Noncollisional ion heating in laboratory and astrophysical plasmas and the mechanism of conversion of magnetic energy to ion thermal energy are not well understood. In the Madison Symmetric Torus reversed-field pinch experiment, ions are heated rapidly during impulsive reconnection, attaining temperatures exceeding hundreds of eV, often well in excess of the electron temperature. The energy budget of the ion heating and its mass scaling in hydrogen, deuterium, and helium plasmas were determined by measuring the fraction of the released magnetic energy converted to ion thermal energy. The fraction ranges from about 10%–30% and increases approximately as the square root of the ion mass. A simple model based on stochastic ion heating is proposed that is consistent with the experimental data.

• Received 29 June 2009

DOI:https://doi.org/10.1103/PhysRevLett.103.145002