Solution to the ion feedback problem in Hybrid Photon Detectors and   Photo Multiplier Tubes
  A general solution to the positive ion feedback problem in Hybrid Photon Detectors (HPD), photo multipliers (PM) and other similar detectors was found in the insertion of a permanent electrostatic potential barrier which prevents drift of positive ions from the anode, or the first dynode, towards the photocathode. In this paper we present the method as applied to the Intevac HPD. 
Daniel Ferenc, Dario Hrupec and Eckart Lorenz