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Tabletop fusion device. Image courtesy of Ben Stein/AIP.
A team of researchers from Lawrence Livermore National Laboratory (LLNL) have converted laser energy into ion kinetic energy to produce nuclear fusion reactions between deuterium atoms on a laboratory tabletop. Speaking during a packed symposium at the APS Centennial Meeting in Atlanta, group leader Todd Ditmire unveiled the device.
Normally fusion research requires the energy of massive lasers, or complex vaccum chambers using superhot gas confined within magnetic fields. The $1 billion stadium-sized National Ignition Facility being built by LLNL is expected to be the world's largest laser, producing bolts of nearly two million joules of energy on a fuel capsule. Ditmire's laser cost less than $1 million. However the number of fusion neutrons produced by Ditmire's device is quite small compared to most large-scale laser fusion experiments.
The LLNL achievement was made possible by the many studies in recent years of how extremely intense pulses of light interact with matter. By amplifying laser pulses with very short temporal durations, ultrahigh intensity studies have become possible with lasers on a tabletop scale. For example, says Ditmire, "These unique lasers can now produce light pulses that are only 20 femtoseconds in duration and, when focused, can produce light intensity above 1017 watts per square centimenter. The last four years in particular have yielded numerous investigations on how such intense pulses of light interact with small balls of atoms ranging in size from 10 to 10,000 atoms per ball, an assemblage known as an atomic cluster. When subjected to very short pulses of high-intensity laser light, these clusters eject very fast ions with energies of many tens of thousands of electron volts, corresponding to ions with temperatures ranging up to 1 billion degress Celsius.
In their experiment, Ditmire and his LLNL colleagues focused an intense 35 femtosecond laser pulse to an intensity of about 1017 watts per square centimeter into a jet of ionized gas containing many clusters of deuterium molecules.
Although the fusion reactions achieved by the LLNL team did release energy, the total was only about 10 millionths of that consumed by the laser. Ditmire believes that the experiment could lead to the production of small-scale, high-repetition rate, affordable neutron sources. Neutron sources are used in a variety of applications, including neutron radiography and materials science studies. Future work will attempt to increase the yield of the fusion neutrons to usable levels
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