Laboratory Experiment Of Electron Scale Magnetic Reconnection

Magnetic reconnection is a fundamental physical process in space plasma.It rapidly converts magnetic energy into plasma kinetic and thermal energy with topology change of oppositely directed magnetic field lines.As an significant energy conversion mechanism,magnetic reconnection plays an important role in explosive phenomenons in space such as solar flare,coronal mass ejection and substorm.Therefore,studies of magnetic reconnection is an important part of space physics research.Satellite observation,numerical simulation and ground-based laboratory experiment are the main methods for studying magnetic reconnection.Laboratory experiment has recieved more and more attention because it has several advantages:the plasma is controllable and the diagnostic is comprehensive,positive,multi-points and high-precision.Laboratory experiments of magnetic reconnection started from the 1970s and has a history of nearly 50 years until now.There are several devices that is studying magnetic reconnection around the world now.These devices respectively have their own unique characteristics and have different ways to generate plasma and drive magnetic reconnection.Therefore,these devices are focus on different physical problems.From 2003,Zhang et al began to study magnetic reconnection with the Keda Linear Magnetized Plasma(KLMP)device in University of Science and Technology of China.We started a series of systematic researches of magnetic reconnection in the KLMP device based on Zhang,s original work from 2016.In the device,an oxide-coated cathode is used to produce uniform and high density plasma column with diameter of 15cm.30G-300G axial magnetic field is produced by a set of coils to constrain the plasma and to work as the guide field of magnetic reconnection.Magnetic field of reconnection is generated by two parallel conductor sticks which is placed in the up and sown side of plasma column and spacing 10cm.Magnetic fields with opposite directions are produced in the middle of the chamber when pulsed currents are conducted in the two sticks.In the rising phase of the pulsed current,magnetic fields generated by the two sticks are pushed towards the center region and drive magnetic reconnection.The magnetic reconnection produced in the experiment is electron-scale collisionless reconnection,but the motion of ions is couples with electrons due to the small size of our device.The main researches of magnetic reconnection in this paper are as follows:(1)Verification of magnetic reconnection in the experimentWe verify that magnetic reconnection is realized in the experiments in two different discharge modes.In the single anode discharge mode,by measuring the out-of-plane current and the radio of common flux versus private flux,the occurrence of magnetic reconnection is preliminary verified.By adding a secondary anode in the end of the device,the discharge mode is changed and the plasma column carries a background current.When the direction of background current is same as the expected current of current sheet,the out-of-plane current increases significantly and the evolution of plasma density shows the existence of reconnection outflow.When the currents are in opposite directions,there is no obvious increase of out-of-plane current and the plasma density remains unchanged.Comparison of the two situation verifies the realization of magnetic reconnection and indicates that the background current greatly influences the magnetic reconnection.(2)Experimental research on scaling rate of magnetic reconnectionWe developed a programmable pulsed current source to provide adjustable pulsed current for the conductor sticks.By changing the rising edge slope of the pulsed current,magnetic reconnections with different drives are studied.The reconnection inflow speed raises as the slope raised which indicates that the drive of magnetic reconnection is indeed adjustable.The effective resistivity ?eff is derived by the measured out-of-plane electric field and current,and it is found to be larger than the calculated classical resistivity ?c which demonstrates the existence of anomalous resistivity.The ratio of effective resistivity and classical resistivity?eff/?cl is used to scaling magnetic reconnection.As the drive of reconnection becomes larger,?eff/?c also increases.When the drive becomes large enough,?eff/?c tends to be saturated.By adding mesh in the first anode of the device,electron temperature of the plasma can be raised from 3.6eV to 9.2eV which means the electron collision becomes weaker.Experimental results shows that when the collision is weaker,the anomalous resistivity significantly raises and the variation tendency with different drive of reconnection stays the same.