67 research outputs found
X-ray powder diffraction as a non-destructive tool for characterising laser wakefield betatron radiation
No full textLaser wakefield accelerators provide a relatively simple means of generating bright X-rays beams which are useful across many fields and industries. To achieve this, a sufficiently powerful laser is required and just a few centimetres of gas. In contrast, other bright X-ray sources, such as synchrotrons, require significantly more infrastructure and can span many hundreds of metres. This thesis looks at a novel diagnostic technique based on X-ray diffraction from a powder in order to characterise X-ray beams with large shot-to-shot variations. The mechanics behind the method, named XCERP diffraction, are presented and the results of simulations are provided as a proof of principle study. XCERP diffraction is shown to be able to retrieve details of the energy spectrum and the beam divergence in a single shot without prior knowledge of the spectral shape. A campaign at the Centre for Advanced Laser Applications in Munich focused on the first experimental demonstration of XCERP diffraction. The results reveal that an increased level of radiation shielding was required as a source of ionising radiation was found to be present whenever an electron beam was generated. This source of radiation is thought to have obscured potential X-ray diffraction. An investigation of the unknown radiation source was performed using GEANT4. From simulations of the CALA target chamber, it was found that the CCD of interest would have been subjected to both bremsstrahlung radiation and stray electrons when an electron beam is steered into a steel floor. Further simulations show potential methods of reducing the flux of particles reaching the CCD. Finally, the designs of an electron spectrometer and filter array are described in detail to measure properties of the electron bunches and betatron radiation produced during an experimental campaign at the Astra Gemini laser facility
Potential to measure quantum effects in recent all-optical radiation reaction experiments
No full textThe construction of 10 PW class laser facilities with unprecedented intensities has emphasized the need for a thorough understanding of the radiation reaction process. We describe simulations for a recent all-optical colliding pulse experiment, where a GeV scale electron bunch produced by a laser wakefield accelerator interacted with a counter-propagating laser pulse. In the rest frame of the electron bunch, the electric field of the laser pulse is increased by several orders of magnitude, approaching the Schwinger field and leading to substantial variation from the classical Landau-Lifshitz model. Our simulations show how the final electron and photon spectra may allow us to differentiate between stochastic and semi-classical models of radiation reaction, even when there is significant shot-to-shot variation in the experimental parameters. In particular, constraints are placed on the maximum energy spread and shot-to-shot variation permissible if a stochastic model is to be proven with confidence
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Measuring signatures in photon angular spectra to distinguish nonlinear Compton scattering models
No full textThe collision of a high-energy electron beam with a laser pulse may be used to study radiation reaction and nonlinear Compton scattering among many other processes in strong-field quantum electrodynamics. Predictions from simulation and theory for these interactions rely on a number of approximations and assumptions that have not been experimentally tested. Here, experimentally measurable signatures are identified that might be able to distinguish between radiation reaction models, i.e., classical or quantum, or between the local constant field and local monochromatic approximations used to calculate the properties of the nonlinear Compton process. These signatures are considered through Monte Carlo simulations of various experimental conditions that are relevant to today's laser facilities. Potential detection schemes for measuring the signatures are proposed. We find that single-photon counting of keV photons to resolve harmonics and scintillator-based detection of MeV photons may allow us to validate nonlinear Compton scattering models and radiation reaction models respectively. This will require electron beams with divergence angles less than 2 mrad and less than 20% energy spread
Self-injection threshold in self-guided laser wakefield accelerators
No full textA laser pulse traveling through a plasma can excite large amplitude plasma waves that can be used to accelerate relativistic electron beams in a very short distance-a technique called laser wakefield acceleration. Many wakefield acceleration experiments rely on the process of wave breaking, or self-injection, to inject electrons into the wave, while other injection techniques rely on operation without self-injection. We present an experimental study into the parameters, including the pulse energy, focal spot quality, and pulse power, that determine whether or not a wakefield accelerator will self-inject. By taking into account the processes of self-focusing and pulse compression we are able to extend a previously described theoretical model, where the minimum bubble size k(p)r(b) required for trapping is not constant but varies slowly with density and find excellent agreement with this model
A Bayesian framework to investigate radiation reaction in strong fields
No full textRecent experiments aiming to measure phenomena predicted by strong-field quantum electrodynamics (SFQED) have done so by colliding relativistic electron beams and high-power lasers. In such experiments, measurements of collision parameters are not always feasible. However, precise knowledge of these parameters is required to accurately test SFQED. Here, we present a novel Bayesian inference procedure that infers collision parameters that could not be measured on-shot. This procedure is applicable to all-optical non-linear Compton scattering experiments investigating radiation reaction. The framework allows multiple diagnostics to be combined self-consistently and facilitates the inclusion of known information pertaining to the collision parameters. Using this Bayesian analysis, the relative validity of the classical, quantum-continuous and quantum-stochastic models of radiation reaction was compared for several test cases, which demonstrates the accuracy and model selection capability of the framework and highlight its robustness if the experimental values of fixed parameters differ from their values in the models
Self-guided wakefield experiments driven by petawatt-class ultrashort laser pulses
No full textWe investigate the extension of self-injecting laser wakefield experiments to the regime that will be accessible with the next generation of petawatt-class ultrashort pulse laser systems. Using nonlinear scalings, current experimental trends, and numerical simulations, we determine the optimal laser and target parameters, i.e., focusing geometry, plasma density, and target length, that are required to increase the electron beam energy (to > 1 GeV) without the use of external guiding structures
On the stability of laser wakefield electron accelerators in the monoenergetic regime
No full textThe effects of plasma density and laser energy on the stability of laser produced monoenergetic electron beams are investigated. Fluctuations in the principal beam parameters, namely, electron energy, energy-spread, charge, and pointing, are demonstrated to be minimized at low densities. This improvement in stability is attributed to the reduced time for pulse evolution required before self-injection occurs; i.e., that the pulse is closest to the matched conditions for these densities. It is also observed that electrons are only consistently produced above a density-dependent energy threshold. These observations are consistent with there being a threshold intensity (a(0)greater than or similar to 3) required for the occurrence of self-injection after accounting for pulse compression. (C) 2007 American Institute of Physics
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