Large grain 9-cell cavities R&D at KEK

We are developing large grain/single crystal niobium material for ILC, collaborating with Tokyo Denkai. These materials are very much promising to obtain high SRF cavity performance with cost-effective production. We have fabricated two 9-cell cavities from large grain materials and made cold test to evaluate the SRF performance. In this paper, we will report cavity fabrications and preparations and cold test results. INTRODUCTION SRF cavity fabrication using large grain (LG) niobium material has a high potential of breakthrough on cavity cost performance. It can be fabricated very efficiently by multi-slicing large grain niobium ingots, which has been already successfully developed at KEK/Tokyo Denkai Corporation Ltd. [1]. The material property becomes more reliable and the cost should be cheaper than fine grain materials based on forging and rolling methods. To produce high gradient, BCP process will be available, which is much cost effective compared with EP. Thus, it will bring another reduction on cavity preparation. The grain size in niobium ingots is getting larger. Heraeus in Germany and CBMM in Brazil already have produced ingots having only several grains in the effective zone of sheet material for 1.3GHz cavity. Tokyo Denkai also has started R&D to produce large grain /single crystal sheets [2]. The regular production of single crystal sheet is getting reality, which could improve the material quality and produce cavity performance more reliably. LG 9-CELL CAVITY FABRICATION EXPERIENCES So far, we have fabricated two LG 9-cell cavities (I9#9 and I9#10) with low loss (LL) shape as ILC R&D. In the experience of I9#9, several difficulties on cavity fabrication were found due to the nonhomogeneity of grain orientations. Some of them were feed-backed to the I9#10 cavity fabrication. Deep Drawing and Trimming We describe the I9#10 experience here. We used LG materials by multi-wire slicing ingot [1]. No annealing carried out for the sheets before deep drawing. Forming was looks nice but some bigger machining error was observed in the cup length ( 140μm) after trimming, compared to the fine grain ( 40μm). The material thickness variation on an end cup is shown in Fig.1. The original thickness is 2.8mm. The iris section is reduced about 10%, the sidewall is no changed and the equator section becomes thicker about 5%. -25 -20 -15 -10 -5 0 5 10 15 0 2 4 6 8 10 12 T hi cn es s v ar ia tio n 2. 8m m [% ] Position Figure 1: Thickness variation in an end cup. Dumbbell Electron Beam Welding (EBW) Stiffener is changed from half rings (I9#9) to complete ring (I9#10), which makes tolerance better. It is important at CBP process later described. Iris EBW was done from inside. EBW shrinkage was measured after dumbbell EBW and was 0.171 0.04mm. We made the off set of +0.20 at trimming for the shrinkage, which was a little too big. Dumbbell Tuning The dumbbells were mechanically tuned with length and plane. Fig.2 shows an example of lengths in one dumbbell measured every 45. Usually dumbbells as received are shrunk about 2mm and the plane is also tilted by the Iris EBW strains, which is seen in Fig.2 (triangles). Dumbbells were tuned up to the target length (116.61 0.1mm) of RF design + a half of the equator EBW shrinkage (0.14mm, here). Grinding at Iris Seam and Cleaning All Iris EBW seams were ground by handy grinder. After that, they were lightly etched all the surfaces about 20μm in KEK. Dumbbells were rinsed with civil water at Al 112 113 114 115 116 117