In Situ Surface Voltage Measurements of Dielectrics Under Electron Beam Irradiation

New instrumentation has been developed for noncontact, in vacuo measurements of the electron beam-induced surface voltage as a function of time and position for nonconductive spacecraft materials in a simulated space environment. The novel compact system uses two movable capacitive sensor electrodes to measure surface charge distributions on samples, using a noncontact method that has little effect on charge dissipation from sample. Design details, calibration, and characterization measurements of the system are presented, with <;1 V to >30 kV surface voltage range, <;0.5 V voltage resolution, and <;1.5-mm spatial resolution. Used in conjunction with the capabilities of an existing ultrahigh vacuum electron emission test chamber, the new instrumentation facilitates measurements of charge accumulation, bulk resistivity, effects of charge depletion and accumulation on yield measurements, electron-induced electrostatic breakdown potentials, radiation-induced conductivity effects, and the radial dispersion of surface voltage. Three types of measurements of surface voltage for polyimide (Kapton HN serve to illustrate the research capabilities of the new system: 1) accumulation using a pulsed electron beam, while periodically measuring the surface voltage; 2) postcharging, as deposited charge dissipated to a grounded substrate; and 3) the evolution of spatial profile resulting from an incident Gaussian beam. Theoretical models for sample charging and discharge are outlined to predict the time, temperature, and electric field dependence of the sample's net surface voltage.