Discovery 24 Science Engineering Technology at AWE September 2013 This issue: - History of High Power Lasers at AWE - The Orion Laser System - Orion Target System - Orion Control System - Buildings - Inertial Confinement Fusion - AWE's Outreach Orion - Shaping the future of plasma physics 2003: - December: Initial design and costed proposal bid submitted to MOD. 2005: - April: Proposal ratified by MOD giving 'authority to proceed'. Progress design for vendor and contractor for build appointed. 2006: - February: Ceremonial ground breaking. - June: First piles. - November: Ground slab complete. 2007: - May: Steel work skeleton erected. - September: Target Hall roof poured and building watertight. - December: Electrical distribution system installed and main air handling units working. 2008: - May: First Capacitor Bank Module arrived. - October: Compressor gratings delivered. - December: Target Chamber installed. 2009: - February: Short pulse laser source operating. - May: Capacitor Bank Modules installed. - July: Compressor chambers installed. 2010: - September: Long pulse and short pulse laser energy in Laser Hall. - November: First light on target. - December: One long pulse and one short pulse beam demonstrated to target. 2011: - July: All beams fired up to required power in the Laser Hall. Achieved Full Target Chamber vacuum requirements. - December: All beams fired to target, demonstrating successful synchronisation of beams to required tolerance. 2012: - March: First petawatt shot fired on target. - September: First academic proposals approved. - December: Aluminium foil compressed to twice-solid density and heated to over 5 million degrees. 2013: - Orion delivers major sub-strategic milestone supporting warhead certification. Discovery 24 Contents: - History of High Power Lasers at AWE (Page 2) - The Orion Laser System (Page 4) - Orion Target System (Page 16) - Orion Control System (Page 22) - Buildings (Page 26) - Inertial Confinement Fusion (Page 28) - AWE's Outreach (Page 34) Cover image: Orion Target Chamber Welcome by Professor Andrew Randewich, Head of Plasma Physics: "It gives me great pleasure to welcome you to the 24th edition of Discovery. This is a special edition focussed on laser plasma physics, and has been produced at a very exciting time for the field. Here at AWE the Orion laser facility has achieved operational status. A packed scientific programme on Orion for 2013 will include two open access academic campaigns. Internationally, the National Ignition Facility (NIF) in the US has produced interesting results in experiments towards sustained fusion, while the HiPER and LIFE programmes for future fusion energy production are now mature and ready to progress. In my role as Head of Plasma Physics, I am particularly proud that the Orion laser facility here at AWE has achieved its final commissioning milestone. AWE has worked in the field of laser plasma physics for over forty years and Orion represents one of the largest capital science investments ever in the UK. Several of the articles herein describe the laser, its development and its importance to AWE programmes for example in the provision of material properties data at extreme conditions of temperature and pressure. The system has now demonstrated a unique capability to heat aluminium to over 5 million degrees at more than twice solid density. These high energy density physics experiments are performed to support AWE’s warhead certification programme. The data are also pertinent to conditions in the cores of large planets and stellar interiors which are of interest to the wider academic community. Orion is attracting significant interest from our international collaborators and UK academia. From 2013, the facility will be available for use by the UK academic community for up to 15% of the available time. A number of submissions were received, and managed through the Central Laser Facility STFC Rutherford Appleton Laboratory (RAL) access panel. We are confident that this represents an excellent opportunity for AWE and the plasma physics community in carrying out novel experiments of importance to science in the UK. Although AWE does not directly contribute to fusion energy programmes, AWE does share existing capabilities in target design and manufacture to aid fusion energy programmes such as HiPER, with the aspiration to delivery clean, carbon zero, energy in the future. Orion became fully operational in April 2013 and there are exciting times ahead. I hope you enjoy the articles in this special edition of Discovery and that you feel the same sense of pride as I do in the fantastic capabilities for plasma physics here at AWE." History of High Power Lasers at AWE A nuclear reaction generates extreme conditions; temperatures reach millions of degrees Celsius, with pressures exceeding millions of atmospheres. Under such conditions, electrons are stripped from atoms, materials turn into plasmas, and we enter the realm of plasma physics. In order to generate such conditions in the laboratory, enormous energy densities are required to turn material from the solid state into the plasma state. In the mid 1970's the Ministry of Defence decided that there was a requirement to investigate the possible uses of high power laser systems for both weapons work and inertial fusion energy experiments. Some work on the use of high power CO2 lasers had already been going for some time, but following the successful development of solid state Neodynium-glass lasers such as ARGUS