Results of the Variable Toroidal Field Ripple Experiments in JET

In all tokamak devices, the finite number and toroidal extension of the Toroidal Field (TF) coils causes a periodic variation of the toroidal field from its nominal value, called toroidal field ripple dBT. Ripple in the toroidal field adversely affects fast ion confinement, and in the ITER design, this is mitigated by including Ferritic Insets (FI) compensation, reducing dBT from ~ 1.2% to ~ 0.5%. With this correction, the magnitude of a particle losses in ITER is expected to be in the 1% region, negligible in terms of a particle confinement and power load to plasma-facing components. On the other hand, experimental results from JT60-U and H-mode dimensionless identity experiments in JET and JT-60U indicated that ripple may also affect the H-mode confinement, pedestal height, ELM size and plasma rotation. This paper describes the results of dedicated experiments carried out in JET, where H-mode plasmas properties were studied for varying levels of toroidal field ripple, in the range from 0.08% (natural dBT for JET) up to ~1%. The experiments were carried out in the ELMy H-mode regime with q95 = 3 to 3.6, to investigate the effect of dBT on pedestal and core properties of the plasma. These experiments show that toroidal field ripple has a clear effect on H-mode properties, although the physics mechanisms at the root of the reduced energy confinement with dBT have not been identified unambiguously. Plasma density pump out and reduction of the global energy confinement is found for dBT ~ 0.5%, but the magnitude of this effect depends on plasma parameters. Ripple may also affect pedestal pressure, as well as size and frequency of ELMs. Plasma toroidal rotation was also strongly affected by ripple: the toroidal velocity is reduced for increased ripple and may become negative at the edge for dBT ~ 1%. These results are discussed and implications for ITER outlined.
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