Density Impact on Toroidal Rotation in Tore Supra: Experimental Observations and Theoretical Investigation

The effect of plasma density on toroidal rotation was examined in density ramp-up experiments at a plasma current of 1 MA and a toroidal magnetic field of 3.6 T in Tore Supra ohmic discharges. Experimental measurements have shown that the toroidal rotation amplitude is reduced at all radii when increasing the plasma density, although it remains in the countercurrent direction. Neoclassical predictions including ripple-induced toroidal friction are in qualitative agreement with such observations. However, in the core (r/a < 0.5) the plasma accelerates at n(l) similar to 3 x 10(19) m(-2) and breaks again above n(l) similar to 3.5 x 10(19) m-2. This rotation bifurcation did not appear to be correlated to a possible influence of the sawtooth activity. Nonetheless, quasi-linear gyrokinetic simulations have shown that the transition of the nature of linear microinstabilities from ITG-TEM to pure ITG occurs at n(l) similar to 3 x 10(19) m-2, which could indicate a change on the turbulence driven contributions and explain the deviation from neoclassical predictions in the plasma core.