E. T. Gumbrell

Comprehensive description of the Orion laser facility

The Orion laser facility at the atomic weapons establishment (AWE) in the UK has been operational since April 2013, fielding experiments that require both its long and short pulse capability. This paper provides a full description of the facility in terms of laser performance, target systems and diagnostics currently available. Inevitably, this is a snapshot of current capability—the available diagnostics and the laser capability are evolving continuously. The laser systems consist of ten beams, optimised around 1 ns pulse duration, which each provide a nominal 500 J at a wavelength of 351 nm. There are also two short pulse beams, which each provide 500 J in 0.5 ps at 1054 nm. There are options for frequency doubling one short pulse beam to enhance the pulse temporal contrast. More recently, further contrast enhancement, based on optical parametric amplification (OPA) in the front end with a pump pulse duration of a few ps, has been installed. An extensive suite of diagnostics are available for users, probing the optical emission, x-rays and particles produced in laser-target interactions. Optical probe diagnostics are also available. A description of the diagnostics is provided.

Intense laser interactions with sprays of submicron droplets

Picosecond laser interaction experiments conducted at peak intensities of 1.5×1017 W cm−2 using a new target medium consisting of a dense spray of 0.5 micron radius ethanol droplets indicate a strong laser–plasma coupling. The laser absorption exceeds that seen in solid targets of greater Z, and remains high over more than four orders of magnitude of intensity. Invariance in the laser absorption with wavelength and polarization is also reported. Together with x-ray spectroscopy studies, absorption measurements have been used to implement nonlocal thermodynamic equilibrium (NLTE) plasma simulations in order to isolate the important features of the droplet heating and explosion dynamics. These simulations show that the interplay of laser heating and energy transport processes is significantly different from those seen in continuous solid target interactions and that a substantial fast electron fraction must be inferred.

High resolution imaging of colliding blast waves in cluster media

Strong shocks and blast wave collisions are commonly observed features in astrophysical objects such as nebulae and supernova remnants. Numerical simulations often underpin our understanding of these complex systems, however modelling of such extreme phenomena remains challenging, particularly so for the case of radiative or colliding shocks. This highlights the need for well-characterized laboratory experiments both to guide physical insight and to provide robust data for code benchmarking. Creating a sufficiently high-energy-density gas medium for conducting scaled laboratory astrophysics experiments has historically been problematic, but the unique ability of atomic cluster gases to efficiently couple to intense pulses of laser light now enables table top scale (1 J input energy) studies to be conducted at gas densities of >1019 particles cm−3 with an initial energy density >5 × 109 J g−1. By laser heating atomic cluster gas media we can launch strong (up to Mach 55) shocks in a range of geometries, with and without radiative precursors. These systems have been probed with a range of optical and interferometric diagnostics in order to retrieve electron density profiles and blast wave trajectories. Colliding cylindrical shock systems have also been studied, however the strongly asymmetric density profiles and radial and longitudinal mass flow that result demand a more complex diagnostic technique based on tomographic phase reconstruction. We have used the 3D magnetoresistive hydrocode GORGON to model these systems and to highlight interesting features such as the formation of a Mach stem for further study.

An in-vacuo optical levitation trap for high-intensity laser interaction experiments with isolated microtargets

We report on the design, construction, and characterisation of a new class of in-vacuo optical levitation trap optimised for use in high-intensity, high-energy laser interaction experiments. The system uses a focused, vertically propagating continuous wave laser beam to capture and manipulate micro-targets by photon momentum transfer at much longer working distances than commonly used by optical tweezer systems. A high speed (10 kHz) optical imaging and signal acquisition system was implemented for tracking the levitated droplets position and dynamic behaviour under atmospheric and vacuum conditions, with ±5 μm spatial resolution. Optical trapping of 10 ± 4 μm oil droplets in vacuum was demonstrated, over timescales of >1 h at extended distances of ∼40 mm from the final focusing optic. The stability of the levitated droplet was such that it would stay in alignment with a ∼7 μm irradiating beam focal spot for up to 5 min without the need for re-adjustment. The performance of the trap was assessed in a series of high-intensity (10(17) W cm(-2)) laser experiments that measured the X-ray source size and inferred free-electron temperature of a single isolated droplet target, along with a measurement of the emitted radio-frequency pulse. These initial tests demonstrated the use of optically levitated microdroplets as a robust target platform for further high-intensity laser interaction and point source studies.

Picosecond optical probing of ultrafast energy transport in short pulse laser solid target interaction experiments

This paper reports observations of rapid energy transport effects resulting from high intensity laser heating of fused silica targets. Picosecond optical probing of these interactions provides information on the kinematics of supersonic ionization fronts driven into the targets. Studies have been conducted as a function of laser intensity, wavelength, and target angle. Additionally, targets with metallic surface layers have been investigated. Characterization of the laser absorption has enabled plasma and radiation hydrodynamics energy transport simulations to be implemented. Although consideration has been given to several energy transport mechanisms, including thermal and suprathermal electron transport, the kinematics are best explained with a radiation transport model. This is confirmed by angled and high and medium Z coated target experiments.

