The High Power laser Energy Research facility (HiPER), is a proposed experimental laser-driven inertial confinement fusion (ICF) device undergoing preliminary design for possible construction in the European Union. As of 2019[update], the effort appears to be inactive.
HiPER was designed to study the "fast ignition" approach to generating nuclear fusion, which uses much smaller lasers than conventional ICF designs, yet produces fusion power outputs of about the same magnitude. This offers a total "fusion gain" that is much higher than devices like the National Ignition Facility (NIF), and a reduction in construction costs of about ten times. This opened a window for a small machine to be rapidly built that would reach ignition before NIF. HiPER and the Japanese FIREX designs intended to explore this approach.
However, research into the fast ignition approach on smaller machines like the Omega laser in the US demonstrated a number of problems with the concept. Another alternative approach, shock ignition, began to take over future development starting around 2012.[1] HiPER and FIREX both appear to have seen no additional development since that time.
HiPER should not be confused with an earlier ICF device in Japan known as "HIPER", which has not been operational for some time.
Inertial confinement fusion (ICF) devices use "drivers" to rapidly heat the outer layers of a "target" to compress it. The target is a small spherical pellet containing a few milligrams of fusion fuel, typically a mix of deuterium and tritium, or "D-T". The heat of the laser burns the surface of the pellet into a plasma, which explodes off the surface. The remaining portion of the target is driven inward due to Newton's Third Law, collapsing into a small point of very high density. The rapid blowoff also creates a shock wave that travels toward the center of the compressed fuel. When it reaches the center of the fuel and meets the shock from the other side of the target, the energy in the center further heats and compresses the tiny volume around it. If the temperature and density of that small spot can be raised high enough, fusion reactions will occur. This approach is now known as "hot-spot ignition" to distinguish it from new approaches.[2]
The fusion reactions release high-energy particles, some of which (primarily alpha particles) collide with the high density fuel around it and slow down. This heats the surrounding fuel, and can potentially cause that fuel to undergo fusion as well. Given the right overall conditions of the compressed fuel – high enough density and temperature – this heating process can result in a chain reaction, burning outward from the center. This is a condition known as "ignition", which can lead to a significant portion of the fuel in the target undergoing fusion, and the release of significant amounts of energy.[3]
To date most ICF experiments have used lasers to heat the targets. Calculations show that the energy must be delivered quickly to compress the core before it disassembles, as well as creating a suitable shock wave. The energy must also be focused extremely evenly across the target's outer surface to collapse the fuel into a symmetric core. Although other drivers have been suggested, notably heavy ions driven in particle accelerators, lasers are currently the only devices with the right combination of features.[4][5]
Description
In the case of HiPER, the driver laser system is similar to existing systems like NIF, but considerably smaller and less powerful.
The driver consists of a number of "beamlines" containing Nd:glass laser amplifiers at one end of the building. Just prior to firing, the glass is "pumped" to a high-energy state with a series of xenon flash tubes, causing a population inversion of the neodymium (Nd) atoms in the glass. This readies them for amplification via stimulated emission when a small amount of laser light, generated externally in a fibre optic, is fed into the beamlines. The glass is not particularly effective at transferring power into the beam, so to get as much power as possible back out, the beam is reflected through the glass four times in a mirrored cavity, each time gaining more power.[6] When this process is complete, a Pockels cell switches the light out of the cavity.[7] One problem for the HiPER project is that Nd:glass is no longer being produced commercially, so a number of options need to be studied to ensure supply of the estimated 1,300 disks.[7]
From there, the laser light is fed into a very long spatial filter to clean up the resulting pulse. The filter is essentially a telescope that focuses the beam into a spot some distance away, where a small pinhole located at the focal point cuts off any "stray" light caused by inhomogeneities in the laser beam. The beam then widens out until a second lens returns it to a straight beam again. It is the use of spatial filters that lead to the long beamlines seen in ICF laser devices. In the case of HiPER, the filters take up about 50% of the overall length. The beam width at exit of the driver system is about 40cm × 40cm.[8]
One of the problems encountered in previous experiments, notably the Shiva laser, was that the infrared light provided by the Nd:glass lasers (at ~1054nm in vaco) couples strongly with the electrons around the target, losing a considerable amount of energy that would otherwise heat the target itself. This is typically addressed through the use of an optical frequency multiplier, which can double or triple the frequency of the light, into the green or ultraviolet, respectively. These higher frequencies interact less strongly with the electrons, putting more power into the target. HiPER will use frequency tripling on the drivers.[9]
