  Metastable (1s)(2s) $^3{\rm S}_1$ helium atoms produced in a supersonic beam
were excited to Rydberg-Stark states (with $n$ in the $27-30$ range) in a
cryogenic environment and subsequently decelerated by, and trapped above, a
surface-electrode decelerator. In the trapping experiments, the Rydberg atoms
were brought to rest in 75~$\mu$s and over a distance of 33~mm and kept
stationary for times $t_{\mathrm{trap}}$ in the $0-525$~$\mu$s range, before
being re-accelerated for detection by pulsed field ionization. The use of a
home-built valve producing short gas pulses with a duration of about 20~$\mu$s
enabled the reduction of losses arising from collisions with atoms in the
trailing part of the gas pulses. Cooling the decelerator to 4.7~K further
suppressed losses by transitions induced by blackbody radiation and by
collisions with atoms desorbing from the decelerator surface. The main
contribution (60\%) to the atom loss during deceleration is attributed to the
escape out of the decelerator moving traps of atoms having energies higher than
the trap saddle point, spontaneous emission and collisions with atoms in the
trailing part of the gas pulses causing each only about 20\% of the atom loss.
At 4.7 K, the atom losses in the trapping phase of the experiments were found
to be almost exclusively caused by spontaneous emission and the trap lifetimes
were found to correspond to the natural lifetimes of the Rydberg-Stark states.
Increasing the temperature to 100 K enhanced the trap losses by transitions
stimulated by blackbody radiation.
