  We present an experimental study of hybrid turnstiles with high charging
energies in comparison to the superconducting gap. The device is modeled with
the sequential tunneling approximation. The backtunneling effect is shown to
limit the amplitude of the gate drive and thereby the maximum pumped current of
the turnstile. We compare results obtained with sine and square wave drive and
show how a fast rise time can suppress errors due to leakage current. Quantized
current plateaus up to 160 pA are demonstrated.
