Spectrographic approach for the diagnosis of rf breakdown in accelerating rf structures

The acceleration gradient of an electron linac is limited by rf breakdown in its accelerating structure. We applied an imaging spectrograph systemto study themechanism of rf breakdown phenomena in accelerating rf structures. Excited gases released from the surfaceemit light during rf breakdownwith the type of gases dependent upon surface treatments and rinsing methods.To study rf breakdown, we used 2-m-long accelerating structures and investigated the effects of a high-pressure ultrapure water rinsing (HPR) treatment applied to these rf structures.We performed experiments to study the gases released fromthe surface of rf structures with quadrupole mass spectroscopy and imaging spectrography of atomic lines. As a result, just after rf breakdown, we could observe instantly increasing signals atmass numbers 2 (H2), 28 (CO), and 44 (CO2), but not 18 (H2O).We also conducted spectral imaging of the light emissions fromthe atoms and ions in a vacuumexcited through rf breakdown. Using an accelerating structure without HPR treatment,we observed atomic lines at 511 nm(Cu I), 622 nm(Cu II), and 711 nm(CI).WithHPRtreatment, the atomic lines were observed at 395 nm(O I), 459 nm(O II), 511 nm(Cu I), 538 nm(CI), 570 nm(Cu I), 578 nm(Cu I), 656 nm(H:Balmer alpha), and 740 nm(Cu II). In an additional surface analysis, we found carbon as the most dominant element, with the exception of copper, on the blackened surface of the rf-conditioned accelerating structure without HPR treatment. Based on these experiments, we concluded that some components of the plasma can affect a copper surface.We also have provided a phenomenological review of our experimental results and a simple explanation of rf conditioning with rf breakdown. # 2004 Elsevier B.V. All rights reserved.