Advances in ion back-flow reduction in cascaded gaseous electron multipliers incorporating R-MHSP elements

A new concept is presented for the reduction of ion back-flow in GEM-based cascaded gaseous electron multipliers, by incorporating Micro-Hole & Strip Plate (MHSP) elements operating in reversed-bias mode (R-MHSP). About an order of magnitude reduction in ion back-flow is achieved by diverting back-drifting ions from their original path. A R-MHSP/2GEM/MHSP cascaded multiplier operated at total gain of ~1.5*10^5 yielded ion back-flow fractions of 0.0015 and 0.0004, at drift fields of 0.5 and 0.1 kV/cm, respectively. A 2R-MHSP/MHSP cascaded multiplier operated at a total gain of ~10^5, yielded an ion back-flow fraction of 0.003. We discuss the concept for trapping back-flowing ions in these cascaded multipliers and the relevance to gaseous photomultiplier and TPC applications; directions for further future developments are outlined.