Determination of the ion distribution function during magnetic reconnection in the versatile toroidal facility with a gridded energy analyzer

Abstract

A gridded energy analyzer (GEA) diagnostic and associated electronics are designed and built to explore the evolution of the ion distribution function during driven magnetic reconnection in the Versatile Toroidal Facility. The temporal evolution of the ion characteristic is measured at different locations throughout the reconnection region, for a number of magnetic field configurations. The measured ion characteristics are found to be in excellent agreement with a theoretical fit constructed from a double Maxwellian distribution, from which the temperatures and drift velocities are found as functions of space and time. It is found that the ion temperature of each Maxwellian exhibit minor temporal variations during reconnection which are negligible in terms of ion heating. Additionally, the temperatures do not significantly change with varying radial position, or magnetic cusp strength. The drift velocities are observed to evolve in time, scale with magnetic cusp strength, and to depend on the exact location throughout the reconnection region. The ions are thus subject to acceleration due to the electric field induced by the ohmic drive. The appearance of a double Maxwellian distribution during the reconnection drive is hypothesized to be due to double ionization of argon atoms. Repetition of the experiment with a hydrogen plasma verified that this scenario is most probable.