Application Of Field Reversed Configuration Formed By Rotating Magnetic Field In KMAX Tandem Mirror

The axial loss of particles in magnetic mirror has posed a major challenge for its feasibility as a magnetic fusion approach.Advanced concepts,such as tandem mirror and field reversed mirror,have thus been proposed in the early days of mirror research to improve the simple mirror's axial confinement.However,those solutions proved to be cumbersome.The purpose of this dissertation is to explore a new method to address this problem while maintain the simplicity of the mirror device.Specifically,a closed magnetic field structures,or field reversed configurations(FRCs),are employed in the two end cells of a magnetic mirror to capture or reflect the axial escaped particles.First,we experimentally elaborate on how to form FRC using rotating magnetic field(RMF)technique and identify the mechanisms to contribute to RMF/FRC formation process.Then,to investigate the influence of RMF/FRCs on the confinement,we calculate the orbits of particles that are lost from central mirror into the RMF regions.Finally,preliminary experimental results are presented.This work has been carried out in the tandem mirror device,Keda Mirror with AXisymmetricity(KMAX).Odd-parity RMF antenna scheme was adopted due to its advantages in the field line closeness.Two RMF antenna suites have been assembled inside two end cells of KMAX separately.RMF is generated by tuning the phase difference between antenna currents to 90 degrees.Each RMF antenna suite composes of four sets of antennas,which are energized by four independent power sources controlled by Insulated Gate Bipolar Transistors to deliver currents upto 1500 A with several ms duration and frequency of 84 kHz.With a seed plasma provided by a washer gun,a significant azimuthal plasma current of?6 kA/m has been successfully driven by RMF,forming a FRC with external magnetic field?50 Gauss,trapped poloidal magnetic flux?0.15 mWb and separatrix radius?17 cm.Using inserted B-dot array and diamagnetic loop array,we demonstrate detailed evolution of 2D FRC topology and verify that radial distribution of magnetic field and density are consistent with rigid rotor model.Moreover,the nonlinear transition from Helicon to FRC mode is also observed by varying RMF strengths.We further investigate different current driving mechanisms by measuring detailed penetration pattern of transverse and axial RMF components(Br?and Bz?).In addition to Hall electric field of even-parity RMF,azimuthal electric field E?? induced by Bz?could also contribute to current driving in the odd-parity RMF.We notice that plasma current driven by the odd-parity RMF is larger than the even-parity RMF even with similar Br?.Besides,we also scan the bias magnetic fields to change penetration depth of RMF,and find that E?? term is responsible for the trend of how plasma current changing with bias magnetic field and plays an important role in driving the plasma currents.Finally,the influence of RMF/FRCs on the dynamics of lost ions from central mirror has been explored via single particle orbit simulation.Two related mechanisms have been identified:(1)Pure FRCs in end cells can reflect back some ions lost from central mirror.This is caused by reflecting force due to the magnetic field gradients along the open field line outside of the FRC,which is equivalent to a mirror consisting of the mirror center around X-Point and mirror throat in the FRC midplane.Such mirror is effective for ions with large perpendicular velocities,but less effective for the ions lost from the central cell with relatively small perpendicular velocities.(2)The inward Hall electric field E,outside of FRC separatrix line can drive ions inside towards FRC,and more importantly,this electric field causes azimuthal drift velocity vi?=-Er/Bz,which yields an axial Lorentz force-evi?Br,pointing back towards the central mirror.These effects favor reflecting back or even capturing lost ions.Near the end,preliminary experimental results are also presented,including enhancement of plasma density and visible light intensity of center plasma and statistically increasing energy confinement time with RMF/FRC in the end cells.Last but not least,RMF away from the center cell has been experimentally demonstrated to be capable of stabilizing the mirror flute instability in the center cell.