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Conference paperChen Y-H, Helle M, Ting A, et al., 2016,
Laser Acceleration of Protons with an Optically Shaped, Near-Critical Hydrogen Gas Target
, 17th Advanced Accelerator Concepts Workshop (AAC), Publisher: AMER INST PHYSICS, ISSN: 0094-243X -
Journal articleSmyth AG, Sarri G, Vranic M, et al., 2016,
Erratum: “Magnetic field generation during intense laser channelling in underdense plasma” [Phys. Plasmas 23, 063121 (2016)]
, Physics of Plasmas, Vol: 23, ISSN: 1089-7674 -
Journal articleSmyth AG, Sarri G, Vranic M, et al., 2016,
Magnetic field generation during intense laser channelling in underdense plasma
, Physics of Plasmas, Vol: 23, ISSN: 1089-7674Channel formation during the propagation of a high-energy (120 J) and long duration (30 ps) laser pulse through an underdense deuterium plasma has been spatially and temporally resolved via means of a proton imaging technique, with intrinsic resolutions of a few μm and a few ps, respectively. Conclusive proof is provided that strong azimuthally symmetric magnetic fields with a strength of around 0.5 MG are created inside the channel, consistent with the generation of a collimated beam of relativistic electrons. The inferred electron beam characteristics may have implications for the cone-free fast-ignition scheme of inertial confinement fusion.
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Conference paperBracco C, Amorim LD, Assmann R, et al., 2016,
AWAKE: A Proton-Driven Plasma Wakefield Acceleration Experiment at CERN
, 37th International Conference on High Energy Physics (ICHEP), Publisher: ELSEVIER SCIENCE BV, Pages: 175-180, ISSN: 2405-6014 -
Journal articleSuttle LG, Hare JD, Lebedev SV, et al., 2016,
Structure of a Magnetic Flux Annihilation Layer Formed by the Collision of Supersonic, Magnetized Plasma Flows
, Physical Review Letters, Vol: 116, ISSN: 1079-7114We present experiments characterizing the detailed structure of a current layer, generated by the collision of two counterstreaming, supersonic and magnetized aluminum plasma flows. The antiparallel magnetic fields advected by the flows are found to be mutually annihilated inside the layer, giving rise to a bifurcated current structure—two narrow current sheets running along the outside surfaces of the layer. Measurements with Thomson scattering show a fast outflow of plasma along the layer and a high ion temperature (Ti∼Z¯Te, with average ionization Z¯=7). Analysis of the spatially resolved plasma parameters indicates that the advection and subsequent annihilation of the inflowing magnetic flux determines the structure of the layer, while the ion heating could be due to the development of kinetic, current-driven instabilities.
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Conference paperRead MP, Kingham RJ, Bissell JJ, 2016,
The influence of magnetised electron transport on thermal self-focusing and channelling of nanosecond laser beams
, 9th International Conference on Inertial Fusion Sciences and Applications (IFSA 2015), Publisher: IOP Publishing, ISSN: 1742-6588The propagation of a nanosecond IR laser pulse through an under-dense (0.01 — 0.1ncr) magnetised laser-plasma is considered. The interplay between magnetised transport, B-field evolution and plasma hydrodynamics in the presence of a dynamically evolving beam are investigated by means of a paraxial wave solving module coupled to CTC, a 2D MHD code including Braginskii electron transport and IMPACT, a 2D implicit Vlasov-Fokker-Planck (VFP) code with magnetic fields. Magnetic fields have previously been shown to improve density channel formation for plasma waveguides however fluid simulations presented here indicate that Nernst advection can result in the rapid cavitation of magnetic field in the laser-heated region resulting in beam defocusing. Kinetic simulations indicate that strong non-local transport is present leading to the fluid code overestimating heat-flow and magnetic field advection and resulting in the recovery of beam channelling for the conditions considered.
