An apparatus allows photons in the infrared, visible, ultraviolet, soft x-ray and hard x-ray energy ranges to pass freely along a line of sight path between two vacuum regions at different pressures, yet maintains the pressure difference between the two regions and, in particular, removes vapor species attempting to pass between the two regions along the line of sight path. The apparatus works by causing the line of sight path to pass through a volume provided with a pumping mechanism which is transparent to the photons. Within this volume the vapor species are ionized and deviated from the line of sight by both magnetic and electric fields. The ionization mechanism is provided by an electron plasma created by the Penning discharge mechanism in an equipotential volume between two sets of electrodes. Vapor species attempting to pass through the electron plasma are ionized by collisions with the electrons in the plasma and removed from the line of sight path by the magnetic and electric fields used to maintain the Penning discharge.