Magnetic FieldLine Curvature
As was mentioned in Section 1.14, the bootstrap current has a destabilizing effect on wide magnetic island chains [2,5,16].
It turns out that the mean curvature of magnetic fieldlines in the inner region has a stabilizing effect on such chains that is similar in magnitude to the destabilizing effect of the bootstrap current [11,15]. Hence, it is
not consistent to include the bootstrap current in our analysis without also including curvature effects. The appropriate
curvature terms are derived in Reference [13]. We shall simply incorporate them into our model,
which generalizes to give:
Here,

(11.56) 
and
, where

(11.57) 
is the magnetic curvature length (i.e., the mean radius of curvature of magnetic fieldlines at the rational surface) [12,17].
Note that the previous expression for is only valid in a largeaspect ratio, low plasma with circular magnetic fluxsurfaces. A more general
expression is given in Section A.8.