The Influence of Toroidal Field Ripple on H-mode Transitions on JET

The finite number of Toroidal Field (TF) coils on tokamaks results in toroidal variation of the magnetic field, TF ripple. The TF ripple is expected to be around d = 0.5 % at the outer separatrix on ITER and it is known that ripple ion losses can lead to significant counter toroidal rotation velocity, vf, at the plasma edge. Any variation in vf or the poloidal rotation velocity, vq, can strongly influence the edge radial electric field, Er, which in turn is thought to play a significant role in turbulence suppression. Therefore, it is important to understand how TF ripple affects the L-H, H-L and ELM phase transitions. JET has the unique capability to vary the TF ripple amplitude from a standard operating value of d ~ 0.08 % up to maximum of 3% and it is therefore an ideal machine on which to perform this study. Results are presented from a recent experiment in which the TF ripple was systematically varied for shots with matched edge density. The effects of increased levels of TF ripple on both global and edge localised parameters over H-mode transitions are shown.
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