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.
High-pressure ultrapure waterrinsing , Particle contamination , Accelerator , rf Breakdown , Imaging spectrograph , outgassing , rf Conditioning , Surface analysis