  Bootstrap current in stellarators can be presented as a sum of a
collisionless value given by the Shaing-Callen asymptotic formula and an
off-set current, which non-trivially depends on plasma collisionality and
radial electric field. Using NEO-2 modelling, analytical estimates and
semi-analytical studies with help of a propagator method, it is shown that the
off-set current in the $1/\nu$ regime does not converge with decreasing
collisionality $\nu_\ast$ but rather shows oscillations over $\log\nu_\ast$
with an amplitude of the order of the bootstrap current in an equivalent
tokamak. The convergence to the Shaing-Callen limit appears in regimes with
significant orbit precession, in particular, due to a finite radial electric
field, where the off-set current decreases as $\nu_\ast^{3/5}$. The off-set
current strongly increases in case of nearly aligned magnetic field maxima on
the field line where it diverges as $\nu_\ast^{-1/2}$ in the $1/\nu$ regime and
saturates due to the precession at a level exceeding the equivalent tokamak
value by ${v_E^\ast}^{-1/2}$ where $v_E^\ast$ is the perpendicular Mach number.
The latter off-set, however, can be minimized by further aligning local
magnetic field maxima and by fulfilling an extra integral condition of
"equivalent ripples" for the magnetic field. A criterion for the accuracy of
this alignment and of ripple equivalence is derived. In addition, the
possibility of the bootstrap effect at the magnetic axis caused by the above
off-set is also discussed.
