Optimal parameters for radiation reaction experiments

As new laser facilities are developed with intensities on the scale of 10^22 - 10^24 W cm^-2 , it becomes ever more important to understand the effect of strong field quantum electrodynamics processes, such as quantum radiation reaction, which will play a dominant role in laser-plasma interactions at these intensities. Recent all-optical experiments, where GeV electrons from a laser wakefield accelerator encountered a counter-propagating laser pulse with a_0 > 10, have produced evidence of radiation reaction, but have not conclusively identified quantum effects nor their most suitable theoretical description. Here we show the number of collisions and the conditions required to accomplish this, based on a simulation campaign of radiation reaction experiments under realistic conditions. We conclude that while the critical energy of the photon spectrum distinguishes classical and quantum-corrected models, a better means of distinguishing the stochastic and deterministic quantum models is the change in the electron energy spread. This is robust against shot-to-shot fluctuations and the necessary laser intensity and electron beam energies are already available. For example, we show that so long as the electron energy spread is below 25%, collisions at a_0 = 10 with electron energies of 500 MeV could differentiate between different quantum models in under 30 shots, even with shot to shot variations at the 50% level.Comment: 12 pages, 7 figure

Realising single-shot measurements of quantum radiation reaction in high-intensity lasers

Collisions between high intensity laser pulses and energetic electron beams are now used to measure the transition between the classical and quantum regimes of light-matter interactions. However, the energy spectrum of laser-wakefield-accelerated electron beams can fluctuate significantly from shot to shot, making it difficult to clearly discern quantum effects in radiation reaction, for example. Here we show how this can be accomplished in only a single laser shot. A millimeter-scale pre-collision drift allows the electron beam to expand to a size larger than the laser focal spot and develop a correlation between transverse position and angular divergence. In contrast to previous studies, this means that a measurement of the beam's energy-divergence spectrum automatically distinguishes components of the beam that hit or miss the laser focal spot and therefore do and do not experience radiation reaction

General features of experiments on the dynamics of laser-driven electron–positron beams

The experimental study of the dynamics of neutral electron–positron beams is an emerging area of research, enabled by the recent results on the generation of this exotic state of matter in the laboratory. Electron–positron beams and plasmas are believed to play a major role in the dynamics of extreme astrophysical objects such as supermassive black holes and pulsars. For instance, they are believed to be the main constituents of a large number of astrophysical jets, and they have been proposed to significantly contribute to the emission of gamma-ray bursts and their afterglow. However, despite extensive numerical modelling and indirect astrophysical observations, a detailed experimental characterisation of the dynamics of these objects is still at its infancy. Here, we will report on some of the general features of experiments studying the dynamics of electron–positron beams in a fully laser-driven setup

Laser Wakefield accelerator modelling with variational neural networks

A machine learning model was created to predict the electron spectrum generated by a GeVclass laser wakefield accelerator. The model was constructed from variational convolutional neural networks which mapped the results of secondary laser and plasma diagnostics to the generated electron spectrum. An ensemble of trained networks was used to predict the electron spectrum and to provide an estimation of the uncertainty on that prediction. It is anticipated that this approach will be useful for inferring the electron spectrum prior undergoing any process which can alter or destroy the beam. In addition, the model provides insight into the scaling of electron beam properties due to stochastic fluctuations in the laser energy and plasma electron density

Application of compact laser-driven accelerator X-ray sources for industrial imaging

X-rays generated by betatron oscillations of electrons in a laser-driven plasma accelerator were characterised and applied to imaging industrial samples. With a 125TW laser, a low divergence beam with 5.2±1.7 × 107photonsmrad−2 per pulse was produced with a synchrotron spectrum with a critical energy of 14.6±1.3keV. Radiographs were obtained of a metrology test sample, battery electrodes, and a damage site in a composite material. These results demonstrate the suitability of the source for non-destructive evaluation applications. The potential for industrial implementation of plasma accelerators is discussed

Laser-wakefield accelerators for high-resolution X-ray imaging of complex microstructures

Laser-wakefield accelerators (LWFAs) are high acceleration-gradient plasma-based particle accelerators capable of producing ultra-relativistic electron beams. Within the strong focusing fields of the wakefield, accelerated electrons undergo betatron oscillations, emitting a bright pulse of X-rays with a micrometer-scale source size that may be used for imaging applications. Non-destructive X-ray phase contrast imaging and tomography of heterogeneous materials can provide insight into their processing, structure, and performance. To demonstrate the imaging capability of X-rays from an LWFA, we have examined an irregular eutectic in the aluminum-silicon (Al-Si) system. The lamellar spacing of the Al-Si eutectic microstructure is on the order of a few micrometers, thus requiring high spatial resolution. We present comparisons between the sharpness and spatial resolution in phase contrast images of this eutectic alloy obtained via X-ray phase contrast imaging at the Swiss Light Source (SLS) synchrotron and X-ray projection microscopy via an LWFA source. An upper bound on the resolving power of 2.7 ± 0.3 µm of the LWFA source in this experiment was measured. These results indicate that betatron X-rays from LWFA can provide an alternative to conventional synchrotron sources for high resolution imaging of eutectics and, more broadly, complex microstructures

An Overview of Recent Progress in Laser Wakefield Acceleration Experiments

The goal of this paper is to examine experimental progress in laser wakefield acceleration over the past decade (2004–2014), and to use trends in the data tounderst and some of the important physical processes. By examining a set of over 50 experiments, various trends concerning the relationship between plasma density, accelerator length, laser power and the final electron beam energy are revealed. The data suggest that current experiments are limited by dephasing and that current experiments typically require some pulse evolution to reach the trapping threshold

Modeling few-cycle shadowgraphy of laser-wakefield accelerators

Ultrafast shadowgraphy utilizes few cycle probe pulses in order to image density gradients in a plasma allowing to probe structures, such as laser-driven wakes, moving close to the speed of light. Here we study the process of shadowgraphic image formation in the interaction of a few cycle probe pulse with a laser-driven wake using particle-in-cell (PIC) simulations. The output of the PIC code is then post-processed by means of Fourier optics in order to take into account the effect of a typical imaging setup. This allows to construct synthetic shadowgrams which can be compared with experimentally recorded ones. This facilitates the correct interpretation of such involved measurements

Measurements of forward scattered spectra from intense laser interactions in the forced laser wake-field regime

International audienceResults of an experiment using an ultra-short laser pulse to create plasma waves through the forced laser wake-field process are described. The transmitted optical spectra are shown to exhibit both red- and blue-shifting, likely due to self-phase modulation through the interaction between the laser pulse and a large amplitude plasma wave. Spectral side-bands shifted by multiples of the plasma frequency associated with the forward Raman instability (FRS) are absent, indicating that the plasma waves which are observed to accelerate electrons are likely not generated through the FRS process. One- and two-dimensional particle-in-cell simulations using similar parameters as the experiment are discussed

Electron Beam Production with an Ultra Short and Intense Laser Pulse: A New Tool for Scientists

International audienceBy focusing a 10 Hz, 30 TW, 30 fs laser beam onto a gas jet, it is now possible to produce an ultra short and high quality electron beam with a maximum energy of up to 200 MeV. The gas is instantaneously ionized by the laser electric field and transformed into plasma, in which accelerating electric fields of the order of 1 TV/m have been generated in the non-linear regime. Some applications of this attractive and compact electron source are presented