Wednesday, February 12, 2014

Closer to ignition

Here’s the original draft of my latest piece for Nature news.


Another milestone is passed on the long road to fusion energy

The usual joke about controlled nuclear fusion, which could provide much ‘cleaner’ nuclear power than fission, is that it has been fifty years away for the past fifty years. But it just got a bit closer. In a report published in Nature today [1], a team of researchers at the US National Ignition Facility (NIF), based at Lawrence Livermore National Laboratory in California, say that their fusion experiments have managed to extract more energy from the nuclear process than was absorbed by the fuel to trigger it.

That’s certainly not the much-sought “break-even” point at which a fusion reactor can generate more energy than it consumes, because there are many other processes that consume energy before it even reaches the nuclear fuel. But it represents “a critical step on the path to ignition”, according to Mark Herrmann of Sandia National Laboratory in Albuquerque, New Mexico, who heads the project on high-energy X-ray pulses there.

While nuclear fission extracts nuclear energy released during breakup of very heavy nuclei such as uranium, nuclear fusion – the process that powers stars – produces energy by the coalescence of very light nuclei such as hydrogen. A tiny part of the masses of the separate hydrogen nuclei is converted into energy during the reaction.

Although the basic physics of fusion is well understood, conducting it in a controlled manner in a reactor – rather than releasing the energy explosively in a thermonuclear hydrogen bomb – has proved immensely difficult, largely because of the challenge of containing the incredibly hot plasma that fusion generates.

There is no agreed way of doing this, and fusion projects in different parts of the world are exploring a variety of solutions. In most of these projects the fuel consists of the heavy hydrogen isotopes deuterium and tritium, which react to produce the isotope helium-4.

A lot of energy must be pumped into the fuel to drive the nuclei close together and overcome their electrical repulsion. At the NIF this energy is provided by 192 high-power lasers, which send their beams into a bean-sized gold container called a hohlraum, in which the fuel sits inside a plastic capsule. The laser energy is converted into X-rays, some of which are absorbed by the fuel to trigger fusion. Most of the energy, however, is absorbed by the hohlraum itself. That’s why obtaining gain (more energy out than in) within the fuel itself is only a step along the way to “ignition”, the point at which the reactor as a whole produces energy.

The fuel is kept in a plastic shell called the ablator. This absorbs the energy in the hohlraum and explodes, creating the high pressure that makes the fuel implode to reach the high density needed to start fusion. But that pressure can burst through the ablator at weak points and destabilize the implosion, mixing the fuel with the ablator plastic and reducing the efficiency of the fusion process.

The NIF team’s success, achieved in experiments conducted between last September and this January, comes from ‘shaping’ the laser pulses to deliver more power early in the pulse. This creates a relatively high initial temperature in the hohlraum which “fluffs up” the plastic shell. “This fluffing up greatly slows down growth of the instability”, says team leader Omar Hurricane.

As a result, the researchers have been able to achieve a “fuel energy gain” – a ratio of energy released by the fuel to energy absorbed – of between 1.2 and 1.9. “This has never been done before in laboratory fusion research”, says Herrmann. “It’s a very promising advance.”

He adds that much of the energy released was produced by self-heating of the fuel through the radiation released in the fusion reactions – an important requirement for sustaining the fusion process.

But fusion energy generation still remains a distant goal, for which Hurricane admits he can’t yet estimate a timescale. “Our total gain – fusion energy out divided by laser energy in – is only about 1%”, he points out.

“This is more than a little progress, but still modest in terms of energy generation”, Hurricane says. “Our goal right now is to more than double the final pressures in our implosion, by making it go faster and improving its shape.”

Meanwhile, other projects, such as the International Thermonuclear Experimental Reactor (ITER) under construction in southern France, will explore different approaches to fusion. “When trying to solve hard problems it is wise to have multiple approaches, as every potential solution has pros and cons”, says Hurricane.

1. Hurricane, O. A. et al., Nature advance online publication doi:10.1038/nature13008 (2014).

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