The Influence of Magnetic Field Ripple on JET Intrinsic Rotation

In tokamaks, a finite number of magnetic field coils creates a variation of the toroidal magnetic field known as "ripple". In the presence of ripple the canonical angular momentum is not conserved and fast ions as well as thermal ions can exchange momentum with the toroidal field coils. It is well known that ripple affects rotation in tokamak plasmas with Neutral Beam Injection (NBI) heating. In JT-60 experiments with perpendicular NBI, the observed edge counter-rotation (i.e. rotation anti-parallel to the plasma current) has been explained by a torque induced by ripple losses of fast-ions. Experiments with NBI at JET have shown that ripple reduced rotation in plasmas with injected momentum parallel to the plasma current and, in some cases, countercurrent rotation was observed at the edge. Fast-ion losses were not enough to completely explain the JET observations, leaving open the question of the role of ripple on thermal ions. In order to separate ripple induced fast ion effects from thermal ion effects, intrinsic plasma rotation (i.e. rotation without momentum input) was measured in JET Ohmic and Ion-Cyclotron Radio Frequency (ICRF) heated plasmas with different ripple levels. Extrapolation to ITER from intrinsic rotation measurements from present day tokamaks is quite uncertain since each machine has a different ripple level. As ITER will have 18 toroidal magnetic field coils, a ripple of 0.5% at the edge, is expected to affect the magnetic confinement of plasma ions as well as the plasma rotation.
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