(F)LIMM deconvolution procedure for on-line measurements of space charge in thin polymeric insulations

On-line measurements are developed to monitor space charge evolution in a polymeric insulation film by the mean of a (Focused) Laser Intensity Modulation Method set-up. The purpose of this paper is to describe how, starting from experimental on-line (F)LIMM currents, one is able to develop a global procedure leading to extract space charge profiles in dedicated test samples. Objects of study are 25$-\mu$m films of PTFE submitted to a protocol of poling starting from OV to 2.5 kV, alternating 6 volt-off and 6 volt-on measurements on each step of 500 V. Each measurement consists in recording a(F)LIMM pyro-current spectrum from 10Hz to 10 kHz hence a whole protocol is composed of 63 acquisitions of such spectra. Once measurements performed on PTFE sample hereinabove described, a normalization process has allowed first to confirm that there was no charge evolution (injection) between two successive Volt-on to Volt-off measurements, and secondly to launch a thermal calibration of the bench that is needed in (F)LIMM experiment. Then a mathematical correction on Volt-on current was operated in order to extract the useful part linked to the internal field produced by the space charge. Finally, significant space charge representations and evolutions were obtained in our test samples by using a mathematical deconvolution procedure.