Numerical Simulation And Experimental Investigation Of Collisionless Magnetic Reconnection

Magnetic reconnection is an important energy conversion mechanism in cosmic interplanetary space and laboratory plasma,which converts the energy stored in the magnetic field into kinetic and thermal energy of the plasma by changing the topological configuration of the magnetic field,and can explain many explosive phenomena such as cosmic energetic particles,solar flares and coronal mass ejections,magnetosphere substorms,and sawtooth collapse in fusion experiments.Since the concept of reconnection was proposed in the 1940s,tremendous studies have been carried out through numerical simulations,satellite observations,and laboratory experiments.Fully understanding of magnetic reconnection remains challenging,including onset mechanism,scale evolution,and particle acceleration,etc.In this thesis,(1)We investigate the onset and scale evolution of magnetic reconnection based on 2D(Two-Dimensional)PIC(Particle-in-Cell)numerical simulations,it is found that the collisionless reconnection onset is mediated by electron dynamics rather than ion dynamics.In addition,the electron outflow plays an important role on scale evolution.(2)We have developed a methodology to ensemble average a number of sawtooth reconnection events in magnetic confinement plasmas(EAST device).It is found that sawtooth reconnection happens much faster than what Kadomtsev full scale reconnection model predicts.(3)We have tested advanced polarimeter-interferometer diagnostics for a new magnetic reconnection platform(KRX)where magnetic reconnection can be driven externally in a well controlled way.Initial test of instrument on bench and forward 2D modeling based on PIC simulation illustrate that new developed instrument is capable of studying reconnection on KRX in future.The main contents of this thesis are given as follows:1.Spontaneous onset of collisionless reconnection and the evolution of collisionless magnetic reconnection from electron scales to ion scales(1)How magnetic reconnection is triggered and what physics is responsible for its occurrence is one of the important issues in reconnection studies.Using 2D PIC numerical simulation,we investigate the spontaneous onset process of collisionless reconnection in an undriven steady-state Harris current sheet.It is found that after the current sheet is destabilized by the collisionless tearing mode,the current layer thinning and gradually develops an X line.Because the seed reconnection electric field is so low that it cannot fully convert the incoming flux,therefore generates a pileup of flux on both sides of the current layer until the reconnection enters a fast growth phase.By changing the mass ratio,it is found that once the current sheet thins to a critical thickness,about 2 electron inertial length,the reconnection begins to grow explosively in the electron current sheet.This study suggests that the collisionless reconnection onset is influenced by electron dynamics rather than ion dynamics.(2)Magnetic reconnection is a multiscale physical process,and although electron only reconnection has been found,most of the reconnections are multiscale coupled.The reconnection onset is in the tiny electron diffusion region but can promote energy transfer and conversion at huge scales,and the question is how the small and large scales are coupled together.By performing two-dimensional numerical simulations,we study the transition of collisoionless reconnection from electron scales to larger ion scales in a Harris current sheet with and without a guide field.After the reconnection enters the fast growth phase from onset phase of the electron scales,the electrons are accelerated by the reconnection electric field and then leave away along the outflow region.In antiparallel reconnection,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 Bz,and this deceleration process further promotes the pileup of Bz.With the increase of Bz,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.2.Sawtooth reconnection events in fusion plasmaSawtooth reconnection can release magnetic energy in the fusion plasma which is unfavorable for plasma confinement,in particular,sawtooth reconnection may provide seed field to trigger neoclassical tearing mode to cause discharge disruption which is not acceptable for nuclear fusion reactor.We perform statistical study of sawtooth collapse in the EAST fusion device by selecting approximately 600 experimental shots with typical sawtooth characteristics during the last three years.Sawtooth collapse time is determined by a rapid drop of ECE diagnostic signals.Statistical results show the collapse time is not in agreement with the reconnection time predicted by the Kadomtsev full scale reconnection model,but more agreement with the two fluids model(Wesson model).It is evidential that the temperature change is well correlated with magnetic field change during the collapse process.Plasma equilibrium profile is obtained by EFIT inversion with using synthetical average of the polarimeter and interferometer diagnostics signals,combined with ECE and SXR diagnostics,the comparison of current profile before and after the sawtooth collapse demonstrates that the internal magnetic energy is reduced during sawtooth reconnection.The safety factor q slightly increases after sawtooth reconnection but less than one,which is further consistent with the characteristics of two-fluid incomplete reconnection model.This will promote further investigation of the sawtooth reconnection in the EAST device.3.Development of polarimeter and interferometer measurements for Keda Reconnection eXperiment(KRX)deviceNeither investigation on magnetic reconnection in space plasmas nor in magnetic fusion plasmas is easy.To investigate magnetic reconnection in laboratory plasma,we have developed an instrument to measure magnetic reconnection in a newly constructed device KRX(Keda Reconnection eXperiment).KRX has many advantages for magnetic reconnection studies with well controllable plasma parameters and excellent reproducibility.KRX reconnection is externally driven by a pair of parallel current plates and the magnetic configuration is similar to 2D PIC model as described before.We develop advanced non-perturbation polarimeter and interferometer diagnostics for KRX.Synthetic forward 2D PIC simulation analysis provides great assistance to design horizontal and vertical line-of-sight measurement on KRX.It is found that we can directly infer the thickness of the reconnection current sheet and estimate the occurrence of reconnection,which is used to further investigate the onset and scale evolution of magnetic reconnection.To ensure the sensitivity of the diagnostics,the 320 GHz solid microwave source with 1 mm wavelength is used to fulfill the high phase difference measurement in such low temperature plasma device.After a series of calibration and optimization,the results of bench test show that the system noise can be controlled within 1 degree,which is only 1%measurement error for KRX plasma.This satisfies the need for accurate reconnection measurement on KRX.