Analysis of three-dimensional effects in laser driven thin-shell capsule implosions

Three-dimensional(3D)hydrodynamic numerical simulations of laser driven thin-shell gas-filled microballoons have been carried out using the computer code MULTI-3D[Ramis et al.,Phys.Plasmas 21,082710(2014)].The studied configuration corresponds to experiments carried at the ORION laser facility[Hopps et al.,Plasma Phys.Controlled Fusion 57,064002(2015)].The MULTI-3D code solves single-temperature hydrodynamics,electron heat transport,and 3D ray tracing with inverse bremsstrahlung absorption on unstructured Lagrangian grids.Special emphasis has been placed on the genuine 3D effects that are inaccessible to calculations using simplified 1D or 2D geometries.These include the consequences of(i)a finite number of laser beams(10 in the experimental campaign),(ii)intensity irregularities in the beam crosssectional profiles,(iii)laser beam misalignments,and(iv)power imbalance between beams.The consequences of these imperfections have been quantified by post-processing the numerical results in terms of capsule nonuniformities(synthetic emission and absorption images)and implosion efficiency(convergence ratio and neutron yield).Statistical analysis of these outcomes allows determination of the laser tolerances that guarantee a given level of target performance.

Inertial confinement fusion and prospects for power production

As our understanding of the environmental impact of fossil fuel based energy production increases, it is becoming clear that the world needs a new energy solution to meet the challenges of the future. A transformation is required in the energy market to meet the need for low carbon, sustainable, affordable generation matched with security of supply. In the short term, an increasing contribution from renewable sources may provide a solution in some locations. In the longer term,low carbon, sustainable solutions must be developed to meet base load energy demand, if the world is to avoid an ever increasing energy gap and the attendant political instabilities. Laser-driven inertial fusion energy(IFE) may offer such a solution.

Irradiation uniformity at the Laser MegaJoule facility in the context of the shock ignition scheme

The use of the Laser MegaJoule facility within the shock ignition scheme has been considered. In the first part of the study, one-dimensional hydrodynamic calculations were performed for an inertial confinement fusion capsule in the context of the shock ignition scheme providing the energy gain and an estimation of the increase of the peak power due to the reduction of the photon penetration expected during the high-intensity spike pulse. In the second part, we considered a Laser MegaJoule configuration consisting of 176 laser beams that have been grouped providing two different irradiation schemes. In this configuration the maximum available energy and power are 1.3 MJ and 440 TW. Optimization of the laser–capsule parameters that minimize the irradiation non-uniformity during the first few ns of the foot pulse has been performed. The calculations take into account the specific elliptical laser intensity profile provided at the Laser MegaJoule and the expected beam uncertainties. A significant improvement of the illumination uniformity provided by the polar direct drive technique has been demonstrated. Three-dimensional hydrodynamic calculations have been performed in order to analyse the magnitude of the azimuthal component of the irradiation that is neglected in twodimensional hydrodynamic simulations.

Pulse fidelity in ultra-high-power(petawatt class)laser systems

There are several petawatt-scale laser facilities around the world and the fidelity of the pulses to target is critical in achieving the highest focused intensities and the highest possible contrast. The United Kingdom has three such laser facilities which are currently open for access to the academic community: Orion at AWE, Aldermaston and Vulcan & Astra-Gemini at the Central Laser Facility(CLF), STFC(Science and Technology Facilities Council)Rutherford Appleton Laboratory(RAL). These facilities represent the two main classes of petawatt facilities: the mixed OPCPA/Nd:glass high-energy systems of Orion and Vulcan and the ultra-short-pulse Ti:Sapphire system of AstraGemini. Many of the techniques used to enhance and control the pulse generation and delivery to target have been pioneered on these facilities. In this paper, we present the system designs which make this possible and discuss the contrast enhancement schemes that have been implemented.

Optimal laser intensity profiles for a uniform target illumination in direct-drive inertial confinement fusion

A numerical method providing the optimal laser intensity profiles for a direct-drive inertial confinement fusion scheme has been developed. The method provides an alternative approach to phase-space optimization studies, which can prove computationally expensive. The method applies to a generic irradiation configuration characterized by an arbitrary number NBof laser beams provided that they irradiate the whole target surface, and thus goes beyond previous analyses limited to symmetric configurations. The calculated laser intensity profiles optimize the illumination of a spherical target.This paper focuses on description of the method, which uses two steps: first, the target irradiation is calculated for initial trial laser intensities, and then in a second step the optimal laser intensities are obtained by correcting the trial intensities using the calculated illumination. A limited number of example applications to direct drive on the Laser Mega Joule(LMJ) are described.

First radiative shock experiments on the SG-II laser

We report on the design and first results from experiments looking at the formation of radiative shocks on the ShenguangII(SG-II)laser at the Shanghai Institute of Optics and Fine Mechanics in China.Laser-heating of a two-layer CH/CH–Br foil drives a∼40 km/s shock inside a gas cell filled with argon at an initial pressure of 1 bar.The use of gas-cell targets with large(several millimetres)lateral and axial extent allows the shock to propagate freely without any wall interactions,and permits a large field of view to image single and colliding counter-propagating shocks with time-resolved,pointprojection X-ray backlighting(∼20µm source size,4.3 keV photon energy).Single shocks were imaged up to 100 ns after the onset of the laser drive,allowing to probe the growth of spatial nonuniformities in the shock apex.These results are compared with experiments looking at counter-propagating shocks,showing a symmetric drive that leads to a collision and stagnation from∼40 ns onward.We present a preliminary comparison with numerical simulations with the radiation hydrodynamics code ARWEN,which provides expected plasma parameters for the design of future experiments in this facility.

