1. ILC Damping Rings Mini-Workshop, KEK, Dec 18-20, 2007 Status and Plans for Impedance Calculations of the ILC Damping Rings Cho Ng Advanced Computations Department Stanford Linear Accelerator Center * Work supported by US DOE ASCR & HEP Divisions under contract DE-AC02-76SF00515

2. Outline  Damping Ring Vacuum Chamber Impedance  SLAC Parallel Modeling Suite  Simulation Status  Schedule & Plans to Facilitate Collaboration

3. Preliminary List of Vacuum Chamber Components (Marco Venturini, LBNL)

4. Damping Ring Impedance Calculations Broadband impedance - Identify major components that contribute to the impedance budget - Calculate short-range wakefields for single-bunch stability studies Narrowband impedance - Identify trapped modes in cavity-type structures - Provide HOM parameters for coupled-bunch stability studies

5. Beam Heating & Engineering Analysis Beam Heating - Identify sources of HOM heating - Investigate damping schemes to mitigate HOM effects Engineering Prototyping - Contribute to integrated analysis including electromagnetic, thermal and structural effects - Include transfer impedances of pickup devices

6. SLAC Parallel Modeling Suite Supported by US DOE SciDAC program, SLAC Parallel Finite Element codes can simulate large problems to high accuracy with near linear speedup using petascale computers at NERSC and NCCS. They include: Omega3P – nonlinear eigensolver to find resonant modes in damped RF cavities T3P – time-domain solver to calculate transients due to external drive and wakefields generated by beam transit (implementation of indirect wakefield integration) TEM3P – multi-physics analysis tool to simulate integrated electromagnetic, thermal and mechanical effects Resources: Zenghai Li, Cho Ng

7. Superconducting RF Cavity Cornell Model – 500 MHz KEK Model – 508 MHz r= 92 mm r= 25 mm f 0 = 650 MHz ILC DR cavity scaled from Cornell model (Sergry Belomestnykh, Cornell)

8. Loss Factor = 1.455 V/pC ABCI calculation Damping Ring Cavity  = 6 mm Loss Factor = 16.17 V/pC  = 0.5 mm Need 20 points per sigma for convergence Used as pseudo-Green’s function Further studies Narrowband impedance and damping Effectiveness of beampipe absorber

9. Damping Ring BPM 10 mm button 25 mm radius Snapshots of beam transit from T3P simulation Scaled model from PEP-II 15 mm button

10. BPM Longitudinal Wakefield Loss Factor = 0.0015 V/pC Effects of trapped modes at the buttons need to be studied for coupled bunch instability and beam heating  = 6 mm  = 1 mm

11. BPM Transfer Impedance Field monitored at coaxial port as a function of time Transfer impedance obtained by Fourier transform Signal sensitivities in x- and y- directions determined by simulations with offset beam excitations

12. TE Mode Propagation TE HOM power propagating in vacuum chamber can couple to BPM, and thus affecting processing signal Ante-chamber lowers the TE mode cutoff frequency TE cutoff at 2.929 GHz TE cutoff at 2.389 GHz Omega3P Calculation (Andy Wolski, Cockcroft)

13. Damping Ring Bellows Scaled model from PEP-II bellows (Preliminary) Loss Factor = 0.0168 V/pC Dominated by step used to shield the bellows  = 6 mm

14. Trapped Modes in Bellows 19.95 GHz6.202 GHz8.724 GHz Examples of trapped modes from Omega3P calculation Trapped modes in bellows convolution are potential sources of excessive heating Excited by HOM power propagating in vacuum chamber

15. Impedance Budget Impedance budget ComponentQuantityLoss factor (V/pC) RF Cavity1826.19 BPM6821.02 Resistive wall12.08 Total39.29 breakdown c.f. PEP-II HER – 2.5 V/pC for 1 cm bunch length NLC DR – 7.67 V/pC for 4 mm bunch length total

16. Schedule of Simulation Effort Year 1 - Impedance modeling using scaled models - Determine longitudinal and transverse wakefields for single- bunch stability studies - Determine HOMs in rf cavity for coupled-bunch stability studies Year 2 - Repeat calculations of broadband impedance using improved models of technical designs - Investigate effectiveness of absorbers in damping HOMs in rf cavity Year 3 - Integrated analysis including rf, thermal and mechanical effects of ring components for optimized technical designs - Finalize impedance calculations using models of engineering prototypes

17. Multi-Physics Analysis for Prototyping Virtual prototyping on computers from CAD model Integrated EM, thermal and mechanical effects Augmented by additional physics - particle effects (emittance, multipacting) - transient and non-linear effects in superconducting rf cavities CAD model of LCLS RF gun Electromagnetic Thermal Mechanical TEM3P

18. Work Packages

19. WP5: Impedance Computation at ANL Resources –Xiaowei Dong (0.25 FTE), Yong-Chul Chae (0.1 FTE) –Linux cluster with 120 cores (4 core/node * 30 nodes) and 480 GB of total memory –Parallelized 3D EM code GdfidL Experience in Computing Wake Potentials –Regular APS storage ring with bunch lengths  z = 1, 2, 5 mm 8.4 cm x 4.2 cm –Reduced APS storage ring chamber with bunch length  z = 5 mm 4.0 cm x 2.0 cm All chamber components scaled by a factor of two in transverse dimension without new design Deliverables –Assuming the APS components in the DR, we will deliver the total wake potential of  z =1 or 2 mm of the ring in the first year by July, 2008 –Refine and update as the mechanical design changes Courtesy of Yong-Chul Chae

20. Plans to Facilitate Collaboration Availability of models of vacuum chamber components from existing machines Standardized CAD format to facilitate information exchange among physicists and engineers Coordination of impedance calculations among different institutions Database to store CAD models and computational results