Tiny fixing key to fusion power

By Ian Parker, freelancer, ianparkerwriter@aol.com

The world needs new, sustainable sources of energy – wind, solar, wave and tide energy might not be enough. Nuclear fission is well established but presents considerable environmental risks. Nuclear fusion has long held the promise of unlimited power from hydrogen, but it is technically very difficult to achieve and sustain.

However, last December a major step forward was achieved with a fusion reaction at the Lawrence Livermore National Laboratory (LLNL) in the USA – it yielded more energy than was required to start it.

Fission reactions involve heavy metals (typically uranium or plutonium) in which large atoms break apart, releasing particles and energy. This happens spontaneously and steps have to be taken to control it. In a conventional nuclear power station, control rods –made of neutron-absorbing materials, such as boron, hafnium and cadmium – in the nuclear pile control the fission, making the energy release manageable.

However, fusion involves light elements (usually hydrogen atoms or their isotopes) that join to form helium, which also releases energy. This is not spontaneous and has to be driven by huge temperatures and pressures. It’s what happens in the cores of stars and keeps them shining.

Energy release from nuclear fusion has been achieved on Earth since the early 1950s in hydrogen bombs. So, the nuclear chemistry is well understood. However, this fusion is catastrophic in its energy release – hence its use as a weapon. The fusion part of the bomb is set off by a fission bomb. For the provision of electricity, the fusion reaction will need to be slower and controlled. This is the big challenge for fusion power.

The two extensively investigated approaches to fusion are Magnetic Confinement Fusion (MCF) and laser driven inertial confinement fusion (laser fusion). One of the questions over MCF is, how do you feed fresh fuel into a reaction chamber? This will require injection into the magnetic bottle or a pulsed system in which the fuel is replenished, and the magnetic bottle switched on again. Laser fusion uses fuel pellets, which are crushed and heated using lasers – producing x-rays in the fixture that holds the pellets.

The LLNL’s National Ignition Facility (NIF) in California is roughly the size of a football stadium and focusses 192 laser beams on the target material. The NIF is using laser fusion to compress a spherical shell of deuterium and tritium, which are hydrogen isotopes and easier to handle and react than hydrogen. The most common hydrogen, protium, has no neutrons; deuterium has one in the nucleus and tritium has two.

The target materials are held in a small fixing called a hohlraum. In radiation thermodynamics, a hohlraum (a non-specific German word for a ‘hollow space’ or ‘cavity’) is a cavity whose walls are in