Magnetic Fields and Non-Local Transport in Laser Plasmas

The first Vlasov-Fokker-Planck simulations of nanosecond laser-plasma interactions – including the effects of self-consistent magnetic fields and hydrodynamic plasma expansion – will be presented. The coupling between non-locality and magnetic field advection is elucidated. For the largest (initially uniform) magnetic fields externally imposed in recent long-pulse laser gas-jet plasma experiments (12T) a significant degree of cavitation of the B-field will be shown to occur (> 40%) in under 500ps. This is due to the Nernst effect and leads to the re-emergence of non-locality even if the initial value of the magnetic field strength is sufficient to localize transport. Classical transport theory may also break down in such interactions as a result of inverse bremsstrahlung heating. Although non-locality may be suppressed by a large B-field, inverse bremsstrahlung still leads to a highly distorted distribution. Indeed the best fit for a 12T applied field (after 440ps of laser heating) is found to be a super- Gaussian distribution – f0 α e−vm – with m = 3.4. The effects of such a distribution on the transport properties under the influence of magnetic fields are elucidated in the context of laser-plasmas for the first time. In long pulse laser-plasma interactions magnetic fields generated by the thermoelectric (‘∇ne × ∇Te’) mechanism are generally considered dominant. The strength of B-fields generated by this mechanism are affected, and new generation mechanisms are expected, when non-locality is important. Non-local B-field generation is found to be dominant in the interaction of an elliptical laser spot with a nitrogen gas-jet

Controlling X-Ray Flux in Hohlraums Using Burn-through Barriers

A technique for controlling X-ray flux in hohlraums is presented. In Indirect Drive Inertial Confinement Fusion (ICF) the soft X-rays arriving at the spherical fuel capsule are required to have a specific temporal profile and high spatial uniformity in order to adequately compress and ignite the fuel. Conventionally this is achieved by modifying the external driver, the hohlraum geometry, and the sites of interaction between the two. In this study a technique is demonstrated which may have utility in a number of scenarios, both related to ICF and otherwise, in which precise control over the X-ray flux and spatial uniformity are required. X-ray burn-through barriers situated within the hohlraum are shown to enable control of the flux flowing to an X-ray driven target. Control is achieved through the design of the barrier rather than by modification of the external driver. The concept is investigated using the one-dimensional (1-D) radiation hydrodynamics code HYADES in combination with a three-dimensional (3-D) time-dependent viewfactor code

Investigation of the performance of mid-Z hohlraum wall liners for producing X-ray drive

M-band transitions (n = 4 to n = 3) in Gold are responsible for a population of X-rays with energy > 1.8 keV in indirect drive inertial fusion. These X-rays can preheat the fuel, cause the ablator-fuel interface to become unstable to Rayleigh-Taylor instabilities, and introduce radiation non-uniformity to the X-ray drive. This work investigates the performance of mid-Z lined hohlraums for producing an efficient drive spectrum absent of M-band X-rays using the two-dimensional lagrangian radiation hydrodynamics code h2d. The removal of the M-band transitions is observed in the Cu-lined hohlraum reducing the total X-ray energy above 1.8 keV to 58% that of the un-lined hohlraum. Total radiation energy in the Cu-lined hohlraum is 93% that of the energy in the pure Au hohlraum for a 1 ns pulse. However, the soft X-ray drive energy (below 1.8 keV) for the lined hohlraum is 98% that of the pure Au hohlraum

Identifying the electron–positron cascade regimes in high-intensity laser-matter interactions

Strong-field quantum electrodynamics predicts electron-seeded electron-positron pair cascades when the electric field in the rest-frame of the seed electron approaches the Sauter-Schwinger field, i.e. $\eta = E_{RF}/E_S \sim 1$. Electrons in the focus of next generation multi-PW lasers are expected to reach this threshold. We identify three distinct cascading regimes in the interaction of counter-propagating, circularly-polarised laser pulses with a thin foil by performing a comprehensive scan over the laser intensity (from $10^{23}$ -- $5\times10^{24}$\ Wcm$^{-2}$) and initial foil target density (from $10^{26}$ -- $10^{31}$\ m$^{-3}$). For low densities and intensities the number of pairs grows exponentially. If the intensity and target density are high enough the number density of created pairs reaches the relativistically-corrected critical density, the pair plasma efficiently absorbs the laser energy (through radiation reaction) and the cascade saturates. If the initial density is too high, such that the initial target is overdense, the cascade is suppressed by the skin effect. We derive a semi-analytical model which predicts that dense pair plasmas are endemic features of these interactions for intensities above $10^{24}$ Wcm$^{-2}$ provided the target's relativistic skin-depth is longer than the laser wavelength. Further, it shows that pair production is maximised in near-critical-density targets, providing a guide for near-term experiments

