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Fast Ignition Boosts Neutron Counts

Photonics Spectra
Jan 2002
Paul Mortensen

In another step toward realizing the dream of fusion energy, researchers investigating fast-ignition inertial confinement fusion have produced 10 times the neutron count compared with that by inertial confinement alone. The work promises clean nuclear fusion reactors that could eliminate nuclear fission plants and their long-lived radioactive decay products.

The fast-ignition approach to inertial confinement fusion promises to lower the energy needed to initiate a reaction. Researchers at Gekko XII in Japan used nine laser beams to compress a 500-µm-diameter, deuterium-filled polystyrene sphere, to which they attached a gold cone that facilitated the delivery of a separate ignition beam to the resulting plasma. Courtesy of Ryosuke Kodama, Osaka University.

In a series of tests at Osaka University's Institute of Laser Engineering in Suita, Japan, researchers from the university and from UK-based Rutherford Appleton Laboratory in Didcot, Imperial College in London and York University used nine of the 12 beams on the Nd:glass Gekko XII laser to compress a 500-µm sphere containing deuterium with 1-ns pulses. Another subpicosecond pulse delivering 60 J ignited the resulting plasma.

The team achieved 1 percent of the temperature rise needed for fusion using 0.1 percent of the theoretically required energy. Counts of the 2.45-MeV neutrons produced in the reaction indicated that the technique yielded 10 times as many neutrons as inertial confinement fusion alone, using only half the energy.

Scientists are studying fusion energy on two fronts: magnetic confinement and inertial confinement. Magnetic confinement seeks to hold a 50 million °C plasma of deuterium and tritium in the fields produced by structures such as a tokamak to achieve continuous burn.

Inertial confinement fusion uniformly compresses and ignites a target filled with deuterium and tritium into a plasma using pulses from a high-power laser or ion source, with the goal to produce a series of tiny explosions.

During the past 10 years, fusion researchers have developed the fast-ignition approach to avoid the technical difficulties that are related to maintaining a symmetrical implosion while delivering the spark. The technique separately compresses and heats the target.

In the experiments at Gekko XII, the researchers attached a gold cone to the deuterium-filled sphere to prevent the plasma in the compressed target from interfering with the ignition beam.

50 kJ to breakeven

Team leader Ryosuke Kodama of Osaka University predicts that the fast-ignition approach will enable researchers in internal confinement fusion to use much less laser power to achieve the same results. But it also may make more realistic the dream of fusion energy in the coming years.

"I estimate breakeven with a total laser energy of less than 50 kJ: 30 kJ for implosion energy and 20 kJ for heating by short-pulse laser," he said.

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