arXiv subjects

S. P. D. Mangles

Publications and source records attributed to S. P. D. Mangles.

At least 37 records · Page 2Linked to original sources

arXiv2018

Temporal Feedback Control of High-Intensity Laser Pulses to Optimize Ultrafast Heating of Atomic Clusters

We describe how active feedback routines can be applied at limited repetition rate (5 Hz) to optimize high-power $>10$ TW) laser interactions with clustered gases. Optimization of x-ray production from an argon cluster jet, using a genetic algorithm, approximately doubled the measured energy through temporal modification of the 150 mJ driving laser pulse. This approach achieved an increased radiation yield through exploration of a multi-dimensional parameter space, without requiring detailed a priori knowledge of the complex cluster dynamics. The optimized laser pulses exhibited a slow rising edge to the intensity profile, which enhanced the laser energy coupling into the cluster medium, compared to the optimally compressed FWHM pulse (40 fs). Our work suggests that this technique can be more widely utilized for control of intense pulsed secondary radiation from petawatt-class laser systems.

physics.plasm-ph↗

arXiv2018

Observation of Laser Power Amplification in a Self-Injecting Laser Wakefield Accelerator

We report on the depletion and power amplification of the driving laser pulse in a strongly-driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from $187 \pm 11$ TW to a maximum of $318 \pm 12$ TW after 13 mm of propagation in plasma density of $0.9 \times 10^{18}$ cm$^{-3}$. The power amplification is correlated with the injection and acceleration of electrons in the nonlinear wakefield. This process is modeled by including localized redshift and subsequent group delay dispersion at the laser pulse front.

physics.plasm-ph↗

arXiv2018

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.

physics.plasm-ph↗

arXiv2018

Ultrafast Imaging of Laser Driven Shock Waves using Betatron X-rays from a Laser Wakefield Accelerator

Betatron radiation from laser wakefield accelerators is an ultrashort pulsed source of hard, synchrotron-like x-ray radiation. It emanates from a centimetre scale plasma accelerator producing GeV level electron beams. In recent years betatron radiation has been developed as a unique source capable of producing high resolution x-ray images in compact geometries. However, until now, the short pulse nature of this radiation has not been exploited. This report details the first experiment to utilise betatron radiation to image a rapidly evolving phenomenon by using it to radiograph a laser driven shock wave in a silicon target. The spatial resolution of the image is comparable to what has been achieved in similar experiments at conventional synchrotron light sources. The intrinsic temporal resolution of betatron radiation is below 100 fs, indicating that significantly faster processes could be probed in future without compromising spatial resolution. Quantitative measurements of the shock velocity and material density were made from the radiographs recorded during shock compression and were consistent with the established shock response of silicon, as determined with traditional velocimetry approaches. This suggests that future compact betatron imaging beamlines could be useful in the imaging and diagnosis of high-energy-density physics experiments.

physics.plasm-ph↗

arXiv2018

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.

physics.plasm-ph↗

arXiv2018

Experimental evidence of radiation reaction in the collision of a high-intensity laser pulse with a laser-wakefield accelerated electron beam

The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today's lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We report on the observation of radiation reaction in the collision of an ultra-relativistic electron beam generated by laser wakefield acceleration ($\varepsilon > 500$ MeV) with an intense laser pulse ($a_0 > 10$). We measure an energy loss in the post-collision electron spectrum that is correlated with the detected signal of hard photons ($γ$-rays), consistent with a quantum (stochastic) description of radiation reaction. The generated $γ$-rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy $\varepsilon_{\rm crit} > $ 30 MeV.

physics.plasm-ph↗

arXiv2017

Experimental observation of a current-driven instability in a neutral electron-positron beam

We report on the first experimental observation of a current-driven instability developing in a quasi-neutral matter-antimatter beam. Strong magnetic fields ($\geq$ 1 T) are measured, via means of a proton radiography technique, after the propagation of a neutral electron-positron beam through a background electron-ion plasma.The experimentally determined equipartition parameter of $ε_B \approx 10^{-3}$, is typical of values inferred from models of astrophysical gamma-ray bursts, in which the relativistic flows are also expected to be pair dominated. The data, supported by Particle-In-Cell simulations and simple analytical estimates, indicate that these magnetic fields persist in the background plasma for thousands of inverse plasma frequencies. The existence of such long-lived magnetic fields can be related to analog astrophysical systems, such as those prevalent in lepton-dominated jets.

