arXiv subjects

Z. Najmudin

Publications and source records attributed to Z. Najmudin.

At least 37 records · Page 2Linked to original sources

arXiv2018

Experimental evidence of radiation reaction in the collision of a high-intensity laser pulse with a laser-wakefield accelerated electron beam

The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, today's lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We report on the observation of radiation reaction in the collision of an ultra-relativistic electron beam generated by laser wakefield acceleration ($\varepsilon > 500$ MeV) with an intense laser pulse ($a_0 > 10$). We measure an energy loss in the post-collision electron spectrum that is correlated with the detected signal of hard photons ($γ$-rays), consistent with a quantum (stochastic) description of radiation reaction. The generated $γ$-rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy $\varepsilon_{\rm crit} > $ 30 MeV.

physics.plasm-ph↗

arXiv2017

Laser Driving Highly Collimated $γ$-ray Pulses for the Generation of $μ^-μ^+$ and $e^-e^+$ Pairs in $γ-γ$ Collider

A scheme to generate highly collimated $γ$-ray pulse is proposed for the production of muon and electron pairs in $γ-γ$ collider. The $γ$-ray pulse, with high conversion efficiency, can be produced as the result of electron phase-locked acceleration in longitudinal electric field through the interaction between an ultra-intense laser pulse and a narrow tube target. Numerical simulation shows that 18\% energy of a 10-PW laser pulse is transferred into the forward $γ$-rays in a divergence angle less than $ 3^\circ$. The $γ$-ray pulse is applied in $γ-γ$ collider, in which muon pairs can be produced and electron pairs can be enhanced by more than 3 orders of magnitude. This scheme, which could be realized with the coming 10PW class laser pulses, would allow the observation of a $γ-γ$ collider for electron and muon pairs in laboratory.

physics.plasm-ph↗

arXiv2017

Experimental observation of a current-driven instability in a neutral electron-positron beam

We report on the first experimental observation of a current-driven instability developing in a quasi-neutral matter-antimatter beam. Strong magnetic fields ($\geq$ 1 T) are measured, via means of a proton radiography technique, after the propagation of a neutral electron-positron beam through a background electron-ion plasma.The experimentally determined equipartition parameter of $ε_B \approx 10^{-3}$, is typical of values inferred from models of astrophysical gamma-ray bursts, in which the relativistic flows are also expected to be pair dominated. The data, supported by Particle-In-Cell simulations and simple analytical estimates, indicate that these magnetic fields persist in the background plasma for thousands of inverse plasma frequencies. The existence of such long-lived magnetic fields can be related to analog astrophysical systems, such as those prevalent in lepton-dominated jets.

physics.plasm-ph↗

arXiv2017

Ultra-brilliance isolated attosecond gamma-ray light source from nonlinear Compton scattering

The explosion in attosecond technology has opened the gate to investigating many unexplored areas which require ultrahigh spatial and temporal resolution. In the area of nuclear physics, using gamma-rays with ultrahigh resolution in time and space will help to investigate intra-nuclear dynamics in an unprecedentedly explicit way. However, the generation of ultrahigh brilliance attosecond gamma-ray pulses with current-generation laser facilities has not been reported. In this letter, we propose a novel method to generate high charge (~1nC) attosecond (<200 attosecond) electron bunch by the near-threshold self-injection in a wakefield accelerator. We demonstrate the ability to generate an ultrahigh-brilliance (> 2*1024 photons s-1mm-2mrad-2 per 0.1%BW) attosecond (<200 attosecond) gamma-ray (Emax > 3 MeV) pulse via nonlinear Compton scattering. To the best of our knowledge, this is the first method reported to generate attosecond gamma-ray photon source using current-generation laser. This is the shortest gamma-ray photon and the highest brilliance photon source in MeV range (orders higher than the results reported). This method can be widely applied for experimental generation of 100 keV to several MeV high brilliance attosecond gamma-ray sources with current ~100 TW laser facilities, which will benefit basic science such as application in ultra-high resolution radiography.

physics.plasm-ph↗

arXiv2017

Laser Wakefield Accelerators

The one-dimensional wakefield generation equations are solved for increasing levels of non-linearity, to demonstrate how they contribute to the overall behaviour of a non-linear wakefield in a plasma. The effect of laser guiding is also studied as a way to increase the interaction length of a laser wakefield accelerator.

physics.acc-ph↗

arXiv2017

Case Studies on Plasma Wakefield Accelerator Design

The field of plasma-based particle accelerators has seen tremendous progress over the past decade and experienced significant growth in the number of activities. During this process, the involved scientific community has expanded from traditional university-based research and is now encompassing many large research laboratories worldwide, such as BNL, CERN, DESY, KEK, LBNL and SLAC. As a consequence, there is a strong demand for a consolidated effort in education at the intersection of accelerator, laser and plasma physics. The CERN Accelerator School on Plasma Wake Acceleration has been organized as a result of this development. In this paper, we describe the interactive component of this one-week school, which consisted of three case studies to be solved in 11 working groups by the participants of the CERN Accelerator School.

