Continuing study on the photoelectron and secondary electron yield of TiN coating and NEG (Ti–Zr–V) coating under intense photon irradiation at the KEKB positron ring

In order to investigate a way to suppress the electron-cloud instability (ECI), the secondary electron and photoelectron yields (SEY and PEY) of a TiN coating were studied with an intense positron beam at the KEK B-Factory (KEKB), following up our previous study on a NEG (Ti–Zr–V) coating. A TiN-coated chamber was installed at an arc section of the KEKB positron ring, where photons with a line density of 6.5×1014 photons m−1 s−1 mA−1 were directly irradiated. The number of electrons around the positron bunches was measured by a special electron-current monitor, up to a stored beam current of about 1700 mA (1284 bunches). The electron current of the TiN-coated chamber was clearly smaller than those of the NEG-coated and the non-coated copper chambers by a factor of 2 for all beam currents. Using the previous results of the NEG-coated and the non-coated copper chambers as well as the TiN-coated one here, the maximum SEY (δmax) and the PEY (ηe) of the TiN coating, the NEG coating and the copper were again estimated based on a simulation. The evaluated δmax values for these three surfaces were 0.8–1.0, 0.9–1.1 and 1.1–1.3, and the ηe values were 0.13–0.15, 0.22–0.27 and 0.28–0.31, respectively. It was found that the TiN coating had an SEY (δmax∼0.9) as low as the NEG coating (δmax∼1.0), but the electron current was clearly smaller than that of the NEG coating, due to its lower photoelectron yield (ηe∼0.14). This study again indicated that the suppression of photoelectrons is required to make effective use of a surface with a low SEY, such as a TiN or a NEG coating. As an application of the simulation code, the electron current of a beam duct with an antechamber was calculated for the case of a NEG coating or a TiN coating ( δ max = 0.9 – 1.0 ). The calculated electron current for a copper duct with an antechamber was about 1/4 of that of a simple circular copper duct ( δ max = 1.2 for both cases) at a high current (∼1700 mA, 1284 bunches), which was in good agreement with the measurement. By combining with a surface with a low SEY ( δ max = 1.0 – 0.9 ), that is, a TiN coating or a NEG coating, a further reduction of the electron current by a factor of about 2 was obtained.