-- Summary highlights --
- The 9-cell EP activity has gotten into full swing, with top-quality performance obtained from a cavity fabricated by a US vendor.
- Collaborations with our Japanese colleagues moved forward with first EP cycle of the ICHIRO 5 cavity.
- A pair of cell thermometry arrays was completed and are ready for use in trouble-shooting performance limitations of 9-cell ILC cavities.
- Integrated contamination analysis tools were put to use to begin characterizing residual contaminants in the cavity production process.
- Analysis of a potential cavity damage mechanism via HPR over-exposure has obtained initial results.
Nb Cavity Electropolishing R&D - 9-cell cavities
First US-industry manufactured 9-cell ILC cavity reached a high gradient of 32.6 MV/m
On November 21, 2007, the cavity AES2 reached an accelerating gradient of 32.6 MV/m (Fig. 1) following the 4th light EP of a 20 microns surface removal. This is the first high gradient achieved by a 9-cell ILC cavity manufactured by the US industry, Advanced Energy System (AES). The cavity performance is limited by quench, instead of field emission. This represents continued success of Jlab’s high gradient capability in pushing high gradient without limited by field emission. It is noticed that the quench behavior of AES2 has been improving as more material is removed (different from that of AES1 and AES3, in which cases the quench limit is insensitive to material removal due to repeated light EP). The quench behavior of AES2 seems to be in agreement with the hypothesis of defects embedded in material, the number or strength of which reduces as more surface material is removed. We anticipate further gradient improvement with even more surface removal after another light EP.
First EP of KEK’s new Ichiro cavity done
On November 27, 2007, the first 20 microns EP of ICHIRO5 was successfully done, demonstrating the capability of Jlab’s EP machine for processing an Ichiro cavity which has a smaller aperture than a TTF cavity. The field flatness was tuned following EP. Another light EP is planned to remove possible contaminants introduced during tuning operation. This is to be followed by a vertical test. Kenji Saito and Fumio Furuta of KEK visited JLab and worked together with JLab colleagues to EP the cavity.
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Nb Cavity Electropolishing R&D - 1-cell cavities
No current activities on this topic - planning has begun
Thermometry
Two sets of single-cell thermometry for Tesla-style cells have been completed. First test this week. They will be available for use in localizing performance-limiting defects in future ILC R&D cavities.
Contamination Control
Field emitters on EP’ed Nb surface revealed and analyzed
Basis studies of properties of niobium surface electropolished together with 9-cell cavities continue. For the first time, individual field emitters revealed by JLab’s Scanning Field Emission SEM are analyzed. Two example field emitters are shown in Fig. 2(a). They show quite different geometrical properties. EDX analysis shows these emitters have chemical composition of Nb and O only (Fig. 2(b)). No detectable S is found.
High Pressure Rinsing R&D
It has been observed that excessive long-duration exposure of a cavity niobium surface to high pressure rinsing with ultra-pure water can have very significant detrimental effects on the rf performance of that surface. This was brought to our attention this past autumn when the JLab high pressure rinse (HPR) system faulted in its vertical translation motion, resulting in multi-hour rinsing of a beampipe near one end of a 9-cell cavity. An easily-observed discoloration mark was made on the cavity and this badly degraded the rf performance, even though it was well outside the high-field region of the cavity. Even slight BCP was adequate to restore the extreme field emission loading.
To investigate this phenomenon, a set of niobium samples were subjected to a range of extended-duration, fixed position HPR exposures. These samples are being examined by a variety of surface characterization tools.
Preliminary indications are that aggressive HPR with 18 Mohm/cm water initially induces an increase in the thickness of the normal Nb pentoxide, as evidenced by repeated optical color interference bands. After some thickness is reached, localized (submicron) fracturing is observed. With further HPR exposure, the damage sites grow and merge, forming what to the eye appears as a grey matte finish. XPS, EDS, and SIMS examination find only Nb and O. Identification of the threshold for onset of this damage mechanism has yet to be worked out. A speculation is that static potential generated by the HPR drives the oxidation chemistry and damaging stresses together with this high potential environment perhaps creates some inclusions of metalic suboxides, which produce the observed non-linear rf losses and/or simply the roughness of the resulting micro surface produces copious field emission sites.
Optical Micrographs:
Sample 3 SEM image in central region:
AFM surface profile texture of Nb sample exposed to 12 hours of fixed HPR:
Improving the characterization of the basic electrochemical process, surface effects, and their association to techniques applied to cavities
- How is the niobium EP process to be characterized in contemporary electochemistry terms?
- What range of local parameters are represented in multi-cell EP as presently practiced?
- How can the evolving surface topology during EP be quantitatively described?
- How does the evolving surface topology depend on local process conditions (polarization potential, temperature, flow, electrolyte composition)?
- How does the maximum sustainable H-field depend on local surface profile?
- What range of EP process conditions consistently produces the surface profile necessary for highest-performance cavities?