C. P. Ridgers

Contemporary particle-in-cell approach to laser-plasma modelling

Particle-in-cell (PIC) methods have a long history in the study of laser-plasma interactions. Early electromagnetic codes used the Yee staggered grid for field variables combined with a leapfrog EM-field update and the Boris algorithm for particle pushing. The general properties of such schemes are well documented. Modern PIC codes tend to add to these high-order shape functions for particles, Poisson preserving field updates, collisions, ionisation, a hybrid scheme for solid density and high-field QED effects. In addition to these physics packages, the increase in computing power now allows simulations with real mass ratios, full 3D dynamics and multi-speckle interaction. This paper presents a review of the core algorithms used in current laser-plasma specific PIC codes. Also reported are estimates of self-heating rates, convergence of collisional routines and test of ionisation models which are not readily available elsewhere. Having reviewed the status of PIC algorithms we present a summary of recent applications of such codes in laser-plasma physics, concentrating on SRS, short-pulse laser-solid interactions, fast-electron transport, and QED effects.

Modelling gamma-ray photon emission and pair production in high-intensity laser-matter interactions

In high-intensity (>10^2^1 Wcm^-^2) laser-matter interactions gamma-ray photon emission by the electrons can strongly affect the electron@?s dynamics and copious numbers of electron-positron pairs can be produced by the emitted photons. We show how these processes can be included in simulations by coupling a Monte Carlo algorithm describing the emission to a particle-in-cell code. The Monte Carlo algorithm includes quantum corrections to the photon emission, which we show must be included if the pair production rate is to be correctly determined. The accuracy, convergence and energy conservation properties of the Monte Carlo algorithm are analysed in simple test problems.

Dense electron-positron plasmas and bursts of gamma-rays from laser-generated quantum electrodynamic plasmas

In simulations of a 12.5 PW laser (focussed intensity I=4×1023Wcm−2) striking a solid aluminum target, 10% of the laser energy is converted to gamma-rays. A dense electron-positron plasma is generated with a maximum density of 1026m−3, seven orders of magnitude denser than pure e− e+ plasmas generated with 1PW lasers. When the laser power is increased to 320 PW (I=1025Wcm−2), 40% of the laser energy is converted to gamma-ray photons and 10% to electron-positron pairs. In both cases, there is strong feedback between the QED emission processes and the plasma physics, the defining feature of the new “QED-plasma” regime reached in these interactions.

Quantum Radiation Reaction in Laser-Electron-Beam Collisions

It is possible using current high-intensity laser facilities to reach the quantum radiation reaction regime for energetic electrons. An experiment using a wakefield accelerator to drive GeV electrons into a counterpropagating laser pulse would demonstrate the increase in the yield of high-energy photons caused by the stochastic nature of quantum synchrotron emission: we show that a beam of 10(9) 1 GeV electrons colliding with a 30 fs laser pulse of intensity 10(22)  W cm(-2) will emit 6300 photons with energy greater than 700 MeV, 60× the number predicted by classical theory.

A review of Vlasov-Fokker-Planck numerical modeling of inertial confinement fusion plasma

The interaction of intense lasers with solid matter generates a hot plasma state that is well described by the Vlasov-Fokker-Planck equation. Accurate and efficient modeling of the physics in these scenarios is highly pertinent, because it relates to experimental campaigns to produce energy by inertial confinement fusion on facilities such as the National Ignition Facility. Calculations involving the Vlasov-Fokker-Planck equation are computationally intensive, but are crucial to proper understanding of a wide variety of physical effects and instabilities in inertial fusion plasmas. In this topical review, we will introduce the background physics related to Vlasov-Fokker-Planck simulation, and then proceed to describe results from numerical simulation of inertial fusion plasma in a pedagogical manner by discussing some key numerical algorithm developments that enabled the research to take place. A qualitative comparison of the techniques is also given.

Proton deflectometry of a magnetic reconnection geometry

Laser-driven magnetic reconnection is investigated using proton deflectometry. Two laser beams of nanosecond duration were focused in close proximity on a solid target to intensities of I∼1×1015 W cm−2. Through the well known ∇ne×∇Te mechanism, azimuthal magnetic fields are generated around each focal spot. During the expansion of the two plasmas, oppositely oriented field lines are brought together resulting in magnetic reconnection in the region between the two focal spots. The spatial scales and plasma parameters are consistent with the reconnection proceeding due to a Hall mechanism. An optimum focal spot separation for magnetic reconnection to occur is found to be ≈400±100 μm. Proton probing of the temporal evolution of the interaction shows the formation of the boundary layer between the two expanding plasma plumes and associated magnetic fields, as well as an instability later in the interaction. Such laboratory experiments provide an opportunity to investigate magnetic reconnection under unique co...

A study of fast electron energy transport in relativistically intense laser-plasma interactions with large density scalelengths

A systematic experimental and computational investigation of the effects of three well characterized density scalelengths on fast electron energy transport in ultra-intense laser-solid interactions has been performed. Experimental evidence is presented which shows that, when the density scalelength is sufficiently large, the fast electron beam entering the solid-density plasma is best described by two distinct populations: those accelerated within the coronal plasma (the fast electron pre-beam) and those accelerated near or at the critical density surface (the fast electron main-beam). The former has considerably lower divergence and higher temperature than that of the main-beam with a half-angle of ∼20°. It contains up to 30% of the total fast electron energy absorbed into the target. The number, kinetic energy, and total energy of the fast electrons in the pre-beam are increased by an increase in density scalelength. With larger density scalelengths, the fast electrons heat a smaller cross sectional are...

Experimental evidence of radiation reaction in the collision of a high-intensity laser pulse with a laser-wakefield accelerated electron beam

The dynamics of energetic particles in strong electromagnetic fields can be heavily influenced by the energy loss arising from the emission of radiation during acceleration, known as radiation reaction. When interacting with a high-energy electron beam, todays lasers are sufficiently intense to explore the transition between the classical and quantum radiation reaction regimes. We present evidence of radiation reaction in the collision of an ultrarelativistic electron beam generated by laser-wakefield acceleration (epsilon > 500 MeV) with an intense laser pulse (a(0) > 10). We measure an energy loss in the postcollision electron spectrum that is correlated with the detected signal of hard photons (gamma rays), consistent with a quantum description of radiation reaction. The generated gamma rays have the highest energies yet reported from an all-optical inverse Compton scattering scheme, with critical energy epsilon(crit) > 30 MeV.

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

With the aid of large-scale three-dimensional quantum electrodynamics (QED)-particle-in-cell simulations, we describe a potential experimental configuration to measure collective effects that couple strong field QED to plasma kinetics and develop a simple analytic model that describes the absorption due to radiation emission. For two counter propagating lasers interacting with a foil at intensities exceeding W cm−2, a near-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.

Signatures of quantum effects on radiation reaction in laser-electron-beam collisions

Two signatures of quantum effects on radiation reaction in the collision of a similar to GeV electron beam with a high intensity (>3 x 10(20) W cm(-2)) laser pulse have been considered. We show that the decrease in the average energy of the electron beam may be used to measure the Gaunt factor g for synchrotron emission. We derive an equation for the evolution of the variance in the energy of the electron beam in the quantum regime, i.e. quantum efficiency parameter eta (sic) 1. We show that the evolution of the variance may be used as a direct measure of the quantum stochasticity of the radiation reaction and determine the parameter regime where this is observable. For example, stochastic emission results in a 25 % increase in the standard deviation of the energy spectrum of a GeV electron beam, 1 fs after it collides with a laser pulse of intensity 10(21) W cm(-2). This effect should therefore be measurable using current high-intensity laser systems.