  Neutral Particle Analyzer (NPA) is one of the crucial diagnostic devices on
Tokamak facilities. Stripping unit is one of the main parts of the NPA. A
windowless gas stripping room with two differential pipes is adopted in a
parallel direction of electric and magnetic fields (E//B) NPA. The pressure
distributions in the stripping chamber are simulated by Ansys Fluent together
with MolFlow+. Based on the pressure distributions extracted from the
simulation, the stripping efficiency of the E//B NPA is studied with GEANT4.
The hadron reaction physics is modified to track the charge state of each
particle in a cross section base method in GEANT4. The transmission rates ($R$)
and the stripping efficiencies $f_{+1}$ are examined for the particle energy
ranging from 20 to 200 keV at the input pressure ($P_0$) ranging from 20 to 400
Pa. According to the combined global efficiency, $R \times f_{+1}$, $P_0$ = 240
Pa is obtained as the optimum pressure for the maximum global efficiency in the
incident energy range investigated.