physics.plasm-ph↗

arXiv2017

Signatures of quantum effects on radiation reaction in laser -- electron-beam collisions

Two signatures of quantum effects on radiation reaction in the collision of a ~GeV electron-beam with a high-intensity (>3x10^20W/cm^2) laser-pulse have been considered. We show that the decrease in the average energy of the electron-beam may be used to measure the Gaunt factor g for synchrotron emission. We derive an equation for the evolution of the variance in the energy of the electron-beam in the quantum regime, i.e. quantum efficiency parameter eta > 0.1$. We show that the evolution of the variance may be used as a direct measure of the quantum stochasticity of the radiation reaction and determine the parameter regime where this is observable. For example, stochastic emission results in a 25% increase in the standard deviation of the energy spectrum of a GeV electron beam, 1 fs after it collides with a laser pulse of intensity 10^21 W/cm^2. This effect should therefore be measurable using current high-intensity laser systems.

physics.plasm-ph↗

arXiv2017

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 to understand 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 en- ergy 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.

physics.acc-ph↗

arXiv2017

Laser-driven plasma acceleration in a regime of strong-mismatch between the incident laser envelope and the nonlinear plasma response

We explore a regime of laser-driven plasma acceleration of electrons where the radial envelope of the laser-pulse incident at the plasma entrance is strongly mismatched to the nonlinear plasma electron response excited by it. This regime has been experimentally studied with the gemini laser using f/40 focusing optics in August 2015 and f/20 in 2008. The physical mechanisms and the scaling laws of electron acceleration achievable in a laser-plasma accelerator have been studied in the radially matched laser regime and thus are not accurate in the strongly mismatched regime explored here. In this work, we show that a novel adjusted-a0 model applicable over a specific range of densities where the laser enters the state of a strong optical shock, describes the mismatched regime. Beside several novel aspects of laser-plasma interaction dynamics relating to an elongating bubble shape and the corresponding self-injection mechanism, importantly we find that in this strongly mismatched regime when the laser pulse transforms into an optical shock it is possible to achieve beam-energies that significantly exceed the incident intensity matched regime scaling laws.

physics.plasm-ph↗

arXiv2015

Modeling ultrafast shadowgraphy in laser-plasma interaction experiments

Ultrafast shadowgraphy is a new experimental technique that uses few cycle laser pulses to image density gradients in a rapidly evolving plasma. It enables structures that move at speeds close to the speed of light, such as laser driven wakes, to be visualized. Here we study the process of shadowgraphic image formation during the propagation of a few cycle probe pulse transversely through a laser-driven wake using three-dimensional particle-in-cell simulations. In order to construct synthetic shadowgrams a near-field snapshot of the ultrashort probe pulse is analyzed by means of Fourier optics, taking into account the effect of a typical imaging setup. By comparing synthetic and experimental shadowgrams we show that the generation of synthetic data is crucial for the correct interpretation of experiments. Moreover, we study the dependence of synthetic shadowgrams on various parameters such as the imaging system aperture, the position of the object plane and the probe pulse delay, duration and wavelength. Finally, we show that time-dependent information from the interaction can be recovered from a single shot by using a broadband, chirped probe pulse and subsequent spectral filtering.

physics.plasm-ph↗

arXiv2015

Direct observation of the injection dynamics of a laser wakefield accelerator using few-femtosecond shadowgraphy

We present few-femtosecond shadowgraphic snapshots taken during the non-linear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma wave period is induced by a lengthening of the first plasma period. Three dimensional particle in cell simulations support our observations.

physics.plasm-ph↗

arXiv2014

Femtosecond-scale Synchronisation of Ultra-Intense Focused Laser Beams

Synchronising ultra-short (~fs) and focussed laser pulses is a particularly difficult task, as this timescale lies orders of magnitude below the typical range of fast electronic devices. Here we present an optical technique that allows for femtosecond-scale synchronisation of the focal planes of two focussed laser pulses. This technique is virtually applicable to any focussing geometry and relative intensity of the two lasers. Experimental implementation of this technique provides excellent quantitative agreement with theoretical expectations. The proposed technique will prove highly beneficial for the next generation of multiple, petawatt class laser systems.

