  The operation of ITER will require reliable simulations in order to avoid
major damage to the device from disruptions. Disruptions are the sudden breakup
of magnetic surfaces across the plasma volume -- a fast magnetic reconnection.
This reconnection can be caused by the growth of perturbations outside of the
plasma core causing an ideal perturbation to the core. This causes an
increasing ratio of the maximum to the minimum separation,
$\Delta_{max}/\Delta_{min}$, between neighboring magnetic surfaces. Magnetic
reconnection becomes a dominant process when magnetic field lines can quickly
interchange connections over a spatial scale $a_r$. This occurs when
$\Delta_{max}/\Delta_{min}\gtrsim a_r/\Delta_d$, where $\Delta_d$ is the scale
over which non-ideal effects make magnetic field lines indistinguishable.
Traditional reconnection theory is fundamentally different. It is a study of
the steady-state cancellation of oppositely directed magnetic field components
across a thin layer. During more than sixty years, mathematical implications of
Faraday's Law have been derived that clarify and constrain the physics of fast
magnetic reconnection. These are reviewed because they are not commonly known
but are needed to understand and to place in context how an ideal magnetic
evolution can cause reconnection to quickly become a dominant process no matter
how small $\Delta_d/a_r$ may be.
