Multi-objective and multi-fidelity Bayesian optimization of laser-plasma acceleration

Beam parameter optimization in accelerators involves multiple, sometimes competing objectives. Condensing these multiple objectives into a single objective unavoidably results in bias towards particular outcomes that do not necessarily represent the best possible outcome for the operator in terms of parameter optimization. A more versatile approach is multi-objective optimization, which establishes the trade-off curve or Pareto front between objectives. Here we present first results on multi-objective Bayesian optimization of a simulated laser-plasma accelerator. We find that multi-objective optimization is equal or even superior in performance to its single-objective counterparts, and that it is more resilient to different statistical descriptions of objectives. As a second major result of our paper, we significantly reduce the computational costs of the optimization by choosing the resolution and box size of the simulations dynamically. This is relevant since even with the use of Bayesian statistics, performing such optimizations on a multi-dimensional search space may require hundreds or thousands of simulations. Our algorithm translates information gained from fast, low-resolution runs with lower fidelity to high-resolution data, thus requiring fewer actual simulations at highest computational cost. The techniques demonstrated in this paper can be translated to many different use cases, both computational and experimental

Tunable X-ray source by Thomson scattering during laser-wakefield acceleration

We report results on all-optical Thomson scattering intercepting the acceleration process in a laser wakefield accelerator. We show that the pulse collision position can be detected using transverse shadowgraphy which also facilitates alignment. As the electron beam energy is evolving inside the accelerator, the emitted spectrum changes with the scattering position. Such a configuration could be employed as accelerator diagnostic as well as reliable setup to generate x-rays with tunable energy

All-optical Compton scattering at shallow interaction angles

All-optical Compton sources combine laser wakefield accelerators and intense scattering pulses to generate ultrashort bursts of backscattered radiation. The scattering pulse plays the role of a short-period undulator in which relativistic electrons oscillate and emit x-ray radiation. To date, most of the working laser-plasma accelerators operate preferably at energies of a few hundreds of MeV and the Compton sources developed so far produce radiation in the range from hundreds of keV to a few MeV. However, for such applications as medical imaging and tomography the relevant energy range is 10-100 keV. In this article, we discuss different scattering geometries for the generation of X-rays in this range. Through numerical simulations, we study the influence of electron beam parameters on the backscattered photons. We find that the spectral bandwidth remains constant for beams of the same emittance regardless of the scattering geometry. A shallow interaction angle of 30 degrees or less seems particularly promising for imaging applications given parameters of existing laser-plasma accelerators. Finally, we discuss the influence of the radiation properties for potential applications in medical imaging and non-destructive testing

Demonstration of relativistic electron beam focusing by a laser-plasma lens

Laser-plasma technology promises a drastic reduction of the size of high energy electron accelerators. It could make free electron lasers available to a broad scientific community, and push further the limits of electron accelerators for high energy physics. Furthermore the unique femtosecond nature of the source makes it a promising tool for the study of ultra-fast phenomena. However, applications are hindered by the lack of suitable lens to transport this kind of high-current electron beams, mainly due to their divergence. Here we show that this issue can be solved by using a laser-plasma lens, in which the field gradients are five order of magnitude larger than in conventional optics. We demonstrate a reduction of the divergence by nearly a factor of three, which should allow for an efficient coupling of the beam with a conventional beam transport line

Hyperspectral Compressive Wavefront Sensing

Presented is a novel way to combine snapshot compressive imaging and lateral shearing interferometry in order to capture the spatio-spectral phase of an ultrashort laser pulse in a single shot. A deep unrolling algorithm is utilised for the snapshot compressive imaging reconstruction due to its parameter efficiency and superior speed relative to other methods, potentially allowing for online reconstruction. The algorithm’s regularisation term is represented using neural network with 3D convolutional layers, to exploit the spatio-spectral correlations that exist in laser wavefronts. Compressed sensing is not typically applied to modulated signals, but we demonstrate its success here. Furthermore, we train a neural network to predict the wavefronts from a lateral shearing interferogram in terms of Zernike polynomials, which again increases the speed of our technique without sacrificing fidelity. This method is supported with simulation-based results. While applied to the example of lateral shearing interferometry, the methods presented here are generally applicable to a wide range of signals, including Shack-Hartmann-type sensors. The results may be of interest beyond the context of laser wavefront characterization, including within quantitative phase imaging

Tango Controls and data pipeline for petawatt laser experiments

The Centre for Advanced Laser Applications in Garching, Germany, is home to the ATLAS-3000 multi-petawatt laser, dedicated to research on laser particle acceleration and its applications. A control system based on Tango Controls is implemented for both the laser and four experimental areas. The device server approach features high modularity, which, in addition to the hardware control, enables a quick extension of the system and allows for automated data acquisition of the laser parameters and experimental data for each laser shot. In this paper we present an overview of our implementation of the control system, as well as our advances in terms of experimental operation, online supervision and data processing. We also give an outlook on advanced experimental supervision and online data evaluation – where the data can be processed in a pipeline – which is being developed on the basis of this infrastructure

