Study of turbulent heating effects in the high beta tokamak Torus II

Abstract

Torus II is a rectangular cross section, high beta tokamak which has a glass vacuum vessel (aspect ratio of 3.5) filled with 12 mTorr helium. The plasma is heated by turbulent poloidal skin currents, induced by a fast (τr=1.7 μsec) reversal of the toroidal field which converts a toroidal Z pinch plasma into a tokamak plasma. By using the broadening of He II 4686 Å and impurity lines, it is found that strong turbulence (Ẽ⩽30 kV/cm microscopic fields at frequency ⩽ωpi≈3×1010 rad/sec) is induced during the heating phase. The turbulent Ẽ fields maximize in the region of maximum induced poloidal skin current (α∂Bφ/∂r). The resulting high beta tokamak plasma, with <β≳v≈10% and ne≈1015 cm−3, has an initially peaked radial temperature profile with Ti≈180 eV at the plasma center. A few microseconds later, the He II-determined ion temperature becomes nearly spatially flat at Ti≈75 eV, and then decreases slowly in time. Impurity ion lines indicated a temperature approximately 100 eV greater than He II. All line profiles indicated a poloidal circulation with Er×Bφ characteristics and Er≈100 V/cm radially inward. The spectroscopy was done in the visible region using a six channel polychrometer having a 0.2 Å resolution, which is capable of radial scans of the plasma. The details of turbulent heating derived from the analysis of ion line profiles are presented.