High-Beta Plasma Confinement and Inward Particle Diffusion in the Magnetospheric Device RT-1

Full text: Ring Trap 1 (RT-1) is a magnetospheric device that confines plasma in a dipole field generated by a levitated superconducting coil. An ultra high-beta state (possibly β> 1) of Alfvenic flowing plasma is theoretically predicted, where the plasma pressure is primarily balanced by the dynamic pressure of the fast flow. The final goal of RT-1 is to generate high-beta flowing plasma in the magnetospheric configuration capable of burning advanced fusion fuels. We have realized stable confinement of high-beta hot-electron plasma generated with electron cyclotron resonance heating (ECH) in RT-1. Geomagnetic field compensation and optimized operation have realized drastic improvements of the plasma properties, and the maximum local beta value has reached 70%. The maximum energy confinement time estimated from the diamagnetic measurements of plasma pressure and input RF power is 80 ms. Grad-Shafranov equilibrium calculation indicates that the pressure profiles have rather steep gradient near the superconducting coil in the strong field region. Cross field particle diffusion into the strong field region has been confirmed by pure electron plasma experiment. The spatial profiles of hot electrons were observed using a soft x-ray CCD camera. When the coil is not levitated, the major loss channel of the high temperature electrons is the coil support structure, from where we observed strong x-ray radiation. (author)