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Journal articlePike OJ, Rose SJ, 2016,
Transport coefficients of a relativistic plasma
, Physical Review E, Vol: 93, ISSN: 1539-3755In this work, a self-consistent transport theory for a relativistic plasma is developed.Using the notation of Braginskii [S. I. Braginskii, in Reviews of Plasma Physics, ed. M. A.Leontovich (1965), Vol. 1, p.174], we provide semi-analytical forms of the electrical resistivity,thermoelectric and thermal conductivity tensors for a Lorentzian plasma in a magnetic field.This treatment is then generalized to plasmas with arbitrary atomic number by numericallysolving the linearized Boltzmann equation. The corresponding transport coefficients arefitted by rational functions in order to make them suitable for use in radiation-hydrodynamicsimulations and transport calculations. Within the confines of linear transport theory andon the assumption that the plasma is optically thin, our results are valid for temperatures upto a few MeV. By contrast, classical transport theory begins to incur significant errors abovekBT ∼ 10 keV, e.g., the parallel thermal conductivity is suppressed by 15% at kBT = 20keV due to relativistic effects
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Journal articleSiminos E, Skupin S, Savert A, et al., 2016,
Modeling ultrafast shadowgraphy in laser-plasma interaction experiments
, Plasma Physics and Controlled Fusion, Vol: 58, ISSN: 1361-6587Ultrafast shadowgraphy is a new experimental technique that uses few cycle laser pulses to imagedensity gradients in a rapidly evolving plasma. It enables structures that move at speeds close tothe speed of light, such as laser driven wakes, to be visualized. Here we study the process of shadowgraphicimage formation during the propagation of a few cycle probe pulse transversely througha laser-driven wake using three-dimensional particle-in-cell simulations. In order to construct syntheticshadowgrams a near-field snapshot of the ultrashort probe pulse is analyzed by means ofFourier optics, taking into account the effect of a typical imaging setup. By comparing syntheticand experimental shadowgrams we show that the generation of synthetic data is crucial for the correctinterpretation of experiments. Moreover, we study the dependence of synthetic shadowgramson various parameters such as the imaging system aperture, the position of the object plane andthe probe pulse delay, duration and wavelength. Finally, we show that time-dependent informationfrom the interaction can be recovered from a single shot by using a broadband, chirped probe pulseand subsequent spectral filtering.
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Journal articleSwadling GF, Lebedev SV, Hall GN, et al., 2016,
Experimental investigations of ablation stream interaction dynamics in tungsten wire arrays: interpenetration, magnetic field advection, and ion deflection
, Physics of Plasmas, Vol: 23, ISSN: 1089-7674Experiments have been carried out to investigate the collisional dynamics of ablation streams produced by cylindrical wire array z-pinches. A combination of laser interferometric imaging, Thomson scattering, and Faraday rotationimaging has been used to make a range of measurements of the temporal evolution of various plasma and flow parameters. This paper presents a summary of previously published data, drawing together a range of different measurements in order to give an overview of the key results. The paper focuses mainly on the results of experiments with tungsten wire arrays. Early interferometric imagingmeasurements are reviewed, then more recent Thomson scattering measurements are discussed; these measurements provided the first direct evidence of ablation stream interpenetration in a wire array experiment. Combining the data from these experiments gives a view of the temporal evolution of the tungsten stream collisional dynamics. In the final part of the paper, we present new experimental measurements made using an imagingFaraday rotationdiagnostic. These experiments investigated the structure of magnetic fields near the array axis directly; the presence of a magnetic field has previously been inferred based on Thomson scattering measurements of ion deflection near the array axis. Although the Thomson and Faradaymeasurements are not in full quantitative agreement, the Faraday data do qualitatively supports the conjecture that the observed deflections are induced by a static toroidal magnetic field, which has been advected to the array axis by the ablation streams. It is likely that detailed modeling will be needed in order to fully understand the dynamics observed in the experiment.
