arXiv subjects

B. Kettle

Publications and source records attributed to B. Kettle.

13 recordsLinked to original sources

arXiv2025

High brightness multi-MeV photon source driven by a petawatt-scale laser wakefield accelerator

We present an experimental demonstration of a bright multi-MeV gamma source driven by a petawatt laser. The source generates on average $(1.2\pm0.6)\times10^9$ photons above 1 MeV per pulse, exceeding those of previous all-optical sources by a hundred times, and reached a peak spectral brightness of $(3.9 \pm 1.5)\times 10^{22}$ photons/mm$^2$/mrad$^2$/s/0.1%BW at $\epsilon_\gamma\approx11$ MeV. The source was produced by inverse Compton scattering of a laser wakefield accelerated GeV electron beam and its back-reflected driving laser pulse. Its performance is well described by a simple model of the laser and electron properties at the collision point that allows quantitative predictions and identifies clear strategies to further enhance radiation efficiency. Our results highlight the promise of this source for fundamental physics studies, as well as for applications of nuclear resonance fluorescence and nuclear transmutation.

physics.plasm-ph↗

arXiv2025

Experimental evidence of production of directional muons from a laser-wakefield accelerator

We report on experimental evidence of the generation of directional muons from a laser-wakefield accelerator driven by a PW-class laser. The muons were generated following the interaction of a GeV-scale high-charge electron beam with a 2cm-thick Pb target and were detected using a Timepix3 detector placed behind a suitable shielding configuration. Data analysis indicates a $(99.1\pm0.5)$% confidence of muon detection over noise, in excellent agreement with numerical modelling. Extrapolation of the experimental setup to higher electron energies and charges suggests the potential to guide and separate from noise approximately $10^4$ muons/s onto cm$^2$-scale areas for applications using a 10 Hz PW laser. These results demonstrate the possibility of generating and transporting directional muon beams using high-power lasers and establish a foundation for the systematic application of laser-driven high-energy muon beams.

physics.acc-ph↗

arXiv2024

Single-pulse Gy-scale irradiation of biological cells at $10^{13}$ Gy/s average dose-rates from a laser-wakefield accelerator

We report on the first experimental characterization of a laser-wakefield accelerator able to deliver, in a single pulse, doses in excess of \unit[1]{Gy} on timescales of the order of a hundred femtoseconds, reaching unprecedented average dose-rates up to \unit[10$^{13}$]{Gy/s}. The irradiator is demonstrated to deliver doses tuneable up to \unit[2.2]{Gy} in a cm$^2$ area and with a high degree of longitudinal and transverse uniformity in a single irradiation. In this regime, proof-of-principle irradiation of patient-derived glioblastoma stem-like cells and human skin fibroblast cells show indications of a differential cellular response, when compared to reference irradiations at conventional dose-rates. These include a statistically significant increase in relative biological effectiveness ($1.40\pm0.08$ at 50\% survival for both cell lines) and a significant reduction of the relative radioresistance of tumour cells. Data analysis provides preliminary indications that these effects might not be fully explained by induced oxygen depletion in the cells but may be instead linked to a higher complexity of the damages triggered by the ultra-high density of ionising tracks of femtosecond-scale radiation pulses. These results demonstrate an integrated platform for systematic radiobiological studies at unprecedented beam durations and dose-rates, a unique infrastructure for translational research in radiobiology at the femtosecond scale.

physics.med-ph↗

arXiv2024

Bounding elastic photon-photon scattering at $\sqrt s \approx 1\,$MeV using a laser-plasma platform

We report on a direct search for elastic photon-photon scattering using x-ray and $\gamma$ photons from a laser-plasma based experiment. A gamma photon beam produced by a laser wakefield accelerator provided a broadband gamma spectrum extending to above $E_\gamma = 200$ MeV. These were collided with a dense x-ray field produced by the emission from a laser heated germanium foil at $E_x \approx 1.4$ keV, corresponding to an invariant mass of $\sqrt{s} = 1.22 \pm 0.22$ MeV. In these asymmetric collisions elastic scattering removes one x-ray and one high-energy $\gamma$ photon and outputs two lower energy $\gamma$ photons. No changes in the $\gamma$ photon spectrum were observed as a result of the collisions allowing us to place a 95% upper bound on the cross section of $1.5 \times 10^{15}\,\mu$b. Although far from the QED prediction, this represents the lowest upper limit obtained so far for $\sqrt{s} \lesssim 1$ MeV.

hep-ex↗

arXiv2023

Generation of photoionized plasmas in the laboratory of relevance to accretion-powered x-ray sources using keV line radiation

