Nuclear fusion, the reaction that burns at the centre of the Sun, could provide safe, carbon free energy and solve the global energy crisis. The catch? Nobody’s quite worked out the finer detail of how to tame fusion reactions here on Earth.
'Fusion is a really efficient way of getting energy, the only downside is that it’s just really difficult to do,’ says fusion scientist Dr Kate Lancaster. She is currently developing technology for the proposed High Power Laser Energy Research Facility (HiPER), which will investigate laser driven fusion.
The theory behind nuclear fusion is well understood. Fusion occurs when tritium and deuterium (isotopes of hydrogen) nuclei combine, producing helium and a neutron. This reaction generates a huge amount of energy, which could be captured and converted into electricity by fusion power plants.
Unlike the fossil fuels we use today, the raw materials for fusion exist in plentiful supply. Deuterium can be extracted from seawater and, although tritium doesn't occur naturally, it can be generated using the metal lithium. ‘Half a bathful of seawater and the lithium in a laptop battery would supply 30 years' worth of energy for one person,’ enthuses Lancaster. Getting to grips with the practicalities of a fusion power plant however is the real challenge.
Building a star
For the reaction to occur, the fuel needs to be confined at a very high temperature, ensuring that the Tritium and Deuterium nuclei get close enough together to react. ‘If the fuel expands, the particles drift too far apart to fuse,’ explains Lancaster.
To complicate things further, at the high temperature required (a sweltering 100 million degrees C), matter becomes a plasma. As Lancaster puts it: ‘the atoms ‘fall apart’ and you’re left with a soup of charged particles’.
Holding the fuel together is therefore a tricky business as any physical container you might want to use either cools down the fuel or becomes a plasma itself. ‘Confining the fuel at 100 million degrees without touching it is very difficult. The Sun does it using its gravitational field, but we can’t make a lab as big as a star! So you can either use electromagnetic fields or lasers,’ says Lancaster.
In fast ignition laser fusion, a collection of long pulse of lasers are fired at the fuel from all directions, squishing it before a short, high power pulse heats it up, acting like a match to spark the reaction. The process is very similar to the workings of a petrol engine, where fuel is compressed and then ignited by a spark plug.
From theory to practice
Laser fusion has not yet been proven to be viable, but it is expected that tests on a very similar system at NIF (the National Ignition Facility) in the USA later this year will back up the theory.
High power lasers gobble up a lot of energy, so in the first instance the aim is to show that we can get more energy out of the reactor than is originally put in. Then, as technology is stepped up, so will the efficiency of future laser fusion reactors. 'You should eventually be able to put in a mega Joule of laser energy and get out 200 mega Joules of fusion energy, which is pretty incredible,’ comments Lancaster.
The next big milestone for the fast ignition community will be the construction of the HiPER testing facility. Lancaster explains, ‘HiPER is going to drive the technology of the lasers because at the moment they’re notoriously inefficient. The point of HiPER is that we’re going to simultaneously develop all the technology required for an eventual working reactor.'
The HiPER facility
In the meantime, ITER, a magnetic confinement fusion reactor, is being built in France, with the aim of proving that magnetic confinement fusion can provide a viable energy gain. Lancaster is reluctant to place bets on which approach will ultimately prove successful. ‘I think they’re complementary,’ she says, ‘you just can’t put all your eggs in one basket. When fission was developed there were many different designs of fission reactors, and quite a few designs are still used today. There’s a lot of overlapping technology anyway.’
Whichever pathway it takes, success will only come after perhaps a few more decades of hard work. And although it brings the promise of cheap energy in the long run, HiPER’s construction is also dependent on participating governments pitching in a total of one billion euros. ‘For long term energy security it’s a small price to pay,’ says Lancaster.
Kate is a NOISEmaker, for more information on NOISE visit www.noisemakers.org.uk
24 August 2009
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