  The effect of cylindrical confinement on the phase behaviour of a system of
parallel hard rods is studied using Onsager's second virial theory. The hard
rods are represented as hard cylinders of diameter $D$ and length $L$, while
the cylindrical pore is infinite with diameter $W$. The interaction between the
wall and the rods is hard repulsive, and it is assumed that molecules are
parallel to the surface of the pore (planar anchoring). In very narrow pores
($D<W<2 D$), the structure is homogeneous and the system behaves as a
one-dimensional Tonks gas. For wider pores, inhomogeneous fluid structures
emerge because of the lowering of the average excluded volume due to the
wall-particle interaction. The bulk nematic-smectic A phase transition is
replaced by a transition between inhomogeneous nematic and smectic A phases.
The smectic is destabilized with respect to the nematic for decreasing pore
width; this effect becomes substantial for $W<10 D$. For $W>100 D$, results for
bulk and confined fluids agree well due to the short range effect of the wall
($\sim 3-4D$).
