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A bright μm-sized source of hard synchrotron x-rays (critical energy Ecrit > 30 keV) based on the betatron oscillations of laser wakefield accelerated electrons has been developed. The potential of this source for medical imaging was demonstrated by performing micro-computed tomography of a human femoral trabecular bone sample, allowing full 3D reconstruction to a resolution below 50 μm. The use of a 1 cm long wakefield accelerator means that the length of the beamline (excluding the laser) is dominated by the x-ray imaging distances rather than the electron acceleration distances. The source possesses high peak brightness, which allows each image to be recorded with a single exposure and reduces the time required for a full tomographic scan. These properties make this an interesting laboratory source for many tomographic imaging applications., Competing Interests: The authors declare no competing financial interests.

Calibration of three scintillators (EJ232Q, BC422Q, and EJ410) in a time-of-flight 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 is shown, measuring the neutron spectrum emitted from a petawatt laser irradiated thin foil.

X-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. Content includes material subject to Dstl (c) Crown copyright (2014). Licensed under the terms of the Open Government Licence except where otherwise stated. To view this licence, visit http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3 or write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email: psi@ nationalarchives.gsi.gov.uk.

The first evidence of x-ray harmonic radiation extending to 3.3 A, 3.8 keV (order n>3200) from petawatt class laser-solid interactions is presented, exhibiting relativistic limit efficiency scaling (eta approximately n{-2.5}-n{-3}) at multi-keV energies. This scaling holds up to a maximum order, n{RO} approximately 8{1/2}gamma;{3}, where gamma is the relativistic Lorentz factor, above which the first evidence of an intensity dependent efficiency rollover is observed. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy hnu>1 keV is found to be into a cone angle approximately 4 degrees , significantly less than that of the incident laser cone (20 degrees ).

The interaction of high intensity short pulse laser beams with plasmas can accelerate electrons to energies in excess of a GeV. These electron beams can subsequently be used to generate short-lived particles such as positrons, muons, and pions. In recent experiments, we have made the first measurements of pion production using 'all optical' methods. In particular, we have demonstrated that the interaction of bremsstrahlung generated by laser driven electron beams with aluminum atoms can produce the long lived isotope of magnesium (27Mg) which is a signature for pion (π+) production and subsequent muon decay. Using a 300TWlaser pulse, we have measured the generation of 150 ± 50 pions per shot.Wealso show that the energetic electron beam is a source of an intense, highly directional neutron beam resulting from (γ, n) reactions which contributes to the 27Mg measurement as background via the (n, p) process. [ABSTRACT FROM AUTHOR]

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, which was produced from a thin deuterated plastic foil target irradiated by a high power laser. [ABSTRACT FROM AUTHOR]

Laser wakefield acceleration experiments were performed using a 30 fs, 1 J laser pulse interacting with an underdense helium plasma. Temporally resolved polarimetry measurements demonstrate the presence of magnetic fields at the ionization front within the plasma which had a peak strength of ~2.8MG and a radial extent of approximately 200µm. The field was seen to vary in strength over picosecond time-scales. The field is likely generated by return current generated in the plasma at the interface between plasma and neutral gas and which is caused by hot electrons produced in the wakefield during formation of a plasma 'bubble' and prior to the time of wave-breaking (beam injection). These effects are confirmed using particle-in-cell simulations. Such measurements can be useful as a diagnostic of bubble formation in laser wakefield accelerators. [ABSTRACT FROM AUTHOR]

As the state of the art for high power laser systems increases from terawatt to petawatt level and beyond, a crucial parameter for routinely monitoring high intensity performance is laser spot size on a solid target during an intense interaction in the tight focus regime (<10 μm). Here we present a novel, simple technique for characterizing the spatial profile of such a laser focal spot by imaging the interaction region in third harmonic order (3ωlaser). Nearly linear intensity dependence of 3ωlaser generation for interactions >1019 Wcm-2 is demonstrated experimentally and shown to provide the basis for an effective focus diagnostic. Importantly, this technique is also shown to allow in-situ diagnosis of focal spot quality achieved after reflection from a double plasma mirror setup for very intense high contrast interactions (>1020 Wcm-2) an important application for the field of high laser contrast interaction science. [ABSTRACT FROM AUTHOR]

Measurements of the bidirectional plasma jets that form at the surface of a solid target during a laser-generated driven magnetic reconnection are presented. Resistivity enhancement of at least 25× the classical Spitzer value is required when applying the Sweet–Parker model of reconnection to reconcile the experimentally observed reconnection time scale. Analytic calculations show that a fast reconnection model, which includes a priori the effects of microturbulent resistivity enhancement, better reproduces the experimental observations. [ABSTRACT FROM AUTHOR]

Experiments to examine the generation of relativistic plasma waves via a high-intensity short-pulse beat-wave scheme are described in detail. The pulse stretcher of the Vulcan chirped-pulse amplification (CPA) laser system was modified to produce two frequency, 3 ps pulses focusable to intensities up to 1018 W cm-2. Short high-intensity pulses were used to avoid limitations to the plasma-wave amplitude due to the modulational instability. Two experiments were undertaken, at 3 and 10 TW, with the generation of plasma waves diagnosed by measuring the sidebands produced in the spectrum of the forward scattered beam. A resonance in the sideband signal was observed for an initial plasma density higher than expected for the given beat frequency. This resonance shift can be attributed to transverse ponderomotive expulsion of plasma electrons from the laser focal region. A monotonically increasing background was also observed, which was due to nonresonant cross-phase modulation. [ABSTRACT FROM AUTHOR]

We show that x-rays from a recently demonstrated table top source of bright, ultrafast, coherent synchrotron radiation [Kneip et al., Nat. Phys. 6, 980 (2010)] can be applied to phase contrast imaging of biological specimens. Our scheme is based on focusing a high power short pulse laser in a tenuous gas jet, setting up a plasma wakefield accelerator that accelerates and wiggles electrons analogously to a conventional synchrotron, but on the centimeter rather than tens of meter scale. We use the scheme to record absorption and phase contrast images of a tetra fish, damselfly and yellow jacket, in particular highlighting the contrast enhancement achievable with the simple propagation technique of phase contrast imaging. Coherence and ultrafast pulse duration will allow for the study of various aspects of biomechanics. [ABSTRACT FROM AUTHOR]

By tailoring the wavefront of the laser pulse used in a laser-wakefield accelerator, we show that the properties of the x-rays produced due to the electron beam’s betatron oscillations in the plasma can be controlled. By creating a wavefront with coma, we find that the critical energy of the synchrotronlike x-ray spectrum can be significantly increased. The coma does not substantially change the energy of the electron beam, but does increase its divergence and produces an energy-dependent exit angle, indicating that changes in the x-ray spectrum are due to an increase in the electron beam's oscillation amplitude within the wakefield. [ABSTRACT FROM AUTHOR]

We report on the generation of a narrow divergence (θγ < 2.5 mrad), multi-MeV (Emax ≈ 18 MeV) and ultrahigh peak brilliance (> 1.8 x 1020 photons s-1 mm-2 mrad-2 0.1% BW) γ-ray beam from the scattering of an ultrarelativistic laser-wakefield accelerated electron beam in the field of a relativistically intense laser (dimensionless amplitude a0 ≈ 2). The spectrum of the generated γ-ray beam is measured, with MeV resolution, seamlessly from 6 to 18 MeV, giving clear evidence of the onset of nonlinear relativistic Thomson scattering. To the best of our knowledge, this photon source has the highest peak brilliance in the multi-MeV regime ever reported in the literature. [ABSTRACT FROM AUTHOR]