried out on the Säntis Mountain in Northeastern Switzerland during the Summer of 2021 with a high repetition rate terawatt laser. The guiding of an upward negative lightning leader over a distance of 50 m was recorded by two separate high-speed cameras. The guiding of negative lightning leaders by laser filaments was corroborated in three other instances by VHF interferometric measurements, and the number of X-ray bursts detected during guided lightning events was significantly increased. While this research field has been very active for more than 20 years with many research groups around the world working to achieve this goal, this result demonstrates lightning guiding by lasers, which may lead to the development of a laser lightning rod. This work paves the way for new atmospheric applications of ultrashort lasers and represents a significant step forward in the development of a laser based lightning protection for airports, launchpads or large infrastructures.

Femtosecond laser filamentation in air creates long columns of weakly ionized plasma that result in a low-density channel over μs timescale [1]. These channels can trigger and guide electric discharges [2]–[3]. Applications have been proposed for this effect, such as the laser lightning rod [4], or the plasma antenna [5]. The main limitation for the later one is the discharge lifetime limited to microseconds [6], since a plasma antenna would require the existence of a permanent conductive channel. To overcome this limitation, we investigate a regime using consecutive guided discharges with prolonged lifetime that could create a quasi-permanent conductive channel.

We report Second Harmonic Generation (SHG) and Third Harmonic Generation (THG) energy conversion efficiencies up to 59% and 27%, respectively, for laser pulses simultaneously delivering high peak power in the sub-TW range and average powers in the sub-kW range. No damage or efficiency decrease is observed after more than 100 h operation time. The resulting high-energy visible and near-UV pulses are suitable for applications, such as lightning control, material analysis and machining, or OPCPA pumping.

Significant efforts have been dedicated to control lightning with lasers [1] to protect sensitive sites like rocket launching pads and airports. We report on a dedicated European large framework project towards this goal, called Laser Lightning Rod (llr-fet.eu) [2] . In this project, a unique laser system has been developed, providing 720 mJ, < 1 picosecond pulses at 1 kHz repetition rate, i.e., TW-class peak power and kW average power [3] . This thin disk laser has been further frequency doubled (SHG) and tripled (THG) to provide high intensity outputs at 1030 nm, 515 nm and 343 nm, with conversion efficiencies up to 45% and 30%, respectively. Nonlinear propagation and filamentation of this laser could be characterized over 130 meters in a dedicated hall (LAL in Orsay), both in a collimated geometry, and using a 40 cm diameter beam expander. The whole system will be transported to the top of the Säntis Mountain (2481 m altitude) in Switzerland in April 2021, one of the locations with the highest lightning occurrence in Europe. The site has been fully equipped to host the laser transmitter in the harsh meteorological conditions encountered on the top of the mountain. The site has also been fully equipped with state of the art lightning detection and measurement devices (a lightning mapping array, an interferometer, high-speed cameras, current sensors, a field mill, etc.) to register the triggering and guiding effects of the laser on the lightning process. This site is especially interesting as it generates mostly upward flashes from the 123 m tall tower communications tower. The laser will be aimed to the top of this tower in order to extend its height by the length of the filaments , realizing a "laser lightning rod".

We study the influence of the laser repetition rate on the generation of low-density channels of air left in the path of femtosecond laser filament. At high repetition rates, we observe the formation of a permanent millimeter-wide low-density channel that exceeds the depth and width of the transient depletion due to a single filament. We also show that this permanent cumulative effect decreases the breakdown voltage between two electrodes and can alter the path of the discharge. By comparing this effect in air and in pure nitrogen, we show that an accumulation of O-2 ions contributes to the reduction in the breakdown voltage. (C) 2021 Author(s).

Lightning is highly destructive due to its high power density and unpredictable character. Directing lightning away would allow to protect sensitive sites from its direct and indirect impacts (electromagnetic perturbations). Up to now, lasers have been unable to guide lightning efficiently since they were not offering simultaneously terawatt peak powers and kHz repetition rates. In the framework of the Laser Lightning Rod project, we develop a laser system for lightning control, with J-range pulses of 1 ps duration at 1 kHz. The project aims at investigating its propagation in the multiple filamentation regime and its ability to control high-voltage discharges. In particular, a field campaign at the Säntis mountain will assess the laser ability to trigger upward lightning.

