Observation of localized ion heating during driven collisionless magnetic reconnection

The Versatile Toroidal Facility (VTF) is a magnetic reconnection experiment, in which plasma is generated in a magnetic cusp field by ECR heating. Reconnection is periodically driven independently from the plasma generation via an additional set of toroidal magnetic coils, which provides highly reproducible conditions and allows for measurements of the ion velocity distribution function (IVDF) with high spatial and temporal resolution by means of laser induced fluorescence (LIF). In the present scheme ArII metastable ions are excited tangentially to the magnetic guide by a diode laser field at a wavelength of 668nm and the fluorescence light at a wavelength of 442nm is observed. The parallel component of the IVDF is recorded with and without driven reconnection at three different horizontal positions: 30 apart the x-line, 3 cm apart the x-line (close to, but outside the reconnection region) and directly at the x-line (inside the reconnection region). While the ion temperature outside the reconnection region remains constant, a significant increase of the ion temperature is found at the x-line during driven reconnection. A detailed analysis on the timescale of the reconnection drive reveals strong ion heating within a reconnection cycle and the heating is found to scale with the reconnection drive amplitude. These findings are consistent with Monte Carlo simulations, which suggest that the observed heating is a consequence of an in-plane electric field that forms around the x-line in response to reconnection