arXiv subjects

Z. Najmudin

Publications and source records attributed to Z. Najmudin.

At least 19 recordsLinked to original sources

arXiv2025

Characterization and automated optimization of laser-driven proton beams from converging liquid sheet jet targets

Compact, stable, and versatile laser-driven ion sources hold great promise for applications ranging from medicine to materials science and fundamental physics. While single-shot sources have demonstrated favorable beam properties, including the peak fluxes necessary for several applications, high repetition rate operation will be necessary to generate and sustain the high average flux needed for many of the most exciting applications of laser-driven ion sources. Further, to navigate through the high-dimensional space of laser and target parameters towards experimental optima, it is essential to develop ion acceleration platforms compatible with machine learning learning techniques and capable of autonomous real-time optimization. Here we present a multi-Hz ion acceleration platform employing a liquid sheet jet target. We characterize the laser-plasma interaction and the laser-driven proton beam across a variety of key parameters governing the interaction using an extensive suite of online diagnostics. We also demonstrate real-time, closed-loop optimization of the ion beam maximum energy by tuning the laser wavefront using a Bayesian optimization scheme. This approach increased the maximum proton energy by 11% compared to a manually-optimized wavefront by enhancing the energy concentration within the laser focal spot, demonstrating the potential for closed-loop optimization schemes to tune future ion accelerators for robust high repetition rate operation.

physics.plasm-ph↗

arXiv2024

A Bayesian Framework to Investigate Radiation Reaction in Strong Fields

Recent experiments aiming to measure phenomena predicted by strong field quantum electrodynamics have done so by colliding relativistic electron beams and high-power lasers. In such experiments, measurements of the collision parameters are not always feasible, however, precise knowledge of these parameters is required for accurate tests of strong-field quantum electrodynamics. Here, we present a novel Bayesian inference procedure which 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 prior or 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 were compared for a series of test cases, which demonstrate the accuracy and model selection capability of the framework and and highlight its robustness in the event that the experimental values of fixed parameters differ from their values in the models.

physics.data-an↗

arXiv2023

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↗

arXiv2023

Plasma density profile reconstruction of a gas cell for Ionization Induced Laser Wakefield Acceleration

Laser-driven plasma wakefields can provide hundreds of MeV electron beam in mm-range distances potentially shrinking the dimension of the actual particle accelerators. The plasma density plays a fundamental role in the control and stability of the acceleration process, which is a key development for the future electron injector proposed by EuPRAXIA. A gas cell was designed by LPGP and LIDYL teams, with variable length and backing pressure, to confine the gas and tailor the gas density profile before the arrival of the laser. This cell was used during an experimental campaign with the multi TW-class laser at the Lund Laser Centre. Ionization assisted injection in a tailored density profile is used to tune the electron beam properties. During the experiment, we filled the gas cell with hydrogen mixed with different concentration of nitrogen. We also varied the backing pressure of the gas and the geometrical length of the gas cell. We used a transverse probe to acquire shadowgraphic images of the plasma and to measure the plasma electron density. Methods and results of the analysis with comparisons between shadowgraphic and interferometric images will be discussed.

physics.plasm-ph↗

arXiv2023

Automated control and optimisation 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 optimisation 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 optimisation. 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 optimisation 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 optimisation of laser-driven proton beams is a crucial step towards deeper physical insight and the construction of future radiation sources.

physics.plasm-ph↗

arXiv2022

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↗

arXiv2022

Mechanisms to control laser-plasma coupling in laser wakefield electron acceleration

Experimental results, supported by precise modelling, demonstrate optimisation of a plasma-based injector with intermediate laser pulse energy ($<1$ J), corresponding to a normalised vector potential $a_0 = 2.15$, using ionisation injection in a tailored plasma density profile. An increase in electron bunch quality and energy is achieved experimentally with the extension of the density downramp at the plasma exit. Optimisation of the focal position of the laser pulse in the tailored plasma density profile is shown to efficiently reduce electron bunch angular deviation, leading to a better alignment of the electron bunch with the laser axis. Single peak electron spectra are produced in a previously unexplored regime by combining an early focal position and adaptive optic control of the laser wavefront through optimising the symmetry of the pre-focal laser energy distribution. Experimental results have been validated through particle-in-cell simulations using realistic laser energy, phase distribution, and temporal envelope, allowing for accurate predictions of difficult to model parameters, such as total charge and spatial properties of the electron bunches, opening the way for more accurate modelling for the design of plasma-based accelerators.

physics.plasm-ph↗

arXiv2020

Automation and control of laser wakefield accelerators using Bayesian optimisation

Laser wakefield accelerators promise to revolutionise many areas of accelerator science. However, one of the greatest challenges to their widespread adoption is the difficulty in control and optimisation of the accelerator outputs due to coupling between input parameters and the dynamic evolution of the accelerating structure. Here, we use machine learning techniques to automate a 100 MeV-scale accelerator, which optimised its outputs by simultaneously varying up to 6 parameters including the spectral and spatial phase of the laser and the plasma density and length. Most notably, the model built by the algorithm enabled optimisation of the laser evolution that might otherwise have been missed in single-variable scans. Subtle tuning of the laser pulse shape caused an 80% increase in electron beam charge, despite the pulse length changing by just 1%.