X-ray computed tomography of adhesive wicking into carbon foam

Laser target components consist of multicomponent porous and nonporous materials that are adhesively bonded together.In order to assess the extent and quantity of adhesive wicking into porous foam, micro X-ray computed tomography(CT)and image processing software have been utilized. Two different laser target configurations have been assessed in situ and volume rendered images of the distribution and quantities of adhesive have been determined for each.

400 TW operation of Orion at ultra-high contrast

The Orion facility at the Atomic Weapons Establishment in the United Kingdom has the capability to operate one of its two 500 J, 500 fs short-pulse petawatt beams at the second harmonic, the principal reason being to increase the temporal contrast of the pulse on target. This is achieved post-compression, using 3 mm thick type-1 potassium dihydrogen phosphate crystals. Since the beam diameter of the compressed pulse is ～600 mm, it is impractical to achieve this over the full aperture due to the unavailability of the large aperture crystals. Frequency doubling was originally achieved on Orion using a circular sub-aperture of 300 mm diameter. The reduction in aperture limited the output energy to 100 J.The second-harmonic capability has been upgraded by taking two square 300 mm × 300 mm sub-apertures from the beam and combining them at focus using a single paraboloidal mirror, thus creating a 200 J, 500 fs, i.e., 400 TW facility at the second harmonic.

Editorial review of HPLSE special issue on laboratory astrophysics

In 2018 the journal High Power Laser Science and Engineering produced a Special Issue on Laboratory Astrophysics.The scope of the special issue was to span the latest research and reviews on the following topics related to laboratory astrophysics and related phenomena.The topics invited for inclusion were:·collisionless shocks;·planetary formation dynamics and planetary interiors;·warm dense matter;·hydrodynamic and magnetohydrodynamic instabilities;·magnetic reconnection;·relativistic plasmas;·magnetic turbulence and magnetic amplification;·nuclear astrophysics;·radiative transfer and radiation hydrodynamics;·target design;·laser-based HED facilities.although this was not meant as an exhaustive list.As is usual with a special issue of this type Guest Editors were invited to lead in sourcing articles.

Laser produced electromagnetic pulses: generation, detection and mitigation

This paper provides an up-to-date review of the problems related to the generation,detection and mitigation of strong electromagnetic pulses created in the interaction of high-power,high-energy laser pulses with different types of solid targets.It includes new experimental data obtained independently at several international laboratories.The mechanisms of electromagnetic field generation are analyzed and considered as a function of the intensity and the spectral range of emissions they produce.The major emphasis is put on the GHz frequency domain,which is the most damaging for electronics and may have important applications.The physics of electromagnetic emissions in other spectral domains,in particular THz and MHz,is also discussed.The theoretical models and numerical simulations are compared with the results of experimental measurements,with special attention to the methodology of measurements and complementary diagnostics.Understanding the underlying physical processes is the basis for developing techniques to mitigate the electromagnetic threat and to harness electromagnetic emissions,which may have promising applications.

Using the ROSS optical streak camera as a tool to understand laboratory experiments of laser-driven magnetized shock waves

Supersonic flows with high Mach number are ubiquitous in astrophysics. High-powered lasers also have the ability to drive high Mach number, radiating shock waves in laboratory plasmas, and recent experiments along these lines have made it possible to recreate analogs of high Mach-number astrophysical flows under controlled conditions. Streak cameras such as the Rochester optical streak system(ROSS) are particularly helpful in diagnosing such experiments,because they acquire spatially resolved measurements of the radiating gas continuously over a large time interval,making it easy to observe how any shock waves and ablation fronts present in the system evolve with time. This paper summarizes new ROSS observations of a laboratory analog of the collision of a stellar wind with an ablating planetary atmosphere embedded within a magnetosphere. We find good agreement between the observed ROSS data and numerical models obtained with the FLASH code, but only when the effects of optical depth are properly taken into account.

Design and fabrication of gas cell targets for laboratory astrophysics experiments on the Orion high-power laser facility

This paper describes the design and fabrication of a range of ‘gas cell' microtargets produced by the Target Fabrication Group in the Central Laser Facility(CLF) for academic access experiments on the Orion laser facility at the Atomic Weapons Establishment(AWE). The experiments were carried out by an academic consortium led by Imperial College London. The underlying target methodology was an evolution of a range of targets used for experiments on radiative shocks and involved the fabrication of a precision machined cell containing a number of apertures for interaction foils or diagnostic windows. The interior of the cell was gas-filled before laser irradiation. This paper details the assembly processes, thin film requirements and micro-machining processes needed to produce the targets. Also described is the implementation of a gas-fill system to produce targets that are filled to a pressure of 0.1–1 bar. The paper discusses the challenges that are posed by such a target.

Review of HPLSE special issue on target fabrication

In 2017 the journal High Power Laser Science and Engineering produced a Special Issue on Target Fabrication.The scope of the special issue was to span the latest developments and reviews on topics related to their deployment on ultrahigh-energy/power laser facilities.The topics invited for inclusion were:·Target assembly·Novel characterization