Charging Characteristics of a Conical Insulating Spacer in Vacuum

Comparison of measured and theoretical results on the influence of contact angle

Abstract

This paper describes characteristics of surface charging on a conical insulating spacer that has various contact angles with a plane cathode. The spacer is made of polymethyl methacrylate resin (PMMA) and has the contact angle between +45 and -45 degrees, where the positive sign indicates that the spacer has an obtuse angle with the cathode surface, and the negative sign an acute angle. The spacer is subjected to a ramped DC voltage, and the charging is observed by using an electrostatic probe located on the surface of the cathode close to the triple junction TJ where the insulator, cathode and vacuum meet. When the contact angle is between -25 and 0 degrees, the spacer surface acquires positive charge, which increases almost linearly with the applied voltage. The spacer with a positive contact angle acquires no charge. These results agree with the theoretical prediction which is obtained from the calculation based on the secondary emission electron avalanche (SEEA) mechanism. On the contrary, the spacer having a contact angle between -30 and -45 degrees acquires very little charge. This result differs from the theoretical prediction in which the surface acquires negative charge that increases with the applied voltage. The disagreement between the results is discussed in terms of the relaxation of electric field at TJ due to the slight negative charge accumulation.