An anomalous phenomenon was observed in the retarding potential analyzer(RPA)measurements of the energy of the ion beam from an 8 cm argon ion source.The current-voltage(7-V) curve,which should theoretically descend,went up as the ion retarding potential was increased.Various explanations,such as the Townsend discharge theory and secondary electron emission etc.were proposed but denied by the theory application condition or the experiment results.An angle of about -10° was found between the axes of the ion beam and the RPA according to the contours of the ion beam density and direction.The particle simulation and experiment of the sum of the collector and wall current were conducted at different incident ion angles.The trends of the Ⅰ-Ⅴ curve in simulation results conformed with the experimental results in most cases.The ion trajectories were simulated at different retarding potentials with an incident angle of-10°.According to these results,the reason for the anomalous phenomenon is that when there is a specific angle between the axes of the ion beam and the RPA,more ions are repelled from the vicinity of the ion retarding grid to avoid striking on the grid as the ion retarding potential increases.These redundant ions reach the plate and thus lead to the formation of an ascendingⅠ-Ⅴ curve.
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