An anomaly detected in a US fusion reactor

 At Lawrence Livermore National Laboratory (USA), experiments with inertial thermonuclear fusion discovered anomalous behavior of ions in the plasma that did not match theoretical predictions. The results of the research carried out by the National Ignition Facility (NIF) have been published in the journal Nature Physics.

In NIF, the fusion reactions are triggered by lasers that heat the so-called Lasers and irradiate the inner wall of the cylinder, which generates thermal X-rays that cause the capsule to explode. Deuterium-tritium fuel is compressed to hundreds of gigabits, creating a hot spot at its center with a temperature of about 10 million Kelvin. The fusion reactions produce alpha particles, the energy of which is able to heat up the rest of the fuel.

Heating alpha leads to an increase in fuel reactivity as the average kinetic energy of the ions in the plasma increases during the capsule explosion. It is assumed that the temperature of ions related to their kinetic energy can be determined by measuring the energy spectra of neutrons produced in a nuclear fusion reaction. Such spectra should contain information about the properties of the heated plasma. For example, shifts in mean neutron energy from nominal 14 MeV are related to ion temperature, mean ion kinetic energy, and plasma velocity.

In another experiment, scientists obtained neutron spectra using time-of-flight spectrometers, which allow for obtaining information about the energy of a particle depending on the time it traveled over a certain distance (about 20 meters) through the medium. Ion temperature was determined from the spread (dispersion) of the neutron kinetic energy spectrum, and the average ion kinetic energy was determined from the shift of the average neutron energy.

For plasma with thermal ions in which there is no fusion heating, their energy distribution corresponded to Maxwell's. However, in the case of burning plasma, a deviation from the expected Maxwell distribution was revealed - the ions at a given temperature had higher energy than the theory predicted, equal to or greater than 10 kilo electronvolts.

There are several possible explanations for this anomaly: unaccounted-for kinetic effects, the effect of plasma hydrodynamics on neutron spectra, and a combustion characteristic that does not allow correct interpretation of plasma properties from neutron spectra. According to the authors, more research is needed to identify the causes of the deviations that may affect the production of fusion energy.