  We investigate low-temperature and low-voltage-bias charge transport in a
superconducting Al single electron transistor in a dissipating environment,
realized as on-chip high-ohmic Cr microstrips. In our samples with relatively
large charging energy values Ec > EJ, where EJ is the energy of the Josephson
coupling, two transport mechanisms were found to be dominating, both based on
discrete tunneling of individual Cooper pairs: Depending on the gate voltage
Vg, either sequential tunneling of pairs via the transistor island (in the open
state of the transistor around the points Qg = CgVg = e mod(2e), where Cg is
the gate capacitance) or their cotunneling through the transistor (for Qg away
of these points) was found to prevail in the net current. As the open states of
our transistors had been found to be unstable with respect to quasiparticle
poisoning, high-frequency gate cycling (at f ~ 1 MHz) was applied to study the
sequential tunneling mechanism. A simple model based on the master equation was
found to be in a good agreement with the experimental data.
