  We construct boundary states for supertubes in the flat spacetime. The T-dual
objects of supertubes are moving spiral D1-branes (D-helices). Since we can
obtain these D-helices from the usual D1-branes via null deformation, we can
construct the boundary states for these moving D-helices in the covariant
formalism. Using these boundary states, we calculate the vacuum amplitude
between two supertubes in the closed string channel and read the open string
spectrum via the open closed duality. We find there are critical values of the
energy for on-shell open strings on the supertubes due to the non-trivial
stringy correction. We also consider supertubes in the type IIA Godel universe
in order to use them as probes of closed timelike curves. This universe is the
T-dual of the maximally supersymmetric type IIB PP-wave background. Since the
null deformations of D-branes are also allowed in this PP-wave, we can
construct the boundary states for supertubes in the type IIA Godel universe in
the same way. We obtain the open string spectrum on the supertube from the
vacuum amplitude between supertubes. As a consequence, we find that the
tachyonic instability of open strings on the supertube, which is the signal of
closed time like curves, disappears due to the stringy correction.
