  A crucial problem concerning a large variety of fusion devices is that the
confinement due to an external magnetic field is lost above a critical density,
while a widely accepted first principles explanation of such a fact is
apparently lacking. In the present paper, making use of standard methods of
statistical mechanics in the Debye--H\"uckel approximation, we give indications
that for a plasma there exists a density threshold corresponding to a
transition from order to chaos, the ordered motions being those in which the
confining Lorentz force on a single electron prevails over the diffusive effect
of the Coulomb forces. The density limit, which is proportional to the square
of the magnetic field, turns out to fit not too badly the empirical data for
plasma collapses in a large set of fusion devices.
