Using particle-in-cell simulations, we investigate the onset of magnetic reconnection from a quiescent Harris current sheet in collisionless plasmas. After the current sheet is destabilized by the collisionless tearing mode instability, it proceeds to onset of reconnection, which manifests spontaneous thinning of current sheet and pileup of upstream magnetic flux. Once the current sheet thins to a critical thickness, about two electron inertial lengths, reconnection begins to grow explosively in this electron current sheet. This study shows that the spontaneous onset of collisionless magnetic reconnection is controlled by electron kinetics.

Motivated by the need of the electron density measurement for the Keda Reconnection eXperiment (KRX) facility which is under development, an interferometer system has been is used to fulfill the high phase difference measurement in such low temperature device. The results of the bench test show that the phase difference is accurately measured. In contrast to tens of degrees of phase shift expected to be measured on the KRX, the system noise (~1o) is low enough for the KRX diagnostics. In order to optimize the system for better performance, we utilize the Terasense Sub-THz imaging system to adjust alignment. The interferometer system has also been calibrated via changing of the optical path length controlled by the piezo inertial motor. Simultaneously, high density polyethylene (HDPE) thin film is introduced successfully to change a tiny phase difference and test the sensitivity of the interferometer system.

It is generally accepted that collisionless magnetic reconnection is initiated on electron scales, which is mediated by electron kinetics. In this paper, by performing a two-dimensional particle-in-cell simulation, we investigate the transition of collisionless magnetic reconnection from electron scales to ion scales in a Harris current sheet with and without a guide field. The results show that after magnetic reconnection is triggered on electron scales, the electrons are first accelerated by the reconnection electric field around the X line, and then leave away along the outflow direction. In the Harris current sheet without a guide field, the electron outflow is symmetric and directed away from the X line along the center of the current sheet, while the existence of a guide field will distort the symmetry of the electron outflow. In both cases, the high-speed electron outflow is decelerated due to the existence of the magnetic field B z , then leading to the pileup of B z . With the increase of B z , the ions are accelerated by the Lorentz force in the outflow direction, and an ion outflow at about one Alfvén speed is at last formed. In this way, collisionless magnetic reconnection is transferred from the electron scales to the ion scales.