Historic milestone in nuclear fusion

The US Department of Energy announced today that a historic milestone in nuclear fusion research has been reached: for the first time, a fusion reaction produced more energy than was needed to start it. This feat was achieved at the Lawrence Livermore National Laboratory (LLNL) in California, using the NIF (National Ignition Facility) laser system, the largest and most energetic in the world, whose size is comparable to three football fields.

The aim of controlled nuclear fusion is to replicate, on Earth, the same mechanism that generates the immense energy of stars such as our Sun. Energy generated by fusion has numerous advantages: it is clean, with a minimal carbon footprint and no radioactive waste; it is safe, as it stops instantly in case of disturbances; and it is economical, relying on cheap, abundant and non-polluting natural resources, even compared to today's renewable energies.

The process of nuclear fusion consists of forcing two atomic nuclei to unite, giving rise to a new element. In stars, for example, pairs of hydrogen isotopes are fused, turning into helium. This is also the combination used in research nuclear reactors.

In order to overcome the huge natural repulsion between the nuclei, one needs to expose them to extreme physical conditions. Researchers have tried two main approaches: the so-called magnetic confinement, in which atoms are squeezed thanks to intense magnetic fields, and inertial confinement, in which high-energy laser beams are used to achieve this compression. In the first process, atomic densities and moderate pressures are used for long times, on the order of several seconds. In inertial fusion, however, very high pressures and densities are used, reproducing the conditions inside stars, but for times of the order of thousandths of millionths of a second.

And it was thanks to the approach of the LLNL researchers with the NIF laser – more precisely, with the 192 beams of this laser – that for a few moments a temperature higher than that of the core of the Sun was attained, reaching ignition conditions: more energy was obtained from fusion than the one that was supplied through the laser beams. Specifically, for 2 million Joules of laser energy, 3 million Joules of fusion energy were obtained, a gain factor of 1.5 times.

In the NIF experiment, laser beams are focused on the inner walls of a small hollow cylinder, the size of a thimble, inside which a tiny spherical capsule containing the hydrogen fuel is suspended. The reaction of the lasers with the cylinder generates a burst of x-rays of such intensity that the capsule is compressed, a process called “indirect irradiation”. The violent shrinking process makes the hydrogen atoms fuse.

This is how it works in theory. In practice, it has proved extremely difficult to get this process to take place in a balanced, homogeneous and sustained way for a long enough time for the fusion reaction to be efficient. Prior to last week's result, hundreds of similar experiments had already been carried out, with the researchers taking advantage of the results in order to refine their knowledge of the physics of inertial fusion. Last August, one of the laser shots led to a fusion reaction with an energy of 1.35 million Joules, on the verge of ignition. But these last results leave no doubt about the overcoming of this crucial point.

“It is a historic achievement”, in the words of LLNL director Kim Budil, during the press conference on Tuesday, in a feeling shared by the several government and scientific representatives present. However, it is recognized that we are still a few decades away (“but not five, nor four”, in her words) from being able to make commercial use of nuclear fusion. In fact, the energy that was needed to generate the laser pulses that start the process is, for the time being, 300 million Joules, that is, the energy generated is just 1% of what was required to start with.

But scientists aren't deterred by these numbers: in fact, NIF was designed more than a couple of decades ago, when laser technology was much less developed than it is today. A laser fusion reactor using current technology could raise this value enough for us to dream of the day when more net energy is obtained than was used.

At the Institute of Plasmas and Nuclear Fusion (IPFN) of the Instituto Superior Técnico (IST), the institution that leads research into nuclear fusion in Portugal, this news was received with great excitement. “The result is absolutely transformative and demonstrates that a path has now been identified to achieve nuclear fusion in the laboratory, also with lasers” in the words of Luís Oliveira e Silva, leader of IPFN’s Group for Lasers and Plasmas.

IPFN scientists have been involved in major European efforts in the area, including the HIPER project – which aimed to develop laser fusion by direct irradiation of the capsule, a more efficient method than the indirect irradiation now demonstrated, and also the activities funded by EURATOM in the field of laser fusion. Some of the scientists who have collaborated with this effort in the USA were trained at IPFN and maintain regular collaborations.

In the laser laboratories of IPFN/IST, relevant research is carried out for the future of laser fusion, whether in the development of innovative laser technologies or in the study of the conditions of laser fusion plasmas and the state of matter associated with it. At the same time, the team is a world leader in the development of numerical models for studying the interaction of lasers with matter and the way energy is deposited.

 IPFN is also the institution that represents Portugal in the world's leading projects in nuclear fusion by magnetic confinement. In the words of IPFN president Bruno Gonçalves, “the scientific and technological challenges to achieve a fusion reactor are enormous; some are common to magnetic confinement nuclear fusion (for instance, fusion materials capable of withstanding very high neutron fluxes) others are specific to laser-driven fusion”. The two approaches have worked together with a view to, in the expected near future, accessing a clean and virtually inexhaustible source of energy, an objective that is not only inspiring but above all essential for the survival of future generations.