arXiv subjects

A. Picksley

Publications and source records attributed to A. Picksley.

13 recordsLinked to original sources

arXiv2026

Experimental demonstration of Flying-Focus enhanced Thomson scattering

We report the experimental demonstration of a spatiotemporally engineered "Flying-Focus" laser pulse for enhanced x-ray generation in relativistic Thomson scattering. A combination of longitudinal chromatic aberration, angular dispersion, and group delay dispersion was applied to an ultrashort relativistically intense laser pulse to control the motion of its focal point. Precise tuning of the group delay dispersion was used to match the velocity of the focus to the trajectory of a counterpropagating electron bunch, produced by a laser wakefield accelerator. This prolonged the Thomson scattering interaction while reducing nonlinear effects, leading to an enhanced x-ray yield. The approach has the potential to increase the spectral density and brightness of the x-ray beam by orders of magnitude compared to equivalent focusing without spatiotemporal control. This experiment establishes a new technique for structured-light control at high intensity, demonstrating the realization of dynamic intensity structures that enhance light-matter interactions and for the generation of ultra-bright radiation sources.

physics.optics↗

arXiv2026

Revealing Laser and Electron Beam Evolution in 10-GeV-class Laser-Plasma Accelerators

Guiding relativistically intense laser pulses in low-density plasmas enables extended acceleration lengths in laser-plasma accelerators (LPAs), allowing for the production of multi-GeV electron beams. Quantitative interpretation of such experiments is often limited by substantial uncertainties in key plasma parameters, particularly the transverse density profile of hydrodynamic optically field-ionized channels. Distinct plasma density distributions can produce similar terminal beam energies, complicating efforts to infer the underlying interaction physics from measurements at the accelerator exit alone. By combining longitudinally resolved electron beam diagnostics with independent measurements of laser spectral evolution in a 10 GeV LPA, we establish a multi-observable constraint on plasma density profiles. Once plasma downramps are taken into account, excellent agreement is observed with simulation over the entire accelerator length for two plasma channel sizes. The validated simulations indicate that extending the accelerator length to 65 cm would increase the electron beam energy to 15 GeV. They also point the way to achieving $\sim$20 GeV electron beams in $\sim$70 cm via linear matching using the same 24 J laser energy.

physics.acc-ph↗

arXiv2025

Stability and optical quality of "windmill"-formed 8CB liquid crystal films for replenishable plasma mirrors

Liquid crystal (LC) film plasma mirrors (PMs) based on 4-octyl-4'-cyanobiphenyl (8CB) are an enabling technology for reflecting high-fluence laser pulses. These freestanding LC films can achieve high optical quality and are well-suited for rep-rated applications, as motorized devices continuously replenish films over an aperture following each destructive laser shot. However, a systematic characterization of film quality as a function of seminal operating conditions had not yet been performed for the LC "windmill" version of the device, which aims to match the repetition rate of an existing "spinning disk" (SDI) version and the angular stability of the "linear slider" (LSTI) version. We determined the 8CB film quality using low-power wavefront measurements, and studied the film-to-film wavefront stability and formation reliability. The film-formation reliability of 8CB LC films demonstrated >97% formation success at 2.7 mm/s film-forming speeds, but decreased to 45% at 10.8 mm/s. These reliability numbers will inform future designs to reach Hz-level repetition rates and beyond. Depending on area-of-interest within the 10 mm diameter film, the added wavefront root-mean-squared (RMS) variation was as small as 12 nm for a 2 mm diameter region, and <50 nm for a 3 mm diameter region. Within the optimal 21-22 degrees C operating regime, pointing fluctuations remained at or below 0.5 mrad. With a maximum effective film formation frequency of approximately 0.25 Hz, these results establish windmill-formed 8CB films as promising candidates to pursue next-iteration improvements towards rep-rated plasma-mirror operation.

physics.optics↗

arXiv2024

Longitudinal tapering in meter-scale gas jets for increased efficiency of laser plasma accelerators

Modern laser plasma accelerators (LPAs) often require plasma waveguides tens of cm long to propagate a high-intensity drive laser pulse. Tapering the longitudinal gas density profile in 10 cm scale gas jets could allow for single stage laser plasma acceleration well beyond 10 GeV with current petawatt-class laser systems. Via simulation and interferometry measurements, we show density control by longitudinally adjusting the throat width and jet angle. Density profiles appropriate for tapering were calculated analytically and via particle-in-cell (PIC) simulations, and were matched experimentally. These simulations show that tapering can increase electron beam energy using 19 J laser energy from ~9 GeV to >12 GeV in a 30 cm plasma, and the accelerated charge by an order of magnitude. This paper was published in Review of Scientific Instruments on April 11, 2025 DOI: https://doi.org/10.1063/5.0250698

physics.acc-ph↗

arXiv2024

Matched Guiding and Controlled Injection in Dark-Current-Free, 10-GeV-Class, Channel-Guided Laser Plasma Accelerators

