In the 2015 review paper 'Petawatt Class Lasers Worldwide' a comprehensive overview of the current status of highpower facilities of >200 TW was presented. This was largely based on facility specifications, with some description of their uses, for instance in fundamental ultra-high-intensity interactions, secondary source generation, and inertial confinement fusion (ICF). With the 2018 Nobel Prize in Physics being awarded to Professors Donna Strickland and Gerard Mourou for the development of the technique of chirped pulse amplification (CPA), which made these lasers possible, we celebrate by providing a comprehensive update of the current status of ultra-high-power lasers and demonstrate how the technology has developed. We are now in the era of multi-petawatt facilities coming online, with 100 PW lasers being proposed and even under construction. In addition to this there is a pull towards development of industrial and multidisciplinary applications, which demands much higher repetition rates, delivering high-average powers with higher efficiencies and the use of alternative wavelengths: mid-IR facilities. So apart from a comprehensive update of the current global status, we want to look at what technologies are to be deployed to get to these new regimes, and some of the critical issues facing their development.

/pdf/petawatt-and-exawatt-class-lasers-worldwide-10zgtun6jp.pdf

Petawatt and exawatt class lasers worldwide

An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.

Energetic Proton Generation in Ultra-Intense Laser-solid Interactions

Experimental data [F. N. Beg et al., Phys. Plasmas 4, 447 (1997)] indicate that for intense short-pulse laser-solid interactions at intensities up to 5 10 Wcm 2 the hot-electron temperature /ðI 2Þ1=3. A fully relativistic analytic model based on energy and momentum conservation laws for the laser interaction with an overdense plasma is presented here. A general formula for the hot-electron temperature is found that closely agrees with the experimental scaling over the relevant intensity range. This scaling is much lower than ponderomotive scaling. Examination of the electron forward displacement compared to the collisionless skin depth shows that electrons experience only a fraction of a laser-light period before being accelerated forward beyond the laser light’s penetration region. Inclusion of backscattered light in a modified model indicates that light absorption approaches 80%–90% for intensity >10 Wcm .

The hot electron temperature and laser light absorption in fast ignition

We report efficient and stable operation of the first multi-joule diode pumped solid state laser delivering 1 kW average power in 105 J, 10 ns pulses at 10 Hz, confirming the power scalability of multi-slab cryogenic gas-cooled amplifier technology.

Kilowatt average power 100 J-level diode pumped solid state laser

We report the design of a metal powder bed fusion system for in-situ monitoring of the build process during additive manufacture. Its open-architecture design was originally determined to enable access for x-rays to the melt pool, but it also provides access to the build area for a range of other in-situ measurement techniques. The system is sufficiently automated to enable single tracks and high-density, multiple layer components to be built. It is easily transportable to enable measurements at different measurement facilities and its modular design enables straightforward modification for the specific measurements being made. We demonstrate that the system produces components with >99% density. Hence the build conditions are representative to observe process fundamentals and to develop process control strategies.

https://pure.hw.ac.uk/ws/files/15577966/1_s2.0_S2214860416303700_main.pdf

An open-architecture metal powder bed fusion system for in-situ process measurements

Pulsed beams of energetic X-rays and neutrons from intense laser interactions with solid foils are promising for applications where bright, small emission area sources, capable of multi-modal delivery are ideal. Possible end users of laser-driven multi-modal sources are those requiring advanced non-destructive inspection techniques in industry sectors of high value commerce such as aerospace, nuclear and advanced manufacturing. We report on experimental work that demonstrates multi-modal operation of high power laser-solid interactions for neutron and X-ray beam generation. Measurements and Monte-Carlo radiation transport simulations show that neutron yield is increased by a factor ~ 2 when a 1mm copper foil is placed behind a 2mm lithium foil, compared to using a 2cm block of lithium only. We explore X-ray generation with a 10 picosecond drive pulse in order to tailor the spectral content for radiography with medium density alloy metals. The impact of using >1ps pulse duration on laser-accelerated electron beam generation and transport is discussed alongside the optimisation of subsequent Bremsstrahlung emission in thin, high atomic number target foils. X-ray spectra are deconvolved from spectrometer measurements and simulation data generated using the GEANT4 Monte-Carlo code. We also demonstrate the unique capability of laser-driven X-rays in being able to deliver single pulse high spatial resolution projection imaging of thick metallic objects. Active detector radiographic imaging of industrially relevant sample objects with a 10ps drive pulse is presented for the first time, demonstrating that features of 200µm size are resolved when projected at high magnification.

/pdf/laser-driven-x-ray-and-neutron-source-development-for-5asue65s4b.pdf

Laser-driven x-ray and neutron source development for industrial applications of plasma accelerators

Point-like sources of X-rays that are pulsed (sub nanosecond), high energy (up to several MeV) and bright are very promising for industrial and security applications where imaging through large and dense objects is required. Highly penetrating X-rays can be produced by electrons that have been accelerated by a high intensity laser pulse incident onto a thin solid target. We have used a pulse length of ~10ps to accelerate electrons to create a bright x-ray source. The bremsstrahlung temperature was measured for a laser intensity from 8.5-12×1018 W/cm2. These x-rays have sequentially been used to image high density materials using image plate and a pixelated scintillator system.

https://www.spiedigitallibrary.org/conference-proceedings-of-spie/9992/99920E/Pulsed-x-ray-imaging-of-high-density-objects-using-a/10.1117/12.2241776.pdf

Pulsed X-ray imaging of high-density objects using a ten picosecond high-intensity laser driver

The bremsstrahlung x-rays from a laser-solid interaction have been investigated for the use of radiography. The scaling of the x-rays as a function of energy has been characterized and modelled and agrees with previous measurements.

/pdf/scaling-of-x-ray-flux-from-high-intensity-laser-solid-9yxbwxv7mr.pdf

P. Oliver

Papers

Laser-driven x-ray and neutron source development for industrial applications of plasma accelerators

Pulsed X-ray imaging of high-density objects using a ten picosecond high-intensity laser driver

Scaling of X-ray flux from high-intensity laser-solid interactions as a function of energy