and SHIVA at Lawrence Livermore Laboratory (LLNL) in the US, the decision was made to start a laser capability at AWRE. A small laser system called MERLIN operating at the AWRE outstation at Foulness was relocated to the Aldermaston site in the late 1970's to help develop an in house knowledge base of how to design, operate and use lasers for plasma physics experiments. At the same time the requirements for a larger system then called AFL-1 (Aldermaston Fusion Laser -1), with around a Terrawatt of power was developed. These requirements were translated into what became the HELEN laser system. The HELEN laser, based on the technology developed for the SHIVA laser at LLNL, comprised two opposing infra red laser beams, 200 mm in diameter, each delivering around 100 J in 100 ps. The facility was opened by Her Majesty The Queen in 1979 and started delivering experimental data in support of the design physics community. Both the MERLIN and HELEN facilities operated through the 1980s each receiving a number of upgrades in both energy and operating wavelength; HELEN was converted to operate in the green region (527 nm) while MERLIN did some experiments in the ultra violet region (351 nm). In the late 1980's HELEN received a further upgrade adding a third smaller backlighter beam. HELEN operated in this configuration for just over 15 years fielding some unique, world leading experiments and developing and implementing advanced laser and diagnostic technologies. At the same time, the requirements for operating the MERLIN facility were reduced and it was closed in the early 1990's. In late 1990's advances in laser design in the U.S provided a route for HELEN to be reconfigured to provide a more robust, capable and compact facility. This rebuild to a multipass configuration was possible due to the provision of additional amplifier assemblies from LLNL. In 2004 HELEN's backlighter beam was converted to provide a short pulse chirped pulse amplification capability. This capability was based on a similar design implemented at the VULCAN laser facility at Rutherford Appleton laboratory in Oxford. HELEN operated very successfully for more than 5 years in this configuration and provided a unique capability to deliver world class plasma physics experimental data. In 2009 HELEN was closed down to enable staff to move to Orion. Orion succeeds HELEN and allows AWE to generate high energy density plasmas. A major area for study is the state of different materials and how they interact under great heat and pressure. The state of a particular material can be described by its equation-of-state (EOS); this relates the density of a material to the temperature and the pressure it is experiencing. Another phenomenon to be explored on Orion will be how heat and energy flows under such conditions; this provides an understanding of opacity in the plasma state. This edition of Discovery provides an overview of the Orion laser system, facilities and possible areas of research that could be explored using Orion. The Orion Laser System: Design and Performance This article describes the design and performance of the ten long pulse and two short pulse beam lines of the Orion facility. Each long pulse beam has a nominal operating specification of 500 J in a 1 ns square pulse at a wavelength of 351 nm (in the near-ultraviolet region of the spectrum). This pulse can be focussed onto a target in a spot less than 100 μm across, resulting in a focussed intensity per beam of the order 10^16 Wcm^-2. The short pulse beams each deliver 500 J at 1053 nm in a pulse of duration 500 fs. An adaptive optics system enables focussing close to the diffraction limit. The resulting focussed intensity is of the order 10^21 Wcm^-2 and the peak power is 1 petawatt in each beam line. This makes Orion a world-class facility for the study of high energy density physics. In this article an overview of the design of the various beam lines is presented, along with some of the system performance data that underpin the system commissioning. Facility Layout and Overview: Figure 1 shows the layout of the Orion lasers. The long pulse beams consist of two stacks of five beam lines, supported on each side of a large space frame structure. Each stack is seeded with a pre-amplifier module (PAM), which amplifies pulses up to the 200 mJ level and spatially shapes the beam. The PAMs themselves are seeded with pulses of energy roughly 50 pJ, using a pulse generation system housed beneath the laser hall, known as Optical Pulse Generation 1 (OPG1). Figure 1 Layout Highlights: - Laser Hall - Long and Short Pulse Generation and Pulsed Power System (8 MJ) on ground floor - 10 Long Pulse Beams - 2 Short Pulse Beams - Long Pulse Preamplifier modules - 1.5 m thick concrete shield wall - Compressors (13 m grating separation) - Target Area The long pulse beam line architecture comprises four 200 mm aperture disk amplifiers, which are four-passed using angular multiplexing. The output at the fundamental wavelength is about 750 J, with a beam diameter of 300 mm, which is then frequency tripled in the target hall, yielding 500 J per beam. The short pulse beam line components are supported on a second space frame, one beam line per side. These are seeded with pre-amplified pulses from Optical Pulse Generator 2 (OPG2), a room below the laser hall which produces roughly 50 mJ pulses. These are amplified in a rod amplifier system and multiple disk amplifiers at increasing aperture to about 700 J in total. The pulses emerging from