When the amplification process is complete the laser light enters the experimental chamber, lying at one end of the building. Here it is reflected off a series of deformable mirrors that help correct remaining imperfections in the wavefront, and then feeds them into the target chamber from all angles. Since the overall distances from the ends of the beamlines to different points on the target chamber are different, delays are introduced on the individual paths to ensure they all reach the center of the chamber at the same time, within about 10picoseconds (ps). The target, a fusion fuel pellet about 1mm in diameter in the case of HiPER, lies at the center of the chamber.[10]
HiPER differs from most ICF devices in that it also includes a second set of lasers for directly heating the compressed fuel. The heating pulse needs to be very short, about 10 to 20ps long, but this is too short a time for the amplifiers to work well. To solve this problem HiPER uses a technique known as chirped pulse amplification (CPA). CPA starts with a short pulse from a wide-bandwidth (multi-frequency) laser source, as opposed to the driver which uses a monochromatic (single-frequency) source. Light from this initial pulse is split into different colours using a pair of diffraction gratings and optical delays. This "stretches" the pulse into a chain several nanoseconds long. The pulse is then sent into the amplifiers as normal. When it exits the beamlines it is recombined in a similar set of gratings to produce a single very short pulse, but because the pulse now has very high power, the gratings have to be large (approx 1m) and sit in a vacuum. Additionally the individual beams must be lower in power overall; the compression side of the system uses 40 beamlines of about 5kJ each to generate a total of 200kJ, whereas the ignition side requires 24 beamlines of just under 3kJ to generate a total of 70kJ. The precise number and power of the beamlines are currently a subject of research.[10] Frequency multiplication will also be used on the heaters, but it has not yet been decided whether to use doubling or tripling; the latter puts more power into the target, but is less efficient converting the light. As of 2007, the baseline design is based on doubling into the green.[11]
Fast Ignition and HiPER
In traditional ICF devices the driver laser is used to compress the target to very high densities. The shock wave created by this process further heats the compressed fuel when it collides in the center of the sphere. If the compression is symmetrical enough the increase in temperature can create conditions close to the Lawson criterion and lead to ignition.
The amount of laser energy needed to effectively compress the targets to ignition conditions has grown rapidly from early estimates. In the "early days" of ICF research in the 1970s it was believed that as little as 1kilojoules (kJ) would suffice,[12][13] and a number of experimental lasers were built to reach these power levels. When they did, a series of problems, typically related to the homogeneity of the collapse, turned out to seriously disrupt the implosion symmetry and lead to much cooler core temperatures than originally expected. Through the 1980s the estimated energy required to reach ignition grew into the megajoule range, which appeared to make ICF impractical for fusion energy production. For instance, the National Ignition Facility (NIF) uses about 420MJ of electrical power to pump the driver lasers, and in the best case is expected to produce about 20MJ of fusion power output.[2] Without dramatic gains in output, such a device would never be a practical energy source.
The fast ignition approach attempts to avoid these problems. Instead of using the shock wave to create the conditions needed for fusion above the ignition range, this approach directly heats the fuel. This is far more efficient than the shock wave, which becomes less important. In HiPER, the compression provided by the driver is "good", but not nearly that created by larger devices like NIF; HiPER's driver is about 200kJ and produces densities of about 300g/cm3. That's about one-third that of NIF, and about the same as generated by the earlier NOVA laser of the 1980s. For comparison, lead is about 11g/cm3, so this still represents a considerable amount of compression, notably when one considers the target's interior contained light D-T fuel around 0.1g/cm3.[10]
Ignition is started by a very-short (~10picoseconds) ultra-high-power (~70kJ, 4PW) laser pulse, aimed through a hole in the plasma at the core. The light from this pulse interacts with the cool surrounding fuel, generating a shower of high-energy (3.5MeV) relativistic electrons that are driven into the fuel. The electrons heat a spot on one side of the dense core, and if this heating is localised enough it is expected to drive the area well beyond ignition energies.[10]
The overall efficiency of this approach is many times that of the conventional approach. In the case of NIF the laser generates about 4MJ of infrared power to create ignition that releases about 20MJ of energy.[2] This corresponds to a "fusion gain" —the ratio of input laser power to output fusion power— of about 5. If one uses the baseline assumptions for the current HiPER design, the two lasers (driver and heater) produce about 270kJ in total, yet generate 25 to 30MJ, a gain of about 100.[10] Considering a variety of losses, the actual gain is predicted to be around 72.[10] Not only does this outperform NIF by a wide margin, the smaller lasers are much less expensive to build. In terms of power-for-cost, HiPER is expected to be about an order of magnitude less expensive than conventional devices like NIF.