We describe laboratory experiments to generate X-ray photoionized plasmas of relevance to accretion-powered X-ray sources such as neutron star binaries and quasars, with significant improvements over previous work. A key quantity is referenced, namely the photoionization parameter. This is normally meaningful in an astrophysical steady-state context, but is also commonly used in the literature as a figure of merit for laboratory experiments that are, of necessity, time-dependent. We demonstrate emission-weighted values of {\xi} > 50 ergcm/s using laser-plasma X-ray sources, with higher results at the centre of the plasma which are in the regime of interest for several astrophysical scenarios. Comparisons of laboratory experiments with astrophysical codes are always limited, principally by the many orders of magnitude differences in time and spatial scales, but also other plasma parameters. However useful checks on performance can often be made for a limited range of parameters. For example, we show that our use of a keV line source, rather than the quasi-blackbody radiation fields normally employed in such experiments, has allowed the generation of the ratio of inner-shell to outer-shell photoionization expected from a blackbody source with ~keV spectral temperature. We compare calculations from our in-house plasma modelling code with those from Cloudy and find moderately good agreement for the time evolution of both electron temperature and average ionisation. However, a comparison of code predictions for a K-beta argon X-ray spectrum with experimental data reveals that our Cloudy simulation overestimates the intensities of more highly ionised argon species. This is not totally surprising as the Cloudy model was generated for a single set of plasma conditions, while the experimental data are spatially integrated.

astro-ph.HE↗

arXiv2023

Extended X-ray absorption spectroscopy using an ultrashort pulse laboratory-scale laser-plasma accelerator

Laser-driven compact particle accelerators can provide ultrashort pulses of broadband X-rays, well suited for undertaking X-ray absorption spectroscopy measurements on a femtosecond timescale. Here the Extended X-ray Absorption Fine Structure (EXAFS) features of the K-edge of a copper sample have been observed over a 250 eV window in a single shot using a laser wakefield accelerator, providing information on both the electronic and ionic structure simultaneously. This unique capability will allow the investigation of ultrafast processes, and in particular, probing high-energy-density matter and physics far-from-equilibrium where the sample refresh rate is slow and shot number is limited. For example, states that replicate the tremendous pressures and temperatures of planetary bodies or the conditions inside nuclear fusion reactions. Using high-power lasers to pump these samples also has the advantage of being inherently synchronised to the laser-driven X-ray probe. A perspective on the additional strengths of a laboratory-based ultrafast X-ray absorption source is presented.

physics.plasm-ph↗

arXiv2023

Monte Carlo modelling of the linear Breit-Wheeler process within the GEANT4 framework

A linear Breit-Wheeler module for the code Geant4 has been developed. This allows signal-to-noise ratio calculations of linear Breit-Wheeler detection experiments to be performed within a single framework. The interaction between two photon sources is modelled by treating one as a static field, then photons from the second source are sampled and tracked through the field. To increase the efficiency of the module, we have used a Gaussian process regression, which can lead to an increase in the calculation rate by a factor of up to 1000. To demonstrate the capabilities of this module, we use it to perform a parameter scan, modelling an experiment based on that recently reported by Kettle et al. [1]. We show that colliding $50\,$fs duration $\gamma$-rays, produced through bremsstrahlung emission of a $100\,$pC, $2\,$GeV laser wakefield accelerator beam, with a $50\,$ps X-ray field, generated by a germanium burn-through foil heated to temperatures $>\,150\,$eV, this experiment is capable of producing $>1\,$ Breit-Wheeler pair per shot.

physics.plasm-ph↗

arXiv2022

Narrow bandwidth, low-emittance positron beams from a laser-wakefield accelerator

The rapid progress that plasma wakefield accelerators are experiencing is now posing the question as to whether they could be included in the design of the next generation of high-energy electron-positron colliders. However, the typical structure of the accelerating wakefields presents challenging complications for positron acceleration. Research in plasma-based acceleration of positrons has thus far experienced limited experimental progress due to the lack of positron beams suitable to seed a plasma accelerator. Here, we report on the first experimental demonstration of a laser-driven source of ultra-relativistic positrons with sufficient spectral and spatial quality to be injected in a plasma accelerator. Our results indicate, in agreement with numerical simulations, selection and transport of positron beamlets containing $N_{e+}\geq10^5$ positrons in a 5\% bandwidth around 600 MeV, with femtosecond-scale duration and micron-scale normalised emittance. Particle-in-cell simulations show that positron beams of this kind can be efficiently guided and accelerated in a laser-driven plasma accelerator, with favourable scalings to further increase overall charge and energy using PW-scale lasers. The results presented here demonstrate the possibility of performing experimental studies of positron acceleration in a plasma wakefield.