OPG2 are chirped, namely the wavelength of the pulse sweeps from longer values at the leading edge of the pulse to shorter at the trailing edge. The amplified output of the beam line is expanded to 600 mm diameter and then directed into a compressor system, which lays the constituent wavelengths back on top of one another in time, thus recovering a pulse with as short a duration as possible, commensurate with its bandwidth, see Box 1. At this point the pulse has power in excess of 1 petawatt, it has the characteristics of a flat sheet of light, 600 mm across and only 1/7th of a millimeter thick. Its intensity is too great to pass through any dielectric medium. Therefore, since the target interaction takes place under vacuum, the compressors, beam transport and target chamber must form a single, large vacuum system. Finally, the beam is focussed to target using a paraboloidal mirror, since it is too intense to pass through a lens. Long Pulse Beam Line Design: A schematic of the long pulse beam line is shown in Figure 2. The original source of light is a commercial, distributed feedback (DFB) fibre laser delivering up to 1 W of CW power at 1053.0 nm. This beam is chopped into a 1 kHz chain of 400 ns pulses using an in-fibre acousto-optic modulator and then split between two outputs. The two chains of pulses are sent to the two, independent, integrated-optic (IO) modulators. These modulators sculpt the 0.1-5 ns shaped pulses and are each driven by an arbitrary waveform generator (AWG), plus a square pulse generator to sharpen the rise/fall time to less than 100 ps. When the two-way split and the inherent system losses are accounted for, the output of the system is roughly 50 mW e.g. 50 pJ for a 1 ns square pulse. This is more than adequate to ensure good signal to noise ratio from the PAM. An overview of OPG1 is shown in Figure 3. The shaped pulses are sent to the PAMs along two long transport fibres. The system is entirely fibre coupled which minimises maintenance/realignment and makes it inherently safer than an equivalent free space system. The PAMs house a regenerative amplifier, based on a flashlamp pumped Nd:YLF rod, a spatial shaping stage, a 2D smoothing by spectral dispersion (2D-SSD) system based on bulk phase modulators, a four-passed 32 mm aperture rod amplifier and a diagnostics package. These systems are mounted on both sides of a vertically mounted optical table with dimensions 5.5 m by 1.5 m. The regenerative amplifier utilizes roughly 50 passes through a self-imaged ring cavity to amplify the OPG1 pulses to a few millijoules. The beam shaping consists of over filling a serrated aperture. The residual dome from the Gaussian beam is used to compensate the slight radial gain non-uniformity in the 32 mm aperture rod amplifier. The SSD system uses phase modulators at 2.45 GHz, and 10.4 GHz to impart spectral sidebands on the beam, which are dispersed in orthogonal directions using diffraction gratings. Used in conjunction with phase plates, just prior to final focussing, this system can smooth out the laser speckle seen in the focal plane. The rod amplifier system engineers four passes of the rod using polarization switching with a quarter waveplate. A permanent magnet Faraday rotator (FR7) switches the beam in and out of the rod amplifier system. The rod amplifier system is angularly multiplexed so that the on-axis path can be blocked at the focal plane of its relay telescopes, so as to inhibit laser oscillation. Although each PAM is capable of delivering multiple Joules, in the current system design they are only required to yield up to 200 mJ. The output from each PAM is split either five or ten ways (there is an option to seed either all ten beam lines with a single front-end, or have independent seeding of each stack of five beams lines). Each long pulse beam line requires about 20 mJ of seed energy to achieve its required 750 J output at 1053 nm. The PAM output is split using polarizer -waveplate combinations and are image relayed into the long, transport spatial filter (TSF). The TSF is a telescope with a square array of four pinholes (roughly 5 mm in diameter) at its focal plane. The beams are injected near this position. Beyond focus, the beam expands in the TSF to 170 mm before injection into the four 200 mm aperture disk amplifiers. Figure 4 is an overview of the TSF and the disk amplifiers. Each amplifier houses three glass slabs, doped with neodymium, and delivers a small-signal gain of up to 2.4. In practice the gain used is about 2.1, so as to control the potential for parasitic lasing in the cavity. The beam is angularly multiplexed such that different passes of the amplifiers can be spatially separated near the pinhole plane of the TSF. After two passes of the four amplifiers, the beam re-enters the TSF and is directed, near the pinhole plane, into a reverser system, where it is re-collimated at 40 mm diameter and returned to the TSF. At this point, the energy is about 4 J. The reverser also houses Pockels cells to isolate the gain from passes 1 and 2 and passes 3 and 4. Two more passes of the disk amplifiers brings the energy to about 750 J. This time it passes straight through the TSF, being expanded to 300 mm, and is directed into the target hall. Frequency tripling is effected by a pair of crystals. The first is a type I doubler with thickness of 14 mm to achieve the optimal 2:1 energy ratio of doubled and fundamental light at the nominal operating point. The second is a type II mixing crystal, of 12 