  MDPCT1 is a four-layer cos-theta Nb3Sn dipole demonstrator developed and
tested at FNAL in the framework of the U.S. Magnet Development Program. The
magnet reached record fields for accelerator magnets of 14.1 T at 4.5 K in the
first test and 14.5 T at 1.9 K in the second test and then showed large
degradation. While its inner coils performed exceptionally well with only two
quenches up to 14.5 T and no evidence of degradation, the outer coils degraded
over the course of testing. By adopting new measurement and analysis techniques
at FNAL we are discussing in detail what happened. Both success and failure in
our diagnostics are discussed. The evolution of techniques over the course of
two tests (and three thermal cycles) shows the path to address challenges
brought by the first four-layer magnet tested at FNAL. This paper presents the
analysis of quench data along with diagnostic features and complementary
measurements taken in support of the magnet performance analysis.
