9 research outputs found
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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 fields could be important in increasing plasma transport coefficients. In the Advanced Toroidal Facility (ATF), an {ell} = 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 {chi}{sub 0} {approx} 0.3, {chi}{sub a} {approx} 1, the effects of electron trapping in the helical stellarator field are expected to be important in plasmas with {bar n}{sub e} {approx} 5 {times} 10{sup 12} cm{sup {minus}3}, T{sub e0} {approx} 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 {rho} = r/a {le} 1/3, the electron anomalous transport is {le}10 times the neoclassical value, while at {rho} = 2/3 it is 10--100 times neoclassical; this is compatible with expectations for transport enhancement due to dissipative trapped-electron modes. 4 refs., 3 figs
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A model for the L-H transition in tokamaks
Fluctuation-driven transport fluxes in the plateau regime are calculated with the methodology of neoclassical transport theory. Particle and heat fluxes are the most sensitive to fluctuations; the modification to plasma resistivity is the least sensitive. The fluctuation-driven bootstrap current and Ware pinch flux are moderately sensitive and depend on the radial mode structure. One of the thermodynamic forces depends on the radial electric field E/sub r/. Changing E/sub r/ can change the fluctuation spectrum and thus the transport fluxes. The effects of E/sub r/ on the fluctuation spectrum are caused by the radial shear of the angular velocity, which is proportional to E/sub r//r. Studies of the dynamic evolution and the saturation of MHD turbulence under the influence of E/sub r/ show that the saturation amplitudes are lower and the confinement is thus better for a more negative value of E/sub r/. A proposed model for the L-H transition is based on the improved confinement with more negative E/sub r/. A scaling for the power threshold P/sub th/ is P/sub th/ ..cap alpha.. N/sup 3/q/(I/sub p//sup 2/M/sub i/), with the N plasma density, q the safety factor, I/sub p/ the plasma current, and M/sub i/ the ion mass. 14 refs., 2 figs