1 EX / 2-2 Non-local Heat Transport , Core Rotation Reversals and Energy Confinement Saturation in Alcator C-Mod Ohmic L-mode Plasmas

Several seemingly unrelated phenomena in Ohmic L-mode tokamak plasmas are shown to be intimately related: non-local heat transport, core toroidal rotation reversals, energy confinement saturation, turbulence changes and up/down impurity density asymmetries. These effects all abruptly transform at a critical value of the collisionality, below which trapped electron modes dominate the turbulence, while above which ion temperature gradient modes prevail. There are several longstanding mysteries in tokamak research: explanation of the observed edge up/down impurity density asymmetries [1,2], the mechanism governing the linear Ohmic confinement (LOC, also known as neo-Alcator scaling) regime and the transition to saturated Ohmic confinement (SOC, L-mode) [3,4], and the underlying cause of 'non-local electron heat transport' following cold pulses [5,6]. A recent mystery to add to this list is the rotation inversion or reversal process [7,8], in which the core toroidal rotation abruptly switches direction, with negligible effect on other macroscopic plasma parameters. Rotation reversals constitute a novel form of momentum transport bifurcation and have been induced by changes in the collsionality through the density, plasma current and magnetic field. Recently, the connection among rotation reversals, the transformation from LOC to SOC, saturation of electron density profile peaking and changes in turbulence characteristics has been demonstrated [9,8,10,11]. This connection is extended to include 'non-local electron heat transport' and up/down impurity density asymmetries. This detailed study in Ohmic L-mode plasmas has been performed on the Alcator C-Mod tokamak [12] (major radius R = 0.67 m, typical minor radius of 0.21 m).Thermal transport has been investigated by means of rapid edge cooling from impurity injection by laser blow-off. Shown in Fig.1are the time histories