Insensitivity of a turbulent laser-plasma dynamo to initial conditions

Abstract

It has recently been demonstrated experimentally that a turbulent plasma created by the collision of two inhomogeneous, asymmetric, weakly magnetized, 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-magnetized plasma jets whose creation is identical save for the addition of a strong external magnetic field imposed by a pulsed magnetic field generator. 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 external magnetic field has a limited effect on the plasma dynamics in the experiment. Although the external magnetic field induces collimation of the flows in the colliding plasma jets and although 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 energies and morphologies of the stochastic magnetic fields post-amplification are indistinguishable. We conclude that, for turbulent laser-plasmas with supercritical magnetic Reynolds numbers, the dynamo-amplified magnetic fields are determined by the turbulent dynamics rather than the seed fields or modest changes in the initial flow dynamics of the plasma, a finding consistent with theoretical expectations and simulations of turbulent dynamos.

MHD turbulence: a biased review

“…2018; Bott et al. 2021 b , 2022). The system eventually saturates with magnetic energy comparable to kinetic, but not, it seems, necessarily equal to it scale by scale – what the final state is remains an unsolved problem, both numerically (due to lack of resolution) and theoretically (due to lack of theoreticians).…”

“…Whereas I have made an effort to keep all results general and applicable to the high- limit, it is, in fact, quite hard to find naturally occurring high- plasmas for which the standard viscoresistive MHD equations are a good model: this would require the particles’ collision rate to be larger than their Larmor frequency, which rarely happens at high temperatures and low densities needed to achieve high (one exception, quite popular these days, is plasmas created in laser experiments: see, e.g., Bott et al. 2021 b , 2022). In fact, most of the interesting (and observed) plasmas in this hot, rarefied category are either ‘dilute’ (an apt term coined by Balbus 2004 to describe plasmas where turbulence is on scales larger than the mean free path, but the Larmor motion is on smaller scales that it – a good example is galaxy clusters; see, e.g., Melville, Schekochihin & Kunz 2016 and references therein) or downright collisionless (i.e., everything happens on scales smaller than the mean free path; the most obvious example is the solar wind: see the mega-review by Bruno & Carbone 2013 or a human-sized one by Chen 2016).…”

“…The experimental dynamo observed in laser plasmas may be in this regime (Bott et al. 2021 b , 2022).…”

“…2018; Bott et al. 2021 b , 2022). Thus, it seems that my scepticism was premature and we ought to tackle the large- dynamo with renewed vigour and sense of relevance.…”

Kinetic stability of Chapman–Enskog plasmas

“…2018; Bott et al. 2021 c , b , 2022) – found evidence for the existence of large-amplitude local temperature fluctuations over a range of scales, a finding that was inconsistent with Spitzer thermal conduction (Meinecke et al. 2022).…”