R&D of thick COBRA GEM for the application of the GEM-based TPC

Ion Back Flow (IBF) suppression is essential to avoid a space-charge distortion of the electric field under a high rate condition for the Time Projection Chamber (TPC). We developed Thick COBRA GEMs to investigate a capability of IBF suppression. It was found that the COBRA GEM can suppress IBF more effectively compared to a standard GEM. IBF reaches about 0.1-0.5% and energy resolution (σ) is ~ 20% with a stack configuration consisting of one standard GEM facing to the drift field and two COBRA GEMs.