KEK, Ibaraki
Eight superconducting accelerating cavities have been stably operated in the KEKB with sufficiently low trip rates. Two superconducting crab cavities were installed in 2007 and soon the crab crossing operation started. Recently the KEKB luminosity reached the world record of 2.1 x 1034 cm-1s-1. Stable operations of the accelerating cavities contributed for the luminosity increase. For the future Super-KEKB, we are developing a high power coupler for an input power of 600 kW and a HOM damper for RF power absorption more than 30 kW. The Super-KEKB requires RF operations with the high beam loading and the low RF voltage than the present KEKB operation. To suppress klystron output powers the external Q value has to be reduced. A new operation was proposed for superconducting cavities. In order to keep high RF voltages in each cavity, some cavities reverse its synchronous beam phase while the total RF voltage is kept as low as the required one. Beam studies were successfully carried out with one cavity reversed its synchronous beam phase.
KEK, Ibaraki
Ultra Finish Technology Co., Ltd., Yokosuka
Sokendai, Ibaraki
Niobium electropolishing for SRF cavities are generally considered to be the best technology today. However, hydrofluoric and sulphuric acid mixture usually used in the EP process is harmful and requires us carefully controlled handling of it and the many additional facilities. In this article, we propose a new application of electrochemical buffing onto niobium SRF cavity. In the method of electrochemical buffing, a rotating disk with abrasive fine particles where electrolyte is supplied is pressed against the workpiece. The disk and the work function as a cathode and an anode, respectively and an aqueous solution of sodium nitrate is used for the electrolyte. This technique brings us a couple of advantages like high etching rate, ultra small surface roughness, cost-effective and environment-compatible polishing.
KEK, Ibaraki
Sokendai, Ibaraki
In development of superconducting radio-frequency niobium cavities, there are problems in low performances of electro-polished (EP) cavities with a fresh EP solution due to stains on the surfaces with discoloration. Although the stain problems have been known from the past researches, the detailed study with surface analysis has not been carried out. In this study, the stains on the niobium surfaces were observed with x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy and scanning electron microscope. According to results of XPS, there are some differences in atomic components at the stained and non-stained surfaces, ex, a little amount of fluorine and no metal oxide were found only at the stained surface. In this article, we will describe the detail of the XPS results.
KEK, Ibaraki
Sokendai, Ibaraki
We have constructed an Electro-polishing (EP) Facility in the Superconducting RF Test Facility (STF) at KEK in 2008. The EP facility has been used for the EP process of Superconducting RF (SRF) 9-cell cavities for more than one year. In the EP facility, the capacity of the EP-electrolyte reservoir tank is 2,000 L. This size is relatively large if compared with EP facilities in other laboratories. It means that the quality control of EP electrolyte is more difficult because the status of EP-electrolyte changes as the aging of EP-electrolyte proceeds. In the real EP-process operations, we circulated the EP electrolyte of 1,100 L which was firstly delivered into the tank in January 2008 and was disposed in May 2009. During this period, we performed the EP processes 40 times and periodically measured the concentration of Nb ,Al ,HF in the EP electrolyte. In this article, we report the detailed results of the EP-electrolyte monitoring as well as the observation of changing electronic current oscillation in the EP processes during this period in the EP facility at STF/KEK.
Sokendai, Ibaraki
KEK, Ibaraki
Buffered chemical polishing (BCP) has been widely used as a final recipe of the surface treatment for niobium cavities and there is still much room to improve this technology since it is environment friendly, cheaper and simpler than electro-polishing. To examine BCPed surface in detail, we carried out BCP experiment followed by various rinsing methods on a series of niobium samples at KEK. As a result of the BCP process some contaminants like fluorine, carbon, etc. have been detected at the surfaces which may be the prominent cause of limiting the performance of SRF cavities. To remove these contaminants, various rinsing processes such as ultra pure water rinse, ultrasonic pure water rinse, alcoholic rinse, detergent rinse, high pressure water rinse (HPR) had been tested after the BCP. The preliminary results show that, only HPR had potential to mitigate these contaminants. In this article, we describe the surface analysis results using X-ray photo electron spectroscopy etc and a comparative study of niobium BCPed samples followed by above mentioned rinsing processes.
KEK, Ibaraki
KEKB is an e-/e+ collider for the study of B physics and is also used for machine studies for future machines. The peak luminosity of KEKB, which is the world-highest value, has been still increasing. This report summarizes recent progress at KEKB.
KEK, Ibaraki
For future high-intensity positron or proton accelerators, beam instability caused by electron cloud is one of the most important problems. Some coatings on inner surface of beam chambers with materials having low secondary emission yields such as titanium nitride (TiN), non-evaporable getter and so on have represented good effects against the electron cloud instability. In this study, diamond like carbon (DLC) and TiN coated chambers, and a copper chamber without coating were installed to an arc section of KEKB LER to make comparisons of total pressure, residual gas components and electron cloud activity during the beam operation under the same condition. Residual gas observation for the DLC coating revealed much higher hydrogen gas desorption because a process gas including hydrogen was used for the film growth. No remarkable hydrocarbon gas desorption was found. On the other hand, a mass peak of amu=14, that is N+ was prominent in the TiN coating. The electron cloud activity in the DLC coating was lower than the TiN coating and the copper chamber.