  We study five sequential solar flares (SOL2015-08-07) occurring in Active
Region 12396 observed with the Goode Solar Telescope (GST) at the BBSO,
complemented by IRIS and SDO observations. The main flaring region is an arch
filament system (AFS) consisting of multiple bundles of dark filament threads
enclosed by semi-circular flare ribbons. We study the magnetic configuration
and evolution of the active region by constructing coronal magnetic field
models based on SDO/HMI magnetograms using two independent methods, i.e., the
nonlinear force-free field (NLFFF) extrapolation and the flux rope insertion
method. The models consist of multiple flux ropes with mixed signs of helicity,
i.e., positive (negative) in the northern (southern) region, which is
consistent with the GST observations of multiple filament bundles. The
footprints of quasi-separatrix layers (QSLs) derived from the extrapolated
NLFFF compare favorably with the observed flare ribbons. An interesting
double-ribbon fine structure located at the east border of the AFS is
consistent with the fine structure of the QSL's footprint. Moreover, magnetic
field lines traced along the semi-circular footprint of a dome-like QSL
surrounding the AFS are connected to the regions of significant helicity and
Poynting flux injection. The maps of magnetic twist show that positive twist
became dominant as time progressed, which is consistent with the injection of
positive helicity before the flares. We hence conclude that these circular
shaped flares are caused by 3D magnetic reconnection at the QSLs associated
with the AFS possessing mixed signs of helicity.
