"I have to" might suggest some laboratory environment. What are the circumstances you have in terms of equipment and resources, plus the means of controlling the high voltages?
you are right: i am an italian student who are doing these experiments for the thesis of the master degree.
My specimens are nanocomposites with resistivity abot 3*10^12 Ohm*m, the maximum electric field will be about 5MV/m, the specimens have a thickness about 0.1 inch (0.2 mm).
i have a picoamperometer, a regulated power supply and a shielded chamber.