We study the use of frequency upconversion schemes of near-IR picosecond laser pulses and compare their ability to guide and trigger electric discharges through filamentation in air. Upconversion, such as Second Harmonic Generation, is favorable for triggering electric discharges for given amount of available laser energy, even taking into account the losses inherent to frequency conversion. We focus on the practical question of optimizing the use of energy from a given available laser system and the potential advantage to use frequency conversion schemes.

We review recent results on multi-wavelength multipulse schemes to control highvoltage discharges with ultrashort pulses, and discuss their implications on lightning control at atmospheric scale. © 2019 The Author(s)

When a flying object becomes supersonic, a concomitant increase in drag leads to a considerable rise in fuel consumption. We show experimentally that an embarked terawatt femtosecond laser can significantly decrease this drag. We measured a 50% transient reduction of drag on a test model placed in a supersonic wind tunnel at Mach 3. This effect was initiated by the thin hot air column created in front of the supersonic object by filamentation of the laser pulse. We also show that this technique offers possibilities for steering.

The goal of this study is to demonstrate experimentally the Laser Spike concept in a supersonic flow. The Laser Spike uses a thin filament of plasma created by a femtosecond laser. This plasma filament formed by the TeraWatt pulse acts as a linear energy deposition upstream of the shock wave created by a blunt body. It has been found that the energy deposition induces a significant transient reduction of the drag. This reduction has been quantified using a drag balance. These experiments have been simulated numerically by modeling the plasma filament as a source term in the energy equation. It is shown that the plasma filament created by the ultra-short pulse forms a low-density heated core that interacts with the detached bow shock. This interaction produces an inflating recirculating bubble and leads to a transient reduction of the drag. These findings demonstrate the interest of ultrashort for flow control. Drag reduction, flow control and trajectory control are among the possible applications of the laser spike concept. Sonic boom alleviation could also be improved with this device.

source in a filament plane and in the plane perpendicular to the filament was measured for different conditions of focalization of the laser beam.In a second step, parameters affecting the efficiency of the opto-acoustic conversion were investigated.In order to get better laser propagation conditions in water the laser wavelength was changed from 800nm to 400nm.This was achieved by using a KDP crystal (second harmonic generation).The influence of optical pulse duration and total laser pulse energy on the level of the received acoustical signals were successively investigated.In this experiment three hydrophones were used, covering all together the frequency band [0-15 MHz].Finally, some tests of sound source generation in a saline solution (35 g/l NaCl) were made in order to predict what would be the source level in a real sea experiment.

Acoustic signals generated by filamentation of TW laser pulses in water are characterized experimentally and numerically revealing a strong influence of the input pulse duration on the shape of the acoustic signal.

We study the propagation of intense, high repetition rate laser pulses of picosecond duration at 1.03 µm central wavelength through air. Evidence of filamentation is obtained from measurements of the beam profile as a function of distance, from photoemission imaging and from spatially resolved sonometric recordings. Good agreement is found with numerical simulations. Simulations reveal an important self shortening of the pulse duration, suggesting that laser pulses with few optical cycles could be obtained via double filamentation. An important lowering of the voltage required to induce guided electric discharges between charged electrodes is measured at high laser pulse repetition rate.

Acoustic signals generated by filamentation of ultrashort terawatt laser pulses in water are characterized experimentally. Measurements reveal a strong influence of input pulse duration on the shape and intensity of the acoustic wave. Numerical simulations of the laser pulse nonlinear propagation and the subsequent water hydrodynamics and acoustic wave generation show that the strong acoustic emission is related to the mechanism of superfilamention in water. The elongated shape of the plasma volume where energy is deposited drives the far-field profile of the acoustic signal, which takes the form of a radially directed pressure wave with a single oscillation and a very broad spectrum.

In this work the non-linear opto-acoustic problem which consists in generating an acoustic signal in water from an intense ultra short laser pulse has been studied. The acoustic source obtained could be related to the phenomenon of filamentation which produces a contraction of the initial beam accompanied by the formation of plasma. Relatively recent work has shown that lasers of this type could be used to produce remote acoustic sources with interesting applications to underwater acoustics. The spectrum of the sound source obtained was investigated and its directivity pattern in both planes (plane of the filament and plane perpendicular to the filament) was measured. The sound level of the source as a function of energy, duration, and wavelength of the laser pulse was also measured.