Properties of thin antimultipactor TiN and Cr2O3 coatings for klystron windows

Abstract

Low secondary electron emission yield (1–1.3), low radio-frequency (rf) loss films are of interest as coatings for alumina ceramic high power klystron windows. Very thin (15–50 Å) Ti and TiN layers have been previously used with success in klystron tubes operating at lower power levels (<35 MW peak power) and short (2.5 μs) pulses but are subject to property changes during air exposure, tube bakeout (550 °C for up to 10 days) and in situ electron bombardment. Higher power tubes (≥50 MW) with longer (5 μs) pulse lengths require coatings that remain stable under these more rigorous conditions in order to avoid multipactoring and window failure due to overheating. Air-oxidized Cr films offer an alternative to TiN. This work shows that they have the required resistivity for low rf loss, combined with stability in the secondary electron emission (SEE) yield under bakeout and electron bombardment. SEE, AES, XPS, ELS, oxidation, bakeout, and electron bombardment results are presented for Cr and TiN layers deposited by sputtering onto high density alumina substrates. The surface of TiN oxidized to a 9 Å thick TiO2 layer which, under bakeout to 550 °C in vacuum, thinned to 6 Å. This extreme layer thinness resulted in a significant drop in the SEE yield which, unfortunately, was reversible upon reoxidation. The oxidized layer on Cr was 17 Å of Cr2O3 which thinned to 13 Å upon bakeout. This thicker oxide layer produced a stable SEE yield which, although not as low as oxidized TiN, is sufficient to prevent electron multipactor. Windows covered by air-oxidized Cr films are at least as good, under tube processing conditions, as the best TiN-coated windows.