Status of developments at CEA-Saclay WP10: SRF Task 10.2: SC Cavities for proton Linacs Status of developments at CEA-Saclay

The team High beta Cavity Design/Study: G. Devanz, J. Plouin & S. Chel / S. Cazaux, Ph. Hardy, A. Mohamed Vertical EP station Study/Installation: F. Eozénou, Y. Gasser & S. Chel / J.-P. Poupeau, T. Vappereau

High b cavity: Sub task breakdown design of b=1 cavities (HE Cav) This presentation purchase of Nb material fabrication of 1 or 2 HE cavity prototype(s) surface preparation of cavity prototype(s) new vertical EP station Fabien’s presentation new HPR station low power RF test of cavity prototype(s) set-up for tuning of field flatness vertical insert of cryostat

Reference Parameters (see CDR SPL II) High b cavity: RF design Reference Parameters (see CDR SPL II) RF frequency 704.4 MHz Gradient 25 MV/m Number of cells/cavity 5 Average pulse current 40 mA Synchronous phase -15° Optimisation of cavity design in 3 steps: Step 1) Find the best cavity shape with respect to r/Q, Ep/Eacc, Bp/Eacc Step 2) Do what is necessary to reach the optimal coupling Qex Step 3) Add stiffeners to keep the LFD under control

RF design: cavity shape SPL Tesla HIPPI Number of cells 5 9 Frequency [MHz] 704.4 1300 Beta 1 0.47 Bpk/Eacc [mT/(MV/m)] 4.20 4.26 5.59 Epk/Eacc 1.99 2 3.36 G [W] 270 161 Cell to cell coupling [%] 1.92 1.87 1.35 r/Q [W] 566 1036 173 Lacc = Ngap.b.l/2 [m] 1.065 1.038 0.5 Operating Temp. [K) 2 K Theor. RBCS @ O.T. [nW] 3.2 11 Q0 /1010 @ O.T. for RBCS 8.4 2.5 RF parameters for the TM010 after iterations between step1 & step2 and optimisation outer ½ cell pick-up side Inner ½ cells outer ½ cell FPC side Equator REQ 190.786 Iris RI 65 64.6 70 Length AL 103.07 106.47 E_ellipse A1 74.45 77.5 E_ellipse B1 83.27 76.89 I_ellipse A2 18.5 22.1 I_ellipse B2 24.9 35.1

safe for b=1 and b=0.65 cavities RF design: coupler location With our set of cavity and beam parameters: Qex,opt=1.2 e6 ; Pbeam=Pin,max=1029 kW 50W – Ø100mm coaxial coupler is free of multipactor levels either around 1MW or 500 kW safe for b=1 and b=0.65 cavities 35 Ø 100 Ø 43.5 144 60 R5 R7 Optimisation of coupler location ØOC = 100mm ØIC = 43.5 mm

Qualification of 700MHz - 1MW coupler Two couplers successfully processed last year on a dedicated RF test stand up to 1.2 MW peak @10% DC in TW for 300hrs Assembly of one coupler on HIPPI cavity (700 MHz, b=0.5) in ISO4 CR, and installation in test cryostat CryHoLab Only short time of RF processing in full reflection was necessary to reach high power levels Cavity and coupler operation at 1MW full DC for several hours Efficient counter-flow GHe cooling of the coupler leading to limited heat transfer to the LHe bath Qualification of 704 MHz power coupler on sc cavity at 2K operated at Ppeak=1.2 MW with tpulse=2 ms and frep=50Hz

Due to the pulsed mode operation, LFD is of major concern RF design: LFD Due to the pulsed mode operation, LFD is of major concern To limit this effect and partially compensate for it, mechanical stiffeners and piezo actuators are used (correction algorithm is optimised in SLHC-PP ) Wall thickness 3mm Eq. weld thickn. 2mm Optimal position of stiffeners: Rring= 91 mm 35 kN/mm Measured tuner stiffness: 35 kN/mm Saclay IV tuner on HIPPI cavity For the cavity+tuner : |KL|  1 Hz/(MV/m)2

RF design Dimensions for cavity fabrication assymetric cavity 3 shapes of ½ cells stiffening rings between adjacent cells large beam tube ø140 mm with a ø100 mm port for power coupler small beam tube ø130 mm with a ø10 mm port for pick-up probe inner diameter of the cavity flanges fixed to 80 mm tapers connect the beam tubes to the cavity flanges each beam tube is equipped with one ø40 mm HOM port outer ½ cell pick-up side inner ½ cells outer ½ cell FPC side External radius (REQ) 190.86 Iris radius (RI) 64.89 64.49 69.90 Length (AL) 103.22 106.62 Equator ellipse a (A1) 74.36 77.41 Equator ellipse b (B1) 83.19 76.80 Iris ellipse a (A2) 18.73 22.33 Iris ellipse b (B2) 25.14 35.35 Stiffening Ring (RR) - 91.0 Dimensions for cavity fabrication before any chemical polishing and at room temp.

