Resistive and sympathetic cooling of highly-charged-ion clouds in a   Penning trap
  We present measurements of resistive and sympathetic cooling of ion clouds confined in a Penning trap. For resistive cooling of a cloud consisting of one ion species, we observe a significant deviation from exponential cooling behavior which is explained by an energy-transfer model. The observed sympathetic cooling of simultaneously confined ion species shows a quadratic dependence on the ion charge state and is hence in agreement with expectations from the physics of dilute non-neutral plasmas. 
M. Vogel, H. H\"affner, K. Hermanspahn, S. Stahl, J. Steinmann and W.   Quint
physics.atom-ph