mm thickness. The result is a third harmonic beam with a polarization the same as the input fundamental beam. Five dichroic transport mirrors each discriminate between residual wavelengths and the 3ω beam with an efficiency of ~100:1, giving a very pure 3ω beam at the target. Their geometry is such that the beams are orientated onto target in two opposing cones of five beams. The beam lines themselves do not oppose each other through the target chamber; the two cones of beams are mirror images of each other about the target plane i.e. a plane at the apex of each cone, orthogonal to their mutual axis. The axis of each individual beam is also rotated by the geometry of the transport mirrors such that the polarization of each beam is in the 'p' state, relative to the target plane, to better couple laser radiation into a typical target plasma. Final focusing of the beams is achieved using the long pulse final optics assembly (LPFOA) which consists of an optional kinoform phase plate, a vacuum window, a 1.2 m focal length lens and a debris shield. Short Pulse Beam Line Design: Figure 5 is an overview of the short pulse beam line. The short pulses are generated by a commercial Titanium-Sapphire oscillator delivering pulses with a bandwidth of about 12 nm around 1054 nm. Immediately these pulses are split to seed each of the two short pulse beam lines. A common oscillator is used to realise the best timing jitter between the two short pulse beams. On each side of the split, the pulses are sent to two independent Offner triplet stretcher systems. These are four-passed systems with an effective grating separation of 3.25 m per pass, with a 1480 mm^-1 diffraction grating at 47.9° angle of incidence. This compensates the compressor with a 13 m grating separation and the same grating specification. Two independent stretchers are used so that each short pulse beam can operate with a different pulse duration, specified to be between 0.5 and 20 ps. The resultant chirp is 300 ps nm^-1. The stretcher imparts a hard spectral clip at 18 nm, so the stretched pulse duration is 6 ns; there is significant power (about 20% of peak) at these pulse extremities. The first stage of amplification also takes place within the OPG2 subsystem. An optical parametric chirped pulse amplifier (OPCPA) is used to conserve pulse bandwidth. OPCPA is a nonlinear optical technique in which a flow of energy can be effected from an intense 'pump' pulse to a much weaker 'signal' pulse. A third pulse, known as the 'idler', is generated as a by-product and discarded. This technique is used here principally because a large gain can be realized across a wide range of wavelengths. The bandwidth is maintained during the first gain stages to a much greater extent than is possible using laser amplification. The OPCPA system for both beam lines is pumped by a single, commercially sourced, pump laser. This is an Nd:YAG system operating at the second harmonic, which was specified to have very flat pulses both spatially and temporally. The pump pulse is of 6 ns duration and so amplifies the full bandwidth from the stretcher. The amplification occurs in three stages, the final two of which exhibit strong pump depletion. The more intense regions of the pulse are driven to the point of back-conversion to achieve optimal output energy stability. In this regime, the signal pulse mimics the spatial and temporal properties of the pump pulse; it becomes a top-hat, square pulse. Each OPCPA stage can produce about 150 mJ, which is much greater than that required to meet the short pulse baseline performance, which results in greater operational flexibility. The OPCPA output is expanded and then apodized at 16 mm diameter such that only the cleanest, central region of the beam is injected into the main short pulse laser chain. This begins with a four-passed mixed glass, 32 mm aperture rod amplifier subsystem. This uses phosphate and silicate glass, which have offset gain spectra, in order to maintain as much bandwidth as possible before injection into the disk amplifiers. Other than the use of two rods, the design is very similar to the rod amplifiers on the long pulse PAMs. This system produces about 2 J for baseline performance with 9 nm bandwidth. The beam is then expanded to 86 mm diameter before it is injected into a 100 mm aperture Faraday rotator. This has two functions; it allows a double pass of the first disk amplifier and secondly it acts as an optical isolator for back reflections. In the double passed disk amplifier the pulse experiences a small signal gain of about 6 per pass. This is somewhat lower than the gain for a monochromatic pulse. The beam is then expanded to 140 mm diameter, where it passes through a 150 mm aperture disk amplifier, with a gain of about 4. The next component is another Faraday rotator, with the sole function of isolation against back reflections. The beam is then expanded to 180 mm and single passes three 200 mm amplifiers of the same design as those on the long pulse beams. The beam line is shown schematically in Figure 5. The result is pulses with about 700 J and 5 nm bandwidth. These are expanded to 600 mm diameter and image-relayed into the compressor vacuum vessels, which contain single pass compressor systems, using the same grating configuration as the stretcher, and a 13 m grating separation. The gratings are gold-coated and have 940 mm aperture in the horizontal plane. There is a slight spectral truncation in the near-field on the second grating. Long