physics.acc-ph↗

arXiv2020

Compton recoil effects in staging of laser wakefield accelerators

Laser plasma accelerators capable of generating >10 GeV electron beams may require plasma mirrors to remove undepleted laser energy at the end of each accelerator stage. Near the plasma mirror surface, the electron bunch can interact with the reflected light, resulting in inverse Compton scattering. For realistic conditions, we show that a significant fraction of electrons emit one or more photons, increasing the energy spread of the electron bunch. We provide an analytical expression for calculating this effect, and use it to estimate the minimum drift space required before the plasma mirror to meet given energy spread specifications. Mitigation strategies, necessary to achieve sub-percent energy spread in multi-GeV laser wakefield electron sources, are proposed and explored.

physics.plasm-ph↗

arXiv2020

The Laser-hybrid Accelerator for Radiobiological Applications

The `Laser-hybrid Accelerator for Radiobiological Applications', LhARA, is conceived as a novel, uniquely-flexible facility dedicated to the study of radiobiology. The technologies demonstrated in LhARA, which have wide application, will be developed to allow particle-beam therapy to be delivered in a completely new regime, combining a variety of ion species in a single treatment fraction and exploiting ultra-high dose rates. LhARA will be a hybrid accelerator system in which laser interactions drive the creation of a large flux of protons or light ions that are captured using a plasma (Gabor) lens and formed into a beam. The laser-driven source allows protons and ions to be captured at energies significantly above those that pertain in conventional facilities, thus evading the current space-charge limit on the instantaneous dose rate that can be delivered. The laser-hybrid approach, therefore, will allow the vast ``terra incognita'' of the radiobiology that determines the response of tissue to ionising radiation to be studied with protons and light ions using a wide variety of time structures, spectral distributions, and spatial configurations at instantaneous dose rates up to and significantly beyond the ultra-high dose-rate `FLASH' regime. It is proposed that LhARA be developed in two stages. In the first stage, a programme of in vitro radiobiology will be served with proton beams with energies between 10MeV and 15MeV. In stage two, the beam will be accelerated using a fixed-field accelerator (FFA). This will allow experiments to be carried out in vitro and in vivo with proton beam energies of up to 127MeV. In addition, ion beams with energies up to 33.4MeV per nucleon will be available for in vitro and in vivo experiments. This paper presents the conceptual design for LhARA and the R&D programme by which the LhARA consortium seeks to establish the facility.

physics.acc-ph↗

arXiv2020

Bright X-ray radiation from plasma bubbles in an evolving laser wakefield accelerator

We show that the properties of the electron beam and bright x-rays produced by a laser wakefield accelerator can be predicted if the distance over which the laser self-focuses and compresses prior to self-injection is taken into account. A model based on oscillations of the beam inside a plasma bubble shows that performance is optimised when the plasma length is matched to the laser depletion length. With a 200~TW laser pulse this results in an x-ray beam with median photon energy of \unit[20]{keV}, $> 6\times 10^{8}$ photons above \unit[1]{keV} per shot and a peak brightness of $\unit[3 \times 10^{22}]{photons~s^{-1}mrad^{-2}mm^{-2} (0.1\% BW)^{-1}}$.

physics.plasm-ph↗

arXiv2019

A New Injection and Acceleration Scheme of Positrons in the Laser-Plasma Bubble Regime

A novel approach for positron injection and acceleration in laser driven plasma wakefield is proposed. A theoretical model is developed and confirmed through PIC simulation. One ring-shaped beam and one co-axially propagating Gaussian beam drive wakefields in a preformed plasma volume filled with both electrons and positrons. The laser's ponderomotive force as well as the charge separation force in the front bucket of the first bubble are utilized to provide the transverse momenta of injected positrons and those positrons can be trapped by the focusing field and then accelerated by the wakefield. The simulation shows that a relatively high-charge, quasi-monoenergetic positron beams can be obtained.

physics.plasm-ph↗

arXiv2019

Simulation of a radiobiology facility for the Centre for the Clinical Application of Particles

The Centre for the Clinical Application of Particles' Laser-hybrid Accelerator for Radiobiological Applications (LhARA) facility is being studied and requires simulation of novel accelerator components (such as the Gabor lens capture system), detector simulation and simulation of the ion beam interaction with cells. The first stage of LhARA will provide protons up to 15 MeV for in vitro studies. The second stage of LhARA will use a fixed-field accelerator to increase the energy of the particles to allow in vivo studies with protons and in vitro studies with heavier ions. BDSIM, a Geant4 based accelerator simulation tool, has been used to perform particle tracking simulations to verify the beam optics design done by BeamOptics and these show good agreement. Design parameters were defined based on an EPOCH simulation of the laser source and a series of mono-energetic input beams were generated from this by BDSIM. The tracking results show the large angular spread of the input beam (0.2 rad) can be transported with a transmission of almost 100% whilst keeping divergence at the end station very low (<0.1 mrad). The legacy of LhARA will be the demonstration of technologies that could drive a step-change in the provision of proton and light ion therapy (i.e. a laser source coupled to a Gabor lens capture and a fixed-field accelerator), and a system capable of delivering a comprehensive set of experimental data that can be used to enhance the clinical application of proton and light ion therapy.

