Modelling of negative ion transport in caesium-seeded volume negative ion sources

Trajectories of H- ions are calculated numerically by solving the 3D motion equation, including effects of collisional destruction, elastic collisions and charge exchange collisions. According to these trajectories, the extraction probability of H- ions produced at any location inside the source and the energy relaxation of extracted H- ions are estimated as a function of gas pressure. Effects of production zone and field intensity of the magnetic filter on extraction probability are also discussed. The extraction probability of surface produced H- ions is much higher than that of volume produced H- ions. The kinetic energy of extracted H- ions is reduced mainly by charge exchange collisions with H in higher pressure region and by elastic collisions with H+ in lower pressure region. We also briefly discuss the pressure dependence of extracted negative ion currents combining the present numerical results and the results of the model calculation with the zero-dimensional code.