CEA, Gif-sur-Yvette
Commisariat à l'Energie Atomique (CEA), Grenoble Cedex 9
NHMFL, Tallahassee, Florida
CNR-INFM-LAMIA, Genova
Best RF bulk niobium accelerating cavities have nearly reached their ultimate limits at rf equatorial magnetic field H ~ 200 mT close to the thermodynamic critical field Hc. In 2006 Gurevich proposed to use nanoscale layers of superconducting materials with high values of Hc > HcNb for magnetic shielding of bulk niobium to increase the breakdown magnetic field inside SC RF cavities. Depositing good quality layers inside a whole cavity is rather difficult; we have made high quality samples with the techniques used for the preparation of superconducting electronics circuits and characterized these samples by PPMS, SQUID, and X-ray reflectivity. The dc magnetisation curves of 250 nm thick Nb film have been measured, with and without a magnetron sputtered coating of a single or multiple stack of 15 nm MgO and 25 nm NbN layers. The Nb samples with/without the coating clearly exhibit different behaviours. Because SQUID measurements are influenced by edge and shape effects we propose to develop a specific local magnetic measurement of HC1 based on ac third harmonic analysis in order to reveal the screening effect of multilayers.
KEYWORDS:
Particle accelerators, niobium, niobium nitride, RF cavities, thin films, superconducting multi-layers
CEA, Gif-sur-Yvette
JLAB, Newport News, Virginia
Buffered Electrochemical Polishing (abbreviated as BEP) is a process developed at JLAB. It was shown that BEP can produce the smoothest niobium surface ever reported [*,**] with a very fast removal rate. Some encouraging results after BEP on single-cell cavities treated by a vertical set-up have also been reported [***]. Complementary experiments on flat Nb samples have been carried out at CEA Saclay with a rotating disc electrode. Electrochemical Impedance Spectroscopy measurements were also performed. Experimental results obtained on BEP were compared with those achieved with the standard EP [****]. Differences in terms of fluorine diffusion and of surface film resistance between BEP and EP have been put forward. Studies are on going to understand if some poor RF test results obtained after BEP with a horizontal set-up as well as with a vertical one [***] are due to the modified electro-chemical mechanisms or the decreased quality of commercial electrolyte used.
*A.T. Wu et al, Applied Surface Science,253(2007)3041
**S. Jin et al, Proc. of 13th SRF Workshop SRF(2007)
***A.T. Wu et al, Proc. of PAC 2009, Vancouver(2009)
****F. Eozénou et al,This conference
CEA, Gif-sur-Yvette
The use of a rotating disc electrode for electrochemical measurements gives precious information concerning mass transport of species. Measurements were performed with a rotating niobium flat sample in HF-H2SO4 media. Intensity is found as a linear function of the square root of the rotation speed ω which is the signature of an electro-polishing (EP) controlled by the diffusion of the fluorine ion. The value of the related diffusion coefficient D has been estimated and D is in agreement with magnitudes reported for ionic species in different EP systems. D has also been found lower in aged mixtures. Electrochemical Impedance Spectroscopy measurements were also performed with a rotating disc electrode. Both voltamperemetric and also EIS measurements [*] prove the central role of fluorine during EP and show that EP mechanisms are modified with the aging of the bath. These criterions can be used to evaluate the quality of acids and to improve the life time of EP mixtures. However, a high fluorine content in niobium could be the origin of decreased performances noticed on single-cell cavities after EP, using modified parameters.
* F. Eozénou et al, CARE-Report-2008-022-SRF
CEA, Gif-sur-Yvette
DESY, Hamburg
First promising results concerning Electro-Polishing at lower voltage of 5V (abbreviated as LV-EP) has previously been reported [*,**]. This effort is being pursued and a 1-cell Tesla Shape 1.3 GHz cavity has been dedicated to LV-EP and has reached improved gradient exceeding 39MV/m. Furthermore, a second cavity has alternately been electro-polished at 5V and 17V. It did not encounter any decrease in performance after LV-EP. This process is then especially promising for the treatment of large cavities for proton applications. Moreover, long-time EP experiments on niobium flat samples show that high-voltage EP is more likely to generate impurities in the EP mixture that might contaminate cavities. Some results will also be presented concerning efficient field emission removal by chloroform rinsing of 1-cell cavities.
* F. Eozénou et al, Proc. of 13th workshop on SRF, China, (2007) TUP80
** F. Eozénou et al, CARE-Report-2008-022-SRF
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.