We measure the high intensity laser propagation throughout meter-scale, channel-guided LPAs by adjusting the length of the plasma channel on a shot-by-shot basis, showing high quality guiding of 500 TW laser pulses over 30 cm in a hydrogen plasma of density $n_0 \approx 1 \times 10^{17} \, \mathrm{cm^{-3}}$. We observed transverse energy transport of higher-order modes in the first $\approx 12 \, \mathrm{cm}$ of the plasma channel, followed by quasi-matched propagation, and the gradual, dark-current-free depletion of laser energy to the wakefield. We quantify the laser-to-wake transfer efficiency limitations of currently available PW-class laser systems, and demonstrate via simulation how control over the laser mode can significantly improve accelerated beam parameters. Using just 21.3 J of laser energy, and triggering localized electron injection into the accelerator, we observed electron bunches with single, quasimonoenergetic peaks, relative energy spreads as low as 3 % and energy up to 9.2 GeV with charge extending beyond 10 GeV.

physics.plasm-ph↗

arXiv2023

Guided mode evolution and ionization injection in meter-scale multi-GeV laser wakefield accelerators

We show that laser wakefield electron accelerators in meter-scale, low density hydrodynamic plasma waveguides operate in a new nonlinear propagation regime where sustained beating of lowest order modes of the ponderomotively modified channel plays a significant role, whether or not the injected pulse is linearly matched to the guide. For a continuously doped gas jet, this mode beating effect leads to ionization injection and a striated multi-GeV energy spectrum of multiple quasi-monoenergetic peaks; the same process in a locally doped jet produces single multi-GeV peaks with <10% energy spread. A 3-stage model of drive laser pulse evolution and ionization injection characterizes the beating effect and explains our experimental results.

physics.plasm-ph↗

arXiv2023

All-optical GeV electron bunch generation in a laser-plasma accelerator via truncated-channel injection

We describe a simple scheme, truncated-channel injection, to inject electrons directly into the wakefield driven by a drive pulse guided by an all-optical plasma channel. We use this approach to generate dark-current-free 1.2 GeV, 4.5 % relative energy spread electron bunches with 120 TW laser pulses guided in a 110-mm-long hydrodynamic optical-field-ionized (HOFI) plasma channel. Our experiments and particle-in-cell simulations show that high-quality electron bunches were only obtained when the drive pulse was closely aligned with the channel axis, and was focused close to the density down-ramp formed at the channel entrance. Start-to-end simulations of the channel formation, and electron injection and acceleration show that increasing the channel length to 410 mm would yield 3.65 GeV bunches, with a slice energy spread $\sim 5 \times 10^{-4}$.

physics.acc-ph↗

arXiv2023

Measurement of the decay of laser-driven linear plasma wakefields

We present the first measurements of the temporal decay rate of one-dimensional, linear Langmuir waves excited by an ultra-short laser pulse. Langmuir waves with relative amplitudes of approximately $6\%$ were driven by $1.7$ J, $50$ fs laser pulses in hydrogen and deuterium plasmas of density $n_{e0} = 8.4 \times 10^{17}$ cm$^{-3}$. The wakefield lifetimes were measured to be $\tau^\mathrm{H_2}_\mathrm{wf} = (9\pm2)$ ps and $\tau^\mathrm{D_2}_\mathrm{wf} = (16\pm8)$ ps respectively for hydrogen and deuterium. The experimental results were found to be in good agreement with 2D particle-in-cell simulations. In addition to being of fundamental interest, these results are particularly relevant to the development of laser wakefield accelerators (LWFAs) and wakefield acceleration schemes using multiple pulses, such as multi-pulse laser wakefield accelerators (MP-LWFAs).

physics.plasm-ph↗

arXiv2021

Multi-GeV electron bunches from an all-optical laser wakefield accelerator

We present the first demonstration of multi-GeV laser wakefield acceleration in a fully optically formed plasma waveguide, with an acceleration gradient as high as 25 GeV/m. The guide was formed via self-waveguiding of <15 J, 45 fs (<~300 TW) pulses over 20 cm in a low density hydrogen gas jet, with accelerated electron bunches simultaneously driven up to 5 GeV in a milliradian divergence quasi-monoenergetic peak of relative energy width ~15% and charge of at least ~10 picocoulombs. Energy gain is inversely correlated with on-axis waveguide density in the range N_e0=(1.3-3.2)x10^17 cm^(-3). We find that shot-to-shot stability of bunch spectra and charge are strongly dependent on the pointing of the injected laser pulse and gas jet uniformity. We also observe evidence of pump depletion-induced dephasing, a consequence of the long optical guiding distance.

