In the theoretical description of the neoclassical tearing mode it is usually assumed that the ion banana width $w_b$ is much smaller than the island width $W$. This assumption is questionable at least for the island size at the mode onset. We show that a significant fraction of the (ion) bootstrap current survives inside the island when $w_b$ is comparable to $W$. This effect also leads to a linear scaling of ${\beta }_{\theta }^{\mathrm{onset}}$ with the normalized ion poloidal gyroradius ${\rho }_{\theta }^{*}$, in agreement with the experimental results of ASDEX Upgrade.

• Received 14 June 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.075001