(ORDO NEWS) — At the Lawrence Livermore National Laboratory (USA), during experiments with inertial thermonuclear fusion, an anomalous behavior of ions in plasma was discovered, which is not consistent with theoretical predictions.
At NIF, fusion reactions are triggered by lasers that heat so-called hohlraums, small golden cylinders that contain a capsule with thermonuclear fuel, a mixture of tritium and deuterium.
The lasers irradiate the inner wall of the cylinder, which generates thermal x-rays that cause the capsule to explode.
Deuterium-tritium fuel is compressed to a pressure of hundreds of gigabar, which creates a hot spot at its center with a temperature of about 10 million kelvins.
Thermonuclear fusion reactions produce alpha particles, the energy of which is capable of heating all the rest of the fuel.
Alpha heating leads to an increase in the reactivity of the fuel, since the average kinetic energy of ions in the plasma formed during the explosion of the capsule increases.
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 the fusion reaction.
Such spectra should contain information about the properties of the heated plasma.
For example, shifts in the average neutron energy from the nominal value of 14 MeV are related to the temperature of the ions, the average kinetic energy of the ions, and the plasma velocity.
During the next experiment, scientists obtained neutron spectra using time-of-flight spectrometers, which allow obtaining information about the energy of a particle depending on the time of its flight of a certain distance (about 20 meters) through the medium.
The 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 in the average neutron energy.
For a plasma with thermal ions, in which thermonuclear heating does not occur, their energy distribution corresponded to the Maxwellian
. However, for the burning plasma, a deviation from the expected Maxwellian distribution was revealed – the ions had a higher energy at a given temperature than the theory predicted, equal to or higher than 10 kiloelectronvolt’s.
There are several possible explanations for this anomaly: unaccounted for kinetic effects, the influence of plasma hydrodynamics on the neutron spectra, and combustion characteristics that do not allow a correct interpretation of plasma properties from neutron spectra.
According to the authors, further research is required to pinpoint the causes of deviations that may affect the production of fusion energy.
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