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

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

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 /

Kinetic modeling of Nernst effect in magnetized hohlraums

We 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

Extended-magnetohydrodynamics in under-dense plasmas

Extended-magnetohydrodynamics (MHD) transports magnetic flux and electron energy in high-energy-density experiments, but individual transport effects remain unobserved experimentally. Two factors are responsible in defining the transport: electron temperature and electron current. Each electron energy transport term has a direct analog in magnetic flux transport. To measure the thermally driven transport of magnetic flux and electron energy, a simple experimental configuration is explored computationally using a laser-heated pre-magnetized under-dense plasma. Changes to the laser heating profile precipitate clear diagnostic signatures from the Nernst, cross-gradient-Nernst, anisotropic conduction, and Righi-Leduc heat-flow. With a wide operating parameter range, this configuration can be used in both small and large scale facilities to benchmark MHD and kinetic transport in collisional/semi-collisional, local/non-local, and magnetized/unmagnetized regimes