Surface charging of insulators in vacuum-comparison of measured and calculated data

Experimental investigations have been carried out to measure, by means of an electrostatic probe, the surface charging of cylindrical insulators in a vacuum. Four different insulator materials have been employed namely PTFE, PMMA, Polyimide and Al₂O₃. Voltage was applied as a step function and the temporal development of charge monitored over the first few minutes after which an equilibrium condition is generally achieved. The axial distribution of charge has been compared with predicted results from both a charge simulation technique for equilibrium conditions and a Monte Carlo technique for time developing conditions, both relying upon a secondary electron emission process. For equilibrium conditions, there is fairly close agreement between measured and calculated data except for positions on the insulator surface close to the cathode. For time developing conditions, disagreement between measurement and prediction suggests that the model for charge development is perhaps not as simple as imagined