Observations of core toroidal rotation reversals in Alcator C-Mod ohmic L-mode plasmas

• Published in: Nuclear fusion 2011-08, Vol.51 (8), p.083005-12
• Main Authors: Rice, J.E, Duval, B.P, Reinke, M.L, Podpaly, Y.A, Bortolon, A, Churchill, R.M, Cziegler, I, Diamond, P.H, Dominguez, A, Ennever, P.C, Fiore, C.L, Granetz, R.S, Greenwald, M.J, Hubbard, A.E, Hughes, J.W, Irby, J.H, Ma, Y, Marmar, E.S, McDermott, R.M, Porkolab, M, Tsujii, N, Wolfe, S.M
• Format: Article
• Language: English
• Subjects: Correlation; Density; Electron density; Magnetic fields; Ohmic; Plasma currents; Plasmas; Ramps
• ISSN: 0029-5515; 1741-4326
• DOI: 10.1088/0029-5515/51/8/083005

Direction reversals of intrinsic toroidal rotation have been observed in Alcator C-Mod ohmic L-mode plasmas following modest electron density or toroidal magnetic field ramps. The reversal process occurs in the plasma interior, inside of the q = 3/2 surface. For low density plasmas, the rotation is in the co-current direction, and can reverse to the counter-current direction following an increase in the electron density above a certain threshold. Reversals from the co- to counter-current direction are correlated with a sharp decrease in density fluctuations with k R ⩾ 2 cm −1 and with frequencies above 70 kHz. The density at which the rotation reverses increases linearly with plasma current, and decreases with increasing magnetic field. There is a strong correlation between the reversal density and the density at which the global ohmic L-mode energy confinement changes from the linear to the saturated regime.