Intrinsic Toroidal Rotation, Density Peaking, and Turbulence Regimes in the Core of Tokamak Plasmas

Abstract

Observations in the ASDEX Upgrade tokamak show a correlation between the gradient of the intrinsic toroidal rotation profile and the logarithmic gradient of the electron density profile. The intrinsic toroidal rotation in the center of the plasma reverses from co- to countercurrent when the logarithmic density gradients are large, and the turbulence is either dominated by trapped electron modes or is at the transition between ion temperature gradient and trapped electron modes. A study based on local gyrokinetic calculations suggests that the dominant trend in the observations can be explained by the combination of residual stresses produced by E × B and profile shearing mechanisms.

Discriminating the trapped electron modes contribution in density fluctuation spectra

“…From [48] we can see that the toroidal rotation amplitude has been found to decrease as the electron density increased. No rotation reversal were observed as it was reported in several machines [15,16,44,49,50]. However, in the core (r/a < 0.5) a transitory acceleration is observed at the LOC-SOC transition.…”

“…If the onset of an ion mode in the SOC regime was proposed to explain the energy confinement time saturation [39,40], it is now widely believed that the electrons play a major role, as suggested in [41,42]. During the last decade, the conventional understanding of turbulence in Ohmic plasma is that the LOC regime is dominated by TEM and the SOC regime by ITG [3,5,14,[43][44][45][46]. The LOC-SOC transition is predicted to occur at n s the so-called Shimomura density (in 10 20 m −3 ) defined by: [47]:…”

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

“…The best explanation appears to be the q-shear effect [9]. With higher ECH power in these same conditions there is a less pronounced reversal and minimal difference in q-shear, indicating the dominant turbulence effect is responsible in this condition [7].…”

“…Near the magnetic axis Te/Ti ~ 3 for this highest Te. Consequently the electron collisionality is over a factor of 2 smaller for the shot with the rotation depression and we postulate that effects other than shear are at work here, such as described in [7].…”

“…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].…”