Insensitivity of a turbulent laser-plasma dynamo to initial conditions

TL;DR

This study shows that in turbulent laser-plasma experiments with high magnetic Reynolds numbers, the final magnetic field characteristics are unaffected by initial seed fields or modest flow differences, highlighting the dynamo's insensitivity.

Contribution

It demonstrates experimentally that the turbulent dynamo in laser-produced plasmas is insensitive to initial magnetic seed fields and flow variations, confirming theoretical predictions.

Findings

External magnetic fields influence initial flow and seed magnetic field strength.

Final stochastic magnetic fields are similar regardless of initial seed fields.

Turbulent dynamics dominate magnetic field amplification in high Reynolds number plasmas.

Abstract

It has recently been demonstrated experimentally that a turbulent plasma created by the collision of two inhomogeneous, asymmetric, weakly magnetised laser-produced plasma jets can generate strong stochastic magnetic fields via the small-scale turbulent dynamo mechanism, provided the magnetic Reynolds number of the plasma is sufficiently large. In this paper, we compare such a plasma with one arising from two pre-magnetised plasma jets whose creation is identical save for the addition of a strong external magnetic field imposed by a pulsed magnetic field generator (`MIFEDS'). We investigate the differences between the two turbulent systems using a Thomson-scattering diagnostic, X-ray self-emission imaging and proton radiography. The Thomson-scattering spectra and X-ray images suggest that the presence of the external magnetic field has a limited effect on the plasma dynamics in the experiment. While the presence of the external magnetic field induces collimation of the flows in the colliding plasma jets and the initial strengths of the magnetic fields arising from the interaction between the colliding jets are significantly larger as a result of the external field, the energy and morphology of the stochastic magnetic fields post-amplification are indistinguishable. We conclude that, for turbulent laser-plasmas with super-critical magnetic Reynolds numbers, the dynamo-amplified magnetic fields are determined by the turbulent dynamics rather than the seed fields and modest changes in the initial flow dynamics of the plasma, a finding consistent with theoretical expectations and simulations of turbulent dynamos.