Marginal β-limit for neoclassical tearing modes in JET H-mode discharges

Abstract

A neoclassical tearing mode (NTM) requires a finite size seed island tobecome unstable. Usually the local pressure gradient is relatively largeat the β-values needed for these seed islands to destabilize theNTMs. Therefore, the island has a large growth rate at mode onset andgrows rapidly to its saturated island width. This width is proportionalto β as long as it is well above the marginal β-limit below whichthe mode is stable. The marginal β-limit is independent of the seedisland trigger mechanism and provides detailed information on thestabilizing terms in the modified Rutherford equation, which are notunambiguously determined theoretically. It is shown that in JET themarginal normalized β-limit for the 3/2 mode, βN,marg, is ofthe order of 0.5-1 for magnetic fields between 3.3 and 1 T, withq95≈3.3, and near the H-L transition. Therefore, all H-modeswith typical q-profiles (q95≈3.3) are metastable in JET to3/2 NTMs. In addition, the marginal island width is of the order of 2-4 cmand the stabilizing terms are such that they influence the saturatedisland width when it is smaller than 4-6 cm in these H-mode discharges.It is also shown that detailed analyses of the time evolution of theisland width with slow β ramp-down suggest that the convective formof the stabilization term due to the `χ⊥ model' is moreappropriate and can explain the island decay between 4 and 6 cm to themarginal island width, while the polarization current model can explainthe rapid stabilization when β<βmarg. The range of valuesof the different stabilizing terms are discussed in detail. Inparticular, it is shown that the mode is stabilized and has a large negative growth rate, when the effect of the stabilizing terms is suchas to reduce the saturated width by a factor of 2.