  In this paper, a detailed investigation has been carried out to understand
the physics behind GEM charging-up and its effects on gain. Experiments have
been performed on both double and triple GEM with the help of 55Fe X-ray source
and a comparative study of these configurations along with the single GEM
results observed in our previous work has been reported. The increase in gain
due to polarization of GEM foil dielectric and reduction in gain due to charge
accumulation on dielectric are studied for various field configurations and
different radiation intensities.
