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Abstract

Understanding magnetic-field generation and amplification in turbulent plasma
is essential to account for observations of magnetic fields in the universe. A
theoretical framework attributing the origin and sustainment of these fields to
the so-called fluctuation dynamo was recently validated by experiments on laser
facilities in low-magnetic-Prandtl-number plasmas ($\mathrm{Pm} < 1$). However,
the same framework proposes that the fluctuation dynamo should operate
differently when $\mathrm{Pm} \gtrsim 1$, the regime relevant to many
astrophysical environments such as the intracluster medium of galaxy clusters.
This paper reports a new experiment that creates a laboratory $\mathrm{Pm}
\gtrsim 1$ plasma dynamo for the first time. We provide a time-resolved
characterization of the plasma's evolution, measuring temperatures, densities,
flow velocities and magnetic fields, which allows us to explore various stages
of the fluctuation dynamo's operation. The magnetic energy in structures with
characteristic scales close to the driving scale of the stochastic motions is
found to increase by almost three orders of magnitude from its initial value
and saturate dynamically. It is shown that the growth of these fields occurs
exponentially at a rate that is much greater than the turnover rate of the
driving-scale stochastic motions. Our results point to the possibility that
plasma turbulence produced by strong shear can generate fields more efficiently
at the driving scale than anticipated by idealized MHD simulations of the
nonhelical fluctuation dynamo; this finding could help explain the large-scale
fields inferred from observations of astrophysical systems.