Dr Charlotte Palmer from the University of Oxford (now at Queen’s University Belfast) in collaboration with a team including scientists from the University of Michigan, Imperial College, University of York and the CLF have published a paper on their work in re-creating a phenomenon called magnetic reconnection, which is often found in astrophysical phenomena. The work was conducted in Target Area West of the Vulcan high power laser.
Magnetic reconnection is a process that occurs in a wide range of astrophysical processes, from the birth of Coronal Mass Ejections to the interface between the solar wind hitting earth’s magnetic field. It also occurs in Magnetic Confinement Fusion experiments. Reconnection occurs when two magnetic fields of opposing directions are forced together in a plasma, leading to a disconnection and reconnection of the magnetic fields. This process leads to a conversion of magnetic energy into kinetic energy, accelerating charged particles to high velocities. Direct measurement of this change in magnetic field arrangement in astrophysical environments is extremely challenging due to the small scale of the reconnection region and often the large distance from which we observed the phenomena.
“Lasers offer a unique way for us to very briefly create environments with similar properties to some of the most extreme in the universe. These extreme conditions only last for a fraction of a second but enable us to directly study processes in the laboratory that can help us to test theories and verify models developed to explain astrophysical phenomena that have been observed or predicted,” says Dr Palmer.
Two lasers can be used to generate magnetic fields that collide as the plasma formed by the lasers expands radially outwards. ‘The expanding plasma has a density and temperature gradient that are not parallel and this leads to the formation of a magnetic field which circles the laser focus. This is known as the Biermann battery and is believed to be responsible for the initial formation of magnetic fields in the cosmos. Two laser spots close together lead to two magnetic fields (of opposing direction) colliding at the midplane.
Vulcan Target Area West was used as it has 2 short pulse beams. One of these was split into two to allow the creation of 2 laser spots and therefore two neighbouring magnetic fields, and the other was used to produce a proton probe. The facilities at CLF are uniquely suited to the experimental need for the proton probe which enables the experimenters to “see” and measure the strength of the magnetic fields during the collision.
As the laser intensity increases, the magnetic field can be generated in a different way, caused by relativistic electrons travelling away from the laser focus at close to the speed of light. The collision of these magnetised relativistic plasmas at the midpoint between the laser foci creates conditions in the laboratory similar to extremely energetic astrophysical systems. Magnetic reconnection in these conditions has not been studied experimentally in much detail as they can only be created by very intense lasers.
This study builds on work by Dr. Louise Willingale (PI) and paves the way for better understanding of relativistic magnetic reconnection relevant to extreme cosmic phenomena as well as terrestrial research programmes exploring inertial confinement fusion.
Link to paper: https://aip.scitation.org/doi/full/10.1063/1.5092733