Emmanuel d'Humieres

Ultrafast laser-driven microlens to focus and energy-select mega-electron volt protons

We present a technique for simultaneous focusing and energy selection of high-current, mega–electron volt proton beams with the use of radial, transient electric fields (107 to 1010 volts per meter) triggered on the inner walls of a hollow microcylinder by an intense subpicosecond laser pulse. Because of the transient nature of the focusing fields, the proposed method allows selection of a desired range out of the spectrum of the polyenergetic proton beam. This technique addresses current drawbacks of laser-accelerated proton beams, such as their broad spectrum and divergence at the source.

Comparative spectra and efficiencies of ions laser-accelerated forward from the front and rear surfaces of thin solid foils

The maximum energy of protons that are accelerated forward by high-intensity, short-pulse lasers from either the front or rear surfaces of thin metal foils is compared for a large range of laser intensities and pulse durations. In the regime of moderately long laser pulse durations (300–850fs), and for high laser intensities [(1−6)×1019W∕cm2], rear-surface acceleration is shown experimentally to produce higher energy particles with smaller divergence and a higher efficiency than front-surface acceleration. For similar laser pulse durations but for lower laser intensities (2×1018Wcm−2), the same conclusion is reached from direct proton radiography of the electric fields associated with proton acceleration from the rear surface. For shorter (30–100fs) or longer (1–10ps) laser pulses, the same predominance of rear-surface acceleration in producing the highest energy protons is suggested by simulations and by comparison of analytical models with measured values. For this purpose, we have revised our previous ...

Laser acceleration of high-energy protons in variable density plasmas

The acceleration of protons, induced by electrons generated by a short-pulse laser, is experimentally investigated when varying the density of the plasma target the laser is interacting with. The experimental results are compared with particle-in-cell (PIC) simulations for which the target conditions are inferred from hydrodynamic simulations. High-energy protons are observed only for the two extreme configurations, namely solid-density foils and near- critical-density plasmas having large gradients. Cold solid foils, however, yield the highest energy protons and best proton beam profiles. As suggested by simulations, near-critical-density plasmas could be optimized to further increase the proton energy.

Laser Triggered Micro-Lens for Focusing and Energy Selection of MeV Protons

We present a novel technique for focusing and energy selection of high-current, MeV proton/ion beams. This method employs a hollow micro-cylinder that is irradiated at the outer wall by a high intensity, ultra-short laser pulse. The relativistic electrons produced are injected through the cylinders wall, spread evenly on the inner wall surface of the cylinder, and initiate a hot plasma expansion. A transient radial electric field (10 7 –10 10 V/m) is associated with the expansion. The transient electrostatic field induces the focusing and the selection of a narrow band component out of the broadband poly-energetic energy spectrum of the protons generated from a separate laser irradiated thin foil target that are directed axially through the cylinder. The energy selection is tunable by changing the timing of the two laser pulses. Computer simulations carried out for similar parameters as used in the experiments explain the working of the micro-lens.

Ultrafast Synchrotron-Enhanced Thermalization of Laser-Driven Colliding Pair Plasmas.

We report on the first self-consistent numerical study of the feasibility of laser-driven relativistic pair shocks of prime interest for high-energy astrophysics. Using a QED-particle-in-cell code, we simulate the collective interaction between two counterstreaming electron-positron jets driven from solid foils by short-pulse (~60 fs), high-energy (~100 kJ) lasers. We show that the dissipation caused by self-induced, ultrastrong (>10^{6} T) electromagnetic fluctuations is amplified by intense synchrotron emission, which enhances the magnetic confinement and compression of the colliding jets.

Investigation of longitudinal proton acceleration in exploded targets irradiated by intense short-pulse laser

It was recently shown that a promising way to accelerate protons in the forward direction to high energies is to use under-dense or near-critical density targets instead of solids. Simulations have revealed that the acceleration process depends on the density gradients of the plasma target. Indeed, under certain conditions, the most energetic protons are predicted to be accelerated by a collisionless shock mechanism that significantly increases their energy. We report here the results of a recent experiment dedicated to the study of longitudinal ion acceleration in partially exploded foils using a high intensity (∼5 × 1018 W/cm2) picosecond laser pulse. We show that protons accelerated using targets having moderate front and rear plasma gradients (up to ∼8 μm gradient length) exhibit similar maximum proton energy and number compared to proton beams that are produced, in similar laser conditions, from solid targets, in the well-known target normal sheath acceleration regime. Particle-In-Cell simulations, p...

Reduction of the fast electron angular dispersion by means of varying-resistivity structured targets

We present novel structured targets capable of collimating laser-generated fast electrons through dense plasmas. The proposed targets are made of narrow high- and low-Z filaments leading to a transversely modulated electrical resistivity profile. When featuring a spatially decreasing density, these targets permit both to guide the fast electrons and reduce their angular dispersion. The principle of our target design is explained by a theoretical model. Two-dimensional particle-in-cell simulations are performed to demonstrate its efficiency.

Scaling Laws for Proton Acceleration from the Rear Surface of Laser-Irradiated Thin Foils

In the last few years, intense research has been conducted on the topic of laser‐accelerated ion sources and their applications. Ultra‐bright beams of multi‐MeV protons are produced by irradiating thin metallic foils with ultra‐intense short laser pulses. These sources open new opportunities for ion beam generation and control, and could stimulate development of compact ion accelerators for many applications, in particular proton therapy of deep‐seated tumours. Here we show that scaling laws deduced from fluid models reproduce well the acceleration of proton beams for a large range of laser and target parameters. These scaling laws show that, in our regime, there is an optimum in the laser pulse duration of ∼200 fs–1 ps, with a needed laser energy level of 30 to 100 J, in order to achieve e.g. 200 MeV energy protons necessary for proton therapy.

Ultra-fast ionization modeling in laser-plasma interaction

A new ionization model for weighted particle simulation of large density scale plasmas has been developed. It is capable of treating both field and impact ionization. The high-order interpolation scheme we have adopted reduces numerical noises of electromagnetic waves, which ionize the target unphysically. This model has been tested in simulations of high intensity laser interaction with both underdense and overdense targets. With the addition of the treatment of collisions, this new code allows us to perform large-scale simulations of laser matter interaction with atomic physics. This model is a very valuable tool for the design of high-energy density experiments with short pulse lasers.

Tree code for collision detection of large numbers of particles applied to the Breit–Wheeler process

Collision detection of a large number N of particles can be challenging. Directly testing N particles for collision among each other leads to N 2 queries. Especially in scenarios, where fast, densely packed particles interact, challenges arise for classical methods like Particle-in-Cell or Monte-Carlo. Modern collision detection methods utilising bounding volume hierarchies are suitable to overcome these challenges and allow a detailed analysis of the interaction of large number of particles. This approach is applied to the analysis of the collision of two photon beams leading to the creation of electron-positron pairs.