  The superconductor to insulator transition is studied in a self-charging
model for a ladder of Josephson-junctions in presence of an external magnetic
field. Path integral Monte Carlo simulations of the equivalent
(1+1)-dimensional classical model are used to study the phase diagram and
critical behavior. In addition to a superconducting (vortex-free) phase, a
vortex phase can also occur for increasing magnetic field and small charging
energy. It is found that an intervening insulating phase separates the
superconducting from the vortex phases. Surprisingly, a finite-size scaling
analysis shows that the field-induced superconducting to insulator transition
is in the KT universality class even tough the external field breaks
time-reversal symmetry.