physics.acc-ph↗

arXiv2017

Laser-driven plasma acceleration in a regime of strong-mismatch between the incident laser envelope and the nonlinear plasma response

We explore a regime of laser-driven plasma acceleration of electrons where the radial envelope of the laser-pulse incident at the plasma entrance is strongly mismatched to the nonlinear plasma electron response excited by it. This regime has been experimentally studied with the gemini laser using f/40 focusing optics in August 2015 and f/20 in 2008. The physical mechanisms and the scaling laws of electron acceleration achievable in a laser-plasma accelerator have been studied in the radially matched laser regime and thus are not accurate in the strongly mismatched regime explored here. In this work, we show that a novel adjusted-a0 model applicable over a specific range of densities where the laser enters the state of a strong optical shock, describes the mismatched regime. Beside several novel aspects of laser-plasma interaction dynamics relating to an elongating bubble shape and the corresponding self-injection mechanism, importantly we find that in this strongly mismatched regime when the laser pulse transforms into an optical shock it is possible to achieve beam-energies that significantly exceed the incident intensity matched regime scaling laws.

physics.plasm-ph↗

arXiv2015

AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN

The Advanced Proton Driven Plasma Wakefield Acceleration Experiment (AWAKE) aims at studying plasma wakefield generation and electron acceleration driven by proton bunches. It is a proof-of-principle R&D experiment at CERN and the world's first proton driven plasma wakefield acceleration experiment. The AWAKE experiment will be installed in the former CNGS facility and uses the 400 GeV/c proton beam bunches from the SPS. The first experiments will focus on the self-modulation instability of the long (rms ~12 cm) proton bunch in the plasma. These experiments are planned for the end of 2016. Later, in 2017/2018, low energy (~15 MeV) electrons will be externally injected to sample the wakefields and be accelerated beyond 1 GeV. The main goals of the experiment will be summarized. A summary of the AWAKE design and construction status will be presented.

physics.acc-ph↗

arXiv2015

Path to AWAKE: Evolution of the concept

This report describes the conceptual steps in reaching the design of the AWAKE experiment currently under construction at CERN. We start with an introduction to plasma wakefield acceleration and the motivation for using proton drivers. We then describe the self-modulation instability --- a key to an early realization of the concept. This is then followed by the historical development of the experimental design, where the critical issues that arose and their solutions are described. We conclude with the design of the experiment as it is being realized at CERN and some words on the future outlook. A summary of the AWAKE design and construction status as presented in this conference is given in [1].

physics.plasm-ph↗

arXiv2015

Direct observation of the injection dynamics of a laser wakefield accelerator using few-femtosecond shadowgraphy

We present few-femtosecond shadowgraphic snapshots taken during the non-linear evolution of the plasma wave in a laser wakefield accelerator with transverse synchronized few-cycle probe pulses. These snapshots can be directly associated with the electron density distribution within the plasma wave and give quantitative information about its size and shape. Our results show that self-injection of electrons into the first plasma wave period is induced by a lengthening of the first plasma period. Three dimensional particle in cell simulations support our observations.

physics.plasm-ph↗

arXiv2015

Calibration of Time Of Flight Detectors Using Laser-driven Neutron Source

Calibration of three scintillators (EJ232Q, BC422Q and EJ410) in a time-of-flight (TOF) arrangement using a laser drive-neutron source is presented. The three plastic scintillator detectors were calibrated with gamma insensitive bubble detector spectrometers, which were absolutely calibrated over a wide range of neutron energies ranging from sub MeV to 20 MeV. A typical set of data obtained simultaneously by the detectors are shown, measuring the neutron spectrum emitted from a petawatt laser irradiated thin foil.

physics.ins-det↗

arXiv2015

Spectral modification of shock accelerated ions using hydrodynamically shaped gas target

We report on reproducible shock acceleration from irradiation of a $λ= 10$ $μ$m CO$_2$ laser on optically shaped H$_2$ and He gas targets. A low energy laser prepulse ($I\lesssim10^{14}\, {\rm Wcm^{-2}}$) was used to drive a blast wave inside the gas target, creating a steepened, variable density gradient. This was followed, after 25 ns, by a high intensity laser pulse ($I>10^{16}\, {\rm Wcm^{-2}}$) that produces an electrostatic collisionless shock. Upstream ions were accelerated for a narrow range of prepulse energies ($> 110$ mJ & $< 220$mJ). For long density gradients ($\gtrsim 40 μ$m), broadband beams of He$^+$ and H$^+$ were routinely produced, whilst for shorter gradients ($\lesssim 20 μ$m), quasimonoenergetic acceleration of proton was observed. These measurements indicate that the properties of the accelerating shock and the resultant ion energy distribution, in particular the production of narrow energy spread beams, is highly dependent on the plasma density profile. These findings are corroborated by 2D PIC simulations.

