| In the Madison Symmetric Torus (MST) reversed field pinch discrete bursts of magnetic reconnection are driven by resonant tearing modes. These events liberate a large amount of energy from the equilibrium magnetic field, of which a significant fraction (10--25%) becomes ion thermal energy. The ion temperature is observed to increase by as much as a factor of four in <100 micros, much faster than the classical ion-electron collision rate.;New experimental measurements reveal previously unknown features of the majority and impurity ion distribution functions during these events and offer insight into the character of the energization process. First, measurements of the D-D fusion neutron flux and charge exchange neutral energy spectra show there to be a suprathermal tail on the majority ion distribution, representing up to several percent of the total ion density. This tail is generated at the reconnection event, decreases in size with plasma density, and is well-described by a power law. Second, a detailed comparison of the local parallel and perpendicular C+6 impurity ion temperature is made with charge exchange recombination spectroscopy. It is found that Tperp > Tpar near in time to the reconnection, implying that the heating mechanism favors the perpendicular degree of freedom. Furthermore, it is observed that the anisotropy increases with plasma density. This counter-intuitive observation is explained by postulating a varying impurity content in the plasma.;Additionally, a review of previous experimental results on MST is presented, and an attempt is made to evaluate existing theoretical models of ion heating based on a synthesis of these results. We find that the collection of observations cannot be explained with a single existing model. |
