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Overview of the recent results of the RT-1 magnetospheric experiment with a levitated superconducting coil

Author: Haruhiko Saitoh
Requested Type: Consider for Invited
Submitted: 2009-12-05 05:00:33

Co-authors: Z. Yoshida, J. Morikawa, Y. Yano, T. Mizushima, S. Kobayashi

Contact Info:
Graduate School of Frontier Sciences, The Universi
5-1-5 Kashiwanoha
Kashiwa, Chiba   277-856
Japan

Abstract Text:
The Ring Trap 1 (RT-1) experiment aims to produce an ultra high-beta plasma in a magnetospheric configuration suitable for burning advanced fusion fuels. RT-1 has taken a hint from a flowing high-beta plasma observed in the Jovian magnetosphere, and plans to confine a plasma using a hydrodynamic pressure due to an Alfvenic flow. We report the recent experimental results of RT-1, especially focusing on the improved confinement properties of the plasmas realized by the coil levitation. In RT-1, a dipole field coil made with Bi-2223 high-temperature superconducting wires is magnetically levitated inside the vacuum chamber, and the plasma is generated by ECH. The ECH plasma in RT-1 has two electron populations with different temperatures. By adjusting the filling neutral gas pressure below approximately 1 mPa, the hot electrons become the major component of the electrons, and the maximum local beta exceeds 40 %. For the measurements of the spatial profiles of the hot electrons, or pressure profiles, we installed a soft x-ray CCD camera. When 8.2 GHz RF is applied, where the ECR layer intersects the case of the dipole field coil, the x-ray emitting region is localized near the coil. Bremsstrahlung by the collisions between the energetic electrons and the coil is also observed, indicating that some of the hot electrons are lost before filling the confinement region. For 2.45 GHz discharges, x-rays are observed over the entire region inside the separatrix. When the coil is not levitated, the major loss channel of the hot electrons is the support structure of the coil.

In order to realize pure magnetic confinement of non-neutral plasmas, we have also conducted experiments on a pure electron plasma in RT-1. A toroidal electron plasma with a density of 10^11 m^-3 is trapped for more than 300 s in RT-1, when the superconducting coil is magnetically levitated and the disturbance is minimized. After the electron injection ended, the growth rate of the electrostatic fluctuation amplitude is close to zero during the stable confinement phase. The observed coherent fluctuation implies a rigid-rotor equilibrium state of the plasma, although the plasma is confined in the strongly inhomogeneous dipole magnetic field. For perturbation-free measurements of the spatial profiles of the plasma, we installed a wall probe array. Each of the wall probe signal was monitored by a current amplifier and an integration circuit, and the strength of a radial electric field was directly measured. We used three wall probes and estimated the confinement regions of the plasma. During the electron gun operation, the outer edge of the electron confinement region approximately agreed with the magnetic separatrix. After the electron supply ended, electrons on the magnetic surfaces that intersected the electron gun structures were rapidly lost. In the stable confinement period, electrons shifted inward and moved to the strong magnetic field region.

Characterization: A3,A5

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