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Journal articleJoglekar AS, Ridgers CP, Kingham RJ, et al., 2016,
Kinetic modeling of Nernst effect in magnetized hohlraums
, Physical Review E, Vol: 93, ISSN: 1539-3755We present nanosecond time-scale Vlasov-Fokker-Planck-Maxwell modeling of magnetized plasma transport and dynamics in a hohlraum with an applied external magnetic field, under conditions similar to recent experiments. Self-consistent modeling of the kinetic electron momentum equation allows for a complete treatment of the heat flow equation and Ohm's law, including Nernst advection of magnetic fields. In addition to showing the prevalence of nonlocal behavior, we demonstrate that effects such as anomalous heat flow are induced by inverse bremsstrahlung heating. We show magnetic field amplification up to a factor of 3 from Nernst compression into the hohlraum wall. The magnetic field is also expelled towards the hohlraum axis due to Nernst advection faster than frozen-in flux would suggest. Nonlocality contributes to the heat flow towards the hohlraum axis and results in an augmented Nernst advection mechanism that is included self-consistently through kinetic modeling.
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Journal articleHaerendel G, Suttle L, Lebedev SV, et al., 2016,
Stop layer: a flow braking mechanism in space and support from a lab experiment
, Plasma Physics and Controlled Fusion, Vol: 58, ISSN: 1361-6587The paper presents short summaries and a synopsis of two completely independent discoveries of a fast flow braking process, one realized by a laboratory experiment (Lebedev et al 2014 Phys. Plasmas 21 056305), the other by theoretical reasoning stimulated by auroral observation (Haerendel 2015a J. Geophys. Res. Space Phys. 120 1697–714). The first has been described as a magnetically mediated sub-shock forming when a supersonic plasma flow meets a wall. The second tried to describe what happens when a high-beta plasma flow from the central magnetic tail meets the strong near-dipolar field of the magnetosphere. The term stop layer signals that flow momentum and energy are directly coupled to a magnetic perturbation field generated by a Hall current within a layer of the width of c/ω pi and immediately propagated out of the layer by kinetic Alfvén waves. As the laboratory situation is not completely collision-free, energy transfer from ions to electrons and subsequent radiative losses are likely to contribute. A synopsis of the two situations identifies and discusses six points of commonality between the two situations. It is pointed out that the stop layer mechanism can be regarded as a direct reversal of the reconnection process.
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Journal articleSherlock M, Rozmus W, Hill EG, et al., 2016,
Sherlock et al. Reply
, Physical Review Letters, Vol: 116, ISSN: 1079-7114 -
Journal articleKononenko O, Lopes NC, Cole JM, et al., 2016,
2D hydrodynamic simulations of a variable length gas target for density down-ramp injection of electrons into a laser wakefield accelerator
, Nuclear Instruments & Methods in Physics Research Section A - Accelerators Spectrometers Detectors and Associated Equipment, Vol: 829, Pages: 125-129, ISSN: 0168-9002In this work, two-dimensional (2D) hydrodynamic simulations of a variable length gas cell were performed using the open source fluid code OpenFOAM. The gas cell was designed to study controlled injection of electrons into a laser-driven wakefield at the Astra Gemini laser facility. The target consists of two compartments: an accelerator and an injector section connected via an aperture. A sharp transition between the peak and plateau density regions in the injector and accelerator compartments, respectively, was observed in simulations with various inlet pressures. The fluid simulations indicate that the length of the down-ramp connecting the sections depends on the aperture diameter, as does the density drop outside the entrance and the exit cones. Further studies showed, that increasing the inlet pressure leads to turbulence and strong fluctuations in density along the axial profile during target filling, and consequently, is expected to negatively impact the accelerator stability.
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Journal articleBehm KT, Zhao TZ, Cole JM, et al., 2016,
Ionization injection effects in x-ray spectra generated by betatron oscillations in a laser wakefield accelerator
, Plasma Physics and Controlled Fusion, Vol: 58, ISSN: 1361-6587 -
Conference paperSymes DR, Najmudin Z, Cole JM, et al., 2016,
High-resolution tomographic imaging using coherent hard x-rays from compact laser driven accelerators
, Compact EUV & X-ray Light Sources 2016, Publisher: OSA PublishingExtremely bright coherent femtosecond x-ray pulses are generated in compact laserdriven electron accelerators. Micro-tomography obtained with the Gemini laser indicates the usefulness of these sources in research and clinical applications.
