fusion
Image: J. ZWEIBACK/LLNL

The promise of building commercial fusion reactors for generating electric power is balanced against a number of technical challenges--chief among them, how to make a material capable of sustaining the constant bombardment of high-energy neutrons. "Any standard material just turns to cottage cheese," says Todd Ditmire of the Lawrence Livermore National Laboratory. To date, researchers have turned to computer simulations to study this destruction, estimating that a single neutron hitting a crystal can melt 10 5 ions, which recrystallize imperfectly within nanoseconds. Over time, the imperfections add up, compromising the material. But in hopes of verifying this problem in the real world, last year Ditmire and his colleages created a tabletop system that produces a tiny taste of it in the form of super-short neutron pulses. In the October 23 issue of Physical Review Letters, they characterize the pulse in detail.

In this laboratory system, the researchers blast a cluster of heavy hydrogen atoms with a quick and powerful laser punch (pink in the image above). This punch blows off the electrons and leaves the remaining mutually-repulsive ions to explode so violently that nearby heavy hydrogen clusters fuse and emit neutrons [white area above]. This neutron pulse is a mere 650 picoseconds wide, and the group's calculations suggest that 100 ps pulses should be possible. They propose using the system to study radiative damage by hitting a solid first with the neutron pulse and then with an x-ray pulse from the same laser to record the aftermath. "You could make a movie of this melting and recrystallization," Ditmire notes. Such a movie might suggest ways to protect the solid from the neutron onslaught.