We present a novel damage detection method for use in high-repetition rate, high pulse energy laser systems. Synthetic dark field imaging involves numerical beam propagation and high-pass spatial filtering of the laser beam profile to simultaneously detect damage and identify the optic responsible within the optical chain. This technique utilises the main beam path and has the capability to monitor damage in situ, across numerous optics with a single image of the beam profile. Synthetic dark field imaging will be trialled in the new 10 Hz repetition rate, 1 PW laser driver currently being commissioned at the Extreme Photonics Applications Centre (EPAC) at the Rutherford Appleton Laboratory, UK.
The Petawatt beamline at the Vulcan laser facility is capable of delivering pulses with 500J of energy in <500fs, and has been operational as a user facility since 2003; being used to study laser matter interactions under extreme conditions. In addition to this short-pulse beamline there is a single long pulse beamline capable of 250J with durations from 0.5 to 6ns. In this paper we present our plans to add an auxiliary beamline to this facility based on Optical Parametric Chirped Pulse Amplification (OPCPA) using LBO as the non-linear crystal. This new beamline will have a dedicated laser area where the seed will be generated, stretched and amplified before being transported to the target area for compression and delivery to target. The beamline will be implemented in 2 phases the first phase will see the development of a 7J <30fs capability with the second phase increasing the delivered energy to 30J. This additional beamline will open up the potential for novel pump probe experiments when operated with the existing PW and long pulse beamlines.
In this paper we demonstrate the implementation of a modified uniformly redundant array (MURA) coded aperture in the x-ray imaging of high power laser produced plasma. We detail the process of design and manufacture of a self-supporting tantalum coded aperture with ~ 50% open area to work in the 1-25 keV x-ray regime. The advantage of using a coded aperture imaging system in this high noise environment in comparison to a standard pinhole aperture is its larger solid angle and increased signal to noise. The increased solid angle allows the aperture and detector to be placed at a further distance from the interaction point. This is beneficial as it reduces the mechanics in the close proximity of the often crowded interaction region and moves the detector which can include sensitive electronics further away from the source of EMP, hard x-rays and secondary sources generated in the interaction. Here we present initial data taken on an experiment using the Vulcan Petawatt Laser at the Central Laser Facility of a prototype x-ray imager.
Many of the new large European facilities that are in the process of coming online will be operating at high power and high repetition rates. The ability to operate at high repetition rates is important for studies including secondary source generation and inertial confinement fusion research. In these interaction conditions, with solid targets, debris mitigation for the protection of beamline and diagnostic equipment becomes of the upmost importance. These facilities have the potential to take hundreds, if not thousands, of shots every day, creating massive volumes of debris and shot materials. In recent testing of the Central Laser Facility’s High Accuracy Microtargetry Supply (HAMS) system on the mid-repetition rate Gemini facility (15 J, 40 fs, 1 shot every 20 seconds), diagnostics were deployed in order to specifically look at the debris emitted from targets designed for high repetition rate experiments. By using a high frame rate camera, it has been possible to observe and characterize some of the debris production, whilst also looking at target fratricide. Alongside these results from Gemini, we also present results of static debris measurements undertaken on the Vulcan Petawatt high energy, high power facility, where the cumulative effects of debris produced by high power laser experiments have been observed.
The increasing demand for high laser powers is placing huge demands on current laser technology. This is now reaching a limit, and to realise the existing new areas of research promised at high intensities, new cost-effective and technically feasible ways of scaling up the laser power will be required. Plasma-based laser amplifiers may represent the required breakthrough to reach powers of tens of petawatt to exawatt, because of the fundamental advantage that amplification and compression can be realised simultaneously in a plasma medium, which is also robust and resistant to damage, unlike conventional amplifying media. Raman amplification is a promising method, where a long pump pulse transfers energy to a lower frequency, short duration counter-propagating seed pulse through resonant excitation of a plasma wave that creates a transient plasma echelon that backscatters the pump into the probe. Here we present the results of an experimental campaign conducted at the Central Laser Facility. Pump pulses with energies up to 100 J have been used to amplify sub-nanojoule seed pulses to near-joule level. An unprecedented gain of eight orders of magnitude, with a gain coefficient of 180 cm−1 has been measured, which exceeds high-power solid-state amplifying media by orders of magnitude. High gain leads to strong competing amplification from noise, which reaches similar levels to the amplified seed. The observation of 640 Jsr−1 directly backscattered from noise, implies potential overall efficiencies greater than 10%.
