The flow of avalanche ions towards the radiation-conversion volume of a Time Projection Chamber (TPC) or the photocathode of a gaseous photomultiplier (GPM) may seriously affect the TPC’s drift field, damage the GPM’s photocathode or severely limit the GPM gain due to ion-induced secondary avalanches. Some applications require the suppression of ion flow by at least a factor of 103-104. We demonstrated that a unique combination of GEM, MHSP and R-MHSP multipliers in cascade suppress the ions’ back-flow (IBF) down to the 10-4 level, while maintaining the full electron detection efficiency. In another new principle, with Coimbra and Aveiro colleagues, we demontrated the "optical gate" concept. in UV-emitting counting gases, a first multiplier operating in avalanche mode transmits avalanche-induced UV photons to the following amplification stage coated with CsI. The phoelectrons are multiplied to form the final signal while ions cannot flow back to the first stage.