Surface charge evolution during vacuum process in dielectric materials used in RF equipment for space applications

The use of dielectric materials in devices for space applications is steadily spreading due to the wide range of benefits that they offer in terms of electrical performance, weight reduction, and durability. Nevertheless, dielectric materials exhibit an inherent limitation that may compromise the device reliability: the accumulation of residual surface charge as a consequence of their low electrical conductivity. This surface charge may arise in orbit from many sources, such as space weather, onboard electronic systems, or the exposure to radiofrequency electromagnetic fields. Furthermore, the mere friction of air evacuation during the rocket launch can also produce different triboelectric surface charging levels depending on the electrical material properties. In this work, the evolution of surface charge in dielectric materials generated by air friction during its evacuation in a vacuum pumping process from atmospheric pressure to the pressure range of 10 -6 mbar is investigated, simulating the pressure profile for a rocket launch. Four dielectric materials extensively used in the space industry were selected: Teflon®, Kapton®, RT/Duroid® and Ferrite. On the one hand, a series of Teflon® samples with different thicknesses was analysed. The results revealed two distinct charge-evolution regimes governed predominantly by Ohm’s and Child’s law, respectively. Regarding the final static generated charge, a variation between negative and the positive value of 4·10 -11 C was observed as the pressure decreases from atmospheric conditions to high vacuum. This charge level was found to be consistent with the experimentally measured saturation charge of Teflon®. On the other hand, a set of samples with similar geometry comprising all four materials was investigated. The results demonstrate a clear relation between the static surface charge level and the electrical conductivity. In summary, this study provides insight into the evolution of the surface charge induced by air friction in dielectric materials involved in radiofrequency devices for space applications. These results contribute to a better understanding and prediction of the Multipactor effect, as surface charging has a significant influence on the secondary electron emission yield and, consequently, on the susceptibility of RF components to Multipactor discharges.