physics.acc-ph↗

arXiv2021

A laser-plasma platform for photon-photon physics

We describe a laser-plasma platform for photon-photon collision experiments to measure fundamental quantum electrodynamic processes such as the linear Breit-Wheeler process with real photons. The platform has been developed using the Gemini laser facility at the Rutherford Appleton Laboratory. A laser wakefield accelerator and a bremsstrahlung convertor are used to generate a collimated beam of photons with energies of hundreds of MeV, that collide with keV x-ray photons generated by a laser heated plasma target. To detect the pairs generated by the photon-photon collisions, a magnetic transport system has been developed which directs the pairs onto scintillation-based and hybrid silicon pixel single particle detectors. We present commissioning results from an experimental campaign using this laser-plasma platform for photon-photon physics, demonstrating successful generation of both photon sources, characterisation of the magnetic transport system and calibration of the single particle detectors, and discuss the feasibility of this platform for the observation of the Breit-Wheeler process. The design of the platform will also serve as the basis for the investigation of strong-field quantum electrodynamic processes such as the nonlinear Breit-Wheeler and the Trident process, or eventually, photon-photon scattering.

physics.plasm-ph↗

arXiv2020

Characterisation of Laser Wakefield Acceleration Efficiency with Octave Spanning Near-IR Spectrum Measurements

We report on experimental measurements of energy transfer efficiencies in a GeV-class laser wakefield accelerator. Both the transfer of energy from the laser to the plasma wakefield, and from the plasma to the accelerated electron beam were diagnosed by simultaneous measurement of the deceleration of laser photons and the acceleration of electrons as a function of plasma length. The extraction efficiency, which we define as the ratio of the energy gained by the electron beam to the energy lost by the self-guided laser mode, was maximised at $19\pm3$\% by tuning of the plasma density and length. The additional information provided by the octave-spanning laser spectrum measurement allows for independent optimisation of the plasma efficiency terms, which is required for the key goal of improving the overall efficiency of laser wakefield accelerators.

physics.plasm-ph↗

arXiv2019

Single-shot multi-keV X-ray absorption spectroscopy using an ultrashort laser wakefield accelerator source

Single-shot absorption measurements have been performed using the multi-keV X-rays generated by a laser wakefield accelerator. A 200 TW laser was used to drive a laser wakefield accelerator in a mode which produced broadband electron beams with a maximum energy above 1 GeV and a broad divergence of $\approx15$ miliradians FWHM. Betatron oscillations of these electrons generated $1.2\pm0.2\times10^6$ photons/eV in the 5 keV region, with a signal-to-noise ratio of approximately 300:1. This was sufficient to allow high-resolution XANES measurements at the K-edge of a titanium sample in a single shot. We demonstrate that this source is capable of single-shot, simultaneous measurements of both the electron and ion distributions in matter heated to eV temperatures by comparison with DFT simulations. The unique combination of a high-flux, large bandwidth, few femtosecond duration X-ray pulse synchronised to a high-power laser will enable key advances in the study of ultra-fast energetic processes such as electron-ion equilibration.

physics.plasm-ph↗

arXiv2019

Multiple Fourier Component Analysis of X-ray Second Harmonic Generation in Diamond

The unprecedented brilliance of X-ray free-electron lasers (XFELs) [1, 2] has enabled first studies of nonlinear interactions in the hard X-ray range. In particular, X-ray-optical mixing [3], X-ray second harmonic generation (XSHG) [4] and nonlinear Compton scattering (NLCS) [5] have been recently observed for the first time using XFELs. The former two experiments as well as X-ray parametric downconversion (XPDC)[6, 7] are well explained by nonlinearities in the impulse approximation[8], where electrons in a solid target are assumed to be quasi free for X-ray interactions far from atomic resonances. However, the energy of the photons generated in NLCS at intensities reaching up to 4 x 1020 W/cm2 exhibit an anomalous red-shift that is in violation with the free-electron model. Here we investigate the underlying physics of X-ray nonlinear interactions at intensities on order of 1016 W/cm2. Specifically, we perform a systematic study of XSHG in diamond. While one phase-matching geometry has been measured in Shwartz et al.[4], we extend these studies to multiple Fourier components and with significantly higher statistics, which allows us to determine the second order nonlinear structure factor. We measure the efficiency, angular dependence, and contributions from different source terms of the process. We find good agreement of our measurements with the quasi-free electron model.

physics.optics↗

arXiv2018

Melting and phase change for laser-shocked iron

Using the LCLS facility at the SLAC National Accelerator Laboratory, we have observed X-ray scattering from iron compressed with laser driven shocks to Earth-core like pressures above 400GPa. The data shows shots where melting is incomplete and we observe hexagonal close packed (hcp) crystal structure at shock compressed densities up to 14.0 gcm-3 but no evidence of a double-hexagonal close packed (dhcp) crystal. The observation of a crystalline structure at these densities, where shock heating is expected to be in excess of the equilibrium melt temperature, may indicate superheating of the solid. These results are important for equation of state modelling at high strain rates relevant for impact scenarios and laser-driven shock wave experiments.

physics.plasm-ph↗