Interview with Markus Roth

The Lawrence Livermore National Laboratory (LLNL) in California recently reported an important advance in laser-fusion research. For the first time, the team has come close to generating almost the same amount of energy from nuclear fusion as was expended in triggering the fusion reaction itself. Physicist Professor Markus Roth, a specialist in laser-fusion research based at the TU Darmstadt, was part of the team employed in this latest experiment. Here, he explains the context of the project, and its significance for fusion research and the future of energy generation.


Can you tell us something about the background to the experiments?

In the case of inertial confinement fusion – the strategy that is being pursued at the Lawrence Livermore National Laboratory – the basic set-up consists of a hollow metal cylinder about 1 cm long in which a spherical capsule with a diameter of about 2 mm is mounted. A mixture of the hydrogen isotopes deuterium and tritium is injected into the capsule, and each end of the cylinder is then irradiated by 96 laser beams. Together, they generate an extremely powerful radiation field within the chamber, which vaporizes the surface of the capsule. Its contents are then concentrically compressed, and the implosion causes the deuterium and tritium particles to collide at velocities of between 300 and 400 kilometers per second. The fuel rapidly reaches a density and temperature sufficient to trigger the fusion of atomic nuclei. The fusion process gives rise to helium, which further raises the temperature of the fuel, and ultimately results in the fusion of a certain fraction of the fuel, before the target is blown apart.


What levels of energy are we talking about here?

In excess of 1300 kilojoules of energy was produced in the experiment. But the process can in principle lead to the generation of very large amounts of energy, and this is why the procedure is of interest as a means of energy production.


What are the obstacles to achieving a yield that exceeds the amount of energy deposited in the target?

In all of previous attempts, instabilities in the fuel or asymmetries in the radiation field prevented ‘ignition’ – the production of excess energy. Meanwhile, great progress has been made in the understanding of the laser-plasma interaction, and this culminated in the experiment carried out at the Lawrence Livermore National Laboratory on the 8th of August, in which the fusion reaction generated approximately ten times more energy than had been produced in any of the earlier experiments.


What can be learned from the insights gained in the experiment?

This result has particularly interesting implications for the civilian use of fusion energy. It testifies to the rapid strides that have been made in understanding the fundamental physics of the ignition process, in the development of laser technology and in the ability to fabricate fusion targets of high quality. This leap forward brings us tantalizingly close to the break-even point – the point at which the amount of energy generated by the fusion reaction is equivalent to the energy delivered to the target.


What do the experiments mean for the future of research on laser-driven fusion?

They represent a milestone in the field of laser-based fusion research, and they will undoubtedly stimulate further research on harnessing fusion reactions for the production of clean energy.