  We present a novel ultrastable superconducting radio-frequency (RF) ion trap
realized as a combination of an RF cavity and a linear Paul trap. Its RF
quadrupole mode at 34.52 MHz reaches a quality factor of $Q\approx2.3\times
10^5$ at a temperature of 4.1 K and is used to radially confine ions in an
ultralow-noise pseudopotential. This concept is expected to strongly suppress
motional heating rates and related frequency shifts which limit the ultimate
accuracy achieved in advanced ion traps for frequency metrology. Running with
its low-vibration cryogenic cooling system, electron beam ion trap and
deceleration beamline supplying highly charged ions (HCI), the superconducting
trap offers ideal conditions for optical frequency metrology with ionic
species. We report its proof-of-principle operation as a quadrupole mass filter
with HCI, and trapping of Doppler-cooled ${}^9\text{Be}^+$ Coulomb crystals.
