1. ELBE SRF Gun Testing Status PeterKneisel (JLAB) For the Rossendorf Team A.Arnold,P.Murcek,J.Teichert,P.Michel,P.Lu, H.Vennekate,R.Xiang T.Kamps (HZB Berlin) I.Will, MBI Berlin

2. DRACO laser 150 -> 500 TW Penelope 1.5 PW laser (2014) ion wake field acceleration CBS x-rays electron wake field acceleration 1. Operation of the 1st SRF gun at ELBE Superconducting RF photoinjector a high-brightness, megahertz rep. rate gun for CW operation the candidate for ERL-based future light sources

3. Review(2) (A.Arnold, SRF2009)

4. 4 cold mass of the first SRF gun (photograph from 2007) 1. Operation of the 1st SRF gun at ELBE designoperation choke filterSC cello.k. half-cellownmultipacting in cathode channel tunersELBE modifiedo.k. HOMs2 x DESY with saphier feed-through o.k. Main couplerELBEuntil 10 kW CW o.k.

5. E acc E peak on AxisE kin CW6.5 MV/m17.5 MV/m3.3 MeV Pulsed RF8 MV/m22 MV/m4.0 MeV measured Q 0 is 10 times lower than in vertical test → cavity pollution during string assembly? Maximum achievable field 1/3 of the design value 50 MV/m) Cavity performance limited by FE & He consumption Good News No Q degradation seen since over 4 years of operation! Same performance of cavity with or w/o cathode! Q vs. E measurement to identify cavity contamination Summary: 1. Operation of the 1st SRF gun at ELBE

6. 6 standard RRR 300 Nb cavity large grain Nb cavity Fabrication of two new cavities to approach the design value of the acceleration gradient collaboration with Jlab (fabrication, treatment and tests) slightly modified design: lower Lorentz force detuning lower microphonics better cleaning and simpler clean room assembly half-cell stiffening larger boring modified choke-cell pick-up 4. STATUS NEW SRF GUN

7. Cavity Fabrication

8. Problems Frequency and field profile was off: cathode cell ~ 2 mm too long very difficult tuning Shipment of cavity to FZDR for tuning, back July 2010 After two tests with LG cavity defects in the cathode cell showed up limiting the performance Cavity was cut apart by wire EDM, defects mechanically ground, rewelded with change in frequency and field profile, after 5 tests,still poor; FG cavity mechanically ground through beampipe, several tests Finally in test #7 FG cavity had good performance after 800C and “in situ” baking (July 6, 2011), subsequent fine tuning at Jlab by Andre and Petr, performance confirmed in test #8 (Aug. 21, 2011) Welding of Helium vessel with TIG before “evacuation “ of Testlab in April 2012, re-test in test #9 Leak detected at a joint between Nb and Ti ---- EBW needed, not before Sept. 2012 First leak eventually fixed with EBW, new leak detected at inner bellows Exchange of bellows in progress

9. Tuning Analysis by A.Arnold of measured field profile and Passband frequencies showed that cathode cell was ~ 2 mm too long. Difficult tuning on bench, only after heat treatment and softening of material possible at 600C Shipment of cavities to Rossendorf for tuning

10. Tuning in Rossendorf (A.Arnold et al, SRF2011)

11. Test Preparation

12. Defects: Repair

13. Add. Problems Large grain cavity  3 more tests after repair, finally found a defect on a weld by using second sound detectors  Defect is in the cathode cell, which is not accessible by a grinding tool  Meanwhile the cavity has been barrel polished by Ari Fine Grain  Defects were remove by grinding through beam pipe  After several more treatments and tests, a decent result was achieved in test #7 after 800C heat treatment

14. Summary, Status SRF 2011

15. Test #8 after final tuning at Jlab by A.Arnold and P.Murcek prior to SRF2011,

16. He vessel welding Very difficult and time consuming because some pre-fabricated parts did not fit and needed to be modified Competing with other projects Only one person qualified for Ti – welding It took 4 months to complete the He- vessel (Spet. 2011 – Dec. 2011) Testing with He- vessel 2/13/2012

17. Test #9 with He-vessel Epeak ~ 42 MV/m

18. Leak Repair Plans Cut out the inner bellows by milling Replace bellows by electron beam welding Leak is gone !! Retest cavity after HPR, then assemble string Wire EDM Mill

19. Summary The replacement of the injector cavity into ELBE has been delayed quite substantially by our difficulties with the replacement cavities Nevertheless, if the performance of the FG cavity can be preserved after elimination of the leak in the helium vessel, an improvement from present operation conditions of Epeak = 17.5 MV/m to ~ 40 MV/m would significantly improve the ELBE machine. The future plans are to assemble the cryo-module subsection at Jlab and ship it to FZD The LG cavity will be completed with Andre’s help at Jlab next year As said before, we totally underestimated the complexity of the project, including the difficulty in qualifying and the logistics of shipping equipment between Germany and the USA Originally we had foreseen 2 qualification tests for each cavity: FG was tested 9 times, LG so far 5 times

20. Acknowledgement Many thanks to our colleagues, who supported this work at Jlab: L.Turlington, G.Slack, B.Clemens, B.Martin, D.Forehand, R. Overton, T. Harris, Chr. Wilson, P.Kushnick, G.Ciovati, P. Dhakal, M.Stirbet, G. Eremeev, B.Rimmer, FZDR crew