  Tau neutrino is the least studied lepton of the Standard Model (SM). The
NA65/DsTau experiment targets to investigate $D_s$, the parent particle of the
$\nu_\tau$, using the nuclear emulsion-based detector and to decrease the
systematic uncertainty of $\nu_\tau$ 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
$10^5~\rm{cm^{-2}}$ 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
$0.98~\rm{cm^{-2}}$ and the density fluctuation of $2.0\pm 0.3$% in the DsTau
2021 run.
