The thermal collapse of a nearly collisionless plasma interacting with a
cooling spot, in which the electron parallel heat flux plays an essential role,
is investigated both theoretically and numerically. We show that such thermal
collapse, which is known as thermal quench in tokamaks, comes about in the form
of propagating fronts, originating from the cooling spot, along the magnetic
field lines. The slow fronts, propagating with local ion sound speed, limit the
aggressive cooling of plasma, which is accompanied by a plasma cooling flow
toward the cooling spot. The extraordinary physics underlying such a cooling
flow is that the fundamental constraint of ambipolar transport along the field
line limits the spatial gradient of electron thermal conduction flux to the
much weaker convective scaling, as opposed to the free-streaming scaling, so
that a large electron temperature and hence pressure gradient can be sustained.
The last ion front for a radiative cooling spot is a shock front where cold but
flowing ions meet the hot ions