Self-heating plasma brings nuclear fusion dream closer to reality

Energy, clean, safe and unlimited. This is the promise of nuclear fusion, a promise in which many see the answer to our current energy crisis. Recent experiments created at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory in California in the USA have validated a way to produce laser-driven nuclear fusion energy, in which a plasma is heated and compressed. This milestone, which is the cover story of this week's Nature, provides crucial evidence that plasma can supply its own heat.

Nuclear fusion mimics the energy of stars, and that's what engineers and physicists involved in ongoing projects around the world are trying to do - they're trying to mimic what happens inside stars. But recreating this process in the laboratory has proven to be a challenge and uses far more energy than it produces.

A critical step toward fusion is to have a net energy generator, a burning plasma in which nuclear fusion is the main source of heat to keep the fuel in a plasma state hot enough to allow further fusion reactions. U.S. center researcher Alex Zylstra and his team have succeeded in taking that step in the laboratory.

The team of U.S. scientists has created a burning plasma using deuterium and tritium, two isotopes of hydrogen. Deuterium can be extracted from seawater and tritium can be produced in a reactor. In a burning plasma, the particles produced when the nuclei fuse become the main source of plasma heating. The researchers have demonstrated in four experiments that generated more than 100 kilojoules of energy. In one case, the authors were able to extract 170 kJ of energy from a millimeter-sized sphere containing less than one milligram of hydrogen isotopes.

The experiment detailed in Nature uses the energy of 192 laser beams to rapidly heat the inside of a hollow cylinder, generating X-rays. The cylinder contains a spherical capsule with the deuterium-tritium fuel. The X-rays uniformly heat the outer regions of the capsule, causing it to expand rapidly and causing the fuel to accelerate inward. In ten billionths of a second, the fuel and capsule are compressed to many thousandths of their volume, and temperatures reach 50 million kelvins at the center.

These combined effects cause the hydrogen isotopes to fuse, producing a neutron and an alpha particle, which is the nucleus of a helium atom. The alpha particles collide with the plasma, self-heating the fuel.

Fission and fusion

Both are nuclear reactions that aim to release energy from the nucleus of an atom that is released in the form of heat. This is what a nuclear power plant does by converting the nuclear energy of atoms into thermal energy. But the difference is that nuclear fission splits the atom and fusion joins two lighter nuclei to form a heavier one. Another important difference is that nuclear fusion energy produces very little radioactive waste.