February 21, 2010

UK Proposes Plan for Commercial Scale Laser Nuclear Fusion System for 2030

HiPER's layout from a preliminary design study. The ignition lasers are on the left, the drivers on the right (just visible in the cutaway). The main amplifiers (in darker blue) are at the upper left end of the building, driven by the capacitor banks in the smaller buildings on either side. Spatial filters are shown in green. The large silver boxes contain diffraction gratings that compress the passage of the light into a very short pulse of about 10 ps. The ignition system is focused onto a single spot entering on the left side of the chamber, whereas the compression beams are reflected to shine into the chamber from all directions.

Timesonline UK reports Research Councils UK (RCUK), which oversees the British government’s spending on science and technology, has said it believes that many of the obstacles to commercial nuclear fusion are close to being overcome.

Research Councils UK (RCUK), which oversees the British government’s spending on science and technology, has said it believes that many of those obstacles are close to being overcome.

Dunne’s vision for Hiper is to feed a continual stream of fuel pellets into the reactor, blasting them with lasers in rapid succession to generate a constant stream of nuclear fusion explosions.

“The lasers will crush the 2mm pellet to a hundredth of its size in a billionth of a second, making it 10 times hotter than the middle of the sun,” he said.

Under such conditions the hydrogen atoms that make up the fuel are ripped apart, creating a “plasma” of electrons and hydrogen nuclei which collide and interact at high speed.

Some of these collisions result in the nuclei fusing, forming another element called helium and ejecting a neutron, a sub-atomic particle, which hurtles outwards at high speed.

When the neutrons reach the wall of the fusion chamber they pass through it but are absorbed by a blanket of lithium, heating it up. This heat is then captured and used to make steam that can in turn be used to drive a turbine.

The RCUK report, written by a group of independent fusion experts, suggests such a plant would be big enough to generate large amounts of power. “We think the first demonstration should be around 500MW, comparable to a commercial power station,” said Dunne.

[Wikipedia] The High Power laser Energy Research facility (HiPER), is an experimental laser-driven inertial confinement fusion (ICF) device undergoing preliminary design for possible construction in the European Union starting around 2010.

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.

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.

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%.

HiPER proposes to build a demonstrator diode-pump 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 Livermore 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 500 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 and a target chamber capable of extracting the power. 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.

It also follows the recent start-up of America’s National Ignition Facility, in California, which has been designed to demonstrate the principle of laser fusion. There, 192 giant lasers have been installed, collectively capable of generating 500 trillion watts — 1,000 times the power of the US national grid — for a fraction of a second.

That energy will be focused on a tiny fuel pellet of frozen hydrogen which, in theory, should be compressed and heated to 100mC — so hot that the atoms within it start to fuse.

“The world is watching and waiting to see what happens at NIF,” said the report, calling this “a seminal moment” in the development of fusion.

Mike Dunne, project co-ordinator for Hiper, agreed. “The NIF laser is performing well — they have already achieved fusion reactions but so far they are putting in much more energy than they are getting out.

“The crucial test will come this autumn when they ramp up the power. In theory they should start generating more power than they put in and if that happens it will be a clear demonstration to the world that laser fusion is ready to be harnessed. So far the signs are very good.”

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