Proton acceleration enhanced by a plasma jet in expanding foils undergoing relativistic transparency

Ion acceleration driven by the interaction of an ultraintense (2x10^20 Wcm^-2) laser pulse with an ultrathin (40nm) 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 (PIC) 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 prole of the laser pulse

Factors confounding the assessment of reflection: a critical review

BACKGROUND: Reflection on experience is an increasingly critical part of professional development and lifelong learning. There is, however, continuing uncertainty about how best to put principle into practice, particularly as regards assessment. This article explores those uncertainties in order to find practical ways of assessing reflection. DISCUSSION: We critically review four problems: 1. Inconsistent definitions of reflection; 2. Lack of standards to determine (in)adequate reflection; 3. Factors that complicate assessment; 4. Internal and external contextual factors affecting the assessment of reflection. SUMMARY: To address the problem of inconsistency, we identified processes that were common to a number of widely quoted theories and synthesised a model, which yielded six indicators that could be used in assessment instruments. We arrived at the conclusion that, until further progress has been made in defining standards, assessment must depend on developing and communicating local consensus between stakeholders (students, practitioners, teachers, supervisors, curriculum developers) about what is expected in exercises and formal tests. Major factors that complicate assessment are the subjective nature of reflection's content and the dependency on descriptions by persons being assessed about their reflection process, without any objective means of verification. To counter these validity threats, we suggest that assessment should focus on generic process skills rather than the subjective content of reflection and where possible to consider objective information about the triggering situation to verify described reflections. Finally, internal and external contextual factors such as motivation, instruction, character of assessment (formative or summative) and the ability of individual learning environments to stimulate reflection should be considered

Simulations of radiation pressure ion acceleration with the VEGA Petawatt laser

The Spanish Pulsed Laser Centre (CLPU) is a new high-power laser facility for users. Its main system,VEGA, is a CPA Ti:Sapphire laser which, in itsfinal phase, will be able to reach Petawatt peak powers inpulses of 30 fs with a pulse contrast of 1:1010at 1 ps. The extremely low level of pre-pulse intensitymakes this system ideally suited for studying the laser interaction with ultrathin targets. We have usedthe particle-in-cell (PIC) code OSIRIS to carry out 2D simulations of the acceleration of ions from ultrathinsolid targets under the unique conditions provided by VEGA, with laser intensities up to 1022Wcm 2impinging normally on 20–60 nm thick overdense plasmas, with different polarizations and pre-plasmascale lengths. We show how signatures of the radiation pressure-dominated regime, such as layercompression and bunch formation, are only present with circular polarization. By passively shaping thedensity gradient of the plasma, we demonstrate an enhancement in peak energy up to tens of MeV andmonoenergetic features. On the contrary linear polarization at the same intensity level causes the targetto blow up, resulting in much lower energies and broader spectra. One limiting factor of RadiationPressure Acceleration is the development of Rayleigh–Taylor like instabilities at the interface of theplasma and photonfluid. This results in the formation of bubbles in the spatial profile of laser-acceleratedproton beams. These structures were previously evidenced both experimentally and theoretically. Wehave performed 2D simulations to characterize this bubble-like structure and report on the dependencyon laser and target parametersWe thank the OSIRIS consortium (UCLA/IST) for the use ofOSIRIS and IST for the use of the their clusters for the production ofthe simulation data. We also thank Prof Paul McKenna and hisgroup of the University of Strathclyde for the fruitful discussions.We acknowledge the EU-funded LA3NET consortium under grantnumber GA-ITN-2011-289191. E.C.J. acknowledges the supportfrom MINECO (FIS2013-44174-P) and from Junta de Castilla y Leon(Project no. SA116U13