Surface TiN coating of TESLA couplers at DESY as an antimultipactor remedy

A development project was carried out at DESY, aimed at reducing secondary electron emission and multipactor effects in TESLA couplers by TiN layers generation on surfaces which were not protected in this way previously. Thin TiN films on ceramic or metallic surfaces were produced by deposition from Ti vapor in low pressure ammonia. Selection of processing parameters and their effect on multipactor suppression in RF field have been studied using a multipactor test resonator at DESY. Appropriate values of deposition rate, substrate temperature, final layer thickness and chemical conversion procedure were selected. A significant reduction of multipactor time during RF tests was reached due to surface coating. Chemical analysis of TiN layers on both ceramic and metallic substrates has been performed using SIMS method. TiN coating of more than 80 wave guide and cylindrical RF windows were performed for TTF2, TTF3 and TTF4 versions of TESLA couplers. Surface processing of flat wave guide windows for TTF2 resulted in significant reduction of multipactor effects and improvement of power transmission. Their good performance remained unchanged after 24-hour exposition to air. TiN coating of all vacuum-facing surface of TTF2 coupler led to reduction of the indispensable RF conditioning time from typically 3 days down to 4-6 hours. (author)