vacuum transport systems send the beam from the compressor vessels to the target chamber, where they are focused with f/3 paraboloidal mirrors. Long Pulse Beam Line Performance: The performance of all the beam lines are closely monitored using laser diagnostics at multiple locations along the laser chain. The principal locations for diagnosis are directly after the TSF output and after the frequency conversion crystals. The former location monitors the spatial characteristics of the beam as it emerges from the TSF (the near-field) and also how well the beam can be focused (the far-field). Closely related to this is the wavefront error on the beam, which is measured using a Shack-Hartmann wavefront sensor. The temporal profile of the pulse is measured using a streak camera at this point. After frequency conversion, the near- and far-field beam profiles are measured again, as is the temporal profile, and the presence of any pre-pulses in advance of the main pulse is monitored using a pair of photodiodes. In addition to these key beam parameters, the pulse energy through the laser chain must be well understood, if laser induced damage is not to be exhibited. The plot in Figure 6 shows the energy produced by one of the long pulse beam lines as a function of the input pulse energy for a given capacitor bank voltage. The effect of gain saturation is visible in the lower gradient at higher energies. The curve is a simple gain saturation model fitted to the data. Short Pulse Laser Performance: There are more properties of the laser pulse that are pertinent to laser performance and so an extended suite of laser diagnostics is required. In addition to the usual near- and far-field images, the spectrum of the pulse is important, as is the level of any pre-pulses present on the beam, post-compression. Since intensity is the principal parameter to be maximised, the requirements on the focal spot of the pulse are much tighter, requiring an active, adaptive optics system to be fielded and optimised. Finally, the duration of the pulse is much shorter than can be measured using conventional electronics and so optical techniques must be deployed to ascertain the pulse shape indirectly. Immediately after the amplifiers a diagnostics station measures the beam profiles, the wavefront, the pulse energy and the pulse spectrum. This determines that the pulse has suitable characteristics for injection into the compressor system. The large compressor gratings are very expensive and difficult to replace. They are also the most vulnerable components to laser induced damage. Great care must be taken to ensure the suitability of the pulse prior to compression. After the compressor, leakage through one of the turning mirrors is used to diagnose the pulse characteristics. As the pulse is short, the intensity can build up in the diagnostics package to levels such that nonlinear optical effects can perturb the beam quality and this must be managed carefully. The near-field profile is dispersed horizontally; this is a consequence of a pulse with significant bandwidth propagating through the compressor system. The far-field is monitored to ensure that the compression process has not disrupted the wavefront of the beam. The pulse spectrum is also measured, as is the pulse duration. The pulse is of order 500 fs long, this is much too short to be measured directly using conventional techniques. A device known as an autocorrelator was designed and built at AWE to make an estimate of the pulse duration. It splits the pulse in two halves, which are then made to shear across each other spatially in a nonlinear optical medium. This technique realises a temporal-to-spatial mapping so that the pulse duration can be inferred by imaging the output of the pulse's self-interaction on a camera. What is measured is not the pulse shape itself, but rather a quantity known as the intensity autocorrelation of the pulse. A key parameter of interest to facility users is the pre-pulse contrast ratio of the short pulses. This is the ratio of the peak power of the pulse to the power at some defined time before the peak. Photodiodes with suitable attenuation were used to measure the pre-pulse contrast a few nanoseconds before the peak of the pulse. This was measured to be between 10^7 and 10^8 for ~3 ns prior to the peak. Although this meets the original specification, for some experiments it is desirable to improve upon this pre-pulse level. A development programme is under way to improve the contrast from OPG2, but the facility is able to offer enhanced contrast using another technique. There is an option to frequency-double the output from one of the short pulse beams, post compression. Since the lower intensity pre-pulse will not double with high efficiency, the contrast of the doubled pulse is greatly enhanced. An additional vacuum vessel is provided after one of the compressors to redirect the beam through a doubling crystal and on to the target chamber. The crystal thickness that is optimal for doubling such an intense pulse is around 3 mm. The aperture of such a thin crystal is limited to 300 mm, so that the SP beam must be apodized to this diameter, thus losing about 70% of the incident energy. In addition a doubling efficiency of about 65% is achieved, leading to a maximum doubled energy of about 100 J. This capability enables novel opportunities for experiments that require high contrast or shorter wavelengths. Summary: The combination of long pulse beam lines with two independent