1. The HiLumi LHC Design Study (a sub-system of HL-LHC) is co-funded by the European Commission within the Framework Programme 7 Capacities Specific Programme, Grant Agreement 284404. Helium Vessel and Tuning for the UK 4Rod Cavity Thomas Jones, STFC, Daresbury Laboratory 10/12/13

2. Contents Helium vessel conceptual design Helium vessel conceptual design Vessel manufacture Vessel manufacture Thermal contraction and Pressure Thermal contraction and Pressure Tuning Tuning Magnetic shielding Magnetic shielding 2

3. Helium vessel conceptual design Helium Vessel made from Grade 2 Titanium Helium 2-Phase line Input coupler port Modified Saclay II tuner Stiffening ribs Additional beam tube required for LHC installation UK 4 Rod Cavity Port for He level probe 3 194mm 550mm 312mm Cryoperm (magnetic shield)

4. HOM coupler port Titanium bellows to allow cavity compression when tuning Nb-Ti transitions e-beam welded during cavity manufacture. Bare cavity prior to He vessel installation LOM coupler port 4 Cavity bracing, could be fiducialised to give external reference. Rectangular flange to required for tuner.

5. Vessel Manufacture 5 316LN Conflat (CERN Spec) Bent 6mm thick Grade 2 Ti sheet 40mm deep x 10mm thick Grade 2 Ti Stiffening ribs 10mm thick Grade 2 Titanium End plates 30mm thick Grade 2 Ti Mounting blocks for tuner All joints in vessel to be TIG welded 316LN Conflat (CERN Spec)

6. 6 Thermal contraction – Preliminary study AttributeTi vessel and flangeSS vessel and Flange Stress at tuning flange174 MPa444 MPa Max stress in cavity180 MPa566 MPa ΔLength of He vessel0.76 mm1.68 mm ΔLength of Cavity in vessel0.79 mm Note: Grade 2 Ti used for analysis, Grade 5 has enough difference in CTE to also give fairly high stresses. Von Mises Stress (MPa) Grade 2 Ti 304 Stainless Steel Tube face fixed Additional bellows would be required for a stainless steel vessel, which may compromise cavity support. Grade 2 Ti should work, the stresses are currently just above an acceptable limit but this design could be optimised. Thanks to Tom Nicol for material data! 600 MPa 0 MPa

7. Pressure vessel analysis – Mesh, Load and Boundary conditions Fixed in X and Y Outer He vessel modelled as Grade 2 Titanium Young’s modulus 113GPa Poisson’s ratio 0.342 Cavity modelled as Niobium Reactor grade Type 1, UNS R04200. Young’s modulus 100GPa, Poisson’s Ratio 0.4 0.26MPa applied to all internal surfaces, to simulate a 2.6 Bar (abs) internal pressure test required by CERN safety code. 7 Fixed in all directions/rotations Stainless steel tuner representation Bellows removed to simplify analysis. CF flanges and 2 phase line removed to simplify FEA.

8. Pressure vessel analysis – Results Max stress in Niobium 59MPa which is below allowable 70MPa calculated by Yield Stress/1.05. [1] Fatigue not considered as no more than 500 full pressure cycles will occur and pressure fluctuations inside the vessel will be <<5%PS. Vessel as designed is suitable for 2.6 Bar (absolute) pressure test. The maximum stress in the model is 181MPa but this occurs in the Grade 2 Titanium vessel which has a Yield Strength of 275MPa (Allowable 262MPa). The largest deformation at 2.6bar (abs) in the Ti vessel is 1.9mm. Largest deformation in cavity 0.87mm (shown below). RF simulations show 70KHz per mm deflection in this region, therefore cavity pressure sensitivity is 23Hz/mbar. ((0.87/2.6) x 70) Ref 1 : BS EN13445-3:2002 +A11:2006 Unfired pressure vessel Part 3: Design 8

9. Tuning with Saclay II design Above images ref: Oliver Kugeler, ERL07, Daresbury, May 21-25 9 Modified Saclay II tuner installed on DICC module built at Daresbury. Stiffness test of modified Saclay II design at Daresbury. Experience with tuner and piezo active tuning system gained on Daresbury International Collaborative Cryomodule project. Current tuner design tested to 7KN with ~25μm average deflection. Tuner has been modified to increase stiffness for increased width.

10. Tuning with Saclay II design Above images ref: Oliver Kugeler, ERL07, Daresbury, May 21-25 Experience with tuner and piezo active tuning system gained on Daresbury International Collaborative Cryomodule project. Current tuner design tested to 7KN with ~25μm average deflection. Tuner has been modified to increase stiffness for increased width. 10 Modified Saclay II tuner installed on DICC module built at Daresbury. Stiffness test of modified Saclay II design at Daresbury.

