Surface charging of insulated materials by negative ion beam bombardment

When a negative ion beam bombards an insulated material such as an isolated electrode or an insulator, a surface voltage is induced by the surface charging and will affect the ion beam trajectories during beam transport. The mechanism of the surface charging of an insulated electrode is related with its secondary electron emission factor and energy distribution. The charging voltage in this case is as low as several volts and its polarity is positive. On the other hand, in case of an insulator, the polarity is negative and its value is several volts because an electric dipole layer is generated just on the surface due to negative ion beam bombardment. These low surface charging voltages lead an advantage in negative ion beam applications as well as in negative ion beam transport