Toroidal Field Ripple Effects on H-modes in JET and Implications for ITER
The main goal of ITER is to produce plasmas with high fusion gain (QDT>10), where a large fraction of the plasma heating is supplied by the a-particles from fusion reactions. To first order, this requires both thermal and fast ion confinement to be at least as good as predictions. The plasma H-mode energy confinement time estimates for ITER are based on extrapolation from a wide database from existing Tokamaks, while the projected a-particle confinement is based mainly on theoretical predictions but also on experimental data. 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. In this paper, we use the definition of dBT = (BFMax ­ BFmin)/(BFMax + Fmin), that is the same adopted by ITER. The dBT values quoted in the paper are always maximum values at the plasma separatrix. It is well known that ripple in the toroidal field adversely affects fast ion confinement, and in the case of ITER, this has been accounted for by including in the design Ferritic Insets (FI) compensation, reducing dBT from ~1.2% to ~0.5%, to reduce first wall power loads. Analysis shows that the main a-particle loss mechanism in ITER will be ripple banana orbit diffusion, and that the magnitude of these losses is expected to be in the 1% region, therefore negligible in terms of a-particle confinement. Recent experimental results from JT60-U and H-mode dimensionless H-mode experiments in JET and JT-60U have indicated that ripple may also affect the H-mode confinement and plasma rotation. Although the physics mechanisms at the root of the reduced energy confinement with dBT was not identified unambiguously, the implication of a reduction of energy confinement on projected ITER performance due to ripple stimulated a series of experiments at JET. This paper reports the results of these experiments and briefly discusses the implications for the ITER design.