Advances in Large Grain Resonators Activities of DESY, W.C. Heraeus and RI

1. Advances in Large Grain Resonators Activities of DESY, W.C. Heraeus and RI • material and fabrication aspects • preparation and RF test results W. Singer, S. Aderhold, J. Iversen, G. Kreps, A. Matheisen, X. Singer, K. Tvarovski, H. Weise (DESY) M. Pekeler (Research Instruments GmbH) F. Schölz, B. Spaniol, E. Stiedl (W.C. HERAEUS) Presented by: B. Spaniol

2. Ingot and disc fabrication W.C. Heraeus: Development of LG disc production was done within the framework of the R&D program of DESY and the W. C. HERAEUS. Large Grain Nb Energy exchange in the electron beam melted Niobium Melting-/cooling behavior was investigated: beam figures (different numbers, position und shape); energy entry (different focusing of the beam and stay time); refrigeration parameters (bottom, crucible wall, split). The complete process is not sufficiently stable in order to create a central crystal of big diameter and required orientation along the whole ingot. Parts of ingots with a big crystal of definite orientation can be reproducibly created

3. Ingot and disc fabrication W.C. Heraeus: Development of LG disc production was done within the framework of the R&D program of DESY and the W. C. HERAEUS. Cutting of the large grain Ingots by wire sawing at W.C.Heraeus

4. Ingot and disc fabrication W.C. Heraeus: Development of LG disc production was done within the framework of the R&D program of DESY and the W. C. HERAEUS. XXX MP1 MP6 MP2 MP5 MP7 MP3 MP4 very small thickness variations by wire sawing process

5. Material and fabrication aspects • Fabrication: • Disc of W.C. HERAEUS with big central crystal of diameter >150 mm cut by diamond saw • No Eddy Current scanning • Deep drawing • Machining • EB welding • Grinding of steps on grain boundaries only on 2 of 11 cavities (AC155 and AC156) • Very smooth (shiny) surface in grain areas after BCP • the steps at grain boundaries are more pronounced as in polycrystalline material DESY fabricated 11 LG 9-cell and several single cell cavities at ACCEL (RI) from HERAEUS material

6. Fabrication aspects • Some spring back after the deep drawing, making the half cells “elliptical”. The same happens after the trimming for EBW • Large single crystal at centre, no problems on iris area for deep drawing • Difficulties on assembly for EBW • In order to overcome the difficulties a special tool has been build by the company. • Successful assembly of dumb bells with new tool and EB welding of all equator welds Each cavity has a main central crystal of definite orientation. Half cells measured by tactile 3D-measurement

7. Deep drawing of LG half cells Tactile 3D-measurement shows deviation from tolerances (tolerance: +/-0,2 mm).

8. Dependence of the half cell shape accuracy from the crystal orientation of the main central crystal Averaged deviation from ideal shape of the half cell for different crystal orientations. Crystallographic plane (100) parallel to the sheet surface produce more shape deviation on deep drawing, compare to (211), (221)

9. Shape accuracy: frequency measurement Deviation of the frequency from defined value of 6 end half cells (L and S) and 48 middle half cells (N) for LG AC112-114. C – LG. W and T - fine grain material. The shape accuracy of LG material is lower as of conventional fine grain. All LG cavities are successfully tuned to the correct resonance frequency

10. Preparation and RF tests of AC112 – AC114 First test Q(Eacc) curve of the LG nine cell cavities AC112- AC114 at 2K after 100µm BCP, 800°C 2h, 20µm BCP, HPR XFEL Spec.

11. Preparation and RF tests of AC112 – AC114 Q(Eacc) curve of the LG nine cell cavities AC113- AC114 at 2K after additional 20-30 µm BCP and 125°C, 50 h baking Very similar behavior, good reproducibility

12. What treatment EP or BCP?? DESY data on one single cell cavity; it seems that EP works better 1AC4 DESY: Q(Eacc) curve of the LG single cell cavity 1AC4 after EP and BCP treatment. EP at Henkel, preparation at DESY (D. Reschke et al)

13. Preparation and RF tests of AC111 – AC114, after EP 3 LG cavities: additional EP of 50 -100 µm, combined with additional “in situ” baking. Enhancing the acceleration gradient by approx. 10 MV/m can be seen on two cavities (AC112 and AC113). AC113 installed into cryomodule PXFEL1 Degradation of Eacc from 28 to 14 MV/m for the cavity AC114 (quench without field emission)

14. T-maps of AC114: for π-mode, the quench in cell 2 above and near equator. In other modes, quench was also found in cell 1 and 9. In addition, many other smaller hot spots were found. Large groups of small craters on the entire surface of all cells found by optical inspection with high resolution camera in AC114 Depends the pits creation on the crystal orientation? More results needed.

15. Recently fabricated 8 LG cavities AC151-AC158.Treatment:100µm BCP, 800°C 2h, 20µm BCP, HPR and 125°C, 50 h baking. 100 µm BCP done at Fa. RI. XFEL No any conspicuousness for AC155 where the grinding of steps on grain boundaries has been done

16. All available at DESY up to now Q(Eacc) of LG 9-cell cavities (AC112 was not baked) XFEL XFEL requirements Eacc =24,3 MV/m fulfilled in first test. Eacc 25 - 30 MV/m stably reachable after ca. 120µm BCP

17. Comparison of the Eacc performance of large grain (LG) 9-cell cavities with similarly treated fine grain TTF cavities. BCP works for LG better compare to fine grain cavities

18. Is LG good enough? Single grain is better (100) (110) (111) Profile and step heights of the three grain sample at the grain boundary intersection AFM image of LG Nb, BCP etched up to 100 µm AFM roughness measurement (X. Singer, A. Dangwal-Pandey). Roughness of fine grain Nb after EP is 251 nm (A. Wu) .

19. Grain boundaries GBs contribute eventually to reduction of the cavity performance Single Crystal Option • Fabrication of TESLA shape single crystal single cell cavities was proposed at DESY • Following aspects have been investigated and taken into consideration during cavity fabrication • Definite enlargement of the discs diameter is possible without destroying the single crystal structure in an existing state. • Appropriate heat treatment will not destroy the deformed single crystal • The single crystals keep the crystallographic structure and the orientations after deep drawing and annealing at 800°C • Two single crystals will grow together by EB welding, if the crystal orientations is taken into account.

20. DESY single crystal cavity 1AC8 build from Heraeus disc by rolling at RWTH, deep drawing and EB welding at ACCEL Q(Eacc) curve after 112 µm BCP and in situ baking 120°C for 6 hrs. Eacc vs. material removal on single crystal single cell cavity (left). Preparation and RF tests of P.Kneisel, JLab

21. One large grain (single crystal) is still a very perspective option that allows stably reach very high gradient by simple BCP treatment Summary of test results on single crystal single cell cavities P. Kneisel et al EPAC2008

22. Summary Fabrication of multi cell cavities from large grain niobium by deep drawing and electron beam welding is feasible. 11 nine - cell LG cavities have been produced in frame of DESY order at company ACCEL from W.C. Heraeus material and treated at DESY Performance up to 30 MV/m can be achieved on the nine cell cavity after only 120 µm BCP treatment. EP preparation on multi-cell LG cavities is ongoing. Up to 41 MV/m was measured on a LG single cell cavities after EP. Several single grain (single crystal) single cell cavities of TESLA shape are build with very encouraging results.

23. Acknowledgments • We would like to thank our colleagues • Ermakov, B. van der Horst, L. Lilje and D. Reschke • for support of this work