short pulse systems gives the Orion facility great flexibility in the range of experiments that can be performed. During 2011 and 2012, the laser was commissioned successfully to yield the advertised performance requirements and is now ready to operate as a fully capable user facility. BOX 1: Chirped Pulse Amplification Ultra-short laser pulses are not difficult to produce at low energies. However amplification to any significant energy is difficult as the intensity is high. This drives non-linear effects in optical materials, disrupting the spatio-temporal profile of the pulse, leading to laser-induced damage. This could be avoided if the laser pulse could be lengthened prior to amplification and recompressed afterwards. This concept forms the basis of chirped pulse amplification (CPA). A generic CPA beam line consists of pulse generation, stretching, amplification and recompression. Focussing to a small spot size produces a high intensity pulse. Any temporal waveform may be expressed as a superposition of single-frequency wave trains of the right amplitudes and phases. Adjusting the amplitudes and phases allows manipulation of the waveform. Adjustment of temporal profile can be seen in the stretcher and compressor of a CPA beam line as these devices apply distortions of spectral phase. It is important that these are controlled with high accuracy. Residual distortion must be minimised to allow good recompression of the pulse. At the compressor pulse energies are high and a large aperture is needed to keep energy density below the threshold for laser-induced damage to the optical components. Compressors typically consist of one or more grating pairs, each arranged with the gratings face to face. The beam size is much smaller and more manageable at the stretcher. The need to produce an identical but conjugate grating geometry demands the use of an optical imaging system. A virtual grating pair is produced with the gratings back to back. Functional block diagram: Pulse generator (1 nJ, 120 fs) -> Stretcher (400 pJ, 4.8 ns) -> Amplifier (700 J, 1.6 ns) -> Compressor (500 J, 500 fs) -> Focussing (500 J, 500 fs) -> Target. BOX 2: Orion Pulse Power System The Orion Pulsed Power System supplies 8 MJ of energy to drive the Orion Laser amplifiers and Faraday Rotators. The main pulsed power system consists of fourteen Capacitor Bank Modules (CBMs) which can be subdivided into ten banks driving 200 mm amplifiers on ten long pulse laser beams, two banks driving 200 mm amplifiers on two short pulse beam lines, one bank driving 100/150 mm amplifiers on the short pulse beam lines and a bank driving all the Faraday Rotators. A CBM typically consists of two halves with each half-bank having eight 150 μF main pulse charge storage capacitors. The half-bank driving Laser Amplifiers have a 25 μF pre-pulse storage capacitor installed to allow the flashlamps to be pre-ionised. A single 25 kV power supply per half-bank, charges the main capacitors with both the front and rear pre-pulse capacitors charged by means of the front unit. The main capacitors in each half are switched by a single air pressurised fast closing spark gap switch (FCS) with the duty of switching both front and rear pre-pulse capacitors undertaken by a FCS mounted in the front half of the CBM. Each FCS is triggered by a 120 kV output Marx generator. The system is timed to switch the pre-pulse capacitors in the region of 300 μs prior to the main pulse with the main pulse then having a total duration 490 μs. The two short-pulse beam lines have additional amplification in the form of 100 mm and 150 mm amplifiers which are driven by a single CBM operated as two independent halves. The CBM driving the Faraday Rotators consists of a half-bank and is configured to drive 100 mm and 150 mm Faraday Rotators. The charging and firing of a half bank is managed by a built in Command and Control Module (CCM) which also monitors the banks output. All of the CBM internal control signals are communicated by compressed air or fibre optics to mitigate the effects of electromagnetic pulses and interference. The CCM communicates with an external Test Controller via fibre optic cables which controls and monitors the capacitor banks' operation. The Pulsed Power system is controlled in conjunction with the laser by means of commands sent to the Test Controller from the Main Laser Control Room. Orion Target System The Orion target system comprises of six major sub-systems that must all work together: - Short Pulse Final Optics Assemblies (SPFOAs) – The final focussing parabolic mirrors in the short pulse beam lines - Long Pulse Final Optics Assemblies (LPFOAs) – The last few optical components in the long pulse beam lines - Target Positioning System (TPS) – The manipulator used to position and orient the target into the required position within the target chamber - Target Inserter (TI) – The device used to insert the experimental target into the target chamber - Ten Inch Manipulator (TIM) – Inserts and positions removable diagnostics into the target chamber - Target Viewer and Illuminator System (TVS) – Provides remote camera views of the target Manipulators: The LPFOAs & SPFOAs in the Orion beamlines both use a class of mechanism called parallel manipulators. These manipulators allow accurate positioning of the optic assemblies. In parallel manipulators each of the actuators is mounted to the same base, unlike a serial mechanism where each actuator is mounted on the output end of the previous actuator. One type