We are currently focusing on the improvement of contrast pedestal (CP) in the compressed laser pulse of PW Ti:Sapphire lasers. In our previous studies, we have identified the stretcher in our laser system as the source of CP. In order to underpin the true origins of CP, we have quantitatively characterised the surface quality of large optics used in the Gemini laser stretcher, where the laser beam is spatially dispersed and the spectral phase noise is induced by the optical surface roughness. We have measured the surface profiles of 2 different gold gratings, the new and old grating, and back mirror to a very high precision (~ a fraction of nm) by using ZYGO Dynafiz, with a spatial resolution of ~50µm over a width up to ~320mm, an unprecedented combination of very high spatial resolution with a very wide field of view. The surface roughness of the large curved mirror was determined experimentally. We have developed a simple physical model to deal with the influence of the surface roughness on the contrast pedestal. Based on the measured surface profiles and by taking the actual laser beam size into account, we are able to determine the spectral phase noise induced by the optical surface roughness in the stretcher. Consequently, we are able to accurately evaluate the impact of individual large optics in the stretcher and an overall impact of the stretcher on the contrast pedestal. The calculated contrast induced by both stretches with the new and old gratings are in an excellent agreement with the experimental results measured by the Sequoia scan. For the stretcher with the old grating, the grating is the dominant impact factor on the contrast. However, for the stretcher with the new gold grating of higher quality, the impact of the curved mirror on the contrast is comparable to that of grating. This implies that the influence of curved mirror on the contrast pedestal becomes more significant when the surface quality of grating is further improved. It is clearly observed that the impact of back mirror on the contrast is more than one order of magnitude lower than that of gratings and also much lower than that of curved mirror.
In conclusion, we have demonstrated a novel method to evaluate the impact of large optics in the stretcher on the contrast pedestal by precisely quantitative characterization of optical surface quality. It is possible to accurately predict the contrast pedestal based on the stretcher configuration and precise characterisation of the optical surface in the stretcher prior to the construction of actual CPA high power laser system.
AWE’s Orion Laser facility contains ten nanosecond beamlines, each employing a series of flash-lamp pumped disk amplifiers capable of generating up to 750 J in 1 ns at 1053 nm. Discharge through the flash-lamps, however, causes unwanted disk heating which induces wavefront aberrations. This immediate effect, referred to as ‘prompt aberration’, alters the onshot wavefront compared to the wavefront of the alignment beams. In addition, a thermal load remains on the disks between shots, causing an evolution of both the alignment and on-shot aberrations over a typical day. The combination of the prompt aberrations and wavefront evolution has limited performance. After approximately six shots the alignment aberrations became so severe that further alignment was impossible. Operations would then cease to allow the disks to cool and the wavefronts to return to normal for the start of the following day. This paper reports on the development and implementation of static wavefront correctors. By mitigating the effect of the prompt aberrations, the alignment beam wavefront better matches the on-shot aberrations, and no longer limits operations, allowing more shots to be fired in a day. The wavefront analysis is discussed and shows the prompt aberration to comprise mainly of ~1 μm of defocus and astigmatism, averaged across many shots and all beamlines. Compensating static correctors are shown to reduce the effect of the prompt aberration to ~0.2 μm. The outcome indicates the possibility of firing more shots in a single day’s operation.
AWE’s Orion Laser Facility comprises ten 500J nanosecond (“long pulse”) beam lines (3ω) and two petawatt (“short pulse”) beam lines, each delivering 500J, 500fs pulses at 1054nm. One short pulse beam can operate at 2ω (at reduced aperture), producing ultrahigh contrast pulses. This paper reports on recent developments and planned future work. Static wavefront correctors have been implemented to mitigate prompt aberrations in the long pulse beams, which alter the onshot wavefront characteristics compared to the CW alignment beams. This mitigates aberration accumulation through the day, increasing the maximum number of shots in one shift. A TIM-mounted wavefront sensor/focal imager has been developed, which is better able to characterise the post-compression system aberrations, resulting in higher focal intensity. A diode-pumped, multi-joule rod amplifier has been prototyped. This is planned to replace the ageing, flashlamp-based ns-OPCPA pump laser, which constitutes a single point failure mode for our short pulse capability. Preliminary design work has commenced for a facility life-extension project, planned for ~2023. The infrared performance will be enhanced to ~1kJ per beam in 300fs, the additional bandwidth being supported by greater use of silicate glass. The two-grating, single-pass compressor systems will be replaced by four-grating compressors, retaining the extant vacuum vessels. The frequency doubling option will be retained. Since the greater near-field intensity inevitably over-drives the doublers, compressor detuning is necessary. We assess a novel, small compressor at the second harmonic. Simulations suggest that up to 500J in 150fs is possible in this configuration.
The use of solid targets irradiated in a vacuum target chamber by focussed high energy, high power laser beams to study the properties of matter at high densities, pressures and temperatures are well known. An undesirable side effect of these interactions is the generation of plumes of solid, liquid and gaseous matter which move away from the target and coat or physically damage surfaces within the target chamber. The largest aperture surfaces in these chambers are usually the large, high specification optical components used to produce the extreme conditions being studied [e.g. large aperture off axis parabolas, aspheric lenses, X ray optics and planar debris shields]. In order to study these plumes and the effects that they produce a set of dedicated experiments were performed to evaluate target by product coating distributions and particle velocities by a combined diagnostic instrument that utilised metal witness plates, polymer witness plates, fibre velocimetry and low density foam particle catchers.
When lasers are used to produce high temperature, high density plasmas from solid targets it is inevitable that the targets
are turned into a variety of products [gas, liquid, solid, sub-atomic particles and electromagnetic radiation] that are
distributed around the surfaces of the vacuum chamber used to field such experiments. These by products are produced
in plumes of debris and shrapnel that depend on the irradiation conditions, target materials and target geometry. We have
monitored the distribution of such plumes by witness plates and used microscopy, photography and spectrophotometry to
determine the physical state of material in the plumes and the spatial distribution from various target geometries. The
impact of this material on the operations of laser optics and plasma physics diagnostics is discussed.
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 10<sup>21</sup> W/cm<sup>2</sup>. 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.
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 10<sup>21</sup> Wcm<sup>-2</sup> 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.
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.