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Radiation Source Rossendorf SRF-Gun Cavity Characteristics Member of the Leibniz Association Bautzner Landstr. 128 01328 Dresden/Germany http://www.fzd.de Contact: André Arnold Institute of Radiation Physics, Radiation Source ELBE Email: a.arnold@fzd.de Phone: +49 351 260 - 3382 Introduction We acknowledge the support of the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003-506395) and the support of the German Federal Ministry of Education and Research grant 05 ES4BR1/8. Acknowledgement A. Arnold 1, H. Buettig 1, D. Janssen 1, U. Lehnert 1, P. Michel 1, K. Moeller 1, P. Murcek 1, Ch. Schneider 1, R. Schurig 1, F. Staufenbiel 1, J. Teichert 1, R. Xiang 1, A. Matheisen 2, B. v. d. Horst 2, J. Stephan 3, T. Kamps 4, V. Volkov 5 1)FZ Dresden Rossendorf, Dresden, Germany 2)DESY, Hamburg, Germany 3)IKS, Dresden, Germany 4)BESSY, Berlin, Germany 5)Budker Institute of Nuclear Physics, Novosibirsk, Russia detuning measured by demodulated LL phase controller signal FFT using one minute time signals show discrete PSD (power spectral density) strong damping of the membrane pumps and its vacuum tubes was needed to achieve stable cavity operation Detuning 3-times higher than TESLA 9-cell cavities – but still sufficient for CW the electric & magnetic surface field causes a Lorentz Force pressure on the cavity wall the induced neg. frequency shift is proportional to the square of gradient Tektronix RSA3408A Rep Rate: 2kHz & 125kHz Beam current: 230nA Bunchcharge: 115pC @ 2kHz and 1.8pC @ 125kHz Gradient: 5MV/m (ca. 2MeV) HOM @ 2.491GHz R/Q = 18.90 Ω → HOM Power @ 1mA = 2.0 W HOM @ 2.822GHz R/Q = 1.87 Ω → HOM Power @ 1mA = 4.7 W R/Q calculation from measured channel power for the marked monopole HOMs results in: Due to the low rep rates down to some ten kHz and the unknown HOM spectra of a new welded cavity, at least one HOM coupler should be preferred In the case of two different tuners a shifting of the HOM spectra with one of them seems possible while the fundamental mode is kept constant using the other one! Monopole HOM tuning lever 3-cell cavity tuning lever 1/2-cell cavity tuning spindle stepping motor & gearbox tuning spankHz± 78± 225 mech. resolution nm/ step 1.32.1 frequency resolution Hz/ step 0.30.9 stored energy U32.5 J quality factor Q 010 dissipated power P c 25.8 W maximum beam power P B 9.4 kW geometry factor G241.9 Ω accel. voltage V acc accel. gradient E acc 9.4 MV 18.8 MV/m shunt impedance R a =V acc ²/2P c 1.72x10 12 Ω R a /Q 0 166.6 Ω E peak /E acc 2.66 B peak /E acc 6.1 mT/(MV/m) ½ cell tuner TESLA cell tuner main coupler e - -beam value s @ Q 0 =5 x10 9 1st BCP / HPR @DESY 2nd HPR @ DESY (Crash) 3rd BCP / HPR @ ACCEL 4th BCP / HPR @ ACCEL (crash) E acc 14.6 MV/m 2.8 MV/m 7.4 MV/m 9.3 MV/m E peak 39.0 MV/m 7.4 MV/m 19.6 MV/m 24.6 MV/m P diss 32 W1 W8 W13 W 1st V-Test (cleaning done @ DESY): - Residual surface resistance R res =3.4nΩ - Strong field emission, two Q-switches (thermal breakdown @ activated FE) Supposed problem: surface pollution out of the choke (cleaning not feasible with lance) 2nd V-Test (cleaning done @ DESY): - Choke rinsing by special HPR lance - HPR lance crashed with cavity during choke cleaning → worst result / limit FE 3rd V-Test (cleaning done @ ACCEL): - Caused by high risk no choke rinsing - No problems occurred but only 20MV/m 4th V-Test (cleaning done @ ACCEL): - Choke rinsing by special HPR lance - Achieved E peak better but also limit FE - To estimate position of FE all 4 passband modes measured cathode sc choke filter (to prevent RF leakage) half Cell FZD coupler & pickup ant. 2 HOM coupler 3 TESLA cell Under consideration of cooldown shrinking, change of permittivity & final BCP cleaning, the field distribution of the operated cavity inside the module can be calculated using the fundamental mode passband frequencies at 2K and the last known cavity state before final cleaning. The SRF-Gun development at FZD has made lot of progress. The installation of the injector module next to the linear accelerator ELBE was finished in September 2007. The cavity characteristics measured during the following commissioning are presented here. On-Axis Field Distribution 9th test after high power processing (HPP) up to E peak =25MV/m results in stable CW operation up to 17.6MV/m, above this level cavity starts to quench maximum achievable field only 1/3 of the designed value of E peak =50MV/m measured Q 0 is 10 times lower than in all vertical tests no Q degradation found during the 1 st year of operation first eight Q vs. E measurements limited by FE & He consumption Q 0 vs. E peak Tuning System Microphonics Microphonics (closed loop σ = 0.055° rms) comparable to ELBE & sufficient for CW CW Lorentz Force Detuning The resolution and the tuning span of both tuners are sufficient but the hysteresis has to be improved within the next maintenance! No problems during 2K cooldown Frequency shift same as expected from TESLA cells But 380kHz to high because change of ∆ε r not considered pressure sensibility (230Hz/mBar) is seven times higher than known from ELBE modules which is probably caused by different cavity design (weak half cell) no problem seen for the LL controller and max. available RF Power due to helium pressure stability of 0.1mbar Pressure Sensibility damped membrane pumps Helium refrigerator 63.1% 100% 96.6% 97.0% electrical field distribution along cavity axis for E z,max =50MV/m desired field flatness: [0.6 - 1 – 1 – 1]

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