of parallel manipulator is known as a hexapod (Figure 2b), which consists of a fixed base platform onto which 6 independent extendable legs are attached via ball joints. The other ends of all 6 legs are mounted on the payload platform also via ball joints. This triangulated mechanism produces an extremely stiff structure. In order to manipulate the output platform, each of the legs must be moved to a new calculated length. Any error in an individual leg plays a smaller part in the resulting final position, resulting in a total error in position less than the error in any one single leg. Short Pulse Final Optics Assembly (SPFOA): The SPFOA is the last optical component in the short pulse laser chain. The mechanics of the SPFOA is a 6 degree of freedom hexapod. The mechanism consists of a fixed base platform onto which 6 independent extendable legs are attached via ball joints in a triangulated arrangement. The other ends of all 6 legs are mounted in a cradle also via ball joints. The parabolic mirror is then mounted into and supported by the cradle. Statistics: - Optic Diameter: 720 mm - Optic Weight: 250 kg - Mechanism Type: 6 axis parallel manipulator - Mechanism Weight: 50 kg - Positioning Range: 35 mm sphere at Target Chamber Center - Angular Range: 0.15° around twice the focal length - Positioning Resolution: < 0.1 μm Long Pulse Final Optics Assembly (LPFOA): The LPFOA comprises of the last few optical components in the laser chain (chamber window, focus lens, debris shield and phase plate) mounted in a parallel positioning mechanism which enables the incoming laser beam to be focused down onto the target. A retraction mechanism is provided to enable easy access to the debris shield for operational expediency. The LPFOA utilises a special parallel manipulator mechanism with 3 actuators connected to the output frame via fixed length bars oriented parallel to each other. This parallel joint constrains rotational degrees of freedom so that the lens can only translate linearly. All 3 sliders can move simultaneously to retract the lens to larger distances outside the chamber wall to change debris shields. Statistics: - Optic Diameter: 350 mm - Mechanism Type: 3 axis parallel manipulator - Positioning Range: 35 mm sphere at Target Chamber centre - Positioning Resolution: < 0.1 μm - Retraction Travel: 1250 mm Target Positioning System (TPS): The TPS positions and orients the target into the required position within the target chamber using a traditional serial stacked slide system with piezo actuators for X, Y, and rotary stages followed by a counterbalanced piezo Z stage. Statistics: - Mechanism Type: Serial Stacked Slides - Linear Travel X/Y: 100 mm - Linear Travel Z: 115 mm - Angular Travel: 320° - Resolution: 0.01° and 0.3 μm Target Inserter (TI): Transfers target from outside the chamber and places it on the target positioner while under vacuum using a telescopic boom mechanism. Statistics: - Mechanism Type: Telescopic boom - Total Travel: 3450 mm - Retracted Size: 2500 mm - Vacuum: Independently pumped from the Target Chamber Ten Inch Manipulator (TIM): Provides a load-lock and insertion mechanism for diagnostic payloads on Orion (six TIMs located around the chamber). Uses a linear slide system for insertion/removal and bipod legs at the rear to change orientation, pivoting around a gimbal just inside the chamber wall. Statistics: - Mechanism Type: Linear Slide & Bipod Legs - Total Travel (In & Out): 2370 mm - Total Travel (Up/Down, Left/Right): 30 mm - Positional Resolution (Z): < 0.5 μm - Positional Resolution (Up/Down, Left/Right): < 0.2 μm - Isolation: Independently pumped from Target Chamber Target Viewer and Illuminator System (TVS): Provides two orthogonal views of the target with back illumination (selectable red and blue light). Features wide view (30 mm circle) and narrow view (5 mm circle). Orion Control System The Orion laser facility has been designed to be capable of firing in excess of six shots onto shot specific targets in one day. The control system provides remote control and configuration for thousands of components including multi-axis mirrors, diagnostic manipulators, hexapods, oscilloscopes, cameras, and diagnostics. Design: Sub-systems include: - Integrated Control System (ICS) - Target Data Acquisition System (TDAS) - Laser Alignment and Diagnostics - Target Alignment and Diagnostics - Pulsed Power (delivering 10 MJ in 600 μs) - Vacuum System - Timing System (sub 10 ps jitter) - Safety Interlock System (SIS) - Building Management System (BMS) Prototyping facility 'Osiris' allowed in-house engineers to develop and test systems early. IT Infrastructure: - 10 gigabit fibre-optic backbone routed through 20 km of multi-mode fibre-optic cable to ~150 network switches in 2-metre high enclosures. - Three Active Directory Domains purposefully segregated: Integrated Control System (ICS) domain, Unclassified Target Data Acquisition System (UTDAS) domain, and Classified Target Data Acquisition System (CTDAS) domain. - 30 terabytes of database storage across three servers. Control System User Interfaces: - SCADA (Supervisory Control And Data Acquisition) application running on operator workstations in the Control Room and classified/unclassified operations rooms. - Over 1200 separate controllable motion axes across 35 km of cabling in containment. Safety Interlock System (SIS): Given hazards of high voltages, intense laser light, and ionising radiation, the SIS is an independent, Safety Integrity Level 2 (SIL 2) rated