physics.acc-ph↗

arXiv2018

Experimental signatures of the quantum nature of radiation reaction in the field of an ultra-intense laser

The description of the dynamics of an electron in an external electromagnetic field of arbitrary intensity is one of the most fundamental outstanding problems in electrodynamics. Remarkably, to date there is no unanimously accepted theoretical solution for ultra-high intensities and little or no experimental data. The basic challenge is the inclusion of the self-interaction of the electron with the field emitted by the electron itself - the so-called radiation reaction force. We report here on the experimental evidence of strong radiation reaction, in an all-optical experiment, during the propagation of highly relativistic electrons (maximum energy exceeding 2 GeV) through the field of an ultra-intense laser (peak intensity of $4\times10^{20}$ W/cm$^2$). In their own rest frame, the highest energy electrons experience an electric field as high as one quarter of the critical field of quantum electrodynamics and are seen to lose up to 30% of their kinetic energy during the propagation through the laser field. The experimental data show signatures of quantum effects in the electron dynamics in the external laser field, potentially showing departures from the constant cross field approximation.

physics.plasm-ph↗

arXiv2018

Temporal Feedback Control of High-Intensity Laser Pulses to Optimize Ultrafast Heating of Atomic Clusters

We describe how active feedback routines can be applied at limited repetition rate (5 Hz) to optimize high-power $>10$ TW) laser interactions with clustered gases. Optimization of x-ray production from an argon cluster jet, using a genetic algorithm, approximately doubled the measured energy through temporal modification of the 150 mJ driving laser pulse. This approach achieved an increased radiation yield through exploration of a multi-dimensional parameter space, without requiring detailed a priori knowledge of the complex cluster dynamics. The optimized laser pulses exhibited a slow rising edge to the intensity profile, which enhanced the laser energy coupling into the cluster medium, compared to the optimally compressed FWHM pulse (40 fs). Our work suggests that this technique can be more widely utilized for control of intense pulsed secondary radiation from petawatt-class laser systems.

physics.plasm-ph↗

arXiv2018

Observation of Laser Power Amplification in a Self-Injecting Laser Wakefield Accelerator

We report on the depletion and power amplification of the driving laser pulse in a strongly-driven laser wakefield accelerator. Simultaneous measurement of the transmitted pulse energy and temporal shape indicate an increase in peak power from $187 \pm 11$ TW to a maximum of $318 \pm 12$ TW after 13 mm of propagation in plasma density of $0.9 \times 10^{18}$ cm$^{-3}$. The power amplification is correlated with the injection and acceleration of electrons in the nonlinear wakefield. This process is modeled by including localized redshift and subsequent group delay dispersion at the laser pulse front.

physics.plasm-ph↗

arXiv2018

Ultrafast Imaging of Laser Driven Shock Waves using Betatron X-rays from a Laser Wakefield Accelerator

Betatron radiation from laser wakefield accelerators is an ultrashort pulsed source of hard, synchrotron-like x-ray radiation. It emanates from a centimetre scale plasma accelerator producing GeV level electron beams. In recent years betatron radiation has been developed as a unique source capable of producing high resolution x-ray images in compact geometries. However, until now, the short pulse nature of this radiation has not been exploited. This report details the first experiment to utilise betatron radiation to image a rapidly evolving phenomenon by using it to radiograph a laser driven shock wave in a silicon target. The spatial resolution of the image is comparable to what has been achieved in similar experiments at conventional synchrotron light sources. The intrinsic temporal resolution of betatron radiation is below 100 fs, indicating that significantly faster processes could be probed in future without compromising spatial resolution. Quantitative measurements of the shock velocity and material density were made from the radiographs recorded during shock compression and were consistent with the established shock response of silicon, as determined with traditional velocimetry approaches. This suggests that future compact betatron imaging beamlines could be useful in the imaging and diagnosis of high-energy-density physics experiments.

physics.plasm-ph↗

arXiv2018

General features of experiments on the dynamics of laser-driven electron-positron beams

The experimental study of the dynamics of neutral electron-positron beams is an emerging area of research, enabled by the recent results on the generation of this exotic state of matter in the laboratory. Electron-positron beams and plasmas are believed to play a major role in the dynamics of extreme astrophysical objects such as supermassive black holes and pulsars. For instance, they are believed to be the main constituents of a large number of astrophysical jets, and they have been proposed to significantly contribute to the emission of gamma-ray bursts and their afterglow. However, despite extensive numerical modelling and indirect astrophysical observations, a detailed experimental characterisation of the dynamics of these objects is still at its infancy. Here, we will report on some of the general features of experiments studying the dynamics of electron-positron beams in a fully laser-driven setup.

physics.plasm-ph↗