The ability to maximise the shot rate of large scale laser facilities is dependent on the turnaround time of the laser,
diagnostics and targetry. In a move to improve the last of these, a combined target mount and carousel are being
implemented on the Vulcan Petawatt facility. The Vulcan Petawatt interaction chamber currently operates with either a
single target or with a target wheel; which has limited positions and varying degrees of subsequent target survivability.
Whenever the target holder needs to be changed the chamber vacuum has to be cycled, delaying shots by up to an hour.
The new carousel design is capable of holding 30 target assemblies at a safe distance from the interaction point, with
each target capable of being dialed in to position on demand. This allows for a whole day’s worth of shots with the
flexibility to choose any target or reference object without having to break vacuum. Here we present the design,
characterisation and implementation of this new target inserter.
The polarization state of a laser has a huge bearing on the physics of laser-plasma interactions and it is often desirable to change between linear and circular. For short pulse high power lasers large beam apertures are necessary for transportation. However, in these extreme conditions transmissive birefringent polarization optics become impractical due to their delicacy and their dispersion of the laser bandwidth which will increase the pulse length, which along with large B-integrals, which arises from the transmission of the high-power beams through optics, can be detrimental to the intensity of the laser. It is therefore necessary to consider reflective optics in order to change the polarization. Modelling has been performed at the Central Laser Facility on a design of a large aperture broadband reflective waveplate suitable for short pulse laser systems.
The requirement from large scale facilities for high repetition rate operations is rapidly approaching, and is increasingly
important for studies into high intensity secondary source generation, QED studies and the push for inertial confinement
fusion. It is envisioned that multiple PW systems at high repetition rates will be built for projects such as the European
Extreme Light Infrastructure project. Depending on the interaction physics involved, a number of differing parameters in
the interaction increase in importance, including positioning accuracy and target surface quality, and to ensure
reproducible optimum interaction conditions, presents a significant problem for accurate target positioning. With these
requirements in mind, a co-ordinated project is underway at the Central Laser Facility amongst the experimental science,
engineering and target fabrication groups, to tackle some of the challenges that we as a community face in working
towards high repetition rate operations. Here we present the latest work being undertaken at the CLF to improve
capability in key areas of this project, specifically in the areas of reliable motion control and rapid target positioning.
KEYWORDS: Modeling, Mirrors, High power lasers, Point spread functions, Adaptive optics, Wavefronts, Motion models, Tolerancing, Off axis mirrors, Near field optics
For many years parabolic mirrors have been used as the primary focusing optics of short pulse high power lasers.
Pushing the boundaries of the highest focused intensities requires not only increases in peak laser power but also
exploring the limits of focal spot size. Modelling has been performed at the Central Laser Facility to evaluate the
performance and tolerance of the alignment of a variety of off-axis parabolic mirrors and their limitations in correcting
beam aberrations. Practical considerations such as debris shields and optic mounting have also been assessed for their
effects on the focal spots.
