JLAB, Newport News, Virginia
We report the progress on high gradient R&D of ILC cavities at Jefferson Lab since the Beijing workshop. Routine 9-cell cavity EP processing and testing has been enhanced with added surface mapping and T-mapping instrumentations. Field emission and quench behaviors of electropolished 9-cell cavities are studied. EP process continues to be optimized, resulting in advanced procedures and hence improved cavity performance. Several 9-cell cavities reached 35 MV/m after the first light EP processing. Field-emission-free performance has been demonstrated in a 9-cell cavity. 1-cell cavity studies explore new techniques for defect removal as well as advanced integrated cavity processing. Surface studies of niobium samples electropolished together with real cavities provide new insight into the nature of field emitters.
JLAB, Newport News, Virginia
The College of William and Mary, Williamsburg
A Surface Impedance Characterization (SIC) system has been proposed at the 2005 SRF workshop and recently updated as detailed at the 2009 PAC conference. Currently the SIC system can measure samples in a temperature range from 2K to 20K exposed to an RF magnetic flux density of less than 3mT. We report on new results of a BCP etched large grain Nb sample measured with this system as compared with previous results of a BCP etched polycrystalline Nb sample. The design of an upgraded SIC system for use at higher magnetic flux densities is on the way to more efficiently investigate correlations between local material characteristics and associated SRF properties, both for preparation studies of bulk niobium and also new thin film SRF developments.
JLAB, Newport News, Virginia
Several 9-cell cavities electropolished and tested at Jefferson Lab are found to be quench limited. Pass-band mode excitation measurements provide the first clue of candidate cells responsible for the quench limit. A second RF test with thermometers attached to the equator region of candidate cells (typically only 2 candidates) reveals a hot spot caused by excessive heating of the operational defect and hence determines its location. High resolution optical tools inspect the RF surface corresponding to the hot spot to image and document the defect. All defects in cavities quench limited ~ 20 MV/m are sub-mm sized irregularities near but outside of the equator EBW. In contrast, no observable irregularities are found in some other cavities that are quench limited ~ 30 MV/m. These two types of quench limited cavities have different response to a second EP processing. In this paper, we will give a summary of the test result and attempts to catalog the observed defects.
JLAB, Newport News, Virginia
KEK, Ibaraki
GUAS/AS, Ibaraki
We report the results of surface characterization of niobium samples electropolished together with single-cell niobium cavities. These witness samples were located in three regions of the cavity, namely at the equator, the iris and the beam-pipe respectively. Auger electron spectroscopy (AES) was utilized to probe the chemical composition of the topmost four atomic layers. Scanning electron microscopy with energy dispersive x-ray for elemental analysis (SEM/EDX) was used to observe the surface topography and chemical composition at the micrometer scale. A few atomic layers of sulfur (S) were found covering the samples non-uniformly. Niobium oxide granules with a sharp geometry were observed on every sample. Some Nb-O granules appeared to also contain sulfur.
JLAB, Newport News, Virginia
We have EP processed several multi-cell cavities previously heavy BCP etched. With a surprisingly light EP removal of less than 50 micron, all cavities have shown significant gradient and Q improvement. So far three cavities including two fine-grain niobium 7-cell CEBAF upgrade prototype cavities and one large-grain niobium 9-cell ILC cavities have been treated and tested. Both 7-cell cavities reached a quench limit without field emission. Another 7-cell cavity has been treated and is under RF test. We give a summary of the test results.
KEK, Ibaraki
Fermilab, Batavia
JLAB, Newport News, Virginia
We have continued high gradient R&D of ICHIRO 9-cell cavities at KEK. ICHIRO 9-cell cavity #5 (I9#5) that has no end groups on beam tube to focus on high gradient sent to Jlab as S0 tight loop study. Surface treatments and vertical test were repeated 3 times at Jlab, and then I9#5 sent back to KEK. We also repeated surface treatments and test at KEK. Maximum gradients were 36.5MV/m at Jlab, and 33.7MV/m at KEK so far. Now we are struggling with the puzzle why the results of singles do not work well on 9-cell cavities.
KEK, Ibaraki
CEA, Gif-sur-Yvette
JLAB, Newport News, Virginia
GUAS/AS, Ibaraki
The Electro-Polishing (EP) process is the best candidate of final surface treatment for the production of ILC cavities. Nevertheless, the broad distribution of the gradient caused by field emitters in cavities is sitll a serious problem for the EP process. A candidate source of field emitter is the sulfur component which is produced in the EP process and remains the inner-surface of cavities. We studied the effect of Ethanole- and degreaser-rinse processes after the EP process by a unique method. Moreover, we tried to test the sponge cleaning as the post-EP process to remove the field emitter inside the cavcity. This article describe the results of series tests of the post-EP process at KEK.
KEK, Ibaraki
JLAB, Newport News, Virginia
GUAS/AS, Ibaraki
In this article, two subjects would be described. the first subject is on the production of stains on the surface of Nb sample plates in Electro-polishing (EP) process and the second subject is on the development of defects/pits in the EP process on the surface of a Nb sample plate. Recently, some 9-cell cavities were treated with new EP acid at KEK and the performance of these cavities were limited by heavy field emissions. On the inside surface of these cavities, brown stains were observed. We made an effort to reproduce the brown stains on Nb sample plates with an EP setup in laboratory with varying the concentration of Nibium in the EP acid. We found that the brown stains would appear only when processed with new EP acid. In the second subject, we made artificial pits on the surface of a Nb-sample plate and observed the development of the pits after each step of 30um-EP process where 120um was removed in total by the EP process. This article describes these series EP-tests with Nb sample plates at KEK.