Local Simulations of Instabilities Driven by Composition Gradients in
  the ICM
  The distribution of Helium in the intracluster medium (ICM) permeating galaxy
clusters is not well constrained due to the very high plasma temperature.
Therefore, the plasma is often assumed to be homogeneous. A non-uniform Helium
distribution can however lead to biases when measuring key cluster parameters.
This has motivated one-dimensional models that evolve the ICM composition
assuming that the effects of magnetic fields can be parameterized or ignored.
Such models for non-isothermal clusters show that Helium can sediment in the
cluster core leading to a peak in concentration offset from the cluster center.
The resulting profiles have recently been shown to be linearly unstable when
the weakly-collisional character of the magnetized plasma is considered. In
this paper, we present a modified version of the MHD code Athena, which makes
it possible to evolve a weakly-collisional plasma subject to a gravitational
field and stratified in both temperature and composition. We thoroughly test
our implementation and confirm excellent agreement against several analytical
results. In order to isolate the effects of composition, in this initial study
we focus our attention on isothermal plasmas. We show that plasma
instabilities, feeding off gradients in composition, can induce turbulent
mixing and saturate by re-arranging magnetic field lines and alleviating the
composition gradient. Composition profiles that increase with radius lead to
instabilities that saturate by driving the average magnetic field inclination
to roughly $45^{\circ}$. We speculate that this effect may alleviate the core
insulation observed in homogeneous settings, with potential consequences for
the associated cooling flow problem.
