Configuration control, fluctuations, and transport in low-collisionality plasmas in the ATF torsatron

In low-collisionality plasmas confined in tokamaks and stellarators, instabilities driven by particles trapped in inhomogeneities of the magnetic field could be important in increasing plasma transport coefficients. In the Advanced Toroidal Facility (ATF), an l=2, M=12 field-period stellarator device with major radius R=2.1 m, average plasma minor radius a=0.27 m, central and edge rotational transforms χ0∼0.3, χa∼1, the effects of electron trapping in the helical stellarator field are expected to be important in plasmas with ne∼5x1012 cm-3, Te0∼1 keV. Such plasmas have already been sustained for long-pulses (20 s) using 150-400 kW of 53.2-GHz ECH power at B=0.95 T. Transport analysis shows that for ρ=r/a≤1/3, the electron anomalous transport is ≤10 times the neoclassical value, while at ρ=2/3 it is 10-100 times neoclassical; this is compatible with expectations for transport enhancement due to dissipative trapped-electron modes. (author) 4 refs., 3 figs