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

Optimal parameters for radiation reaction experiments

As new laser facilities are developed with intensities on the scale of 10^22 - 10^24 W cm^-2 , it becomes ever more important to understand the effect of strong field quantum electrodynamics processes, such as quantum radiation reaction, which will play a dominant role in laser-plasma interactions at these intensities. Recent all-optical experiments, where GeV electrons from a laser wakefield accelerator encountered a counter-propagating laser pulse with a_0 > 10, have produced evidence of radiation reaction, but have not conclusively identified quantum effects nor their most suitable theoretical description. Here we show the number of collisions and the conditions required to accomplish this, based on a simulation campaign of radiation reaction experiments under realistic conditions. We conclude that while the critical energy of the photon spectrum distinguishes classical and quantum-corrected models, a better means of distinguishing the stochastic and deterministic quantum models is the change in the electron energy spread. This is robust against shot-to-shot fluctuations and the necessary laser intensity and electron beam energies are already available. For example, we show that so long as the electron energy spread is below 25%, collisions at a_0 = 10 with electron energies of 500 MeV could differentiate between different quantum models in under 30 shots, even with shot to shot variations at the 50% level.Comment: 12 pages, 7 figure

Incorporating nonlocal parallel thermal transport in 1D ITER SOL modelling

Accurate modelling of the thermal transport in the ‘Scrape-Off-Layer’ (SOL) is of great importance for assessing the divertor exhaust power handling in future high-power tokamak devices. In conditions of low collisionality and/or steep temperature gradients that will be charateristic of such devices, classical local diffusive transport theory breaks down, and the thermal transport becomes nonlocal, depending on conditions in distant regions of the plasma. An advanced nonlocal thermal transport model is implemented into a 1D SOL code ‘SD1D’ to create ‘SD1D-nonlocal’, for the study of nonlocal transport in tokamak SOL plasmas. The code is applied to study typical ITER steady-state conditions, to assess the relevance of nonlocality for ITER operating scenarios. Results suggest that nonlocal effects will be present in the ITER SOL, with strong sensitivity in simulation outputs observed for small changes in upstream density conditions, and drastically different temperature profiles predicted using local/nonlocal transport models in some cases. Global flux limiters are shown to be inadequate to capture the spatially and temporally changing SOL conditions. Introducing impurity seeding, under conditions where detached divertor operation is achieved using the flux-limited Spitzer-H ̈arm models used in standard SOL codes, simulations using the nonlocal thermal transport model under equivalent conditions were found to not reach detachment. An analysis of the connection between SOL collisionality and nonlocality suggests that nonlocal effects will be significant for future devices such as DEMO as well. The results motivate further work using nonlocal transport models to study disruption events and low collisionality regimes for ITER, to further improve accuracy of the nonlocal models employed in comparison to kinetic codes, and to identify more appropriate boundary conditions for a nonlocal SOL model

Realising single-shot measurements of quantum radiation reaction in high-intensity lasers

Collisions between high intensity laser pulses and energetic electron beams are now used to measure the transition between the classical and quantum regimes of light-matter interactions. However, the energy spectrum of laser-wakefield-accelerated electron beams can fluctuate significantly from shot to shot, making it difficult to clearly discern quantum effects in radiation reaction, for example. Here we show how this can be accomplished in only a single laser shot. A millimeter-scale pre-collision drift allows the electron beam to expand to a size larger than the laser focal spot and develop a correlation between transverse position and angular divergence. In contrast to previous studies, this means that a measurement of the beam's energy-divergence spectrum automatically distinguishes components of the beam that hit or miss the laser focal spot and therefore do and do not experience radiation reaction

Erratum : Author correction: Relativistic doppler-boosted γ-rays in high fields (Scientific reports (2018) 8 1 (9155))

A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper