Pair plasma cushions in the hole-boring scenario

Pulses from a 10 PW laser are predicted to produce large numbers of gamma-rays and electron-positron pairs on hitting a solid target. However, a pair plasma, if it accumulates in front of the target, may partially shield it from the pulse. Using stationary, one-dimensional solutions of the two-fluid (electron-positron) and Maxwell equations, including a classical radiation reaction term, we examine this effect in the hole-boring scenario. We find the collective effects of a pair plasma "cushion" substantially reduce the reflectivity, converting the absorbed flux into high-energy gamma-rays. There is also a modest increase in the laser intensity needed to achieve threshold for a non-linear pair cascade.Comment: 17 pages, 5 figures. Accepted for publication in Plasma Physics and Controlled Fusion. Typos corrected, reference update

The effect of non-linear quantum electrodynamics on relativistic transparency and laser absorption in ultra-relativistic plasmas

With the aid of large-scale three-dimensional QED-PIC simulations, we describe a realistic experimental configuration to measure collective effects that couple strong field quantum electrodynamics to plasma kinetics. For two counter propagating lasers interacting with a foil at intensities exceeding $10^{22}$ Wcm$^{-2}$, a binary result occurs; when quantum effects are included, a foil that classically would effectively transmit the laser pulse becomes opaque. This is a dramatic change in plasma behavior, directly as a consequence of the coupling of radiation reaction and pair production to plasma dynamics

Numerical calculations of a high brilliance synchrotron source and on issues with characterizing strong radiation damping effects in non-linear Thomson/Compton backscattering experiments

A number of theoretical calculations have studied the effect of radiation reaction forces on radiation distributions in strong field counter-propagating electron beam-laser interactions, but could these effects - including quantum corrections - be observed in interactions with realistic bunches and focusing fields, as is hoped in a number of soon to be proposed experiments? We present numerical calculations of the angularly resolved radiation spectrum from an electron bunch with parameters similar to those produced in laser wakefield acceleration experiments, interacting with an intense, ultrashort laser pulse. For our parameters, the effects of radiation damping on the angular distribution and energy distribution of \emph{photons} is not easily discernible for a "realistic" moderate emittance electron beam. However, experiments using such a counter-propagating beam-laser geometry should be able to measure such effects using current laser systems through measurement of the \emph{electron beam} properties. In addition, the brilliance of this source is very high, with peak spectral brilliance exceeding $10^{29}$ photons$\,$s$^{-1}$mm$^{-2}$mrad$^{-2}(0.1$% bandwidth$)^{-1}$ with approximately 2% efficiency and with a peak energy of 10 MeV.Comment: 11 figures, 11 page

Multi-stage scheme for nonlinear Breit-Wheeler pair-production utilising ultra-intense laser-solid interactions

Multi-petawatt (PW) lasers enable intensities exceeding 1023 W cm-2, at which point quantum electrodynamics (QED) processes, such as electron-positron pair-production via the nonlinear Breit-Wheeler process, will play a significant role in laser-plasma interactions. Using 2D QED-particle-in-cell simulations, we present a two-stage scheme in which nonlinear pair-production is induced via an ultra-intense laser-solid interaction. The first stage is the generation of a γ-ray beam, through the interaction of an ultra-intense laser pulse with a thick target, whose features are found to be strongly dependent on collective plasma effects. This compact, high energy γ-ray beam (characterised by a divergence half-angle ∼10° and average photon energy ∼10 MeV) then interacts with two counter-propagating laser pulses. By varying the laser polarisation and angle of incidence, we show that in the case of two circularly polarised laser pulses propagating at an angle equal to the divergence half-angle of the γ-ray beam, the produced positron distribution is highly anisotropic compared to the case of a standard head-on collision

What factors are associated with adolescents\u27 school break time physical activity and sedentary time?

Purpose Adolescents\u27 physical activity levels during school break time are low and understanding correlates of physical activity and sedentary time in this context is important. This study investigated cross-sectional and longitudinal associations between a range of individual, behavioural, social and policy/organisational correlates and objectively measured school break time physical activity and sedentary time.Methods In 2006, 146 adolescents (50% males; mean age = 14.1&plusmn;0.6 years) completed a questionnaire and wore an accelerometer for &ge;3 school days. Time spent engaged in sedentary, light (LPA) and moderate-to-vigorous physical activity (MVPA) during school break times (recess and lunchtime) were calculated using existing cut-points. Measures were repeated in 2008 among 111 adolescents. Multilevel models examined cross-sectional and longitudinal associations.Results Bringing in equipment was cross-sectionally associated with 3.2% more MVPA during break times. Females engaged in 5.1% more sedentary time than males, whilst older adolescents engaged in less MVPA than younger adolescents. Few longitudinal associations were observed. Adolescents who brought sports equipment to school engaged in 7.2% less LPA during break times two years later compared to those who did not bring equipment to school.Conclusion These data suggest that providing equipment and reducing restrictions on bringing in sports equipment to school may promote physical activity during school recess. Strategies targeting females\u27 and older adolescents\u27, in particular, are warranted.<br /

Objectively assessed recess physical activity in girls and boys from high and low socioeconomic backgrounds

BackgroundThe school environment influences children&rsquo;s opportunities for physical activity participation. The aim of the present study was to assess objectively measured school recess physical activity in children from high and low socioeconomic backgrounds.MethodsFour hundred and seven children (6&ndash;11 years old) from 4 primary schools located in high socioeconomic status (high-SES) and low socioeconomic status (low-SES) areas participated in the study. Children&rsquo;s physical activity was measured using accelerometry during morning and afternoon recess during a 4-day school week. The percentage of time spent in light, moderate, vigorous, very high and in moderate- to very high-intensity physical activity were calculated using age-dependent cut-points. Sedentary time was defined as 100 counts per minute.ResultsBoys were significantly (p&thinsp;&lt;&thinsp;0.001) more active than girls. No difference in sedentary time between socioeconomic backgrounds was observed. The low-SES group spent significantly more time in light (p&thinsp;&lt;&thinsp;0.001) and very high (p&thinsp;&lt;&thinsp;0.05) intensity physical activity compared to the high-SES group. High-SES boys and girls spent significantly more time in moderate (p&thinsp;&lt;&thinsp;0.001 and p&thinsp;&lt;&thinsp;0.05, respectively) and vigorous (p&thinsp;&lt;&thinsp;0.001) physical activity than low-SES boys.ConclusionsDifferences were observed in recess physical activity levels according to socioeconomic background and sex. These results indicate that recess interventions should target children in low-SES schools.<br /

Potential to measure quantum effects in recent all-optical radiation reaction experiments

The construction of 10 PW class laser facilities with unprecedented intensities has emphasized the need for a thorough understanding of the radiation reaction process. We describe simulations for a recent all-optical colliding pulse experiment, where a GeV scale electron bunch produced by a laser wakefield accelerator interacted with a counter-propagating laser pulse. In the rest frame of the electron bunch, the electric field of the laser pulse is increased by several orders of magnitude, approaching the Schwinger field and leading to substantial variation from the classical Landau-Lifshitz model. Our simulations show how the final electron and photon spectra may allow us to differentiate between stochastic and semi-classical models of radiation reaction, even when there is significant shot-to-shot variation in the experimental parameters. In particular, constraints are placed on the maximum energy spread and shot-to-shot variation permissible if a stochastic model is to be proven with confidence

Proton radiography in background magnetic fields

Proton radiography has proved increasingly successful as a diagnostic for electric and magnetic fields in high-energy-density physics experiments. Most experiments use target-normal sheath acceleration sources with a wide energy range in the proton beam, since the velocity spread can help differentiate between electric and magnetic fields and provide time histories in a single shot. However, in magnetized plasma experiments with strong background fields, the broadband proton spectrum leads to velocity-spread-dependent displacement of the beam and significant blurring of the radiograph. We describe the origins of this blurring and show how it can be removed from experimental measurements, and we outline the conditions under which such deconvolutions are successful. As an example, we apply this method to a magnetized plasma experiment that used a background magnetic field of 3 T and in which the strong displacement and energy spread of the proton beam reduced the spatial resolution from tens of micrometers to a few millimeters. Application of the deconvolution procedure accurately recovers radiographs with resolutions better than 100 µm, enabling the recovery of more accurate estimates of the path-integrated magnetic field. This work extends accurate proton radiography to a class of experiments with significant background magnetic fields, particularly those experiments with an applied external magnetic field

Models of thermal conduction and non-local transport of relevance to space physics with insights from laser–plasma theory

Models of solar and space plasmas require an accurate model for thermal transport. The simplest such model is to assume that the fluid approach is valid and that local transport models can be used. These local transport coefficients are derived under the assumption that the electron mean-free path is “small” compared to the temperature scale length. When this approximation breaks down, non-local transport models or thermal flux limiters must be used to maintain a physically realistic model. This article will review the background theory of how small is “small” for the mean-free path and what options there are for including non-local transport within the fluid framework. Much of this recent work has been motivated by laser–plasma theory, where mean-free paths can be large and the Spitzer–Harm approach is never used