Mark Girling

Overview of laser systems for the Orion facility at the AWE

The commissioning of the Orion laser facility at the Atomic Weapons Establishment (AWE) in the UK has recently been completed. The facility is a twelve beam Nd:glass-based system for studying high energy density physics. It consists of ten frequency-tripled beam-lines operating with nanosecond pulses, synchronized with two beam-lines with subpicosecond pulses, each capable of delivering 500 J to target. One of the short pulse beams has the option of frequency doubling, at reduced aperture, to yield up to 100 J at 527 nm in a subpicosecond pulse with high temporal contrast. An extensive array of target diagnostics is provided. This article describes the laser design and commissioning and presents key performance data of the facilitys laser systems.

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.

Ultrahigh contrast from a frequency-doubled chirped-pulse-amplification beamline

This paper describes frequency-doubled operation of a high-energy chirped-pulse-amplification beamline. Efficient type-I second-harmonic generation was achieved using a 3 mm thick 320 mm aperture KDP crystal. Shots were fired at a range of energies achieving more than 100 J in a subpicosecond, 527 nm laser pulse with a power contrast of 10(14).

Contrast enhancements to petawatt lasers using short pulse optical parametric amplifiers and frequency doubling

This paper describes the integration of a short pulse optical parametric amplifier into the chirped pulse amplification beam lines of the Orion laser facility. This enables Orion to generate petawatt laser pulses at 1054 nm with a nanosecond contrast of >10(10). By combining this with frequency-doubling post compression, we can generate 100 J, 500 fs laser pulses with a nanosecond contrast calculated to be ∼10(18).

Overview of Project Orion

Project Orion will provide a facility for performing high energy density plasma physics experiments at AWE. The laser consists of ten, nanosecond beam lines delivering a total of 5kJ with 0.1-5ns temporally shaped pulses and two short pulse beam lines, each producing 500J in 0.5ps with intensity > 10^21 W/cm^2. The performance of the Orion laser is reported as the first phase of commissioning (one short and one long pulse beam) concludes. Target shots with all beam lines will begin in 2012.

Improving the intensity of the HELEN Laser at AWE

The HELEN laser is a three-beam, large aperture Nd:glass laser, used for plasma physics studies at the Atomic Weapons Establishment in the UK. Two of the beams nominally deliver 500 J each in 1 ns at the second harmonic (527 nm). The third beam, the “backlighter”, has recently been upgraded to operate as a chirped pulse amplification system and it now routinely delivers 70 J to target in 500 fs. Optimal focal spot performance is achieved using a closed-loop adaptive optics system, which ensures good wavefront characteristics, irrespective of whether previous firing of the amplifiers has induced refractive index variations in the laser glass. The system uses a 32 element bimorph mirror with 98 mm aperture, roughly half way through the laser chain. A Shack-Hartman wavefront sensor, positioned at the output of the laser is the diagnostic used to provide feedback to the deformable mirror. Correction of the static and slowly varying aberrations on the beam has been demonstrated. The fast aberrations induced during the flashlamp discharge have been evaluated. The improved focal spot characteristics result in an intensity on target of significantly greater than 1019 Wcm-2.

Operational experience of contamination and damage of the larger aperture optics in the HELEN laser facility vacuum system at 1053 and 527 nanometres

In this paper we review contamination and damage that has occurred since the HELEN laser was converted for use as a chirped pulse amplification (CPA) system. We concentrate on the largest components in the facility that operated in a vacuum environment in the short pulse (500fs) parts of the system. This experience will be valuable for planning commissioning and operations on the successor facility, ORION that is currently being constructed. The optical components located in the vacuum sections will be described. We have an interest in laser-induced damage and contamination from the long pulse only (~1ns), short pulse only (500fs) and combined regimes. For most of the operations the CPA system has operated at a wavelength of 1053nm (1ω). Some experiments have also been conducted with 527nm (2ω) CPA light derived by the use of a KDP doubling crystal. Damage to the infra red pulse compression gratings has occurred infrequently with a minority of high-energy shots. Contamination of multilayer dielectric plane turning mirrors has arisen from target disassembly. Focussing systems using off axis parabolic mirrors have sustained contamination from debris and a dielectric protected silver reflector used for green light suffered laser induced damage. Debris shields placed between the target and the parabolas have been used on selected experiments. The shields were anti-reflection coated with single layer, sol gel silica. These shields became contaminated on the target facing sides and in the case of 2ω operation also from blow off of the damaged mirror coating that caused a two-pass transmission loss in the system. A number of characterisation methods were used to evaluate and quantify the damage and contamination. These include macroscopic photography, microscopy, reflectometry and transmission spectroscopy.

Orion facility status update

The Orion laser facility at AWE in the UK began operations at the start of 2012 to study high energy density physics. It consists of ten nanosecond beam lines and two sub-picosecond beam lines. The nanosecond beam lines each deliver 500 J per beam in 1ns at 351nm with a user-definable pulse shape between 0.1ns and 5ns. The short pulse beams each deliver 500J on target in 500fs with an intensity of greater than 1021 Wcm-2 per beam. All beam lines have been demonstrated, delivering a pulse to target as described. A summary of the design of the facility will be presented, along with its operating performance over the first year of experimental campaigns. The facility has the capability to frequency-double one of the short pulse beams, at sub aperture, to deliver a high contrast short pulse to target with up to 100J. This occurs post-compression and uses a 3mm thick, 300mm aperture KDP crystal. The design and operational performance of this work will be presented. During 2012, the laser performance requirements have been demonstrated and key diagnostics commissioned; progress of this will be presented. Target diagnostics have also been commissioned during this period. Also, there is a development program under way to improve the contrast of the short pulse (at the fundamental) and the operational efficiency of the long pulse. It is intended that, from March 2013, 15% of facility operating time will be made available to external academic users in addition to collaborative experiments with AWE scientists.

Orion: a commissioned user facility

The Orion Laser Facility at AWE in the UK consists of ten nanosecond beamlines and two sub-picosecond beamlines. The nanosecond beamlines each nominally deliver 500 J at 351 nm in a 1 ns square temporal profile, but can also deliver a user-definable temporal profile with durations between 0.1 ns and 5 ns. The sub-picosecond beamlines each nominally deliver 500 J at 1053 nm in a 500 fs pulse, with a peak irradiance of greater than 1021 W/cm2. One of the sub-picosecond beamlines can also be frequency-converted to deliver 100 J at 527 nm in a 500 fs pulse, although this is at half the aperture of the 1053 nm beam. Commissioning of all twelve beamlines has been completed, including the 527 nm sub-picosecond option. An overview of the design of the Orion beamlines will be presented, along with a summary of the commissioning and subsequent performance data. The design of Orion was underwritten by running various computer simulations of the beamlines. Work is now underway to validate these simulations against real system data, with the aim of creating predictive models of beamline performance. These predictive models will enable the user’s experimental requirements to be critically assessed ahead of time, and will ultimately be used to determine key system settings and parameters. The facility is now conducting high energy density physics experiments. A capability experiment has already been conducted that demonstrates that Orion can generate plasmas at several million Kelvin and several times solid density. From March 2013 15% of the facility operating time will be given over to external academic users in addition to collaborative experiments with AWE scientists.