Bremsstrahlung scattering of fast electrons on ions can be enhanced by the microwave radiation present in the solar corona. It can account for the electron diffusive transport along magnetic loops and high precipitation rates. This process can also dominate the transport of thermal electrons confined in such loops. The influence of stimulated Bremsstrahlung scattering on the electron transport is studied, with focus on the return current induced by the fast electron population trapped in magnetic loops. Overall, transport coefficients are reevaluated in the radiation-dominated plasma, characterized by the stimulated action of radiation on the Bremsstrahlung electron-ion collision, down to thermal velocities. We develop a theoretical framework for electron transport driven by a large bandwidth, bright low-frequency part of the photon spectrum and compute a set of radiation-enhanced transport coefficients. UV, XEUV and hard X-ray signals from flares, evidencing anomalous resistivity, thermal conduction inhibition and high precipitation rates of fast electrons, are reinterpreted. The anomalous resistivity due to stimulated Bremsstrahlung scattering is found to dominate the classical resistivity in flares. The runaway effect due to Coulomb collisions is suppressed. Thermal conduction is inhibited compared to the Spitzer conduction, in agreement with coronal seismology of slow-mode waves. Stimulated Bremsstrahlung scattering is found to be a key collisional process in flaring events. It can explain the above loop-top hard X-ray signal due to the fast electrons, and the measured electrical conductivity due to the thermal electrons. As a perspective, the corresponding transport coefficients can be used in radiation MHD codes. The radiation model could also be applied to stimulate large-angle electron scattering in the kinetic or hybrid models used to study the solar corona.

We demonstrate a very strong laser gain in the near UV, at 391 nm, involving one- and two-photon transitions between initial and final levels of the transition. This opens the way to the realization of all-optical lasers at short wavelengths. The amplifying medium is N2+ at low density, pumped by an intense laser source at a longer wavelength, i.e., 800 nm. It is probed by measuring with femtosecond precision the absorption of XUV high harmonics through the gas, displaying the evolution of the relevant electronic levels during the amplification process. We show the role of a third level forming a "V scheme" with the initial and final levels.

We demonstrate that a single-color ultrashort optical pulse generating a short plasma string in air can emit THz radiation along any direction with respect to its propagation axis. The emission angle can be adjusted by the flying focus technique [1], [2], which determines the speed and direction of the ionization front.

We demonstrate that pulsed THz radiation produced in air by a focused ultrashort laser pulse can be steered to large angles or even in the backward direction with respect to the laser propagation axis. The emission angle is adjusted by the flying focus technique, which determines the speed and direction of the ionization front created by the single-color laser pulse. This easily adjustable THz source, being well separated from the intense laser, opens exciting applications for remote THz spectroscopy.

The ion temperature in laser-heated foam materials can be considerably higher than the electron temperature due to the internal collisions of the plasma flows originating from the heterogeneous foam microstructure. Recently, we have developed a novel hybrid multiscale model for laser-foam interaction that successfully reproduces the experimentally measured heat front propagation in laser-heated subcritical foams of various densities. However, when applied to undercritical foams with average density closer to critical, the hybrid model simulations predict an ion-electron temperature ratio much larger than in any previously reported measurements and suggest that the influence of foam microstructure is more impactful for larger average densities. For such foams, the laser-driven heat front velocity was measured many times, but the ion temperature received much less attention. To investigate the ion temperature, the laser interaction with 10 mg/cm^3 undercritical chlorine-doped TMPTA foams has been studied at the PALS facility, using an extended diagnostic complex emphasizing the X-ray time-resolved studies of the plasma wave propagation inside the foam and the distribution of macroscopic plasma parameters via high-resolution X-ray spectroscopy. The ion and electron temperatures have been measured from Doppler broadening and the relative intensity ratio of chlorine X-ray spectral lines. The averaged ion and electron temperature ratio ranges from 2 to 4 depending on the laser pulse energy. The simulations agree reasonably well with the experimental results.

The interaction of high-power laser pulses with undercritical foams produced by different techniques but with the same average density is studied at the PALS laser facility. The spatial–temporal evolution of X-ray emission is observed using an X-ray streak camera, electron and ion temperatures are measured by X-ray spectroscopy, and hot-electron production is characterized by monochromatic X-ray imaging. Transmission of a femtosecond laser probe pulse through foams is observed in the near and far fields. In spite of large differences in pore size and foam structure, the velocity of ionization front propagation is quite similar for all the foams studied and is slower than that in a homogeneous material of the same average density. The ion temperature in the plasma behind the ionization front is a few times higher than the electron temperature. Hot-electron production in plastic foams with small pores is strongly suppressed compared with that in solid targets, whereas in foams produced by additive manufacturing, it is significantly increased to the level observed in bare copper foil targets.

