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 destabilise 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 Beta 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 stabilising terms in the modified Rutherford equation, which are not unambiguously determined theoretically. It is shown that in JET the marginal normalised 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) approximately equal to 3.3, and near the H-L transition. Therefore, all H-modes with typical q-profiles (q(95) approximately equal to 3.3) are metastable in JET to 3/2 NTMs. In addition, the marginal island width is of the order of 2-4cm and the stabilising terms are such that they influence the saturated island width when it is smaller than 4-6cm in these H-mode discharges. The range of values of the different stabilising terms are discussed in detail. In particular, it is shown that the mode is stabilised and has a large negative growth rate, when the effect of the stabilising terms is such as to reduce the saturated width by a factor of 2.
Bibliographic Reference: An article published in: Plasma Physics and Controlled Fusion, Vol 44 (September 2002), 1999-2019
Availability: This article can be accessed online by subscribers, and can be ordered online by non-subscribers, at: http://stacks.iop.org/0741-3335/44/1999
Record Number: 200215493 / Last updated on: 2002-11-07
Original language: en
Available languages: en