A. Ermakov, W. Singer, X. Singer

1. Unloaded Quality FactorQoof Prototype European XFEL Cavities, Large Grainand Hydroformed Cavities TUP024 A. Ermakov, W. Singer, X. Singer Deutsches Elektronen-Synchrotron, DESY, Notkestrasse 85, 22607 Hamburg, Germany. Abstract The comparative analysis of unloaded quality factor Q0 (at 1 MV/m; 5 MV/m), residual surface resistance Rres, medium field (5-20 MV/m) Q slope γof different types of cavities (Fine Grain, Large Grain, Single Crystal, Hydroformed - 1, 9-cell cavities) of different treatment procedures (BCP, EP, Baking, HPR etc.), different Nb supplier and cavities producers was done based on available RF data (DESY Database) [1]. The purpose is to find the relationship between the parameters describing the behavior of Q0 and main treatments procedures as well as the different types of cavities and Nb suppliers. Introduction The curves Q0 vs. Eacc show 3 distinct regions. At low fields below 5 MV/m often can be seen an increase of Q0 in fields below 20-25 MV/m followed by slow degradation of Q0 (medium field Q slope) then by rapid decrease of quality factor (Q drop). Medium field Q-slope in field range 5-20 MV/m was estimated according to model of Halbritter [2]. The reasons for increasing of the surface resistance with increasing of RF field according to his model is the thermal impedance between the inner surface of the cavity and the thermal bath, cooling the outer surface of the cavity. Surface resistance Rs can be expressed as quadratic function of Bp (Bp=k*Eacc): Rs (T, Bp)= Rs(T)[ 1+γ(T)(Bp/Bc)2+O(Bp4)], where Rs=Rbcs(T)+Rres, Bc=200 mT- thermodynamiccritical field at T=0. The medium field Q-slope is presented by the parameter γ(T)≈Rbcs(T)Bc2Δ/2kT2 (d/k+Rk), where k and Rk are the niobium thermal conductivity and Kapitza resistance respectively, d is the wall thickness. The distribution of Q0 (1;5 MV/m), Rres and medium field Q-slope γ (in fields below 20 MV/m) isstatisticallycompared through the whole set of studied cavities. Fine grain (FG) 9-cells cavities (EXFEL Prototype) 35 fine grain 9-cells cavities series AC115-150 (Research Instruments GmbH, RI) and Z130-144 (Ettore Zanon S.p.A., EZ) designed as prototype EXFEL cavities having the following treatment sequence according to EXFEL recipe: EP removal of a 110-140 µm, outside BCP etching, following by final EP of 40-50 μm, ultra-pure high-pressure water rinsing (HPR), 120°C bake or alternatively a final Flash BCP of 10 µm, HPR and 120°C bake [3]. Large grain (LG) 9-cells cavities (EXFEL Prototype) 11 large grain cavities 1.3 GHz TESLA shape cavities series AC112-AC114 & AC151-AC158 (producer RI; Nb supplier Heraeus). Sequence of treatments applied in DESY: BCP of inside surface, final BCP and EPremoval of inside surface followed by baking [4]. Cavity AC114 removed from analysis due to its strong performance degradation after EP. Fine grain, Large Grain & Single Crystal 1-cells cavities 18 fine grain 1 cell cavities series 1DE1-1DE18 have the similar treatment recipe (BCP; EP, Baking, HPR). 2 single crystal 1-cell cavities: BCP; EP (1AC6 and1AC8) (producers: RI; Nb suppliers: CBMM & Heraeus) and 6 LG 1 cells cavities 1AC3-1AC7, 1DE20-21. The similar treatment sequence (EP & BCP) for cavities 1DE20-21; 1AC3-1AC5; 1AC7. Hydroformed 9-cells cavities 3 hydroformed by DESY 9 cell cavities (Z145, Z163, Z164): BCP, EP, BCP Flash (Z145); BCP, EP (Z163); BCP, EP (+250 µm removed during first preparation stage) (Z164) [5]. Unloaded quality factor Q0, surface residual resistance Rres and medium field Q slopeγ vs. different treatments (EP, BCP, final EP, BCP flash), different Nb