Multi-Device Scaling of Neoclassical Tearing Mode Onset with Beta

摘要

The islands from tearing modes driven unstable and sustained by helically211u001eperturbed neo-classical bootstrap current at high beta often provide the 211u001epractical limit to long-pulse, high confinement tokamak operation (1,2). The 211u001edischarges studied are ELMy H-mode single-null divertor (SND) at q(sub 95) 211u001e(approx) > 3. Periodic sawteeth with m/n = 1/1 and 2/2 are observed to induce m/n 211u001e= 3/2 neoclassical tearing modes (NTMs) in the tokamaks Asdex-Upgrade (3), DIII-D 211u001e(4) and JET (5); confinement can drop by up to 30%, constituting a 'soft' beta 211u001elimit. Data for the onset of these modes was obtained by slowly raising beta on a 211u001etime scale longer than the sawteeth period and observing the beta value at onset. 211u001eComparison of the measured critical beta to a model for the critical beta is made 211u001ein terms of dimensionless parameters. This modeling is then used for 211u001eextrapolation/prediction to a reactor-grade tokamak.