Electron Heating and Acceleration during Magnetic Reconnection in the PHASMA Experiment

Magnetic reconnection, a universal process converting magnetic energy into thermal and kinetic energy in space and laboratory plasmas, is often governed by processes that operate at kinetic scales. Measurements of particle velocity distribution functions (VDFs) at kinetic scales are routinely acquired in space plasmas and numerical simulations, but are still relatively rare in space-relevant laboratory experiments. One incoherent Thomson scattering system was implemented in the PHASMA (PHAse Space MApping) facility to enable non-perturbative and localized measurements of electron VDF (EVDF) during reconnection at the electron kinetic scale. We present EVDFs for electron-only magnetic reconnection between two argon flux ropes, showing electron heating of 0.5-1.0 eV. The electron energy gain corresponds to > 50% of the available reconnecting magnetic energy, which is larger than is normally observed in standard magnetic reconnection and consistent with the electron-only magnetic reconnection observations without the ion coupling in the terrestrial magnetosheath [Phan et al., Nature 557, 202, 2018]. As predicted for a finite guide field, electron heating is stronger along one separatrix, with Hall magnetic field weakening the guide field. Electron beams with velocities around the electron Alfvén speed are observed. We argue the existence of beams rather than bulk flows results from the marginally collisionless regime of PHASMA and possible electron trapping mechanisms. Particle-in-cell simulations designed specifically for PHASMA conditions will be introduced and compared to experimental results. Particle VDF measurements in laboratory reconnection complement observations from satellites and numerical simulations and provide new opportunities for validating theory and simulation via systematically and controllably varying related parameters.