Finding the origin of magnetic fields starting from unmagnetized plasmas constitutes a long-standing quest in plasma physics [1,2]. This fundamental and intriguing problem has implications not only in astrophysics but also in laboratory plasmas. In fact, one of the main unanswered questions regards the genesis of the magnetic fields found throughout the universe. On the other hand, in a laboratory, long-lived magnetic fields can be created via the interaction of intense lasers with plasmas, in what is considered one of the most exciting emerging new branches of plasma physics. Laboratory experiments will indeed allow reproducing astrophysical mechanisms under controlled conditions favoring a physical insight that would be otherwise inaccessible.

The Biermann battery has often been invoked as a possible mechanism for magnetic field generation in unmagnetized plasmas [3]. The Biermann battery acts in an unmagnetized plasma when its temperature and pressure gradients are perpendicular to each other. In such plasmas, due to their small inertia, electrons will expand away from the front side of the target, while ions, due to their higher inertia, will not move. As a result, a net electric field is created, whose integral over a closed loop is non-zero, thus generating a magnetic flux via Ampère’s equation. This animation illustrates what happens when a 30fs, 1µm laser with intensity 7 x 10¹⁷ W/cm^-2 is shined onto a target. It shows the laser field in light orange impacting on the target surface (blue). This causes expansion (density gradient) and heating of the plasma (temperature gradient) in directions that are perpendicular to each other. As a result, a toroidal magnetic field (in dark orange) is generated. The movie shows also the so-called fountain effect due to the electron current (blue arrows) along the gradients, which is self-consistent with the magnetic field creation.

The simulation has been performed with the particle-in-cell code OSIRIS [4], which was run on the supercomputer Marconi (Cineca, Italy). It represents the first three-dimensional kinetic simulation of the laser-driven Biermann battery effect.

References:
[1] D. Uzdensky and S. Rightley, Reports on Progress in Physics 77, 036902 (2014).
[2] L. O. Silva, et al., Physics of Plasmas 9, 2458 (2002).
[3] D. Schoeffler et al., Physical Review Letters 112, 175001 (2014); K. Schoeffler et al., Physics of Plasmas 23, 056304 (2016).
[4] R. A. Fonseca et al., Lecture Notes in Computer Science 2331, 342 (2002); R. A. Fonseca et al., Plasma Physics and Controlled Fusion 50, 124034 (2008)

For more information, see N. Shukla, K. Schoeffler, J. Vieira, R. A. Fonseca, and L. O. Silva, Weibel magnetic field competes with Biermann fields in laser-solid interactions, NP10.00141, 58th Annual Meeting of the APS Division of Plasma Physics, October 31–November 4 (2016), more details here.