11. Saclay II design integration Piezo units Tuner width increased to suit He Vessel geometry Eccentric cam Stepper motor in its own magnetic shield Ribs support He vessel under tuning loads 11 The cavity has been thinned in certain regions to increase tuning range and limit the force required by the tuners whilst remaining within the pressure code. Studs between tuner and flange can be used to allow for thermal contractions. Material added here to increase tuner stiffness

12. 12 Tuning analysis – Mesh, Load and Boundary conditions Side wall not shown for clarity, but used for FEA. Same material properties as pressure analysis used. 3500N applied here for slow tuner, zero applied for piezo analysis. Force equal to sum of top and bottom applied here. Fixed in X and Y Fixed in all directions/rotations Active tuning is only in one direction as Piezos need to always be in compression. 3500N applied here for slow tuner, 500N applied for piezo analysis.

13. The maximum stress with 7kN load is 103MPa. The FERMILAB Summary of Niobium material properties states that the acceptable stress limit for Nb at 2K is 137MPa. The danger with using this limit is that if the tuner ‘sticks’ at cryogenic temperatures when the module warms up and the yield stress of Nb returns to 75MPa the cavity will be plastically deformed. This, however, may not be a disadvantage as it can be used to permanently tune the cavity. The total deformation is 0.76mm, however, for tuning of the UK 4 Rod it is the lateral deflection that is critical (next slide). This result gives a cavity stiffness of 9.2kN/mm. Von-Mises equivalent stress Total deformation 13 Tuning analysis – Results

14. Maximum lateral movement is 719 μm. This is a 0.3° rotation. This reduces to 700 μm once the tuner stiffness is considered. Tuner stiffness is 320kN/mm. Frequency tuning was studied in a simplified model. The frequency shift per mm of transverse offset is 0.3 MHz/mm so tuning range with current design is 210 kHz. Tuning range required by specification is +/- 60kHz, therefore this design is acceptable with some additional capacity. Tuning analysis – Results 7000N on Tuner frame gives 21.5μm maximum deformation. 14

15. 15 Tuning analysis – Piezo analysis 500N load on one side of the Tuner only gives a total deformation of 55μm. The deformation is fairy symmetrical about the ZX symmetry plane. The 53μm in the lateral direction gives a tuning range of +/-16kHz. Further work will be required to access the level of preloading for the piezos. The maximum force on the two piezos will be 7000N. Reference: P. Bosland, Bo Wu, DAPNIA – CEA Saclay. Mechanical Study of the Saclay Piezo tuner

16. Magnetic Shielding 16 1 layer of 2mm thick Mu Metal 1 layer of 1mm thick Cryoperm Analysis by Magnetic Shields LTD Shielding shown gives 36x attenuation of the external magnetic fields. Considering the Earths magnetic field only this gives a <14mG resultant field at the cavity. A field map for the SPS area (both beam on and off) is required for more detailed analysis.

17. Conclusions and Further work The concept for the dressed UK 4 Rod cavity has been further developed. The concept for the dressed UK 4 Rod cavity has been further developed. The concept uses a TIG welded grade 2 Titanium Helium tank with Nb-Ti transition pieces e-beam welded to the cavity. The concept uses a TIG welded grade 2 Titanium Helium tank with Nb-Ti transition pieces e-beam welded to the cavity. This is to minimise stress induced on cool down but also has the advantage of saving weight, therefore minimising the load on the cavity string support. This is to minimise stress induced on cool down but also has the advantage of saving weight, therefore minimising the load on the cavity string support. The cavity has been reinforced, and is therefore now suitable for the 2.6 bar absolute test pressure. The cavity has been reinforced, and is therefore now suitable for the 2.6 bar absolute test pressure. The cavity has been optimised for tuning and initial analysis shows that the use of a modified Saclay II type tuner is viable. The cavity has been optimised for tuning and initial analysis shows that the use of a modified Saclay II type tuner is viable. The anticipated tuning range is +210kHz which meets specification. The anticipated tuning range is +210kHz which meets specification. Fast acting Piezo tuners have been investigated and seem viable, however, more work is required to assess piezo/tuner pre-load to account for thermal contraction. Fast acting Piezo tuners have been investigated and seem viable, however, more work is required to assess piezo/tuner pre-load to account for thermal contraction. A scheme for the magnetic shielding has been devised, however, a field map of the SPS area is required for more thorough analysis. A scheme for the magnetic shielding has been devised, however, a field map of the SPS area is required for more thorough analysis. 17