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Journal articlePogorelsky IV, Babzien M, Ben-Zvi I, et al., 2016,
Extending laser plasma accelerators into the mid-IR spectral domain with a next-generation ultra-fast CO<sub>2</sub> laser
, PLASMA PHYSICS AND CONTROLLED FUSION, Vol: 58, ISSN: 0741-3335- Author Web Link
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- Citations: 9
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Conference paperGschwendtner E, Adli E, Amorim L, et al., 2016,
AWAKE, The Advanced Proton Driven Plasma Wakefield Acceleration Experiment at CERN
, 2nd Workshop on European Advanced Accelerator Concepts (EAAC), Publisher: ELSEVIER SCIENCE BV, Pages: 76-82, ISSN: 0168-9002 -
Conference paperKing M, Gray RJ, Powell HW, et al., 2016,
Ion acceleration and plasma jet formation in ultra-thin foils undergoing expansion and relativistic transparency
, 2nd Workshop on European Advanced Accelerator Concepts (EAAC), Publisher: Elsevier, Pages: 163-166, ISSN: 0168-9002At sufficiently high laser intensities, the rapid heating to relativistic velocities and resulting decompression of plasma electrons in an ultra-thin target foil can result in the target becoming relativistically transparent to the laser light during the interaction. Ion acceleration in this regime is strongly affected by the transition from an opaque to a relativistically transparent plasma. By spatially resolving the laser-accelerated proton beam at near-normal laser incidence and at an incidence angle of 30°, we identify characteristic features both experimentally and in particle-in-cell simulations which are consistent with the onset of three distinct ion acceleration mechanisms: sheath acceleration; radiation pressure acceleration; and transparency-enhanced acceleration. The latter mechanism occurs late in the interaction and is mediated by the formation of a plasma jet extending into the expanding ion population. The effect of laser incident angle on the plasma jet is explored.
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Journal articleDover N, Cook N, Tresca O, et al., 2016,
Optical shaping of gas targets for laser plasma ion sources
, Journal of Plasma Physics, Vol: 82, ISSN: 1469-7807We report on the experimental demonstration of a technique to generate steep densitygradients in gas jet targets of interest to laser plasma ion acceleration. By using anintentional low energy prepulse, we generated a hydrodynamic blast wave in the gas toshape the target prior to the arrival of an intense CO2 (λ ≈ 10 µm) drive pulse. Thistechnique has been recently shown to facilitate the generation of ion beams by shockwaveacceleration (Tresca et al. 2015). Here, we discuss and introduce a model to understandthe generation of these blast waves and discuss in depth the experimental realisationof the technique, supported by hydrodynamics simulations. With appropriate prepulseenergy and timing, this blast wave can generate steepened density gradients as short asl ≈ 20 µm (1/e), opening up new possibilities for laser-plasma studies with near-criticalgaseous targets.
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Journal articleDover N, Palmer CAJ, Streeter MJV, et al., 2016,
Buffered high charge spectrally-peaked proton beams in the relativistic-transparency regime
, New Journal of Physics, Vol: 18, ISSN: 1367-2630Spectrally-peaked proton beams of high charge (Ep ≈ 8 MeV, ∆E ≈4 MeV, N ≈ 50 nC ) have been observed from the interaction of an intense laser(> 1019 Wcm−2) with ultrathin CH foils, as measured by spectrally-resolved full beamprofiles. These beams are reproducibly generated for foil thicknesses 5-100 nm, andexhibit narrowing divergence with decreasing target thickness down to ≈ 8◦for 5 nm.Simulations demonstrate that the narrow energy spread feature is a result of bufferedacceleration of protons. The radiation pressure at the front of the target results inasymmetric sheath fields which permeate throughout the target, causing preferentialforward acceleration. Due to their higher charge-to-mass ratio, the protons outrun acarbon plasma driven in the relativistic transparency regime.
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Journal articleCaldwell A, Adli E, Amorim L, et al., 2016,
Path to AWAKE: Evolution of the concept
, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Vol: 829, Pages: 3-16, ISSN: 0168-9002This paper 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 Gschwendtner et al. [1].
