Simulation of the time evolution of 1Â MeV proton microbeam transmission through an insulating macrocapillary

We present a classical trajectory Monte Carlo simulation of the transmission of 1 MeV H+ microbeam through an insulating macrocapillary. The focused proton beam was launched from the capillary entrance under 1° angle of incidence with respect to the capillary axis. The beam first hit the inner surface of the capillary and the protons were either implanted into the wall of the capillary target or scattered back mostly in forward direction. The formed charge patch on the wall started to deflect the beam due to the Coulomb repulsion. When enough charge accumulated, the deflection angle became large enough to guide the protons through the capillary without close collisions with the surface. During the simulation the charge migration on the capillary surface due to the finite resistivity of the insulator material is taken into account. The time evolution of the transmitted beam was investigated using several incident beam intensities. We found that the efficiency of the guiding depends on the incident beam intensity. In a certain incident intensity range, stable and efficient guiding is observable, which agrees with the experimental observation.