suppliers and different types of cavities (SC, FG, LG) is presented on Fig. 1-4. Figure 1: Large grain, hydroformed 9-cells, large & fine grain 1 cell cavities series AC112-114, 151-158; 1DE3-1DE18; 1DE20-21; 1AC3-1AC7 after BCP+EP. Different Nb supplier. Summary Statistical analysis of Q0, Rres and medium field Q slope γof FG, LG-1;9 cellcavities (Fig.1) after BCP+EP shows a relatively high valuesofQ0(5 MV/m) (< Q0 >=2,74*1010) for large grain cavities in comparison with fine and large grain 1 cell cavities. Average value <Q0 >= 2,18*1010 for FG 1-cell CVsisa bit lower as for LG 1-cell CVs (<Q0 >= 2,38*1010). Q0 values for 2 hydroformed cavities (Z145, 163)liesin the middle values field. For this type of comparison we assume that cavities producer and Nb supplier have no influence on cavity performance. Surface residual resistance doesn’t change significantly for whole set except cavity AC158 consequently observed the relatively high value of γ for this cavity. Q slope γ also shows a high values for AC154 & AC157 and for 1 cell FG CVs 1DE8 & 1DE11. For hydroformed cavities Z145, 163 medium field Q slope has a low values. Fig. 2 shows the same set of cavities limited only by one Nb supplier (Heraeus). Average values of quality factor (1; 5 MV/m) of FG & LG 1-cell cavities have a bit lower magnitude in compare with data for Nb suppliers (PLANSEE, Cabot, Ningxia OTIC) (see Fig.1). The low field Q increase clearly come out to be small for 1 cell FG cavities. Different types of treatments (EP+BCP flash or EP+final EP) for EXFEL prototype cavities (EZ & RI; Nb supplier – Fa. Tokyo Denkai) show the same range of quality factors at Eacc=1; 5 MV/m (Fig. 3). Residual surface resistance and Q slope γ for cavities after final EP shows a bit high scattering compared to cavities after BCP flash. The quality factor Q0 doesn’t depend significantly on Nb-supplier and CVs producer for 1;9-cell cavities (Fig. 4). SC 1-cell cavity 1AC8 shows slightly low value of Q0(Nb: Heraeus) while another SC cavity 1AC6 (Nb: CBMM) have a high value compared with another LG & FG 1-cell cavities. Parameter γhas a wide distribution for LG 9-cell cavities in comparison to 1-cell cavities having the same treatment type and Nb supplier. Figure 2: Large grain 9-cells; fine & large grain 1 cell cavities series AC112-114, AC151-158, 1DE3-1DE7; 1DE20-21, 1AC3-1AC7. Nb - Fa. Heraeus. Figure 3 (a,b): FG 9-cell cavities series Z131-144 (EZ), AC116-150 (RI) after EP final (a) & BCP flash (b). Nb-Fa. Tokyo Denkai References [1] P.D. Gall, A. Gössel, V. Gubarev, A Database for Superconducting Cavities for the TESLA Test Facility,Proceedings of the 11th Workshop on RF Superconductivity, Lübeck/Travemünder, Germany, 2003, TUP12 [2] J. Halbritter, “RF Residual Losses, High Electric and Magnetic RF Fields in Superconducting Cavities”, 38th Eloisitron Workshop, Erice, Italy, 1999. [3] W. Singer et al., Preparation Phase for 1.3 GHz Cavity Production of the European XFEL, IPAC2010, Kyoto, Japan, May 23–28, THOARA02 [4] W. Singer, S. Aderhold, A. Ermakov, J. Iversen, D. Kostin, G. Kreps, A. Matheisen, W.-D. Moller, D. Reschke, X. Singer, K. Twarowski, H. Weise, H.-G. Brokmeier, Development of Large Grain Cavities, Phys. Rev. STAB 16 (2013) , 012003-1 [5] W. Singer, A. Ermakov, G. Kreps, A. Matheisen, X. Singer, K. Twarowski, I. Zhelezov, P. Kneisel,. R. Crooks, Nine - Cell Tesla Shape Cavities Producedfrom Hydroformed Cells, 15th International Conference on RF Superconductivity (SRF 2011), Chicago, July 25-29, USA Figure 4: SC, FG & LG 1-cell & FG, LG 9-cell cavities (BCP+EP; EP+BCP flash): different Nb-suppliers & CVs producers.