physics.acc-ph↗

arXiv2021

Demonstration of kilohertz operation of Hydrodynamic Optical-Field-Ionized Plasma Channels

We demonstrate experimentally that hydrodynamic optical-field-ionized (HOFI) plasma channels can be generated at kHz-scale pulse repetition rates, in a static gas cell and for an extended period. Using a pump-probe arrangement, we show via transverse interferometry that the properties of two HOFI channels generated \SI{1}{ms} apart are essentially the same. We demonstrate that HOFI channels can be generated at a mean repetition rate of \SI{0.4}{kHz} for a period of 6.5 hours without degradation of the channel properties, and we determine the fluctuations in the key optical parameters of the channels in this period. Our results suggest that HOFI and conditioned HOFI channels are well suited for future high-repetition rate, multi-GeV plasma accelerator stages.

physics.plasm-ph↗

arXiv2020

Meter-Scale, Conditioned Hydrodynamic Optical-Field-Ionized Plasma Channels

We demonstrate through experiments and numerical simulations that low-density, low-loss, meter-scale plasma channels can be generated by employing a conditioning laser pulse to ionize the neutral gas collar surrounding a hydrodynamic optical-field-ionized (HOFI) plasma channel. We use particle-in-cell simulations to show that the leading edge of the conditioning pulse ionizes the neutral gas collar to generate a deep, low-loss plasma channel which guides the bulk of the conditioning pulse itself as well as any subsequently injected pulses. In proof-of-principle experiments we generate conditioned HOFI (CHOFI) waveguides with axial electron densities of $n_\mathrm{e0} \approx 1 \times 10^{17} \; \mathrm{cm^{-3}}$, and a matched spot size of $26 \; \mathrm{μm}$. The power attenuation length of these CHOFI channels is $L_\mathrm{att} = (21 \pm 3) \; \mathrm{m}$, more than two orders of magnitude longer than achieved by HOFI channels. Hydrodynamic and particle-in-cell simulations demonstrate that meter-scale CHOFI waveguides with attenuation lengths exceeding 1 m could be generated with a total laser pulse energy of only $1.2$ J per meter of channel. The properties of CHOFI channels are ideally suited to many applications in high-intensity light-matter interactions, including multi-GeV plasma accelerator stages operating at high pulse repetition rates.

physics.acc-ph↗

arXiv2020

Guiding of high-intensity laser pulses in 100mm-long hydrodynamic optical-field-ionized plasma channels

Hydrodynamic optically-field-ionized (HOFI) plasma channels up to 100mm long are investigated. Optical guiding is demonstrated of laser pulses with a peak input intensity of $6\times10^{17}$ W cm$^{-2}$ through 100mm long plasma channels with on-axis densities measured interferometrically to be as low as $n_{e0} = (1.0\pm0.3)\times10^{17}$cm$^{-3}$. Guiding is also observed at lower axial densities, which are inferred from magneto-hydrodynamic simulations to be approximately $7\times10^{16}$cm$^{-3}$. Measurements of the power attenuation lengths of the channels are shown to be in good agreement with those calculated from the measured transverse electron density profiles. To our knowledge, the plasma channels investigated in this work are the longest, and have the lowest on-axis density, of any free-standing waveguide demonstrated to guide laser pulses with intensities above $>10^{17}$ W cm$^{-2}$.

physics.acc-ph↗

arXiv2019

Low-Density Hydrodynamic Optical-Field-Ionized Plasma Channels Generated With An Axicon Lens

We demonstrate optical guiding of high-intensity laser pulses in long, low density hydrodynamic optical-field-ionized (HOFI) plasma channels. An axicon lens is used to generate HOFI plasma channels with on-axis electron densities as low as $n_e(0) = 1.5\times 10^{17}\, \mathrm{cm}^{-3}$ and matched spot sizes in the range $ 20 μ\mathrm{m} \lesssim W_M \lesssim 40 μ\mathrm{m}$. Control of these channel parameters via adjustment of the initial cell pressure and the delay after the arrival of the channel-forming pulse is demonstrated. For laser pulses with a peak axial intensity of $4 \times 10^{17}\, \mathrm{W\,cm}^{-2}$, highly reproducible, high-quality guiding over more than 14 Rayleigh ranges is achieved at a pulse repetition rate of 5 Hz, limited by the available channel-forming laser and vacuum pumping system. Plasma channels of this type would seem to be well suited to multi-GeV laser wakefield accelerators operating in the quasi-linear regime.

physics.plasm-ph↗