  An electrostatic cryogenic storage ring, CSR, for beams of anions and cations
with up to 300 keV kinetic energy per unit charge has been designed,
constructed and put into operation. With a circumference of 35 m, the ion-beam
vacuum chambers and all beam optics are in a cryostat and cooled by a
closed-cycle liquid helium system. At temperatures as low as (5.5 $\pm$ 1) K
inside the ring, storage time constants of several minutes up to almost an hour
were observed for atomic and molecular, anion and cation beams at an energy of
60 keV. The ion-beam intensity, energy-dependent closed-orbit shifts
(dispersion) and the focusing properties of the machine were studied by a
system of capacitive pickups. The Schottky-noise spectrum of the stored ions
revealed a broadening of the momentum distribution on a time scale of 1000 s.
Photodetachment of stored anions was used in the beam lifetime measurements.
The detachment rate by anion collisions with residual-gas molecules was found
to be extremely low. A residual-gas density below 140 cm$^{-3}$ is derived,
equivalent to a room-temperature pressure below 10$^{-14}$ mbar. Fast atomic,
molecular and cluster ion beams stored for long periods of time in a cryogenic
environment will allow experiments on collision- and radiation-induced
fragmentation processes of ions in known internal quantum states with merged
and crossed photon and particle beams.
