1. Experimental Comparison at KEK of High Gradient Performance of Different Single Cell Superconducting Cavity Designs. F. Furuta, K. Saito, T. Saeki, H. Inoue, Y. Morozumi, T. Higo, Y. Higashi, H. Matsumoto, S. Kazakov, H. Yamaoka, K. Ueno, Y. Kobayashi, a R. S. Orr and b J. Sekutowicz, KEK, a Univ. of Toronto/JSPS, b DESY EPAC 2006 MOPLS084 1

2. Abstract We have performed a series of vertical tests of three different designs of single cell Niobium superconducting cavities at 2 degrees Kelvin. These tests aimed at establishing that an accelerating gradient of 45 MV/m could be reached in any of the designs, while using the standard KEK surface preparation. The designs tested were the Cornel re-entrant shape (RE), the DESY/KEK Low Loss shape (LL), and the KEK ICHIRO series. The cavities underwent surface preparation consisting of centrifugal barrel polishing, light chemical polishing, electropolishing, and final a high-pressure water rinse. All three kinds cavities were used in a series of vertical tests to investigate details of the surface treatment. When using ultra-pure water for the high pressure rinse, the LL cavity reproducibly exceeded a gradient of 45 MV/m, the RE design reproducibly reached a gradient of between 50 MV/m and 52 MV/m, and three of the six ICHIRO cavities reached a gradient of between 45 MV/m and 51 MV/m.

3. High gradient limitation of Type-2 SRF cavity 2

4. TESLALLREIS Diameter [mm]70606661 Ep/Eacc2.02.362.212.02 Hp/Eacc [Oe/MV/m]42.636.137.635.6 R/Q [W]113.8133.7126.8138 G[W]271284277285 Eacc max41.148.546.549.2 Principle of 50MV/m Cavity shape designs with low Hp/Eacc TTF: TESLA shape Reentrant (RE): Cornell Univ. Low Loss(LL): JLAB/DESY Ichiro ｰ Single(IS): KEK from J.Sekutowicz lecture Note 3

5. Ichiro Single Low Loss Reentrant Diameter [mm]60 Ep/Eacc2.36 Hp/Eacc [Oe/MV/m]36.1 R/Q [W]133.7 G[W]284 Eacc max48.5 Diameter [mm]66 Ep/Eacc2.21 Hp/Eacc [Oe/MV/m]37.6 R/Q [W]126.8 G[W]277 Eacc max46.5 Diameter [mm]61 Ep/Eacc2.02 Hp/Eacc [Oe/MV/m]35.6 R/Q [W]138 G[W]285 Eacc max49.2 Done the Principle proof of the 50MV/m 4

6. Eacc vs. Year 2 nd Breakthrough! 1 st Breakthrough! 5

7. Reentrant@ 2k Treatment Eacc,max Qo @ Eacc,max Re-evacuation 51.2 0.59E10 Re-HPR(UPW) 52.3 0.97E10 Re-evacuation 51.9 1.11E10 Warm-up only 52.4 1.21E10 Re-HPR(UPW) 50.0 0.98E10 Eacc max Ave. 51.6MV/m, Std. 1.0 MV/m Q0 = Ave. 0.97e10, Std. 0.24e10 Low Loss @ 2K Treatment Eacc,max Qo @ Eacc,max EP(30um)+HPR(UPR) 46.5 1.20E10 Re-evacuation 47.3 1.13E10 Re-HPR(UPR) 46.6 1.50E10 Warm-up only 45.0 1.03E10 Re-evacuation 44.0 1.20E10 Eacc max Ave. 45.9MV/m, Std. 1.3MV/m, Qo Ave. 1.21e10, Std. 0.18 e10 HPR and evacuation were repeated. Reproducibility 6

8. Temperature dependence of Eacc max 7

9. Field Emission Analysis cavity  FE onset RE6039 LL3045 IS1649  :field enhancement factor Field enhancement factor of IS shape is very small. 8

10. 9 RF critical field analysis New shapes

11. Summary Proof of reproducibility was done. See poster MOPLS087 New cavity shapes with low Hp/Eacc made a breakthrough in high gradient of 50MV/m. From the analysis of field enhancement factor  it was also shown that the high gradient performance of IS shape is superior to other shapes. Establishment of recipe is ongoing Cavities prepared by KEK recipe Mean=44.3MV/m, Sigma=6.9 by R.S.Orr 10