physics.optics↗

arXiv2014

Optimisation of the pointing stability of laser-wakefield accelerated electron beams

Laser-wakefield acceleration is a promising technique for the next generation of ultra-compact, high-energy particle accelerators. However, for a meaningful use of laser-driven particle beams it is necessary that they present a high degree of pointing stability in order to be injected into transport lines and further acceleration stages. Here we show a comprehensive experimental study of the main factors limiting the pointing stability of laser-wakefield accelerated electron beams. It is shown that gas-cells provide a much more stable electron generation axis, if compared to gas-jet targets, virtually regardless of the gas density used. A sub-mrad shot-to-shot fluctuation in pointing is measured and a consistent non-zero offset of the electron axis in respect to the laser propagation axis is found to be solely related to a residual angular dispersion introduced by the laser compression system and can be used as a precise diagnostic tool for compression oprtimisation in chirped pulse amplified lasers.

physics.plasm-ph↗

arXiv2014

Ultra-high brilliance multi-MeV $γ$-ray beam from non-linear Thomson scattering

We report on the generation of a narrow divergence ($θ\approx 2.5$ mrad), multi-MeV ($E_\text{MAX} = 18$ MeV) and ultra-high brilliance ($\approx 2\times10^{19}$ photons s$^{-1}$ mm$^{-2}$ mrad $^{-2}$ 0.1\% BW) $γ$-ray beam from the scattering of an ultra-relativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude $a_0\approx2$). The spectrum of the generated $γ$-ray beam is measured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear evidence of the onset of non-linear Thomson scattering. The photon source has the highest brilliance in the multi-MeV regime ever reported in the literature.

physics.optics↗

arXiv2014

Multi-Pulse Laser Wakefield Acceleration: A New Route to Efficient, High-Repetition-Rate Plasma Accelerators and High Flux Radiation Sources

Laser-driven plasma accelerators can generate accelerating gradients three orders of magnitude larger than radio-frequency accelerators and have achieved beam energies above 1 GeV in centimetre long stages. However, the pulse repetition rate and wall-plug efficiency of plasma accelerators is limited by the driving laser to less than approximately 1 Hz and 0.1% respectively. Here we investigate the prospects for exciting the plasma wave with trains of low-energy laser pulses rather than a single high-energy pulse. Resonantly exciting the wakefield in this way would enable the use of different technologies, such as fibre or thin-disc lasers, which are able to operate at multi-kilohertz pulse repetition rates and with wall-plug efficiencies two orders of magnitude higher than current laser systems. We outline the parameters of efficient, GeV-scale, 10-kHz plasma accelerators and show that they could drive compact X-ray sources with average photon fluxes comparable to those of third-generation light source but with significantly improved temporal resolution. Likewise FEL operation could be driven with comparable peak power but with significantly larger repetition rates than extant FELs.

physics.acc-ph↗

arXiv2013

Probing electron acceleration and X-ray emission in laser-plasma accelerator

While laser-plasma accelerators have demonstrated a strong potential in the acceleration of electrons up to giga-electronvolt energies, few experimental tools for studying the acceleration physics have been developed. In this paper, we demonstrate a method for probing the acceleration process. A second laser beam, propagating perpendicular to the main beam is focused in the gas jet few nanosecond before the main beam creates the accelerating plasma wave. This second beam is intense enough to ionize the gas and form a density depletion which will locally inhibit the acceleration. The position of the density depletion is scanned along the interaction length to probe the electron injection and acceleration, and the betatron X-ray emission. To illustrate the potential of the method, the variation of the injection position with the plasma density is studied.

physics.plasm-ph↗

arXiv2012

Tuning the electron energy by controlling the density perturbation position in laser plasma accelerators

A density perturbation produced in an underdense plasma was used to improve the quality of electron bunches produced in the laser-plasma wakefield acceleration scheme. Quasi-monoenergetic electrons were generated by controlled injection in the longitudinal density gradients of the density perturbation. By tuning the position of the density perturbation along the laser propagation axis, a fine control of the electron energy from a mean value of 60 MeV to 120 MeV has been demonstrated with a relative energy-spread of 15 +/- 3.6%, divergence of 4 +/- 0.8 mrad and charge of 6 +/- 1.8 pC.

physics.plasm-ph↗