Vacuum Evacuation Effect on Ichiro 9cell Cavities during Vertical Test

We have continued high gradient R&D of ICHIRO 9cell cavities at KEK. The maximum gradient of ICHIRO 9-cell cavity #5 that has no end group on beam tubes was still limited around 36MV/m so far. The 9-cell performances were sometimes limited by triggered field emission (FE) by multipactings. We suspected the residual gas in the cavity might be one of the sources of triggered FE. The cavity was closed during vertical test in our system. Other labs evacuates cavity during vertical test. In order to improve the vacuum of cavity during vertical test, we made evacuation system in our cavity test stand. The comparison of results for vertical test with and without evacuation will be reported. INTRODUCTION We have demonstrated the principle proof of 50MV/m with ICHIRO single cell cavities [1]. The both centre and end cell shapes of ICHIRO have no problem on RF design for 50MV/m [2]. For the 9-cell cavity, we have taken 2 steps. The step-1 focused on the proof of 50MV/m on 9-cell using bare 9-cell cavities, which had no end group on beam tubes. End group means HOM couplers, RF input coupler port, and RF pick-up antenna port. The step-2 aims the actual ILC. Figure 1 shows bare and full ICHIRO 9-cell cavities. CURRENT STATUS OF ICHIRO 9-CELL CAVITIES Surface preparations Our current surface preparation recipe for ICHIRO cavities are consists of centrifugal barrel polishing (CBP, ~100μm), light chemical polishing (CP, 10μm), annealing (750C x 3hrs), electropolishing (EP, 80+20μm), flash EP (3μm, fresh acid, no circulation), post EP cleaning, HPR, and baking (120C x 48hrs). We will describe about post EP cleaning later. This recipe garantees 45MV/m: 46.7± 1.9MV/m with ICHIRO centre cell singles [3]. S0 study on ICHIRO#5, bare cavity We sent ICHIRO 9-cell #5 to Jlab as S0 tight loop study. In S0 study, cavities will be exchanged and tested at each laboratory. We can cross check the cavity performance, the yield and also compare the facilities. KEK staffs visited and joined the activities of ICHIRO#5 at Jlab. Figure 2 shows the summary of S0 study on ICHIRO#5 done by Jlab and KEK. The maximum gradient 36.5MV/m had achieved at Jlab and 33.7MV/m at KEK so far. Average Eacc are similar, so we confirmed that our KEK LL facilities are not so different from Jlab. [4] S0 study on ICHIRO#7, full cavity We fabricated two full ICHIRO 9-cell cavities, I9#7 and I9#8 which have full end group on beam tubes. I9#7 once measured at KEK but limited by FE. After VT, we found a defect on beam tube; it might be a source of FE. We ground it by mini-grinder, and then sent ICHIRO#7 to Jlab for S0 study. VT as received was already done at Jlab. Cavity was just rinsed at Jlab, no chemical treatments. Figure 3 shows the results of VT as received. Gradient was improved from 13MV/m at KEK to 21MV/m at Jlab. Figure 1: ICHIRO 9cell cavities, Top: ICHIRO#5, bare cavity, Bottom: ICHIRO#7, full cavity. ___________________________________________ #[email protected] 10 10 10 10 0 10 20 30 40 50 60 Qo N um er Eacc[MV/m] 1 2 3 4 N=4 at Jlab. ave. Eacc max=32.5+-4.2MV/m