arXiv subjects

Zulfikar Najmudin

Publications and source records attributed to Zulfikar Najmudin.

7 recordsLinked to original sources

arXiv2026

Observation of quantum effects on radiation reaction in strong fields

Radiation reaction, the force experienced by an accelerated charge due to radiation emission, has long been the subject of extensive theoretical and experimental research. Experimental verification of a quantum, strong-field description of radiation reaction is fundamentally important, and has wide-ranging implications for astrophysics, laser-driven particle acceleration, next-generation particle colliders and inverse-Compton photon sources for medical and industrial applications. However, the difficulty of accessing regimes where strong field and quantum effects dominate inhibited previous efforts to observe quantum radiation reaction in charged particle dynamics with high significance. We report the first high significance (> 5σ) observation of strong-field radiation reaction on electron spectra where quantum effects are substantial. We obtain the first, quantitative, strong evidence favouring the quantum-continuous and quantum-stochastic models over the classical model; the quantum models perform comparably. The lower electron energy losses predicted by the quantum models accounts for their improved performance. Model comparison was performed using a novel Bayesian framework which has widespread utility for laser-particle collision experiments, including those utilising conventional accelerators, where some collision parameters cannot be measured directly.

hep-ph↗

arXiv2024

Laser Interactions with Gas Jets: EMP Emission and Nozzle Damage

Understanding the physics of electromagnetic pulse emission and nozzle damage is critical for the long-term operation of laser experiments with gas targets, particularly at facilities looking to produce stable sources of radiation at high repetition rate. We present a theoretical model of plasma formation and electrostatic charging when high-power lasers are focused inside gases. The model can be used to estimate the amplitude of gigahertz electromagnetic pulses (EMPs) produced by the laser and the extent of damage to the gas jet nozzle. Looking at a range of laser and target properties relevant to existing high-power laser systems, we find that EMP fields of tens to hundreds of kV/m can be generated several metres from the gas jet. Model predictions are compared with measurements of EMP, plasma formation and nozzle damage from two experiments on the VEGA-3 laser and one experiment on the Vulcan Petawatt laser.

physics.plasm-ph↗

arXiv2023

Measurement of magnetic cavitation driven by heat flow in a plasma

We describe the direct measurement of the expulsion of a magnetic field from a plasma driven by heat flow. Using a laser to heat a column of gas within an applied magnetic field, we isolate Nernst advection and show how it changes the field over a nanosecond timescale. Reconstruction of the magnetic field map from proton radiographs demonstrates that the field is advected by heat flow in advance of the plasma expansion. This changes the dynamics of high energy density plasmas, in which heat flows and fields are strongly coupled, and may disrupt magnetised inertial confinement fusion schemes.

physics.plasm-ph↗

arXiv2021

Spectrally peaked proton beams shock accelerated from an optically shaped overdense gas jet by a near-infrared laser

We report on the generation of impurity-free proton beams from an overdense gas jet driven by a near-infrared laser ($λ_L=1.053$ $\mathrmμ m$). The gas profile was shaped prior to the interaction using a controlled prepulse. Without this optical shaping, a 30$\pm$4 nCsr$^{-1}$ thermal spectrum was detected transversely to the laser propagation direction with a high energy 8.27$\pm$7 MeV, narrow energy spread (6$\pm$2 %) bunch containing 45$\pm$7 pCsr$^{-1}$. In contrast, with optical shaping the radial component was not detected and instead forward going protons were detected with energy 1.32$\pm$2 MeV, 12.9$\pm$3 % energy spread, and charge 400$\pm$30 pCsr$^{-1}$. Both the forward going and radial narrow energy spread features are indicative of collisionless shock acceleration of the protons.

physics.plasm-ph↗

arXiv2019

Plasma Wakefield Accelerator Research 2019 - 2040: A community-driven UK roadmap compiled by the Plasma Wakefield Accelerator Steering Committee (PWASC)

The acceleration gradients generated in a laser- or beam-driven plasma wakefield accelerator are typically three orders of magnitude greater than those produced by a conventional accelerator, and hence plasma accelerators can open a route to a new generation of very compact machines. In addition, plasma-based accelerators can generate beams with unique properties, such as tens of kiloamp peak currents, attosecond bunch duration, ultrahigh brightness and intrinsic particle beam-laser pulse synchronization. In this roadmap we review the status of plasma accelerator research in the UK. We outline potential applications, describe the research and development required to enable those applications, and discuss synergies with related areas of research. We also set-out the resources required to realise these ambitions and provide a timeline for advances in the key areas.

physics.acc-ph↗

arXiv2018

Laser wakefield acceleration with high-power, few-cycle mid-IR lasers

The study of laser wakefield electron acceleration (LWFA) using mid-IR laser drivers is a promising path for future laser driven electronaccelerators, when compared to traditional near-IR laser drivers uperating at 0.8-1 μm central wavelength (λlaser), as the necessary vector potential a_0 for electron injection can be achieved with smaller laser powers due to the linear dependence on λlaser. In this work, we perform 2D PIC simulations on LWFA using few-cycle high power (5-15 TW) laser systems with λlaser ranging from 0.88-10 μm. Such few-cycle systems are currently under development, aiming at Gas High Harmonics Generation applications where the favourable λlaser^2 scaling extends the range of XUV photon energies. We keep a_0 and n_e/n_cr (n_e being the plasma density and n_cr being the critical density for each λlaser) as common denominators in outr simulations, allowing for comparisons between drivers of different λlaser, with respect to the accelerated electron beam energy, charge, and conversion efficiency. While the electron energies are mainly dominated by the plasma dynamics, the laser to electron beam energy conversion efficiency shows significant enhancement with longer wavelength laser drivers.

physics.plasm-ph↗

arXiv2012

The self-injection threshold in self-guided laser wakefield accelerators

A laser pulse traveling through a plasma can excite large amplitude plasma waves that can be used to accelerate relativistic electron beams in a very short distance---a technique called laser wakefield acceleration. Many wakefield acceleration experiments rely on the process of wavebreaking, or self-injection, to inject electrons into the wave, while other injection techniques rely on operation without self-injection. We present an experimental study into the parameters, including the pulse energy, focal spot quality and pulse power, that determine whether or not a wakefield accelerator will self-inject. By taking into account the processes of self-focusing and pulse compression we are able to extend a previously described theoretical model, where the minimum bubble size required for trapping is not constant but varies slowly with density and find excellent agreement with this model.

physics.plasm-ph↗