Recent studies on photoelectron and secondary electron yields of TiN and NEG coatings using the KEKB positron ring

摘要

In order to obtain a method to suppress electron-cloud instability (ECI), the photoelectron and the secondary electron yields (PEY and SEY) of a TiN coating and an NEG (Ti-Zr-V) coating on copper have been studied so far by using the KEK B-factory (KEKB) positron ring. Recently, test chambers with these coatings were installed at a straight section of the ring where the irradiated photon density was considerably smaller than that at the arc section of a previous experiment. The number of electrons around beams was measured by an electron current monitor; this measurement was performed up to a stored beam current of approximately 1700mA (1389 bunches). For the entire range of the beam current, the electron currents of the NEG-coated and the TiN-coated chambers were clearly smaller as compared to those of the uncoated copper chamber by the factors of 2-3 and 3-4, respectively. The small photon density, that is, the weak effect of photoelectrons, elucidated the differences in the SEYs of these coatings when compared to the measurements at the arc section. By assuming almost the same PEY (η_e) values obtained in the previous study, the maximum SEY (δ_(max)) for the TiN and NEG coatings and the copper chamber was again estimated based on a previously developed simulation. The evaluated δ_(max) values for these three surfaces were in the ranges of 0.8-1.0, 1.0-1.15, and 1.1-1.25, respectively. These values were consistent with the values obtained so far. As an application of the simulation, the effective η_e, η_(e-eff) (which included the geometrical effect of the antechamber) and δ_(max) values were also estimated for copper chambers with one or two antechambers. These chambers were installed in an arc section and a wiggler section, respectively. The evaluated η_(e-eff) and δ_(max) values were approximately 0.008 and 1.2, and 0.04 and 1.2, respectively, where η_e = 0.28 was assumed on the side wall. As expected, the η_(e-eff) values were considerably smaller than those obtained in the case of a simple circular chamber (η_e = 0.28-0.3). Further, the δ_(max) values were consistent with those obtained so far. With regard to the uncertainty in the simulation, the effect of the SEY spectrum on the estimation of δ_(max) values is briefly discussed. As the next step in our study, we plan to combine beam ducts with antechambers and TiN coatings; this combination is the most promising solution to ECI at present.