Tau neutrino is the least studied lepton of the Standard
Model (SM). The NA65/DsTau experiment targets to investigate
Ds
, the parent particle of the ντ
, using the nuclear
emulsion-based detector and to decrease the systematic uncertainty
of ντ
flux prediction from over 50 % to 10 % for future
beam dump experiments. In the experiment, the emulsion detectors
are exposed to the CERN SPS 400 GeV proton beam. To provide
optimal conditions for the reconstruction of interactions, the
protons are required to be uniformly distributed over the detector's
surface with an average density of 105 cm-2 and the
fluctuation of less than 10%. To address this issue, we developed
a new proton irradiation system called the target mover. The new
target mover provided irradiation with a proton density of
1.01 × 105 cm-2 and the density fluctuation of
1.9 ± 0.3% in the DsTau 2021 run.