A Study On The Driving Of Plasma Rotation And Confinement In Tandem Mirror

Rotation is perhaps an intrinsic feature of a plasma system,which plays an important role in the evolution of plasma.In magnetic confinement fusion plasma,rotation is found to be of particular importance to the improvement of the radial plasma confinement in toroidal devices as well as the axial plasma confinement in linear open traps.Contrary to other devices,in this paper,a novel method is explored to drive the rotation of the plasma,namely Rotating Magnetic Field（RMF）.The principle is that the oscillating magnetic field initially excites the axial shielding current Jz within the plasma,and then the current interacts with the radial magnetic field Br to generate the Hall term<JzBr>/ne to drive the angular rotation of electrons,finally the electrons transfer angular momentum to ions via electron-ion collision.The experiments are carried out at the Keda Mirror with AXisymmetricity（KMAX）.The typical density and electron temperature of plasma generated by two similar medium-sized washer guns attached at the end of KMAX are ne～3-5 × 10¹⁸m^-3 and Te～5-15 eV,respectively.Four sets of "U" type copper antennas are installed at the axial position Z=0 m to generate a rotating magnetic field.Each set of antennas is independently fed by an 88 kHz pulse power supply with a maximum current of 1.8 kA.In RMF-related experiments,the rotational velocity of the plasma is diagnosed using the homemade Mach probe.Firstly,experiments are carried out on the driving of plasma rotation.In the covered area of the RMF antenna,it is found that when the intensity of the rotating magnetic field reaches a certain threshold so that the magnetization parameter is greater than the penetration parameter,a significant angular rotation can be observed in the plasma,and the rotation velocity of the plasma at the edge can be up to 0.3 Mach.By measuring the rotation velocity under different axial magnetic fields,it is demonstrated that the mechanism of RMF driving plasma rotation is that RMF first drives the rotation of electrons,and then the rotational momentum is transferred to ions through electron-ion collisions.In the non-antenna region,it is found that RMF can also drive plasma rotation in other axial positions of the central chamber,which is not weaker than that in the central plane.In experiments using a single-side washer gun for plasma injection,rotation was also observed in the plasma of the downstream end cell.By analyzing the start time of rotation and radial distribution of rotation velocity,it is determined that the driving mechanism is that the magnetic disturbance caused by the rotating magnetic field propagates to the non-antenna region,and then drives the angular rotation of the plasma.It also provides the theoretical and experimental basis for the subsequent application of RMF to control plasma rotation remotely.Finally,the influence of RMF on the confinement of plasma is also investigated.When RMF is on,the density profile changes significantly,specifically,the radial density gradient scale length decreases from 11 cm to 6 cm.A numerical calculation using the diffusion model proves that the radial diffusion mode has changed from the Bohm diffusion to the classical diffusion.It is worth noting that RMF also has a significant suppressive effect on flute,that is,the fluctuation of the density is weakened and the plasma core no longer deviates from the axis of KMAX.At the same time,the axial confinement of the plasma is also enhanced when RMF is turned on.