Observations of anomalous momentum transport in Alcator C-Mod plasmas with no momentum input

Abstract

Anomalous momentum transport has been observed in Alcator C-Mod tokamak plasmas. The time evolution of core impurity toroidal rotation velocity profiles has been measured with a tangentially viewing crystal x-ray spectrometer array. Following the L-mode to EDA (enhanced D α ) H-mode transition in both Ohmic and ICRF heated discharges, the ensuing co-current toroidal rotation velocity, which is generated in the absence of any external momentum source, is observed to propagate in from the edge plasma to the core with a time scale of order of the observed energy confinement time, but much less than the neo-classical momentum confinement time. The ensuing steady state toroidal rotation velocity profiles in EDA H-mode plasmas are relatively flat, with V φ ∼ 50 km/s, and the momentum transport can be simulated with a simple diffusion model. Assuming the L-H transition produces an instantaneous edge source of toroidal torque (which disappears at the H-to L-mode transition), the momentum transport may be characterized by a diffusivity, with values of ∼ 0.07 m 2 /s during EDA H-mode and ∼ 0.2 m 2 /s in L-mode. These values are large compared to the calculated neoclassical momentum diffusivities, which are of order 0.003 m 2 /s. Velocity profiles of ELM-free H-mode plasmas are centrally peaked (with V φ (0) exceeding 100 km/s in some cases), which suggests the workings of an inward momentum pinch; the observed profiles are consistent with simulations including an edge inward convection velocity of ∼ 10 m/s. In EDA H-mode discharges which develop internal transport barriers, the velocity profiles become hollow in the center, indicating the presence of a negative radial electric field well in the vicinity of the barrier foot. Upper single null diverted and inner wall limited L-mode discharges exhibit strong counter-current rotation (with V φ (0) ∼ −60 km/s in some cases), which may be related to the observed higher Hmode power threshold in these configurations. For plasmas with locked modes, the toroidal rotation is observed to stop (V φ ≤5 km/s). 1

Observations of counter-current toroidal rotation in Alcator C-Mod LHCD plasmas

“…Modeling indicates that about 250 kA were driven by LH waves in this case. The time scale for the change in the velocity is on the order of hundreds of ms, similar to the current relaxation time (τ CR =1.4a 2 κT e 1.5 /Z ef f ∼ 250 ms), but much longer than the energy [27] or momentum [28,29] confinement times (∼35 ms). The time evolutions of the internal inductance and the toroidal rotation velocity are similar, suggesting a connection between the two quantities.…”

Ohmic energy confinement saturation and core toroidal rotation reversal in Alcator C-Mod plasmas

“…There was a delay in the response of the rotation to the current ramps. This delay is between the momentum confinement time (∼30ms) [28] and the current relaxation time. The dependence of the core toroidal rotation velocity on plasma current is shown in Fig.10, from a shot by shot scan at fixed magnetic field (5.2 T) and electron density (0.8-1.2×10 20 /m 3 ).…”

Residual parallel Reynolds stress due to turbulence intensity gradient in tokamak plasmas

“…7,9,10 In contrast, H mode rotation is usually in the co-current direction, and the on-axis plasma velocity appears to increase with stored energy W p and decrease with total plasma current I p . 11 This empirical scaling, which has come to be called the Rice scaling, particularly manifests itself strongly in Alcator C-Mod and D-IIID, 1,2 and if taken at face value gives rather optimistic predictions for ITER's rotation speed. 11 There are indeed different mechanisms to drive rotation without any external torque.…”