  The ATLAS experiment at the Large Hadron Collider (LHC) employs a trigger
system consisting of a first-level hardware trigger (L1) and a software-based
high-level trigger. The L1 muon trigger system selects muon candidates, assigns
them to the correct LHC bunch crossing and classifies them into one of six
transverse-momentum threshold classes. The L1 muon trigger system uses
resistive-plate chambers (RPCs) to generate the muon-induced trigger signals in
the central (barrel) region of the ATLAS detector. The ATLAS RPCs are arranged
in six concentric layers and operate in a toroidal magnetic field with a
bending power of 1.5 to 5.5 Tm. The RPC detector consists of about 3700 gas
volumes with a total surface area of more than 4000 m$^2$. This paper reports
on the performance of the RPC detector and L1 muon barrel trigger using 60.8
fb$^{-1}$ of proton-proton collision data recorded by the ATLAS experiment in
2018 at a centre-of-mass energy of 13 TeV. Detector and trigger performance are
studied using $Z$ boson decays into a muon pair. Measurements of the RPC
detector response, efficiency, and time resolution are reported. Measurements
of the L1 muon barrel trigger efficiencies and rates are presented, along with
measurements of the properties of the selected sample of muon candidates.
Measurements of the RPC currents, counting rates and mean avalanche charge are
performed using zero-bias collisions. Finally, RPC detector response and
efficiency are studied at different high voltage and front-end discriminator
threshold settings in order to extrapolate detector response to the higher
luminosity expected for the High Luminosity LHC.
