  Observational evidence in space and astrophysical plasmas with long
collisional mean free path suggests that more massive charged particles may be
preferentially heated. One possible mechanism for this is the turbulent cascade
of energy from injection to dissipation scales, where the energy is converted
to heat. Here we consider a simple system consisting of a magnetized plasma
slab of electrons and a single ion species with a cross-field density gradient.
We show that such a system is subject to an electron drift wave instability,
known as the universal instability, which is stabilized only when the electron
and ion thermal speeds are equal. For unequal thermal speeds, we find that the
instability gives rise to turbulent energy exchange between ions and electrons
that acts to equalize the thermal speeds. Consequently, this turbulent heating
tends to equalize the component temperatures of pair plasmas and to heat ions
to much higher temperatures than electrons for conventional mass-ratio plasmas.
