Momentum Transport Studies in JET H-Mode Discharges with an Enhanced Toroidal Field Ripple
The study of plasma rotation and momentum transport has gained interest over the last few years as rotation is thought to play an important role in the stability of Tokamak plasmas while it may also affect transport properties via the stabilization of turbulence. A proper understanding of all aspects that affect the rotation of Tokamak plasmas, in particular rotation sources and momentum transport, is important if one wants to make an accurate prediction of the rotation in ITER. The assumption that momentum transport is similar to the turbulence driven ion heat transport is a strong simplification and usually in JET it is found that the effective momentum diffusivity is significantly smaller than the ion heat diffusivity. This observation can be attributed to the existence of an inward momentum pinch. Clear experimental evidence for such a momentum pinch in JET was obtained in earlier experiments. JET has the unique capability to change its Toroidal Field (TF) ripple. It was shown that an increased TF ripple has a strong impact on the overall toroidal rotation at JET. This paper will not deal in detail with mechanisms of TF ripple induced torque on the plasma, which is described elsewhere, but utilize TF ripple as a tool, in order to reveal the impact of the momentum pinch on the rotation profiles in H-mode JET discharges. Besides this an estimation of the magnitude of the momentum pinch can be made for each entry in the JET rotation database , which enables us to look into the basic scaling of this parameter.