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 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. 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.
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 40 cm × 40 cm.
One of the problems encountered in previous experiments, notably the Shiva laser, was that the infrared light provided by the Nd:glass lasers (at ~1054 nm 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.
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 10 ps. 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). 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 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 now has very high power, the gratings have to be large (approx 1 m) 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 5 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 subject of research. 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.
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