physics.plasm-ph↗

arXiv2015

Generation of a neutral, high-density electron-positron plasma in the laboratory

We report on the laser-driven generation of purely neutral, relativistic electron-positron pair plasmas. The overall charge neutrality, high average Lorentz factor ($γ_{e/p} \approx 15$), small divergence ($θ_{e/p} \approx 10 - 20$ mrad), and high density ($n_{e/p}\simeq 10^{15}$cm$^{-3}$) of these plasmas open the pathway for the experimental study of the dynamics of this exotic state of matter, in regimes that are of relevance to electron-positron astrophysical plasmas.

physics.plasm-ph↗

arXiv2014

Femtosecond-scale Synchronisation of Ultra-Intense Focused Laser Beams

Synchronising ultra-short (~fs) and focussed laser pulses is a particularly difficult task, as this timescale lies orders of magnitude below the typical range of fast electronic devices. Here we present an optical technique that allows for femtosecond-scale synchronisation of the focal planes of two focussed laser pulses. This technique is virtually applicable to any focussing geometry and relative intensity of the two lasers. Experimental implementation of this technique provides excellent quantitative agreement with theoretical expectations. The proposed technique will prove highly beneficial for the next generation of multiple, petawatt class laser systems.

physics.optics↗

arXiv2014

Characterisation of deuterium spectra from laser driven multi-species sources by employing differentially filtered image plate detectors in Thomson spectrometers

A novel method for characterising the full spectrum of deuteron ions emitted by laser driven multi-species ion sources is discussed. The procedure is based on using differential filtering over the detector of a Thompson parabola ion spectrometer, which enables discrimination of deuterium ions from heavier ion species with the same charge-to-mass ratio (such as C6+, O8+, etc.). Commonly used Fuji Image plates were used as detectors in the spectrometer, whose absolute response to deuterium ions over a wide range of energies was calibrated by using slotted CR-39 nuclear track detectors. A typical deuterium ion spectrum diagnosed in a recent experimental campaign is presented.

physics.plasm-ph↗

arXiv2014

Optimisation of the pointing stability of laser-wakefield accelerated electron beams

Laser-wakefield acceleration is a promising technique for the next generation of ultra-compact, high-energy particle accelerators. However, for a meaningful use of laser-driven particle beams it is necessary that they present a high degree of pointing stability in order to be injected into transport lines and further acceleration stages. Here we show a comprehensive experimental study of the main factors limiting the pointing stability of laser-wakefield accelerated electron beams. It is shown that gas-cells provide a much more stable electron generation axis, if compared to gas-jet targets, virtually regardless of the gas density used. A sub-mrad shot-to-shot fluctuation in pointing is measured and a consistent non-zero offset of the electron axis in respect to the laser propagation axis is found to be solely related to a residual angular dispersion introduced by the laser compression system and can be used as a precise diagnostic tool for compression oprtimisation in chirped pulse amplified lasers.

physics.plasm-ph↗

arXiv2014

Ultra-high brilliance multi-MeV $γ$-ray beam from non-linear Thomson scattering

We report on the generation of a narrow divergence ($θ\approx 2.5$ mrad), multi-MeV ($E_\text{MAX} = 18$ MeV) and ultra-high brilliance ($\approx 2\times10^{19}$ photons s$^{-1}$ mm$^{-2}$ mrad $^{-2}$ 0.1\% BW) $γ$-ray beam from the scattering of an ultra-relativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude $a_0\approx2$). The spectrum of the generated $γ$-ray beam is measured, with MeV resolution, seamlessly from 6 MeV to 18 MeV, giving clear evidence of the onset of non-linear Thomson scattering. The photon source has the highest brilliance in the multi-MeV regime ever reported in the literature.

physics.optics↗

arXiv2014

Buffered spectrally-peaked proton beams in the relativistic-transparency regime

Spectrally-peaked proton beams ($E_{p}\approx 8$ MeV, $ΔE\approx 4$ MeV) have been observed from the interaction of an intense laser ($> 10^{19 }$ Wcm$^{-2}$) with ultrathin CH foils, as measured by spectrally-resolved full beam profiles. These beams are reproducibly generated for foil thicknesses (5-100 nm), and exhibit narrowing divergence with decreasing target thickness down to $\approx 8^\circ$ for 5 nm. Simulations demonstrate that the narrow energy spread feature is a result of buffered acceleration of protons. Due to their higher charge-to-mass ratio, the protons outrun a carbon plasma driven in the relativistic transparency regime.

physics.plasm-ph↗