Frequency sensitivity SPL Nominal wall thickness [mm] 3 Cavity stiffness Kcav [kN/mm] 3.84 Tuning sensitivity Df/Dz [kHz/mm] 164 Stress per mm of tuning [MPa/mm] 25 KL with fixed ends [Hz/(MV/m)2] -0.55 KL with free ends [Hz/(MV/m)2] -5 KL with realistic bound. cond. [Hz/(MV/m)2] -1 Sensitivity to He pressure [Hz/mbar] (fixed ends) 1.2 15 MPa 25 MPa 1 mm A frequency shift of ~ 500 kHz is induced by a cavity shrinkage of ~ 3 mm (standard value for the Saclay IV tuner) Maximum cavity stress ~ 75 MPa remains far below the yield strength of Nb at 2K

Mechanical design At this step, fabrication of cavity prototype could start and it would fit on our frames for surface preparation and vertical test. To perform qualification tests in horizontal cryostat or cryomodule, studies of a fully equipped cavity (with tuner, coupler, tank, …) are necessary: power coupler port cooled by LHe one beam tube inside the He tank with FPC and HOM port opposite beam tube under vacuum with PU and HOM port lateral frequency tuner (Saclay IV type) located on the beam tube under vacuum symetric load with one piezo actuator each beam tube is equipped with one ø40 mm HOM port fabrication of cavity and He tank without brazing Helium tank made of Ti Flanges made of NbTi

Tuner: status Design principle: The tuner load is applied on the Ti flange while keeping He tank at the same position If Ti flange and He tank are stiff enough, the tuner load is almost entirely applied on the cavity Pre-design of cavity+tuner is necessary to: - Optimise the relative stiffness of the components - Reserve enough room to HOM and flanges - Determine the assembly sequence Special ass’y tool used in CR for cavity leak check Tool removed out of CR after assembly of the tuner on the cavity

Tuner: He tank Efficiency of the tuner strongly depends on the tank stiffness Thickn. = 3 mm Thickn. = 4 mm Goal: stiffness = 70 kN/mm (2x tuner stiffness) Stiffness = 25.7 kN/mm Optimisation still going on: shape of lateral rounded disk Ti flange (radius, thickness) wings (location, shape, number) thickness Thickn. = 4 mm Thickn. = 4 mm Stiffness = 43.4 kN/mm

To be fixed before fabrication Before cavity fabrication: relative angles between couplers (main and HOM) should be fixed HOM Power coupler Before tank fabrication: Fix position/angle of Helium ports Add smooth holes in some pieces welded to He Tank Optimisation of the tank stiffness (> 70 kN/mm) Optimisation of the Ti flange and ass’y tool Can be done in parallel with cavity fabrication

Achievements Cavity design complies with the requirements of the SPL b = 1 cavities (peak fields, r/Q, Qex) The beam tube cooled by LHeII insures thermal stability with safety margin After mechanical optimization and considering the results of tests done in the frame of SLHC-PP, LFD should not be an issue A lateral tuner (Saclay IV type) should provide the required tuning range with limited mechanical stress Most of the flanges and interfaces are now fixed With this design, existing and qualified coupler and tuner fit the b=1 cavity After measurement of fundamental parameters (Eacc, Qo) in a vertical cryostat, this cavity could be tested in an horizontal cryostat (CryHoLab) or be operated in a complete cryomodule

Plans for the next 12 months Detailed drawings of b=1 cavities (HE Cav) Purchase of Nb material Call for offers for cavity fabrication Fabrication of diabolos for 1 or 2 HE cavity prototype(s) Delivery of cavity w/o LHe tank Tuning and field flatness (modification of the tuning set-up) Surface preparation of cavity prototype(s) Electropolishing (vertical EP station) HPR (new rinsing station) Low power RF test of cavity prototype(s) modification of vertical insert of cryostat (not assessed yet)

Schedule

Task 10.2: CEA expenses 2009 Total Cost : 227k€ (tbc)

The End