The landscape of fusion research has shifted significantly. Recent breakthroughs in both inertial and magnetic confinement fusion, along with early insights into the physics of burning plasmas, now challenge the long-standing joke that fusion is always 50 years away. Growing political interest in the light of climate change, the quest for the energy supply in future and the emergence of start-ups are some aspect that have brought the fusion community into an unprecedented public spotlight. However, this development comes with two contrasting sides. On one hand, fusion research is receiving increased support, including private investment. On the other hand, our community faces growing pressure, and scientists are concerned about unrealistic expectations. Young colleagues feel unsure but see plenty of opportunities at the same time.

The quality of the proton beam produced by Target Normal Sheath Acceleration (TNSA) with high power lasers can be significantly improved with the use of helical coils. While they showed promising results in terms of focusing, their performances in terms of the of cutoff energy and bunching stay limited due to the dispersive nature of helical coils. A new scheme of helical coil with a tube surrounding the helix is introduced, and the first numerical simulations and an analytical model show a possibility of a drastic reduction of the current pulse dispersion for the parameters of high power laser facilities. The helical coils with tube strongly increase bunching, creating two collimated narrow-band proton beams from a broad and divergent TNSA distribution. The analytical model provides scaling of proton parameters as a function of laser facility features.

Porous materials offer unique possibilities for the production of plasmas with controlled density profiles for experiments on laser–matter interaction. They are of growing relevance to many applications, such as inertial confinement fusion, fundamental research, and secondary sources. Understanding the processes of transformation of a porous solid into a plasma is of fundamental interest and is needed for producing materials with desired properties.

Laser-driven ion acceleration is well established using solid targets mainly in the target normal sheath acceleration regime. To follow the increasing repetition rate available on high-intensity lasers, the use of high-density gas targets has been explored in the past decade. When interacting with targets reaching densities close to the critical one, the laser pulse can trigger different acceleration mechanisms such as Collisionless Shock Acceleration (CSA) or hole boring. Particle-in-cell simulations using ideal target profiles show that CSA can accelerate a collimated, narrow energy spread and few hundreds of megaelectronvolts ion beam on the laser axis. Nevertheless, in real experiments, the laser will not only interact with an overcritical, thin plasma slab with sharp density gradients, but also with lower density regions surrounding the core of the gas jet, extending to several hundreds of micrometres. The interaction of the laser with these lower density wings will lead to nonlinear effects that will reduce the available energy to drive the shock in the high-density region of the target. Optically tailoring this target could mitigate that issue. Recent experiments conducted on different laser facilities aimed at testing several tailoring configurations. We first tested a scheme with a copropagating picosecond prepulse to create a lower density plasma channel to facilitate the propagation of the main pulse, while the second one was a transverse tailoring driven by nanosecond laser pulses to generate blast waves and form a high-density plasma slab. The main results will be presented here and the methods compared.

Understanding the physics of electromagnetic pulse emission and nozzle damage is critical for the long-term operation of laser experiments with gas targets, particularly at facilities looking to produce stable sources of radiation at high repetition rate. We present a theoretical model of plasma formation and electrostatic charging when high-power lasers are focused inside gases. The model can be used to estimate the amplitude of gigahertz electromagnetic pulses (EMPs) produced by the laser and the extent of damage to the gas jet nozzle. Looking at a range of laser and target properties relevant to existing high-power laser systems, we find that EMP fields of tens to hundreds of kV/m can be generated several metres from the gas jet. Model predictions are compared with measurements of EMP, plasma formation and nozzle damage from two experiments on the VEGA-3 laser and one experiment on the Vulcan Petawatt laser.

Fast electron scattering on plasma ions due to stimulated Bremsstrahlung is investigated and modeled. Comparison with Coulomb scattering suggests that stimulated Bremsstrahlung scattering can be dominant in low-density, radiation-driven plasmas, provided that the radiation spectrum has a sufficiently high brightness temperature in the neighborhood of the plasma frequency. While stimulated Bremsstrahlung scattering cannot be easily observed in laboratory plasmas due to their small size, it should operate in large-scale astrophysical plasmas, such as those met in the flaring solar corona. The effect of the solar microwave radiation on fast-electron scattering is evaluated through a parameterized flaring corona model. We find that stimulated Bremsstrahlung greatly enhances the fast-electron scattering frequency in the flare magnetic loop, leading the transport of deka-keV electrons to occur in the diffusion regime, characterized by significant precipitation rates. This prediction is consistent with the interpretation of the above-loop-top hard x-ray and microwave emissions from the X3.1 flare of August 24, 2002. Our analysis indicates that stimulated Bremsstrahlung may play an essential role in the dynamics of fast electrons trapped in solar flare loops.

