Electric and Photoelectric Gates for ion backflow suppression in   multi-GEM structures
  A new approach to suppress ion backflow in multi-GEM structures is suggested. In this approach, the potential difference applied across the gap between two adjacent GEMs is reversed compared to the standard configuration. In such a gap structure, called Electric Gate, a signal transfer from the first to second GEM is presumably provided by the small residual field still existing at small gate voltages and connecting the holes of the two GEMs. On the other hand, ion backflow between the GEMs turned out to be substantially reduced. We also consider another configuration, called Photoelectric Gate, in which in addition to the Electric Gate configuration, a CsI photocathode is deposited on the second GEM. In the Photoelectric Gate, ion backflow through the gap is fully suppressed and the signal transfer through the gap is provided by the photoelectric mechanism due to either avalanche scintillations in the first GEM or proportional scintillations in the electroluminescence gap replacing the first GEM. The idea of the Electric Gate might find applications in the field of TPC detectors and gas photomultipliers. The idea of the Photoelectric Gate is more relevant in the field of two-phase avalanche detectors. 
A. Buzulutskov, A. Bondar