INERTIAL FUSION: Europeans HiPER about laser fusion

A European program called HiPER (high-power laser-energy-research facility) aims to develop laser-fusion technology to a point where it is commercially attractive for powerplant use.

Aug 1st, 2007
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A European program called HiPER (high-power laser-energy-research facility) aims to develop laser-fusion technology to a point where it is commercially attractive for powerplant use. In the program, researchers will develop the “fast ignition” fusion process, which requires a smaller laser than conventional laser-fusion approaches and which should significantly ease tolerances on both the laser and the fuel-containing capsules.

First posited as early as the 1960s, laser compression of capsules to create the conditions for fusion was first demonstrated in the 1970s. Through inertial fusion, the two heavier forms of hydrogen-deuterium and tritium-produce helium, a neutron, and a net release of energy. This process mimics the mechanism that powers the Sun and other stars. It is intrinsically clean and resource-efficient, the fuel used derives from seawater, and it has attracted much international research effort over the past 30 years. The latest generation of lasers-the National Ignition Facility (NIF) at Lawrence Livermore National laboratory (LLNL; Livermore, CA) and Laser MegaJoule (LMJ; Bordeaux, France)-promise a self-sustaining fusion reaction, one which releases more fusion energy from the capsule than is delivered by the laser system. Current plans predict this transformational event in the period from 2010 to 2012.

Professor Mike Dunne, director of the HiPER project at the U.K.’s RAL (Rutherford Appleton Laboratory), likens this standard process to a diesel engine; fuel is steadily compressed to a point at which it ignites. By contrast, he likens fast ignition to a gas engine. In this scenario, the fuel does not need to be compressed as much, which releases the requirement to have ultraprecise laser profiles and near-perfectly shaped fuel pellets, both of which are likely to hinder routes to a commercial reactor.

Attractive physics

First, a laser is used to partially implode the fusion capsule (this requires on the order of 0.2 to 0.3 MJ in a few nanoseconds, compared to 2 MJ for NIF), explains Dunne (see figure). Then, a high-power short-pulse laser (70 to 100 kJ in 10 ps) is used as a “spark plug” to ignite the fuel. The physics behind fast ignition is less certain than for the approach being adopted by NIF, but it is highly attractive from an energy-generation perspective, says Dunne. Active research to underpin fast ignition is under way across the world-including at LLNL and at the Laboratory for Laser Energetics (Rochester, New York), in Japan, and in Europe.

In the first step of fast-ignition fusion, a capsule of deuterium-tritium (DT) fuel with an imbedded cone of gold is irradiated by many symmetrically arranged laser beams (top left). The light heats a thin layer of the capsule, causing it to expand rapidly and forcing the fuel to implode. The material converges around the tip of the gold cone; the density of the DT is now hundreds of times the density of solid material (top right). An ultraintense laser beam is fired into the gold cone (bottom left). When the laser interacts with the tip of the gold cone, a large number of energetic electrons is produced. The electrons travel into the dense DT fuel, depositing their energy and raising the fuel temperature to 108ºC, hot enough to initiate fusion (bottom right). (Courtesy of HiPER)
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The HiPER project already has formal involvement from seven European nations, along with involvement by scientists from eight other nations (including the U.S.). Its principal mission is to generate an energy gain of a factor of 100. The project has a conceptual design and is now entering a new stage to decide on construction, a stage the team expects to be funded by the European Commission and the partner countries and to take three years. The U.K. is the leading contender to host the HiPER laser facility.

“The HiPER laser technology could go one of two ways, which will be assessed during this next phase,” says Dunne. “We could adopt a similar technology to NIF and LMJ, with some modifications to increase the shot rate for fusion studies. This would be a low-risk, relatively low-cost solution. Alternatively, we could press forward with the development of diode-pumped solid-state laser technology to provide multikilojoule beamlines with repetition rates on the order of a hertz. This would be a high-cost solution, but would mark a huge step toward an eventual power plant.”

One technical hurdle to overcome is the coherent combination of large-aperture, multikilojoule beams to form a single effective beam, notes Dunne. “This is the laser equivalent of the segmented mirror arrays used in large optical telescopes such as Keck and ELT. Another problem is that the short-pulse (10 ps) beam should ideally operate at the second or third harmonic, but frequency conversion of such broadband, high-intensity beams has yet to be achieved. Research in both these areas is under way.”

Bridget Marx

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