KEYWORDS: Optical amplifiers, Picosecond phenomena, Near field optics, Mirrors, Diagnostics, Near field, Silicates, Pulsed laser operation, Glasses, High power lasers
We present details of a refurbishment and development programme that we have undertaken on the Vulcan Nd:Glass laser system to improve delivery to its two target areas. For target area petawatt in addition to replacing the gratings in the compressor chamber we have installed a new diagnostic line for improved pulse length measurement and commissioned a high energy seed system to improve contrast. In target area west we have replaced a grating on the high energy short pulse line and improved the focal spot quality. Both areas have been re-commissioned and their laser parameters measured showing that the pulse in petawatt has been measured below 500fs and focused to a spot size of 4μm the two short pulse beam lines in target area west have been measured as short as 1ps and have been focused to 5μm.
L. Volpe, D. Batani, B. Vauzour, Ph. Nicolai, J. Santos, F. Dorchies, C. Fourment, S. Hulin, C. Regan, F. Perez, S. Baton, M. Koenig, K. Lancaster, M. Galimberti, R. Heathcote, M. Tolley, Ch. Spindloe, P. Koester, L. Labate, L. Gizzi, C. Benedetti, A. Sgattoni, M. Richetta
Generation of high intensity and well collimated multi energetic proton beams from laser-matter
interaction extend the possibility to use protons as a diagnostic to image imploding target in Inertial
Confinement Fusion experiments. An experiment was done at the Rutherford Appleton Laboratory
(Vulcan Laser Petawatt laser) to study fast electron propagation in cylindrically compressed targets,
a subject of interest for fast ignition. This was performed in the framework of the experimental road
map of HiPER (the European High Power laser Energy Research facility Project). In the experiment,
protons accelerated by a ps-laser pulse were used to radiograph a 220 m diameter cylinder (20 m wall,
filled with low density foam), imploded with 200 J of green laser light in 4 symmetrically incident
beams of pulse length 1 ns. Point projection proton backlighting was used to get the compression
history and the stagnation time. Detailed comparison with 2D numerical hydro simulations has
been done using a Monte Carlo code adapted to describe multiple scattering and plasma effects
and with those from hard X-ray radiography. These analysis shows that due to the very large mass
densities reached during implosion processes, protons traveling through the target undergo a very large
number of collisions which deviate protons from their original trajectory reducing proton radiography
resolution. Here we present a simple analytical model to study the proton radiography diagnostic
performance as a function of the main experimental parameters such as proton beam energy and target
areal density. This approach leads to define two different criteria for PR resolution (called "strong"
and "weak" condition) describing different experimental conditions. Finally numerical simulations
using both hydrodynamic and Monte Carlo codes are presented to validate analytical predictions.
The evolution of the transmission of extreme ultra-violet (EUV) light from a germanium backlighter through heated thin iron targets has been measured at laser irradiances of about 8×1016 W cm-2. A rapid increase in transmission from 0 to 30% in 20 ps was observed. A two dimensional radiation hydrodynamics model was used to simulate the heating of the plasma and the transmission of EUV light as a function of time. The tamped iron targets were heated up to an average electron temperature of about 55 eV and a mass density of approximately 0.6 g cm-3. The transmission measurements are in reasonable agreement with modelling results. The experimental layout is similar to an X-ray laser experiment and therefore, for relatively low plasma temperatures, these kinds of experiments can be done in combination with X-ray laser experiments, giving transmission data for a range of wavelengths rather than a single X-ray laser wavelength.
Progress in building high-energy, short pulse laser systems with peak powers in the 100 to 1000 TW regime and applying them to plasma physics experiments has highlighted the need for debris mitigation solutions compatible with high intensity pulses [1]. Mitigation schemes ideally need to protect focusing optics for a number of laser pulses at reasonable cost without degrading beam quality. In this paper we describe preliminary experiments performed at the VULCAN laser facility to address some of these issues. The short pulse beam was passed through a thin optical shield at intensities up to 4 x 1012 W/cm2. The transmission of the shield was measured as a function of intensity along with the near and far field beam quality. Transmission losses occurred at the highest intensities used and these were related to the start of laser damage of the shield. The morphology of the damage features on the surface and in the bulk material was studied by a combination of white light interferometry as well as optical and scanning electron microscopy.
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