Development of an S-band rf window for linear colliders

We report on our research and development of an S-band (2856 MHz) alumina rf window for a high power transmission which exceeds 100 MW. We have studied the mechanism of rf breakdowns of alumina disks theoretically with a computer simulation of multipactor phenomena, and also experimentally with a test facility based on a resonant ring. It was confirmed that at relatively low power transmission (<40 MW) the multipactor process is pronounced. However, for discharge breakdowns at a higher power transmission (>50 MW), impurities within alumina due to sintering binder materials and internal voids were shown to be mostly responsible. We have developed a technique to address those problems by using disks made from high-purity (99.9%) fine alumina powder, and by treating them with a hot isostatic pressing (HIP) method. An experiment at a resonant ring has shown that the new rf window based on this technique can successfully withstand a maximum peak transmission-power of 310 MW with a pulse width 2.5 μs at 10 Hz repetition rate, and a maximum average transmission-power of 300 MW with pulse width 2.5 μs at 50 Hz. (orig.)