Laser heating of large noble gas clusters: from the resonant to the relativistic interaction regimes

Wide-ranging measurements of sub-picosecond laser interactions with large noble gas cluster targets have been conducted in order to help clarify the nature and extent of the underlying laser?plasma heating. Within the sub-relativistic vacuum irradiance range of 1016?1017?W?cm-2, we find that electron temperatures measured with continuum x-ray spectroscopy exhibit a pronounced multi-keV enhancement. Analysis indicates this behaviour to be consistent with collisional or collisionless resonant heating mechanisms. We also present the first measurements of laser-to-cluster energy deposition at relativistic vacuum irradiances, our data demonstrating absorption fractions of 90% or more. Optical probing was used to resolve the onset of a supersonic ionization front resulting from this very high absorption, and shows that despite significant pre-focus heating, the greatest plasma energy densities can be generated about the vacuum focus position. Electron energy spectra measurements confirm that laser?plasma super-heating occurs, and together with ion data establish that relativistic laser?plasma coupling in atomic clusters can take place without significant MeV particle beam production. In conjunction with optical self-emission data, the optical probing also indicates laser pre-pulse effects at peak vacuum irradiance of 5 ? 1019?W?cm-2. Laser absorption, plasma heating and energy transport data are supported throughout with analytical and numerical modelling.

ORION laser target diagnostics.

The ORION laser facility is one of the UKs premier laser facilities which became operational at AWE in 2010. Its primary mission is one of stockpile stewardship, ORION will extend the UKs experimental plasma physics capability to the high temperature, high density regime relevant to Atomic Weapons Establishments (AWE) program. The ORION laser combines ten laser beams operating in the ns regime with two sub ps short pulse chirped pulse amplification beams. This gives the UK a unique combined long pulse/short pulse laser capability which is not only available to AWE personnel but also gives access to our international partners and visiting UK academia. The ORION laser facility is equipped with a comprehensive suite of some 45 diagnostics covering optical, particle, and x-ray diagnostics all able to image the laser target interaction point. This paper focuses on a small selection of these diagnostics.

A high spatio-temporal resolution optical pyrometer at the ORION laser facility

A streaked pyrometer has been designed to measure the temperature of ≈100 μm diameter heated targets in the warm dense matter region. The diagnostic has picosecond time resolution. Spatial resolution is limited by the streak camera to 4 μm in one dimension; the imaging system has superior resolution of 1 μm. High light collection efficiency means that the diagnostic can transmit a measurable quantity of thermal emission at temperatures as low as 1 eV to the detector. This is achieved through the use of an f/1.4 objective, and a minimum number of reflecting and refracting surfaces to relay the image over 8 m with no vignetting over a 0.4 mm field of view with 12.5× magnification. All the system optics are highly corrected, to allow imaging with minimal aberrations over a broad spectral range. The detector is a highly sensitive Axis Photonique streak camera with a P820PSU streak tube. For the first time, two of these cameras have been absolutely calibrated at 1 ns and 2 ns sweep speeds under full operational conditions and over 8 spectral bands between 425 nm and 650 nm using a high-stability picosecond white light source. Over this range the cameras had a response which varied between 47 ± 8 and 14 ± 4 photons/count. The calibration of the optical imaging system makes absolute temperature measurements possible. Color temperature measurements are also possible due to the wide spectral range over which the system is calibrated; two different spectral bands can be imaged onto different parts of the photocathode of the same streak camera.

PPPS-2013: Design study into a very high temporal resolution, 2D imaging, modular UV and x-ray diagnostic: The orion time dilation imager (TIDI)

Summary form only given. A design study is carried out for the implementation of advanced target diagnostic capability on the Orion laser facility at AWE. An Orion Time-Dilation Imager (TIDI) is planned to provide either UV or x-ray, very high temporally, and spatially, resolved images from high-energy density physics experiments.Topics explored include the principles behind the novel technique of pulse-dilation (the acceleration of a photoelectron pulse with a time varying potential resulting in image temporal magnification); past use of pulse-dilation to produce the Dilation X-ray Imager (DIXI), which has been fielded and characterized on NIF; the potential obstacles, requirements, and design constraints for developing a device capable of very high temporal resolution (~10ps) and 2D imaging of laser-plasma experiments. We also assess the scope for utilizing this diagnostic in previously unobtainable regimes in high-energy density physics experiments. The potential experimental applications inform design decisions, for example; the necessity of a modular design for UV and x-ray sensitivity, ensuring the device is compatible with a ten-inch manipulator, and the benefits and trade-offs of certain critical design parameters like the magnetic field and the voltage sweep.

Energy deposition and transport dynamics in plasmas produced by intense, short pulse irradiation of atomic clusters

We have explored the dynamics of laser energy deposition and subsequent energy transport in plasmas produced by the intense ionization of gases composed of large atomic clusters. We find that the laser energy absorption can be extremely efficient, despite the low average density of the gas. We have used time-resolved interferometery to examine the spatial and temporal evolution of the plasmas created in this manner. We find that, due to the high temperatures achieved, a supersonic ionization wave is driven by hot electron thermal energy transport.