2022 Cecilia Payne-Gaposchkin Medal and Prize
Professor Gianluca Gregori for pioneering experiments that have established laboratory astrophysics as a tool to study turbulent magnetised plasmas, particularly dynamo amplification, particle acceleration and heat conduction.
Planets, stars, galaxies and clusters of galaxies all have magnetic fields. Numerous processes have been suggested that could create seed magnetic fields, but the resulting fields are orders of magnitude smaller than the fields we observe today. The most widely invoked mechanism for amplifying them to present day values is the turbulent dynamo.
For many decades, experimental plasma physicists sought to produce the turbulent dynamo in the laboratory, but did not succeed. Achieving fully developed turbulence with magnetic Reynolds numbers high enough that the turbulent dynamo can operate proved to be very challenging. It requires high velocities and high temperatures. The advent of high-power lasers opened a new possible path to achieving these conditions. However, these conditions must be sustained long enough for the turbulent dynamo to amplify the seed magnetic fields. Moreover, the volume of the turbulent plasma must be large enough for charged particles crossing it to experience enough interactions with the magnetic fields for their diffusion to change and experience stochastic acceleration.
Using a novel target design and the advanced diagnostics capabilities available at the largest laser facilities in the world, Professor Gianluca Gregori was able to produce a large-scale turbulent plasma and fully characterise the plasma properties and the magnetic fields. He was able to demonstrate the amplification of seed magnetic fields by the turbulent dynamo – a long-sought goal of experimental plasma physics, as well the evolution of the field beyond the initial nonlinear growth. Later, an analogue to the diffusion of ultra-high energy cosmic rays interacting with the magnetic field of the Galaxy was studied by firing charged particles (protons) through the magnetised turbulent plasma produced by the turbulent dynamo and measuring its effect on their trajectories. In more recent experiments, Gregori's team investigated the suppression of classical heat transport in such turbulent and magnetised plasmas; again, a process that is believed to play an important role in hot and diffuse gas that fills clusters of galaxies (and more generally in high-beta plasmas) where it regulates the overall energy balance.