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Journal articleCole JM, Wood JC, Lopes NC, et al., 2016,
Tomography of human trabecular bone with a laser-wakefield driven x-ray source
, Plasma Physics and Controlled Fusion, Vol: 58, ISSN: 1361-6587A laser-wakefield driven x-ray source is used for the radiography of human bone. The betatron motion of accelerated electrons generates x-rays which are hard (critical energy ${{E}_{\text{crit}}}>30$ keV), have small source size (<3 μm) and high average brightness. The x-rays are generated from a helium gas cell which is near-instantly replenishable, and thus the average photon flux is limited by the repetition rate of the driving laser rather than the breakdown of the x-ray source. A tomograph of a human bone sample was recorded with a resolution down to 50 μm. The photon flux was sufficiently high that a radiograph could be taken with each laser shot, and the fact that x-ray beams were produced on 97% of shots minimised failed shots and facilitated full micro-computed tomography in a reasonable time scale of several hours, limited only by the laser repetition rate. The x-ray imaging beamline length (not including the laser) is shorter than that of a synchrotron source due to the high accelerating fields and small source size. Hence this interesting laboratory-based source may one day bridge the gap between small microfocus x-ray tubes and large synchrotron facilities.
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Conference paperCook N, Tresca O, Dover NP, et al., 2016,
Hydrodynamic Shaping of Gas Jets for Laser Driven Shock Acceleration of Helium Ions
, 16th Advanced Accelerator Concepts Workshop, Publisher: AMER INST PHYSICS, ISSN: 0094-243X -
Journal articleTurcu ICE, Negoita F, Jaroszynski DA, et al., 2016,
HIGH FIELD PHYSICS AND QED EXPERIMENTS AT ELI-NP
, ROMANIAN REPORTS IN PHYSICS, Vol: 68, Pages: S145-S231, ISSN: 1221-1451- Author Web Link
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- Citations: 89
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Conference paperNajmudin Z, Fiuza F, Fernandez JC, 2016,
Summary of Working Group 6: Ion Acceleration with Lasers
, 16th Advanced Accelerator Concepts Workshop, Publisher: AMER INST PHYSICS, ISSN: 0094-243X- Author Web Link
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- Citations: 1
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Journal articleSuzuki-Vidal F, Lebedev SV, Ciardi A, et al., 2015,
BOW SHOCK FRAGMENTATION DRIVEN BY A THERMAL INSTABILITY IN LABORATORY ASTROPHYSICS EXPERIMENTS
, Astrophysical Journal, Vol: 815, ISSN: 1538-4357The role of radiative cooling during the evolution of a bow shock was studied in laboratory-astrophysics experiments that are scalable to bow shocks present in jets from young stellar objects. The laboratory bow shock is formed during the collision of two counterstreaming, supersonic plasma jets produced by an opposing pair of radial foil Z-pinches driven by the current pulse from the MAGPIE pulsed-power generator. The jets have different flow velocities in the laboratory frame, and the experiments are driven over many times the characteristic cooling timescale. The initially smooth bow shock rapidly develops small-scale nonuniformities over temporal and spatial scales that are consistent with a thermal instability triggered by strong radiative cooling in the shock. The growth of these perturbations eventually results in a global fragmentation of the bow shock front. The formation of a thermal instability is supported by analysis of the plasma cooling function calculated for the experimental conditions with the radiative packages ABAKO/RAPCAL.
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Journal articleDeas RM, Wilson LA, Rusby D, et al., 2015,
A laser driven pulsed X-ray backscatter technique for enhanced penetrative imaging
, Journal of X-Ray Science and Technology, Vol: 23, Pages: 791-797, ISSN: 1095-9114X-ray backscatter imaging can be used for a wide range of imaging applications, in particular for industrial inspection and portal security. Currently, the application of this imaging technique to the detection of landmines is limited due to the surrounding sand or soil strongly attenuating the 10s to 100s of keV X-rays required for backscatter imaging. Here, we introduce a new approach involving a 140 MeV short-pulse (< 100 fs) electron beam generated by laser wakefield acceleration to probe the sample, which produces Bremsstrahlung X-rays within the sample enabling greater depths to be imaged. A variety of detector and scintillator configurations are examined, with the best time response seen from an absorptive coated BaF2 scintillator with a bandpass filter to remove the slow scintillation emission components. An X-ray backscatter image of an array of different density and atomic number items is demonstrated. The use of a compact laser wakefield accelerator to generate the electron source, combined with the rapid development of more compact, efficient and higher repetition rate high power laser systems will make this system feasible for applications in the field.