Compression is already a fairly well-understood problem, and HiPER is primarily interested in exploring the precise physics of the rapid heating process. It is not clear how quickly the electrons stop in the fuel load; while this is known for matter under normal pressures, it's not for the ultra-dense conditions of the compressed fuel. To work efficiently, the electrons should stop in as short a distance as possible, to release their energy into a small spot and thus raise the temperature (energy per unit volume) as high as possible.
How to get the laser light onto that spot is also a matter for further research. One approach uses a short pulse from another laser to heat the plasma outside the dense "core", essentially burning a hole through it and exposing the dense fuel inside. This approach will be tested on the OMEGA-EP system in the US. Another approach, tested successfully on the GEKKO XII laser in Japan, uses a small gold cone that cuts through a small area of the target shell; on heating no plasma is created in this area, leaving a hole that can be aimed into by shining the laser into the inner surface of the cone. HiPER is currently planning on using the gold cone approach, but will likely study the burning solution as well.[10]
Related research
In 2005 HiPER completed a preliminary study outlining possible approaches and arguments for its construction. The report received positive reviews from the EC in July 2007, and moved onto a preparatory design phase in early 2008 with detailed designs for construction beginning in 2011 or 2012.[citation needed]
In parallel, the HiPER project also proposes to build smaller laser systems with higher repetition rates. The high-powered flash lamps used to pump the laser amplifier glass causes it to deform, and it cannot be fired again until it cools off, which takes as long as a day. Additionally only a very small amount of the flash of white light generated by the tubes is of the right frequency to be absorbed by the Nd:glass and thus lead to amplification, in general only about 1 to 1.5% of the energy fed into the tubes ends up in the laser beam.[14]
Key to avoiding these problems is replacing the flash lamps with more efficient pumps, typically based on laser diodes. These are far more efficient at generating light from electricity, and thus run much cooler. More importantly, the light they do generate is fairly monochromatic and can be tuned to frequencies that can be easily absorbed. This means that much less power needs to be used to produce any particular amount of laser light, further reducing the overall amount of heat being generated. The improvement in efficiency can be dramatic; existing experimental devices operate at about 10% overall efficiency, and it is believed "near term" devices will improve this as high as 20%.[15]
Current status
Further research in the fast ignition approach cast serious doubt on its future. By 2013, the US National Academy of Sciences concluded that it was no longer a worthwhile research direction, stating "At this time, fast ignition appears to be a less promising approach for IFE than other ignition concepts."[16]
Inertial confinement fusion (ICF) is a fusion energy process that initiates nuclear fusion reactions by compressing and heating targets filled with fuel. The targets are small pellets, typically containing deuterium (2H) and tritium (3H).
This timeline of nuclear fusion is an incomplete chronological summary of significant events in the study and use of nuclear fusion.
The National Ignition Facility (NIF) is a laser-based inertial confinement fusion (ICF) research device, located at Lawrence Livermore National Laboratory in Livermore, California, United States. NIF's mission is to achieve fusion ignition with high energy gain. It achieved the first instance of scientific breakeven controlled fusion in an experiment on December 5, 2022, with an energy gain factor of 1.5. It supports nuclear weapon maintenance and design by studying the behavior of matter under the conditions found within nuclear explosions.
The Laboratory for Laser Energetics (LLE) is a scientific research facility which is part of the University of Rochester's south campus, located in Brighton, New York. The lab was established in 1970 with operations jointly funded by the United States Department of Energy, the University of Rochester and the New York State government. The Laser Lab was commissioned to investigate high-energy physics involving the interaction of extremely intense laser radiation with matter. Scientific experiments at the facility emphasize inertial confinement, direct drive, laser-induced fusion, fundamental plasma physics and astrophysics using the Omega Laser Facility. In June 1995, OMEGA became the world's highest-energy ultraviolet laser. The lab shares its building with the Center for Optoelectronics and Imaging and the Center for Optics Manufacturing. The Robert L. Sproull Center for Ultra High Intensity Laser Research was opened in 2005 and houses the OMEGA EP laser, which was completed in May 2008.
