Hi PER - Fast Ignition and HiPER

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, leading to significant fusion energy production. If the resulting fusion rate is high enough, the energy released 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 from early estimates. In the "early days" of ICF research in the 1970s it was believed that as little as 1 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 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 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 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 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 still represents a considerable amount of compression, notably when one considers the target's interior contained light D-T fuel around 0.1 g/cm3.

Ignition is started by a very-short (~10 picoseconds) ultra-high-power (~70 kJ, 4 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. Considering a variety of losses, the actual gain is predicted to be around 72. Not only does this 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. 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.