High order modes of intense second harmonic light produced from a plasma aperture

Because of their ability to sustain extremely high-amplitude electromagnetic fields and transient density and field profiles,plasma optical components are being developed to amplify,compress,and condition high-power laser pulses.We recently demonstrated the potential to use a relativistic plasma aperture—produced during the interaction of a high-power laser pulse with an ultrathin foil target—to tailor the spatiotemporal properties of the intense fundamental and second-harmonic light generated[Duff et al.,Sci.Rep.10,105(2020)].Herein,we explore numerically the interaction of an intense laser pulse with a preformed aperture target to generate second-harmonic laser light with higher-order spatial modes.The maximum generation efficiency is found for an aperture diameter close to the full width at half maximum of the laser focus and for a micrometer-scale target thickness.The spatial mode generated is shown to depend strongly on the polarization of the drive laser pulse,which enables changing between a linearly polarized TEM01 mode and a circularly polarized Laguerre–Gaussian LG01 mode.This demonstrates the use of a plasma aperture to generate intense higher-frequency light with selectable spatial mode structure.

Optimization and control of synchrotron emission in ultraintense laser–solid interactions using machine learning – CORRIGENDUM

Due to an isolated error in the 3D simulation parameters,the laser energy and intensity(calculated using the energy)values were incorrectly stated as 10.9 J and 3×10^(22) W cm^(−2),respectively,in Sections 3.3,7 and 8.The correct values are 39.8 J and 1.1×10^(23) W cm^(−2).Similarly,the values stated for the higher energy case,109 J and 3×10^(23) W cm^(−2) in Section 7,should be 398 J and 1.1×10^(24) W cm^(−2),respectively.

Optimization and control of synchrotron emission in ultraintense laser–solid interactions using machine learning

The optimum parameters for the generation of synchrotron radiation in ultraintense laser pulse interactions with planar foils are investigated with the application of Bayesian optimization,via Gaussian process regression,to 2D particle-incell simulations.Individual properties of the synchrotron emission,such as the yield,are maximized,and simultaneous mitigation of bremsstrahlung emission is achieved with multi-variate objective functions.The angle-of-incidence of the laser pulse onto the target is shown to strongly influence the synchrotron yield and angular profile,with oblique incidence producing the optimal results.This is further explored in 3D simulations,in which additional control of the spatial profile of synchrotron emission is demonstrated by varying the polarization of the laser light.The results demonstrate the utility of applying a machine learning-based optimization approach and provide new insights into the physics of radiation generation in laser-foil interactions,which will inform the design of experiments in the quantum electrodynamics(QED)-plasma regime.

Automated control and optimization of laser-driven ion acceleration

The interaction of relativistically intense lasers with opaque targets represents a highly non-linear,multi-dimensional parameter space.This limits the utility of sequential 1D scanning of experimental parameters for the optimization of secondary radiation,although to-date this has been the accepted methodology due to low data acquisition rates.High repetition-rate(HRR)lasers augmented by machine learning present a valuable opportunity for efficient source optimization.Here,an automated,HRR-compatible system produced high-fidelity parameter scans,revealing the influence of laser intensity on target pre-heating and proton generation.A closed-loop Bayesian optimization of maximum proton energy,through control of the laser wavefront and target position,produced proton beams with equivalent maximum energy to manually optimized laser pulses but using only 60%of the laser energy.This demonstration of automated optimization of laser-driven proton beams is a crucial step towards deeper physical insight and the construction of future radiation sources.

Versatile tape-drive target for high-repetition-rate laser-driven proton acceleration

We present the development and characterization of a high-stability,multi-material,multi-thickness tape-drive target for laser-driven acceleration at repetition rates of up to 100 Hz.The tape surface position was measured to be stable on the sub-micrometre scale,compatible with the high-numerical aperture focusing geometries required to achieve relativistic intensity interactions with the pulse energy available in current multi-Hz and near-future higher repetition-rate lasers(>kHz).Long-term drift was characterized at 100 Hz demonstrating suitability for operation over extended periods.The target was continuously operated at up to 5 Hz in a recent experiment for 70,000 shots without intervention by the experimental team,with the exception of tape replacement,producing the largest data-set of relativistically intense laser–solid foil measurements to date.This tape drive provides robust targetry for the generation and study of high-repetitionrate ion beams using next-generation high-power laser systems,also enabling wider applications of laser-driven proton sources.

Role of magnetic field evolution on filamentary structure formation in intense laser–foil interactions

Filamentary structures can form within the beam of protons accelerated during the interaction of an intense laser pulse with an ultrathin foil target. Such behaviour is shown to be dependent upon the formation time of quasi-static magnetic field structures throughout the target volume and the extent of the rear surface proton expansion over the same period.This is observed via both numerical and experimental investigations. By controlling the intensity profile of the laser drive,via the use of two temporally separated pulses, both the initial rear surface proton expansion and magnetic field formation time can be varied, resulting in modification to the degree of filamentary structure present within the laser-driven proton beam.

Reflection of intense laser light from microstructured targets as a potential diagnostic of laser focus and plasma temperature

The spatial-intensity profile of light reflected during the interaction of an intense laser pulse with a microstructured target is investigated experimentally and the potential to apply this as a diagnostic of the interaction physics is explored numerically. Diffraction and speckle patterns are measured in the specularly reflected light in the cases of targets with regular groove and needle-like structures, respectively, highlighting the potential to use this as a diagnostic of the evolving plasma surface. It is shown, via ray-tracing and numerical modelling, that for a laser focal spot diameter smaller than the periodicity of the target structure, the reflected light patterns can potentially be used to diagnose the degree of plasma expansion, and by extension the local plasma temperature, at the focus of the intense laser light. The reflected patterns could also be used to diagnose the size of the laser focal spot during a high-intensity interaction when using a regular structure with known spacing.

Effects of target pre-heating and expansion on terahertz radiation production from intense laser-solid interactions

The first experimental measurements of intense(～7 × 1019 W cm-2) laser-driven terahertz(THz) radiation from a solid target which is preheated by an intense pulse of laser-accelerated protons is reported. The total energy of the THz radiation is found to decrease by approximately a factor of 2 compared to a cold target reference. This is attributed to an increase in the scale length of the preformed plasma, driven by proton heating, at the front surface of the target,where the THz radiation is generated. The results show the importance of controlling the preplasma scale length for THz production.