HiPER
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HiPER is an experimental laser-driven inertial confinement fusion (ICF) device currently undergoing preliminary design for possible construction in the European Union starting around 2010. HiPER is the first experiment designed specifically to study the "fast ignition" approach, which uses much smaller lasers than conventional 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, and an order of magnitude reduction in overall cost.
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[edit] Description
The basic goal of any ICF system is to quickly heat the outer layers of a "target" with a "driver laser" (or in some cases, heavy ions). The energy deposited by the laser explosively vaporizes the outer surface of the target and heats it into a plasma. The rapid expansion of the plasma "explodes" off the target, pushing the rest of the target in the opposite direction due to Newton's Third Law. This compresses the interior of the target to very high density (many times the density of lead for instance), while at the same time creating a shock wave traveling into the center. When the shock wave reaches the center of the target its energy further heats and compresses the core of the compressed fuel, raising the temperature at that spot to hundreds of millions of kelvins. The combination of heat and compression can create the conditions required for fusion reactions to occur.
In the case of HiPER, this driver laser system is fairly conventional, but seemingly undersized. 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 niobium 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 in order 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. When this process is complete, a pockels cell "switches" the light out of the cavity.
From there it is fed into a very long spatial filter to clean up the resulting pulse. The filter is effectively a telescope that focuses the beam into a spot some distance away, where a small pinhole located at the focal point cuts of any "stray" light caused by inhomogeneities in the laser beam. 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 from the beamlines is about 20 cm across.
One of the problems encountered in previous experiments, notably the Shiva laser, was that the infrared light provided by the Nd:glass lasers tends to dump a considerable amount of energy into the hot electrons around the target, as opposed to 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 couple much less strongly with the electrons, putting more power into the target. HiPER will also use a frequency multiplier on its driver, but it has not yet been decided whether to use doubling or tripling; the later puts more power into the target, but is less efficient converting the light.
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 of 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 distance from the ends of the beamlines to different points on the target chamber is different, delays are introduced on the individual paths to ensure they all reach the center of the chamber at the same time, within about a picosecond. The target, a fusion fuel pellet about 1 mm in diameter in the case of HiPER, lies at the center of the chamber.
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 20 ps 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). Here, you start with a short-pulse broad-bandwidth laser source, as opposed to the driver which uses a fairly monochromatic source. Light from this initial pulse is split into different colors 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 is now very high power, the gratings have to be large (approx 1 m) and sit in vacuum. Additionally the individual beams must be lower power overall; the compression side of the system uses 40 beamlines of about 50 kJ each to generate a total of 200 kJ, whereas the ignition side requires 24 beamlines of just under 3 kJ to generate a total of 70 kJ. The precise number and power of the beamlines are currently a matter of some amount of research.
[edit] 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 heats the interior of the compressed fuel. If the compression is symmetrical enough the temperature can rise enough to create conditions close to the Lawson criterion, leading to significant fusion energy production. If the rate is high enough, the energy created in these reactions will heat the surrounding fuel to similar temperatures, causing them to undergo fusion as well. In this case, known as "ignition", a significant portion of the fuel will undergo fusion and release large amounts of energy. Ignition is the basic goal of any fusion device.
The amount of laser energy needed to effectively compress the targets to ignition conditions has grown rapidly. In the "early days" of ICF research in the 1970s it was believed that something on the order of 100 kilojoules (Kj) would suffice, and a number of experimental lasers were built in order 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 that 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 330 MJ of electrical power to pump the driver lasers, and in the best case is expected to produce about 20 MJ of fusion power output. Without dramatic gains in output, such a device would never be a practical generator.
The fast ignition approach attempts to avoid these problems to a large degree. 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 largely unimportant. 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 200 kJ and produces densities of about 300 g/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 11 g/cm³, so this represents a considerable amount of compression, notably when one considers the target's interior contained light D-T fuel.
Ignition is started by a very-short (~10 picoseconds) ultra-high-power (~70 kJ, 1 PW) laser pulse, aimed through a hole in the plasma at the core. The light from this pulse interacts with the fuel, generating a shower of high-energy (3.5 MeV) 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 localized enough it is expected to drive the area well beyond ignition energies.
The overall efficiency of this approach is many times that of the conventional approach. In the case of NIF the laser generates about 4 MJ of infrared power to create ignition that releases about 20 MJ of energy. 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 270 kJ in total, yet generate 25 to 30 MJ, a gain of about 100. Not only does this theoretically outperform NIF by a wide margin, the smaller lasers are much less expensive to build as well. 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, in order 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. Another approach, tested successfully on the GEKKO XII laser in Japan, uses a small gold cone that protects a small area of the target from the plasma, leaving a hole in the plasma mechanically. HiPER is currently planning on using the gold cone approach, but will likely study the burning solution as well.
[edit] Current Status
HiPER has now completed a preliminary study, outlining possible approaches and arguments for its construction. If successful, which will be known in mid-2007, a three-year period of detailed design would start.
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. Key to avoiding these problems is replacing the flash lamps with more efficient pumps, typically based on laser diodes that not only generate light more efficiently, and thus run cooler, but are also tuned to produce most of their light in the correct frequency instead of the white light from the flash tubes, of which only a small amount is absorbed in the laser glass. HiPER proposes to build a demonstrator system producing 10 kJ at 1 Hz or 1 kJ at 10 Hz depending on a design choice yet to be made. The best high-repetition lasers currently operating are much smaller; MERCURY at LLNL is about 70 J, HALNA in Japan at ~20 J, and LUCIA in France at ~100 J. HiPER's demonstrator would thus be between 10 and 1000 times as powerful as any of these.
In order to make a practical commercial power generator, the high-gain of a device like HiPER would have to be combined with a high-repetition rate laser. Additional areas of research for post-HiPER devices include practical methods to carry the heat out of the target chamber for power production, protecting the device from the neutron flux generated by the fusion reactions, and the production of tritium from this flux in order to produce more fuel for the reactor.
[edit] References
- HiPER: a laser fusion facility for Europe
- E-mail with Dr. AM (Mike) Dunne, one of the HiPER investigators.
