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Update on LHC 800MHz Crab Cavity Conceptual Design

Update on LHC 800MHz Crab Cavity Conceptual Design. Liling Xiao, Zenghai Li Advanced Computations Department, SLAC. Presented at LARP-CM12, April 9, 2009. Outline. Baseline Design (CM11) Cell Shape Optimization : (R/Q)_T, Bpeak<100mT for TM110

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Update on LHC 800MHz Crab Cavity Conceptual Design

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  1. Update on LHC 800MHz Crab Cavity Conceptual Design Liling Xiao, Zenghai Li Advanced Computations Department, SLAC Presented at LARP-CM12, April 9, 2009

  2. Outline • Baseline Design (CM11) Cell Shape Optimization : (R/Q)_T, Bpeak<100mT for TM110 LOM/SOM/HOM Coupler Design: Qext<200 for LOM/SOM • Alternative Cavity Design • Multipacting Study • Summary L. Xiao, Z. Li, CM12

  3. TM110-pi Baseline Design (CM11) Cell Shape Optimization • Cell_l = 187.5 mm, R_disk = 70mm • VT > 2.5MV, Maximum kick gradient limited by Bpeak • Optimize disk parameters for optimal Epeak and Bpeak Squash ratio:0.8 Squash or racetrack L. Xiao, Z. Li, CM12

  4. Bp Ep Crab Cavity RF Parameters TM110-pi mode B-field E-field TESLA TDR cavity peak fields for comparison (Eacc: 25-30MV/m): Ep=50-60MV/m, Bp=107-128mT L. Xiao, Z. Li, CM12

  5. LOM/SOM LOM/SOM Coax-to-Coax Design d • Strongly damps unwanted modes in a compact structure because no cutoff for TEM mode in the coax • Node at the electric field rejects operating mode in the vertical plane L. Xiao, Z. Li, CM12

  6. Qext vs. Coax Position Gap=10mm There are two additional LOM modes due to the coupling of the cavity modes to the coaxial beampipe TEM modes. Coupler can be optimized to achieve Qext less than 100 for LOM/SOM with a smaller gap. L. Xiao, Z. Li, CM12

  7. HOM Coupler Design • Damp the dipole modes at the horizontal plane and reject the operating TM110 mode at 800MHz. • A two-stub antenna instead of the coupling loop is proposed. LOM Opt. L. Xiao, Z. Li, CM12

  8. LOM/SOM FPC HOM Drawback in Baseline Design There is significant coupling of input power to the LOM/SOM couplers in this baseline design L. Xiao, Z. Li, CM12

  9. LOM/SOM LOM/SOM WG: TM110-0 (TE10:Fc=750MHz) HOM Coax: TM110-pi HOM Alternative Design • Input coupler is between of the two cells to avoid the cross coupling • Coax TM110-pi coupler rejects operating mode using field symmetry. • LOM/SOM couplers reject operating mode using electric field node. • HOM coupler rejects operating mode using filter. L. Xiao, Z. Li, CM12

  10. Rejection of Operating Mode LOM/SOM couplers reject operating mode using electric field node. Field symmetry is very important. The center conductors of LOM/SOM couplers will be slightly bent so that the rotating flange can adjust the tips of the center conductors to lie exactly along the E-field node of the operating mode field. shift L. Xiao, Z. Li, CM12

  11. Rejection of Operating Mode TM110-π coupler rejects operating mode using field symmetry. TM110-0: (R/Q)_T=100Ω/cavity TM110-π: (R/Q)_T=0.3Ω/cavity The misalignment of π-mode coupler should be less than 1mm to avoid the cross coupling between the FPC and pi-mode coupler. L. Xiao, Z. Li, CM12

  12. Monopole & Vertical Dipole Modes Damping requirements for the LOM and SOM modes: Qext < 200. L. Xiao, Z. Li, CM12

  13. Horizontal Dipole Modes L. Xiao, Z. Li, CM12

  14. Track3P - Multipacting Studies MP in the Cell - Resonant particles with impact energy between 50~2000eV TM110-0 mode VT=0.1~1.25MV E-BC eV y 2mm z x L. Xiao, Z. Li, CM12 Avoid E: 200ev~600ev

  15. Multipacting at the Disk 2-points MP, 0.5,1, 1.5 order VT=2.3MV In the disk area at the short axis of the KEKB 509MHz crab cavity, MP were found but it can be processed through. L. Xiao, Z. Li, CM12

  16. MP vs. Disk Profile R_bp=70mm r r=13.75mm, Bs=124mT Smaller disk radius can suppress MP but is at risk of higher Bpeak. Larger disk radius is hard to push the MP barrier beyond VT=2.5MV. L. Xiao, Z. Li, CM12

  17. MP vs Local Geometry E-field Radius=2mm Radius=4mm B-field Adding grooves along the short axis without changing the operating mode’s RF parameters can suppress MP activities. L. Xiao, Z. Li, CM12

  18. MP in LOM Coupler & Coax BP There is no MP in the coaxial beampipe. With fully rounded tip of LOM center conductor, MP can be suppressed. L. Xiao, Z. Li, CM12

  19. MP in HOM Coupler There are MP events in the gap of HOM notch filter. eV y eV x VT z L. Xiao, Z. Li, CM12

  20. MP in Input & πMode Couplers Assuming: FPC coupling Qext~1e6 VT=2.5MV, (R/Q)_T=100Ω/cavity Pin=63KW There is no MP in FPC and πmode couplers below Pin=100KW. L. Xiao, Z. Li, CM12

  21. Summary • Baseline 800MHz crab cavity designhas been proposed. With optimum cell shape, Bpeak=83mT @ VT=2.5MV (Critical Bpeak~100mT) Using coax-to-coax couplers, Qext @ LOM/SOM < 150. (Requirement Qext<200) • StrongMP in disk area at the short axis of normal cavity shape. Adding a small groove along the cavity surface at the short axis can suppress MP • No MP barriers found in LOM/SOM/FPC couplers and coaxial beampipe. If needed, further optimization can be done to fully suppress MP in HOM coupler. • Cavity model put on wiki page for cross checkand cryostat study. https://twiki.cern.ch/twiki/bin/view/Main/CrabCavityRF • Search for eigenmodes below 2GHz in progress. • Further simulations planned to analyze RF/thermal/mechanical effects. Thanks to members of LHC Crab Cavity Collaboration for valuable discussions. L. Xiao, Z. Li, CM12

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