White dwarfs are very high-density stars that often have a very strong magnetic field; they can sometimes absorb the matter from nearby companion stars. This matter is then concentrated and captured by the white dwarf, channelled in a very narrow region above its magnetic pole, thus forming accretion columns with dimensions typically from 100 to 1000 kilometres. These areas are far too small to be observed directly using telescopes. To study the physical phenomena that occur around the magnetic pole of the white dwarf, the researchers needed to reproduce them in a laboratory.
To this end, for a few nanoseconds, they concentrated all the energy from the Orion laser on a surface of a few square millimetres. They were then able to produce a hot plasma flow moving at a velocity of 200 km/s. When this plasma flow struck a steel obstacle, it mimicked the same phenomena that occur at the surface of white dwarfs. By employing a second laser beam, they were able to use X-rays to probe the dynamic of the accretion column. This experiment, the first of its kind anywhere in the world, made it possible to produce a laboratory model of an astrophysical object.
Better understanding of white dwarfs and their dynamics is crucial for astrophysicists and cosmologists because they are considered as the possible progenitors of thermonuclear supernovae, celestial bodies used to measure the expansion of the Universe. Experiments of this kind could be boosted in the near future by a far more powerful LMJ laser.
About high-power lasers
Orion is a laser facility based in the United Kingdom, managed by the Atomic Weapons Establishment (AWE). It combines 10 long-pulse beams (nanosecond scale, 10-9 s) and two ultra-short beams (less than picoseconds, 10-12 s). This type of experiment, which first began a few years ago at the Luli200 facility (Laboratoire pour l'utilisation des lasers intenses) at the École Polytechnique, could soon benefit from much more powerful LMJ and Petal lasers. The LMJ-Petal facility (in the Gironde region) is coordinated by the CEA and will combine the 176 beams from the laser Megajoule with the Petal petawatt beam.