An ion treatment system for space traveling wave tube (TWT) multistage depressed collectors (MDCs) has been built. Processes were investigated using oxygen-free, high-conductivity (OFHC) copper plates and MDCs in terms of the measurement of secondary electron emission ratio (delta) and scanning electron microscope (SEM) analysis of surface texture. Measured delta for ion treated OFHC Cu plates, was found to be closely related to Mo deposition rate and spike texture observed on the SEM micrographs. For the optimized process, maximal d from OFHC Cu plates was reduced to 0.6, less than a half of that of the untreated, while measured maximal delta from inner wall of MDCs varies from 0.64 to similar to 1.0 depending on the geometry of the collector. With the treated four staged-MDCs, Ka-band TWTs have demonstrated a few percent of improvement in total efficiency. (C) 2008 Elsevier B. V. All rights reserved.