Counter-current rotation and ITB formation in Alcator C-Mod LHCD plasmas

Tokamak plasmas with internal transport barriers (ITBs) are attractive for their potential high fusion performance and large bootstrap current. Most ITBs are generated with external rotation and/or current drive applied during the plasma current ramp. Of particular interest are ITBs formed during the current flat top and under reactor relevant conditions, with equilibrated electrons and ions, and without external momentum input. Following application of LHCD during the current flat top, the toroidal rotation in Alcator C-Mod L- and H-mode plasmas is found to increment in the counter-current direction in conjunction with a decrease in the plasma internal. Along with drops in l_i and the core V_Tor, there are increases in the electron density and central ion and electron temperatures. The mechanism giving rise to counter-current rotation is unknown, but it is certainly not due to energetic electron loss, which would induce co-current rotation. During this evolution, transport barriers form in the density, momentum and energy channels. These ITBs develop over a time scale similar to the current relaxation time but slow compared to the energy and momentum confinement times. For most conditions, the ITB foot is located near r/a ∼ 0.4 for electron density and temperature, and for ion (impurity) rotation, temperature and density. These discharges exhibit sawtooth oscillations throughout, with an inversion radius well inside the ITB foot. The role of magnetic shear in the ITB formation is under investigation, in addition to techniques for manipulating the foot location and increasing the strength of the barrier. The magnitude of the changes in the central rotation velocity and the internal inductance is correlated and found to increase with increasing LHCD power and decreasing electron density. The maximum effect is found with a phasing of 60 deg. (n_parallel = 1 .6), with a smaller magnitude at 120 deg. (n_parallel = 3 .1), and no effect for negative or heating phasing. These results are consistent with the current drive efficiency scaling as P_LH /(n_e n_parallel²). Regardless of the plasma parameters and n_parallel, there is a strong correlation between the rotation velocity and l_i changes, possibly providing a clue for the underlying mechanism. Since the magnitude of the ITB scales with the input LHCD power, the future prospects for this technique look promising. (author)