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Vol. 344 ,
- 17 April 2014 12:48 pm , Vol. 344 , #6181
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Fusion "Breakthrough" at NIF? Uh, Not Really …
10 October 2013 12:15 pm
One unintended effect of the U.S. federal shutdown is that helpful press officers at government labs are not available to provide a reality check to some of the wilder stories that can catch fire on the Internet. They would have come in handy this week, when a number of outlets jumped on a report on the BBC News website. The National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California, it reported, had passed a "nuclear fusion milestone." NIF uses the world's highest energy laser system to crush tiny pellets containing a form of hydrogen fuel to enormous temperature and pressure. The aim is to get the hydrogen nuclei to fuse together into helium atoms, releasing energy.
The BBC story reported that during one experiment last month, "the amount of energy released through the fusion reaction exceeded the amount of energy being absorbed by the fuel - the first time this had been achieved at any fusion facility in the world." This prompted a rush of even more effusive headlines proclaiming the "fusion breakthrough." As no doubt NIF's press officers would have told reporters, the experiment in question certainly shows important progress, but it is not the breakthrough everyone is hoping for.
A memo sent out on 29 September to collaborating labs from NIF Director Ed Moses—which has been seen by Science—describes a fusion shot that took place at 5:15 a.m. on 28 September. It produced 5x1015 neutrons, 75% more than any previous shot. Neutrons are a product of fusion reactions, so they are used as a measure of success.
For fusion experiments, NIF directs 192 laser beams from all directions at the fusion target in a pulse that carries 1.8 million joules (MJ) of energy. The outer part of the target is a tiny metal can the size of a pencil eraser, called a hohlraum, at the center of which sits a plastic sphere smaller than a peppercorn containing frozen fusion fuel—a mixture of the hydrogen isotopes deuterium and tritium, known as DT. The ultraviolet beams are fired into the hohlraum through holes at each end but not directly at the fuel capsule. Instead they hit the inner walls of the hohlraum, heating it so much that it emits a pulse of x-rays. The x-rays cause the plastic capsule to explode, driving the fuel inward toward its center.
If all goes according to plan, the fuel—compressed to 100 times the density of lead—will ignite a fusion reaction, but the laser-driven implosion does not provide enough energy to burn all the DT fuel. Some energy from the fusion reactions is needed to keep the burn going. DT fusion reactions produce two products: helium nuclei (aka alpha particles), which carry 20% of the reaction energy as kinetic energy; and neutrons, which carry the rest. For fusion to work as an energy source, the alpha particles must efficiently heat up the fuel to keep the reaction running.
To achieve this, NIF researchers have been experimenting with the shape of the laser pulse to make it deliver more power near the beginning. In his 29 September memo, Moses says these improvements had led to alpha-particle heating that doubled the energy yield—"a clear demonstration of the mechanism that is needed to achieve ignition," he wrote. Ignition is the goal of a self-sustaining, alpha-heated fusion burn producing more energy than the laser put in. Moses also says the energy yield (carried by the neutrons and estimated at 14 kilojoules) was more than the x-ray energy absorbed to implode the capsule, a milestone he refers to as "scientific breakeven."
“It is a good experiment,” says Michael Campbell, a former director of NIF who now works for Logos Technologies in Fairfax, Virginia. “From a science standpoint, the target worked well enough for alpha particles to heat some of the fuel.” But Campbell is concerned about overhyping each step in what is bound to be a long haul toward fusion as an energy source. The energy yield in last month’s experiment is still a very long way from ignition, the goal—enshrined in NIF's name—that the facility was expected to reach a year ago. NIF is now partway through a 3-year campaign to nail down why it is struggling to reach that goal. “It’s a science-based program now. They are trying to identify some of the obstacles to getting to ignition,” Campbell says.
One requirement for ignition is that energy output should exceed the energy input from the laser, i.e., that gain (output divided by input) should be greater than 1. NIF's laser input of 1.8 MJ is roughly the same as the kinetic energy of a 2-tonne truck traveling at 160 km/h (100 miles/h). The output of the reaction—14 kJ—is equivalent to the kinetic energy of a baseball traveling at half that speed. Numerically speaking, the gain is 0.0077. The experiment “is a good and necessary step, but there is a long way to go before you have energy for mankind,” Campbell says.