The generation of large-scale strong magnetic fields remains an unsolved mystery in modern astrophysics and plasma physics. The turbulent dynamo model based on magnetohydrodynamics (MHD) have shown that large-scale magnetic structures can be generated in helically-driven turbulent plasmas, with the underlying physics being the α-effect that characterizes symmetry breaking and the corresponding inverse transfer of magnetic energy. However, MHD models are only applicable to low-temperature, dense, and strongly collisional astrophysical environments. For some astrophysical environments such as the intracluster medium and parts of the interstellar medium, which are hot and tenuous, whether helical driving can excite the generation of large-scale magnetic fields remains an open question.
Here, we explore the large-scale turbulent dynamo process in weakly collisional environments through numerical simulations and laboratory experiments based on high-power lasers. In the numerical simulations, we find that the ubiquitous velocity shear in turbulence can excite pressure anisotropy through the kinetic phase-mixing mechanism, which in turn dissipates small-scale magnetic fields to generate net magnetic helicity and promote the inverse transfer of magnetic energy, ultimately forming macroscopic magnetic structures at the system scales [1]. We have also confirmed the theoretically predicted scaling law: the efficiency of this inverse transfer of magnetic energy is proportional to the square of the turbulent velocity and inversely proportional to the injection scale of turbulence. Additionally, we conduct helical dynamo experiments in laser produced hot plasmas, using lasers to irradiate target alternately to generate helical plasma flows rotating in opposite directions (clockwise and counterclockwise), and guiding the two plasma flows to collide to generate helical turbulence to excite the large-scale turbulent dynamo. Proton radiography provides experimental evidence for the formation of macroscopic magnetic structures, and the reconstructed magnetic power spectra give preliminary support for the inverse transfer of magnetic field energy.
In summary, through simulations and experiments, we have demonstrated that the helical dynamo process can also be excited in astrophysical-like hot and tenuous plasmas to generate large-scale magnetic structures, observing the precursor of large-scale turbulent dynamo in laser produced plasmas and revealing the potential role of kinetic effects in circumventing the “dynamo quench” in conventional MHD models.

turbulence,dynamo,magnetic field,Laboratory astrophysics