Marginal beta-limit for neoclassical tearing modes in JET H-mode discharges
A neoclassical tearing mode (NTM) requires a finite size seed island to become unstable. Usually the local pressure gradient is relatively large at the beta-values needed for these seed islands to destabilize the NTMs. Therefore, the island has a large growth rate at mode onset and grows rapidly to its saturated island width. This width is proportional to as long as it is well above the marginal beta-limit below which the mode is stable. The marginal beta-limit is independent of the seed island trigger mechanism and provides detailed information on the stabilizing terms in the modified Rutherford equation, which are not unambiguously determined theoretically. It is shown that in JET the marginal normalized beta-limit for the 3/2 mode, beta(N,marg), is of the order of 0.5-1 for magnetic fields between 3.3 and 1T, with q(95) approximate to 3.3, and near the H-L transition. Therefore, all H-modes with typical q-profiles (q(95) approximate to 3.3) are metastable in JET to 3/2 NTMs. In addition, the marginal island width is of the order of 2-4 cm and the stabilizing terms are such that they influence the saturated island 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 the island width with slow beta ramp-down suggest that the convective form of the stabilization term due to the 'chi(perpendicular to) model' is more appropriate and can explain the island decay between 4 and 6 cm to the marginal island width, while the polarization current model can explain the rapid stabilization when beta < beta(marg). The range of values of the different stabilizing terms are discussed in detail. In particular, it is shown that the mode is stabilized and has a large negative growth rate, when the effect of the stabilizing terms is such as to reduce the saturated width by a factor of 2.