  The future High Luminosity upgrade of the Large Hadron Collider (HL-LHC) at
CERN will include the low-beta inner triplets (Q1, Q2a/b, Q3) for two LHC
insertion regions. The Q1, Q3 components consist of eight 10 m-long LMQXFA
cryo-assemblies fabricated by the HL-LHC Accelerator Upgrade Project. Each
LMQXFA Cold mass contains two Nb3Sn magnets connected in series. A
stainless-steel shell is welded around the two magnets before the insertion
into the cryostat. There is a limit on how much coil preload increase induced
by the shell welding is allowed. Distributed Rayleigh backscattering fiber
optics sensors were used for the first time to obtain a strain map over a wide
area of a Nb3Sn magnet cold mass shell. Data were collected during welding of
the first LMQXFA cold mass and the results confirm that the increase of the
coil pole azimuthal pre-stress after welding do not exceed requirements.