Progress in hybrid plasma wakefield acceleration

Plasma wakefield accelerators can be driven either by intense laser pulses (LWFA) or by intense particle beams (PWFA). A third approach that combines the complementary advantages of both types of plasma wakefield accelerator has been established with increasing success over the last decade and is called hybrid LWFA→PWFA. Essentially, a compact LWFA is exploited to produce an energetic, high-current electron beam as a driver for a subsequent PWFA stage, which, in turn, is exploited for phase-constant, inherently laser-synchronized, quasi-static acceleration over extended acceleration lengths. The sum is greater than its parts: the approach not only provides a compact, cost-effective alternative to linac-driven PWFA for exploitation of PWFA and its advantages for acceleration and high-brightness beam generation, but extends the parameter range accessible for PWFA and, through the added benefit of co-location of inherently synchronized laser pulses, enables high-precision pump/probing, injection, seeding and unique experimental constellations, e.g., for beam coordination and collision experiments. We report on the accelerating progress of the approach achieved in a series of collaborative experiments and discuss future prospects and potential impact

Fuentes de luz sincrotrón basadas en la interacción láser-plasma: investigación y desarrollo hacia mejor control, estabilidad, ganancia y brillantez

[ES] La tecnología de la interacción láser-plasma tiene el potencial de producir nuevas fuentes de rayos X, brillantes, compactas y ultracortas que reemplacen o sean una alternativa viable a las fuentes convencionales existentes. Experimentos recientes han demostrado los principios básicos de estas fuentes y su increíble potencial, pero también su pobre estabilidad y poco control, lo que limita el alcance de sus aplicaciones. La tecnología de aceleradores convencionales ha logrado un alto grado de control en las características de las fuentes de radiación ionizante mejorando progresivamente cada uno de sus elementos como la inyección, la aceleración el transporte del haz y la generación de radiación. En este trabajo utilizamos esta metodología de optimización individual para las fuentes láser-plasma y reportamos avances en cada uno de estos elementos. El manuscrito está organizado de la siguiente forma: Empezamos con un discurso sobre nuevos métodos de inyección controlada para crear haces de electrones, explorando en partilular la inyección por choque y la inyección por ionización. En la segunda parte presentamos los primeros resultados experimentales acerca de la evolución de la energía de un haz de electrones en un acelerador con un perfil de densidad previamente diseñado. Se demuestra también la posibilidad de focalizar un haz de electrones ultracorto en una lente completa- mente óptica, utilizando la estela lineal del pulso en un plasma de baja densidad. La última parte del texto está dedicada a la generación de radiación. En particular reportamos avances significativos en una fuente betatrón. Demostramos la producción de una fuente estable y polarizada de rayos X producida por electrones inyectados por medio de ionización retrasada. Finalmente reportamos la producción optimizada de rayos X en canales de plasma sobre perfiles de plasma diseñados. Además, hemos estudiado la radiación de frenado y la retrodispersión Compton, centrándonos en sus aplicaciones en imagen de rayos X

Source X issue d'interaction laser-plasma: Études et developpements pour augmenter contrôle, stabilité, gain et brillance

Laser-plasma technology has the potential to provide compact sources of bright femtosecond X-ray, which may soon serve as an alternative to their conventional counterparts. Proof-of- principle experiments have impressively demonstrated the sources’ prospects, yet the poor stability and tunability drastically limit their scope of applicability. Conventional systems have achieved their remarkable control over the source by progressive improvement of the discrete stages of injection, acceleration, beam transport and radiation generation. In this work we have adapted this approach for laser-plasma sources and made advances on all individual parts of the source.Les progrès réalisés dans le domaine de l’interaction laser-plasma au cours des dix dernières années ont permis de produire de nouvelles sources de rayonnement X pouvant rivaliser avec les conventionnels synchrotron et tubes X. Ces nouvelles sources ont un fort potentiel mais leur domaine d’applications reste très limitées en raison d’importantes fluctuations et du peu de contrôle de leurs propriétés. Ces sources sont basées sur le même principe qu’un Synchroton. Il n’agit d’accélérer des électrons jusqu’à des vitesses relativistes et de les faire osciller de manière à ce qu’ils émettent efficacement du rayonnement X. Afin de obtenir un meilleur contrôle de la source nous avons étudié les différentes étapes conduisant à la production de rayonnement : l’injection d’électrons dans l’onde plasma créée dans le sillage du laser, l’accélération et le transport de ces électrons puis les méthodes permettant de les faire osciller. Le manuscrit présente les progrès réalisés dans ces domaines