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Journal articleBurdiak GC, Lebedev SV, Harvey-Thompson AJ, et al., 2015,
Characterisation of the current switch mechanism in two-stage wire array Z-pinches
, Physics of Plasmas, Vol: 22, ISSN: 1089-7674In this paper, we describe the operation of a two-stage wire array z-pinch driven by the 1.4 MA,240 ns rise-time Magpie pulsed-power device at Imperial College London. In this setup, an inversewire array acts as a fast current switch, delivering a current pre-pulse into a cylindrical load wirearray, before rapidly switching the majority of the generator current into the load after a100–150 ns dwell time. A detailed analysis of the evolution of the load array during the pre-pulse ispresented. Measurements of the load resistivity and energy deposition suggest significant bulk heatingof the array mass occurs. The 5 kA pre-pulse delivers 0.8 J of energy to the load, leaving itin a mixed, predominantly liquid-vapour state. The main current switch occurs as the inverse arraybegins to explode and plasma expands into the load region. Electrical and imaging diagnostics indicatethat the main current switch may evolve in part as a plasma flow switch, driven by the expansionof a magnetic cavity and plasma bubble along the length of the load array. Analysis ofimplosion trajectories suggests that approximately 1 MA switches into the load in 100 ns, correspondingto a doubling of the generator dI/dt. Potential scaling of the device to higher currentmachines is discussed. V
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Journal articleTurrell A, Sherlock M, Rose SJ, 2015,
Ultra-fast collisional ion heating by electrostatic shocks
, Nature Communications, Vol: 6, ISSN: 2041-1723High intensity lasers can be used to generate shockwaves which have found applications in nuclear fusion, proton imaging, cancer therapies, and materials science. Collisionless electrostatic shocks are one type of shockwave widely studied for applications involving ion acceleration. Here we show a novel mechanism for collisionlesselectrostatic shocks to heat small amounts of solid density matter to temperatures of ∼ keV in tens of femtoseconds. Unusually, electrons play no direct role in the heating, and it is the ions which determine the heating rate. Ions are heated due to an interplay between the electric field of the shock, the local density increaseduring the passage of the shock, and collisions between different species of ion. In simulations, these factors combine to produce rapid, localised heating of the lighter ion species. Although the heated volume is modest, this would be one of the fastest heating mechanisms discovered if demonstrated in the laboratory.
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Journal articlePowell HW, King M, Gray RJ, et al., 2015,
Proton acceleration enhanced by a plasma jet in expanding foils undergoing relativistic transparency
, New Journal of Physics, Vol: 17, ISSN: 1367-2630Ion acceleration driven by the interaction of an ultraintense (2 × 1020 W cm−2) laser pulse with an ultrathin ($\leqslant 40$ nm) foil target is experimentally and numerically investigated. Protons accelerated by sheath fields and via laser radiation pressure are angularly separated and identified based on their directionality and signature features (e.g. transverse instabilities) in the measured spatial-intensity distribution. A low divergence, high energy proton component is also detected when the heated target electrons expand and the target becomes relativistically transparent during the interaction. 2D and 3D particle-in-cell simulations indicate that under these conditions a plasma jet is formed at the target rear, supported by a self-generated azimuthal magnetic field, which extends into the expanded layer of sheath-accelerated protons. Electrons trapped within this jet are directly accelerated to super-thermal energies by the portion of the laser pulse transmitted through the target. The resulting streaming of the electrons into the ion layers enhances the energy of protons in the vicinity of the jet. Through the addition of a controlled prepulse, the maximum energy of these protons is demonstrated experimentally and numerically to be sensitive to the picosecond rising edge profile of the laser pulse.
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