system. It uses a complex custom logic solver continually monitoring airlocks, emergency stops, and laser key switches, feeding a marshalling panel of safety relays. A custom key exchange system ensures laser controlled areas (LCAs) are systematically searched and vacated prior to shots. Buildings The lasers and process equipment within Orion require a clean, stable and tightly controlled environment. Key Infrastructure Highlights: - Vibration Isolation: Outer shell of Orion is physically separated from the central slabs supporting the lasers; concrete piles are over 30 m deep. - Environmental Stability: Ventilation maintains temperature in main laser areas steady to better than ± 0.2 °C and humidity to better than ± 5%. Clean rooms utilize 339 fan filter units with HEPA filters, reducing dust levels to 1/1000th of typical office air. - Building Management System (BMS): Over 550 sensors running >350 separate programmes monitor and adjust operating parameters. - Radiation & EMP Protection: Target hall features 1.5 m thick concrete walls with comprehensive Electromagnetic Pulse (EMP) shielding. - Sustainability: Awarded an 'Excellent' rating by BREEAM (Building Research Establishment Environmental Assessment Method). Inertial Confinement Fusion and Prospects for Power Production The primary use of the Orion laser facility is for underwriting the safety and performance of the UK's nuclear deterrent, while making 15% of available time accessible for academic programmes, including Inertial Fusion Energy (IFE) research. Physics of Fusion: Extreme laser heating of deuterium and tritium (D-T) yields an alpha particle (3.5 MeV) and a neutron (14.1 MeV), releasing ~17.6 MeV per reaction: D + T -> He4 (3.5 MeV) + n (14.1 MeV) Hotspot ignition condition requires trapping the alpha particle within the core: areal density ρ·r ~ 0.3 g cm^-2. The required laser energy scales inversely with the square of the density: E = [ (4/3) π Ti Cv (ρ·r)^3 ] / ρ^2 = k / ρ^2 Compressing the fuel to ~500 g cm^-3 reduces the required laser energy into attainable regimes. Challenges Ahead: 1. Laser Driver: Transition to Diode Pumped Solid State Lasers (DPSSL) for high repetition rate (>10 Hz) and wall-plug efficiency (>10%). Prototype DPSSLs at RAL operate at 10% efficiency (100 J / 10 Hz prototype under construction). 2. Energy Capture and Tritium Breeding Blanket: Using lithium to breed tritium: - 7% Li6 + n (slow) -> He4 + T3 + 4.8 MeV - 93% Li7 + n (fast) -> He4 + T3 + n (slow) - 2.466 MeV Target Tritium Breeding Ratio (TBR) > 1.1. 3. Fuel Capsule Production: Producing ~1 million fuel capsules per day at low cost. BOX 1: The HiPER Project: HiPER (High Power Laser Energy Research) is a pan-European ESFRI project targeting laser fusion energy on the 2050 timescale, coordinated by STFC at RAL, collaborating with US LIFE and European LMJ facilities. AWE contributes expertise in numerical physics modeling, target design, and fabrication. AWE's Outreach: Major Events and Collaborative Activities UK PONI: - Second annual UK Project On Nuclear Issues conference held on 10 May 2012 aboard HMS President, opened by Lord Hutton of Furness (Chairman of Nuclear Industries Association), marking the 50th anniversary of the Cuban Missile Crisis. Nitrocellulose Symposium: - 5th International Nitrocellulose Symposium co-sponsored by AWE with Nitrochemie and Armasuisse in Spiez, Switzerland (17-18 April 2012), drawing >90 delegates from 18 nations. Insensitive Munitions and Energetic Materials Technology Symposium: - May 2012 in Las Vegas, USA, with 250 global attendees; AWE presented papers on novel high-energy insensitive plasticisers and testing regimes. Materials Science Exhibition: - Held 30-31 May 2012 at Aldermaston showcasing materials R&D, forensics, treaty verification, and aging diagnostics. Plutonium Futures – The Science: - Held at University of Cambridge on 15-20 July 2012, co-chaired by AWE, CEA (France), JRC-ITU (Germany), and NNL (UK), gathering ~230 scientists. Enhanced Detection of Special Nuclear Material: - Held 1-2 November 2012 at Lancaster House, London, co-sponsored by GICNT, FCO, AWE, DTRA, and MOD; opened by Alistair Burt MP. Defence Science and Technology Showcase: - 12 December 2012 at MOD Main Building, London. Heat Flow Calorimetry Symposium on Energetic Materials: - 8th symposium held 29-31 October 2012 at Wokefield Park, Reading. Fusion Energy Presentation: - Joint presentation by Prof Peter Roberts OBE (AWE Chief Scientist) and Prof Steve Cowley (Culham) on 4 April 2012 to IOP Plasma Group at St Hugh's College, Oxford. High Value Manufacturing Catapult: - Visit by Will Barton (TSB) in May 2012 to discuss UK manufacturing strategy. Discovery 24 Editorial Details Editor: - Doctor Graeme Nicholson Editorial Board: - David Chambers - Doctor David Geeson - Doctor Norman Godfrey - Doctor Katherine Grant - Eamonn Harding - Rashad Hussain - John Roberson - Nana Noi Graphic Design and Illustration & Photography: - AWE Media Group Contributors: - Jason Carlisle, Simon Daykin, Doctor Chris Edwards, Philip Elvin, Doctor Tim Goldsack, Doctor Nick Hopps, Tim Layland, Professor Andrew Randewich, Paula Rosen, Mathew Smith, Mark Williamson Contact: - Corporate Communications Office, Building F161.2, AWE Aldermaston, Reading, Berkshire, RG7 4PR - Email: [email protected] - Website: www.awe.co.uk Publication Notice: - Ref: MG/32039 - © British Crown Owned Copyright 2013/AWE - AWE plc is operated by a joint venture of Jacobs Engineering, Lockheed Martin, and Serco.