Observation of Co and Counter Rotation Produced by Lower Hybrid Waves in Alcator C-Mod

Lower hybrid waves launched uni‐directionally into tokamak plasmas impart momentum to the electrons. This momentum can be transferred to the ions, leading to substantial counter current rotation. Observations of LH‐induced counter rotation have been previously reported [1], and the initial rate of increase has been found to be consistent with the calculated rate of wave momentum injection [2]. However, in recent experiments in Alcator C‐Mod it has been found that application of LH waves to relatively low current (Ip∼0.4–0.6 MA) plasmas can result in a co‐current change of rotation, which implies a different mechanism than that described above. This appears to be linked to the so‐called intrinsic rotation commonly observed in Alcator C‐Mod and other tokamaks [3]. In addition to the change in direction at low current, some dependence on the magnetic configuration (USL vs. LSN) has been observed.