This paper explores the capabilities of generative artificial intelligence (AI) for target optimization in inertial confinement fusion for energy production. For demonstration purposes, the focus is on optimizing the laser illumination temporal profile assuming a spherical implosion and a given target structure. An optimization protocol is based on the generative AI tool and a dataset for a shock ignition scheme produced with a reference hydrodynamic code. In a first optimization process, the generative AI proposed a family of laser power profiles by introducing a plateau before the shock that doubles the energy gain value of the reference configuration. In a second optimization process, the number of parameters defining the laser power profile is increased according to the results of the first step. The generative AI then suggested more general solutions including multiple plateaus and classical profiles without shock that further double the gain for half the laser energy required. The suggested optimization method can be extended to other configurations of laser-target interaction. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

We present measurements of magnetic fields generated in laser-driven coil targets irradiated by laser pulses of nanosecond duration, 1.053 μm wavelength, 500 J energy, and ∼1015 W/cm2 intensity, at the LULI2000 facility. Using two perpendicular probing axes, proton deflectometry is used to characterize the coil current and static charge at different times. Results reveal various deflection features that can be unambiguously linked to a looping quasi-steady current of well-understood polarity or to a static charging of the coil surface. Measured currents are broadly consistent with predictions from a laser-driven diode-current source and lumped circuit model, supporting the quasi-steady assessment of the discharges. Peak magnetic fields of ∼50 T at the center of 500-μm-diameter coils, obtained at the moderate laser intensity, open up the use of such laser-driven coil targets at facilities worldwide to study numerous phenomena in magnetized high-energy-density plasmas, and its potential applications.

High power femtosecond laser pulses launched in air undergo nonlinear filamentary propagation, featuring a bright and thin plasma channel in air with its length much longer than the Rayleigh length of the laser beam. During this nonlinear propagation process, the laser pulses experience rich and complex spatial and temporal transformations. With its applications ranging from supercontinuum generation, laser pulse compression, remote sensing to triggering of lightning, the underlying physical mechanism of filamentation has been intensively studied. In this review, we will focus on the fluorescence and cavity-free lasing effect of the plasma filaments in air. The different mechanisms underlying the fluorescence of the excited neutral nitrogen molecules will be throughly examined and it is concluded that the electron collision excitation is the dominant channel for the formation of the excited nitrogen molecules. The recently discovered "air lasing" effect, a cavity-free bidirectional lasing emission emitted by the filaments, will be introduced and its main properties will be emphasized. The applications of the fluorescence and lasing effect of the neutral nitrogen molecules will be introduced, with two examples on spectroscopy and detection of electric field. Finally, we discuss the quenching effect of the lasing effect in atmosphere and the mechanisms responsible will be analyzed. An outlook for the achievement of backward lasing in air will be briefly presented.

Inertial confinement represents one of two viable approaches for producing energy from the fusion of hydrogen isotopes. Scientists have now achieved a record yield of fusion energy when directly irradiating targets with only 28 kilojoules of laser energy.

Air plasma induced by ultrafast laser pulses is an extraordinary source of electromagnetic waves, emitting microwave, terahertz (THz) radiation, and cavityless lasing in the near-infrared and visible ranges. The temporal dynamics of the electron density have been revealed by optical pump-probe techniques, while the evolution of the electron temperature remains elusive due to a lack of suitable methods. Here, it is demonstrated that the intense THz-field-enhanced fluorescence emission from the excited molecules of nitrogen is a novel tool that allows to explore the complex dynamics of the plasma density and electron temperature simultaneously. Two relaxation times of electrons in air plasma are observed and interpreted as a competition between the excitation of a triplet state by laser or THz-field-heated electrons and the dissociative recombination of nitrogen molecular ions. Based on the theoretical simulations, the tens of picoseconds relaxation process is attributed to the ultrafast temperature decrease, while the longer relaxation in the range of hundreds of picoseconds is ascribed to the decay of electron density. The temporal relaxation of both the electron density and temperature revealed by applying an intense THz field provides further insights into the laser-air plasma interaction and will benefit the engineering of this exceptional source. The transient temporal evolution of density, temperature, and collision processes in air plasma needs further understanding and appropriate measurements. In this study, an intense terahertz field is used to enhance the fluorescence emission from the excited molecules of nitrogen in air plasma, providing picosecond resolution to explore the dynamics of the plasma density and electron temperature simultaneously. image

Quantum interference occurs frequently in the interaction of laser radiation with materials, leading to a series of fascinating effects such as lasing without inversion, electromagnetically induced transparency, Fano resonance, etc. Such quantum interference effects are mostly enabled by single-photon resonance with transitions in the matter, regardless of how many optical frequencies are involved. Here, we report on quantum interference driven by multiple photons in the emission spectroscopy of nitrogen ions that are resonantly pumped by ultrafast infrared laser pulses. In the spectral domain, Fano resonance is observed in the emission spectrum, where a laser-assisted dynamic Stark effect creates the continuum. In the time domain, the fast-evolving emission is measured, revealing the nature of free-induction decay arising from quantum radiation and molecular cooperativity. These findings clarify the mechanism of coherent emission of nitrogen ions pumped with mid-infrared pump laser and are found to be universal. The present work opens a route to explore the important role of quantum interference during the interaction of intense laser pulses with materials near multiple photon resonance.