The Z Pulsed Power Facility, informally known as the Z machine or Z, is the largest high frequency electromagnetic wave generator in the world and is designed to test materials in conditions of extreme temperature and pressure. It was originally called the PBFA-II and was created in 1985. Since its refurbishment in October 1996 it has been used primarily as an inertial confinement fusion (ICF) research facility. Operated by Sandia National Laboratories in Albuquerque, New Mexico, it gathers data to aid in computer modeling of nuclear weapons and eventual fusion pulsed power plants.
A fusion energy gain factor, usually expressed with the symbol Q, is the ratio of fusion power produced in a nuclear fusion reactor to the power required to maintain the plasma in steady state. The condition of Q = 1, when the power being released by the fusion reactions is equal to the required heating power, is referred to as breakeven, or in some sources, scientific breakeven.
The Shiva laser was a powerful 20-beam infrared neodymium glass laser built at Lawrence Livermore National Laboratory in 1977 for the study of inertial confinement fusion (ICF) and long-scale-length laser-plasma interactions. Presumably, the device was named after the multi-armed form of the Hindu god Shiva, due to the laser's multi-beamed structure. Shiva was instrumental in demonstrating a particular problem in compressing targets with lasers, leading to a major new device being constructed to address these problems, the Nova laser.
Nova was a high-power laser built at the Lawrence Livermore National Laboratory (LLNL) in California, United States, in 1984 which conducted advanced inertial confinement fusion (ICF) experiments until its dismantling in 1999. Nova was the first ICF experiment built with the intention of reaching "ignition", a chain reaction of nuclear fusion that releases a large amount of energy. Although Nova failed in this goal, the data it generated clearly defined the problem as being mostly a result of Rayleigh–Taylor instability, leading to the design of the National Ignition Facility, Nova's successor. Nova also generated considerable amounts of data on high-density matter physics, regardless of the lack of ignition, which is useful both in fusion power and nuclear weapons research.
An optical frequency multiplier is a nonlinear optical device in which photons interacting with a nonlinear material are effectively "combined" to form new photons with greater energy, and thus higher frequency. Two types of devices are currently common: frequency doublers, often based on lithium niobate (LN), lithium tantalate (LT), potassium titanyl phosphate (KTP) or lithium triborate (LBO), and frequency triplers typically made of potassium dihydrogen phosphate (KDP). Both are widely used in optical experiments that use lasers as a light source.
Inertial Fusion Energy is a proposed approach to building a nuclear fusion power plant based on performing inertial confinement fusion at industrial scale. This approach to fusion power is still in a research phase. ICF first developed shortly after the development of the laser in 1960, but was a classified US research program during its earliest years. In 1972, John Nuckolls wrote a paper predicting that compressing a target could create conditions where fusion reactions are chained together, a process known as fusion ignition or a burning plasma. On August 8, 2021, the NIF at Livermore National Laboratory became the first ICF facility in the world to demonstrate this. This breakthrough drove the US Department of Energy to create an Inertial Fusion Energy program in 2022 with a budget of 3 million dollars in its first year.
Laser Mégajoule (LMJ) is a large laser-based inertial confinement fusion (ICF) research device near Bordeaux, France, built by the French nuclear science directorate, Commissariat à l'Énergie Atomique (CEA).
Argus was a two-beam high power infrared neodymium doped silica glass laser with a 20 cm (7.9 in) output aperture built at Lawrence Livermore National Laboratory in 1976 for the study of inertial confinement fusion. Argus advanced the study of laser-target interaction and paved the way for the construction of its successor, the 20 beam Shiva laser.
The Gekko XII Laser (激光XII号レーザー) is a high-power, 12-beam, neodymium-doped glass laser at the Osaka University's Institute for Laser Engineering (大阪大学レーザーエネルギー学研究センター) completed in 1983, which is used for high energy density physics and inertial confinement fusion research. The name refers to the twelve individual beamlines used to amplify the laser energy.
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The Vulcan laser is an infrared, 8-beam, petawatt neodymium glass laser at the Rutherford Appleton Laboratory's Central Laser Facility in Oxfordshire, United Kingdom. It was the facility's first operational laser.
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LIFE, short for Laser Inertial Fusion Energy, was a fusion energy effort run at Lawrence Livermore National Laboratory between 2008 and 2013. LIFE aimed to develop the technologies necessary to convert the laser-driven inertial confinement fusion concept being developed in the National Ignition Facility (NIF) into a practical commercial power plant, a concept known generally as inertial fusion energy (IFE). LIFE used the same basic concepts as NIF, but aimed to lower costs using mass-produced fuel elements, simplified maintenance, and diode lasers with higher electrical efficiency.
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