Experimental Study Of Collisional-Merging FRC In KMAX

The Field reversed configuration(FRC)is believed to have the potential of becoming an attractive fusion reactor due to its high β property and engineering simplicity,where the βrefers to the ratio of thermal to the magnetic pressure.As a method to generate highperformed FRC,collisional-merging FRCs have been hotly pursued and studied by a number of domestic and abroad experimental devices recently.Even so,the merging of two colliding FRCs has not been verified directly in an experiment.The KMAX-FRC is an FRC experiment with θ-pinch coils installed inside the central cell of a tandem mirror device named KMAX(Keda Mirror with AXisymmetry).To reveal the fine magnetic structure of FRC during merging,a few minor upgrades were incorporated to improve the reliability and repeatability of the KMAX-FRC experiment.Specifically,we have significantly reduced the electromagnetic interference caused by FRC discharge by replacing the capacitor-to-electrode transmission line with new coaxial cables;achieved fast,controllable,and steady pulse gas injection by employing newly developed rapid electromagnetic valves;and built a series of plasma diagnostics such as magnetic probes,triple probes,bolometers,etc.,to meet the physical study of FRC merging through measured FRC parameters.In the dissertation,the formation and translation of KMAX-FRC were first investigated,and then the performance of translated FRC near the device’s mid-plane was optimized.Finally,the FRCs in the north and south formation sections have been successfully tuned up with similar parameters.After that,the collisional-merging FRC experiment was carried out.A home-made 2D magnetic probe array was utilized to record the 2D magnetic field of FRC on the R-Z plane,which has successfully demonstrated the high-speed translation of FRC,the axial compression of FRC,and the collisional-merging of two single-translated FRCs.For the first time,we revealed the merging process of two super-Alfvenic FRCs through magnetic reconnection in the 2D plane during 2-3 radial Alfven times.During the collision,two single-translation FRCs were decelerated and underwent severe axial compression and radial expansion,followed by an increase in toroidal current density and poloidal flux.Furthermore,the electron temperature at the reconnection region was observed to be heated by axial compression and magnetic reconnection.Based on the heating mechanisms described above,the research discovered that the increase in average electron temperature of collisional-merging FRC may be limited by the maximum radial expansion.The result not only explains why the electron temperature was substantially lower than the ion temperature in the previous experiment,but also provides a guideline for the quasi-steady-state operation of collisional-merging FRC in the future.In addition,the toroidal magnetic profiles of KMAX-FRCs were measured.The polarity of the internal toroidal field in two translation FRCs was found to be the opposite.The structure of the toroidal field in each single-translation FRC was plausible explained by the Hall effect.Preliminary experiments on KMAX collisional-merging FRC revealed that the merged FRC preserved a significant toroidal field.Moreover,the possible generation of shock waves and the inferred ion temperature above the theoretical prediction were also observed experimentally.At the same time,deuterium FRC,with stronger kinetic effects,was found to be less prone to tilt instability than hydrogen FRC.The parameters of a well-formed single-translated FRC in the translation section were:ne～2-4×1019 m-3,Te～8eV,Ti～5eV,Rs～0.2m,ls～0.6 m,and φp(RR)～0.2mWb.The typical parameters of merged hydrogen FRC were:Rs～0.3-0.4 m,ne～1.5 × 1019 m-3,Tt～25eV,φp～0.6mWb and the typical parameters of merged deuterium FRC were:Rs～0.3-0.4m,ne～1×1019 m-3,Tt～35-40eV.Finally,we also conducted a novel FRC axial compression experiment with highspeed translated θ-pinch plasma in the KMAX device,achieving about 3 times the axial adiabatic compression of FRC and identifying an increase in the internal temperature and density of FRC.The experiment demonstrates the potential of plasma jets as compression drivers for the magnetized targets and may contribute to the development of the innovative magneto-inertial fusion concept based on plasma jets.