  Magnetic flux ropes (MFRs) are believed to be the core structure in solar
eruptions, nevertheless, their formation remains intensely debated. Here we
report a rapid buildup process of an MFR-system during a confined X2.2 class
flare occurred on 2017 September 6 in NOAA AR 12673, three hours after which
the structure erupted to a major coronal mass ejection (CME) accompanied by an
X9.3 class flare. For the X2.2 flare, we do not find EUV dimmings, separation
of its flare ribbons, or clear CME signatures, suggesting a confined flare. For
the X9.3 flare, large-scale dimmings, separation of its flare ribbons, and a
CME show it to be eruptive. By performing a time sequence of nonlinear
force-free fields (NLFFFs) extrapolations we find that: until the eruptive
flare, an MFR-system was located in the AR. During the confined flare, the
axial flux and the lower bound of the magnetic helicity for the MFR-system were
dramatically enhanced by about 86% and 260%, respectively, although the mean
twist number was almost unchanged. During the eruptive flare, the three
parameters were all significantly reduced. The results evidence the buildup and
release of the MFR-system during the confined and the eruptive flare,
respectively. The former may be achieved by flare reconnection. We also
calculate the pre-flare distributions of the decay index above the main
polarity inversion line (PIL) and find no significant difference. It indicates
that the buildup of the magnetic flux and helicity of the MFR-system may play a
role in facilitating its final eruption.
