  We investigate the self-consistent particle acceleration physics associated
with the development of the kink instability (KI) in nonrelativistic,
electron-ion plasma jets. Using 3D fully kinetic particle-in-cell (PIC)
simulations, we show that the KI efficiently converts the initial toroidal
magnetic field energy into energetic ions. The accelerated ions form a
nonthermal power-law tail in the energy spectrum, containing $\simeq10\%$ of
the initial magnetic field energy, and with the maximum ion energy extending to
the confinement energy of the jet. We find that the ions are efficiently
accelerated by the concerted action of the motional electric field and highly
tangled magnetic field that develop in the nonlinear phase of the KI: fast
curvature drift motions of ions across magnetic field lines enable their
acceleration along the electric field. We further investigate the role of
Coulomb collisions on the ion acceleration efficiency, and identify the
collisional threshold above which nonthermal ion acceleration is suppressed.
Our results reveal how energetic ions may result from unstable nonrelativistic
plasma jets in space and astrophysics, and provide constraints on the plasma
conditions required to reproduce this acceleration mechanism in laboratory
experiments.
