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

Abstract

Direction reversals of intrinsic toroidal rotation have been observed in Alcator CMod 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.

Magnetohydrodynamically generated velocities in confined plasma

“…An important feature is the change of sign in the volume averaged toroidal angular momentum, found also in experimental observations [22,23,24], when the toroidal magnetic field, hence the safety factor, is varied (Fig. 31).…”

“…We recall that for all the simulations presented in this manuscript the magnetic Prandtl number is equal to one, P r = 1. In this case the viscous Lundquist number varies because the reference magnetic field used for its calculation is the imposed toroidal component and to modify the safety factor the magnitude of this field is changed, as also done in experiments [22,23]. The parameter q takes four different values.…”

Discriminating the trapped electron modes contribution in density fluctuation spectra

“…Indeed, spectra analysis has evidenced that TEM are linked with quasi-coherent (QC) modes observed by reflectometry in the core/edge region [6][7][8]. Besides the link established between QC modes and TEM, spectra modifications have also been reported during transitions toward expected TEM-dominated regimes in DIII-D [9][10][11][12][13], and Alcator C-mod [13][14][15][16][17] tokamaks, and in the MST reversed field pinch [18].…”

Electron cyclotron heating and core intrinsic rotation reversal in DIII-D

“…One of the dramatic experimental aspects of core rotation reversals is the "spontaneity" of the phenomenon when it is studied by slowly changing a parameter in time, such as density [6]. At a critical density the rotation in the core reverses, on a rapid time scale compared to the changing parameter in a shot.…”

“…The rotation closer to the outboard edge of the plasma remains in the same direction while the core direction undergoes a rapid reversal. Several theoretical explanations have been put forward for these reversals, including the details of the dominant plasma core turbulence [6,7], the neoclassically predicted reversal of poloidal velocity direction with collisionality regime [8], and an effect due to turbulence coupled to the shear in the q-profile [9].…”