Laser fusion put on slow burn
The US National Ignition Facility
rethinks its strategy on achieving thermonuclear fusion in the lab, but
fails to silence critics.
11 December 2012
government's new plan, revealed to Nature, calls for a slower, more
deliberate approach to achieving ignition: the point at which more
energy is produced by a fusion reaction than is consumed. Many
physicists believe that this would be an important proof of concept for
The plan sets a new course for the laser at
the National Ignition Facility at the Lawrence Livermore National
Laboratory in California. It also promotes the exploration of several
alternative ways to reach ignition, including one not involving the
laser. And it is more tentative than the previous strategy: it sets a
three-year deadline for finding out whether ignition is possible at all,
whereas the last one aimed to demonstrate actual fusion...
US$3.5-billion NIF uses lasers to crush a 2-millimetre pellet of
hydrogen fuel to the point of fusion. Rather than irradiating the fuel
directly, the lasers shine into a cylindrical capsule. The capsule walls
then emit X-rays that squeeze the fuel pellet until it explodes.
indirect approach mimics the ignition system in a thermonuclear weapon,
which uses radiation from a fission 'primary' stage to squeeze hydrogen
isotopes in the fusion 'secondary' — creating a powerful explosion.
NIF's main mission is to gather laboratory data on the process to help
weapons scientists to care for the ageing US nuclear stockpile. The
United States has adhered to a voluntary moratorium on testing nuclear
weapons since 1992, so nuclear scientists must use computer simulations
to check that the weapons still work, and NIF data feed into these
Physicists at the NIF also hope that the process might pave the way for producing electrical power through thermonuclear fusion.
latest plan was drafted by the NNSA, which oversees the lab, in
response to a congressional request for a strategy for achieving
Over the next three years, researchers will conduct
reduced-power tests to refine their computer simulations and understand
why ignition has been so elusive. They will also look at possible
improvements to the capsule design.
Other promising approaches to
be studied include using lasers to ignite the hydrogen fuel inside the
pellet directly, and using a machine called the Z-pinch to squeeze the
fuel inside a magnetic field.
By October 2015, the NNSA hopes, it
will be able to say whether ignition can be achieved using the NIF or
the Z-pinch. Failure, it warns, could have serious implications for the
The more sedate approach follows
"common sense", says Ricardo Betti, a physicist at the University of
Rochester in New York. But he worries that the plan does not give enough
time to ignition experiments, focusing instead on other nuclear-weapons
experiments and fundamental science. Devoting less time to ignition
reduces the probability of success, he warns...
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