Spontaneous core toroidal rotation in Alcator C-Mod L-mode, H-mode and ITB plasmas

Abstract

Spontaneous toroidal rotation, self-generated in the absence of external momentum input, exhibits a rich phenomenology. In L-mode plasmas, the rotation varies in a complicated fashion with electron density, magnetic configuration and plasma current, and is predominantly in the counter-current direction. The rotation depends sensitively on the balance between upper and lower null, and plays a crucial role in the H-mode power threshold. Rotation inversion between the counter-and co-current direction has been observed following small changes in the electron density and plasma current, with very distinct thresholds. In contrast, the intrinsic rotation in H-mode plasmas has a relatively simple parameter dependence, with the rotation velocity proportional to the plasma stored energy normalized to the plasma current, and is nearly always directed co-current. In plasmas with internal transport barriers, formed either with offaxis ICRF heating or LHCD, the rotation velocity inside of the ITB foot increments in the counter-current direction as the barrier evolves.

How turbulence fronts induce plasma spin-up

“…This fluctuation momentum can be scattered into the core via the spreading process, and can result in a coincrement in the core flow. This is consistent with the observation [6], which indicates that the core toroidal flows are less in the counterdirection (sometimes in the codirection) in the favorable configuration. When we change the magnetic geometry from the favorable to the unfavorable configuration, SOL flows and edge toroidal flow shear change their sign, and consequently the fluctuation momentum is in the counterdirection P k ∝ k /k θ ∝ v φ | a < 0.…”

“…While the understanding of the local acceleration mechanisms has been advanced [4], it is now becoming increasingly clear that the nonlocal (global) momentum budget is a key for understanding the overall dynamics [5,6]. For example, the jet sharpening on the planetary atmosphere [2,7] involves not only the local acceleration of flows in the stirring region (the midlatitude on the Earth), but also the response of flows in the equator and the pole regions.…”

“…For example, the jet sharpening on the planetary atmosphere [2,7] involves not only the local acceleration of flows in the stirring region (the midlatitude on the Earth), but also the response of flows in the equator and the pole regions. In the case of toroidal plasmas [5,6], the change of flows in the main plasmas (i.e., plasmas in the core) mimics that of flows in the peripheral plasmas (i.e., in the edge). In these experiments, flows in the edge plasma reverse its direction upon changing the magnetic geometry, and core flows also systematically respond to the change in edge flows.…”

“…Here, for simplicity, we assume that fluctuation energy can propagate from the edge to the core, although a more detailed analysis will be required to characterize the radial extent. As an example, we consider the correlation between core toroidal flows and edge magnetic geometry [5,6]. When the magnetic drift is toward the x point of the magnetic separatrix, namely, for a favorable configuration, SOL flows are in the codirection and the edge toroidal flow shear is positive [5,36].…”

Experimental Evidence of Momentum Transport Induced by an Up-Down Asymmetric Magnetic Equilibrium in Toroidal Plasmas

“…This up-down asymmetry effect is observed in plasmas with similar pressure profiles and global parameters, comparable MHD levels, no external momentum input, and a negligible magnetic field ripple effect. From previous studies, scrape-off layer (SOL) flows depend on the position of the X point [23,24] or plasma wall contact point [25] and can affect the rotation profile by changing the boundary condition. In the present experiments, the effect of the updown asymmetry on the toroidal rotation gradient is still observed when the rotation at $ 0:85 is matched (see Fig.…”