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Prospects for simulating complete ILC modules and eliminating trapped modes

Prospects for simulating complete ILC modules and eliminating trapped modes. CI All-Hands 30/03/07. Dr Ian Shinton The University of Manchester; Cockcroft Institute, Daresbury, UK. A TESLA module. I. Shinton CI All-hands 30/03/07. Reasons for modelling entire ILC structure.

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Prospects for simulating complete ILC modules and eliminating trapped modes

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  1. Prospects for simulating complete ILC modules and eliminating trapped modes CI All-Hands 30/03/07 Dr Ian ShintonThe University of Manchester; Cockcroft Institute, Daresbury, UK

  2. A TESLA module I. Shinton CI All-hands 30/03/07

  3. Reasons for modelling entire ILC structure • Alignment and machining errors in real modules • Every TESLA cell will have some effect on its neighbours • Trapped modes in various parts of the structure I. Shinton CI All-hands 30/03/07

  4. Alignment errors and trapped modes... A trapped mode is a resonant mode (HOM) which is non- propagating and is strongly localised in part of the accelerating structure. Alignment and machining errors are simply a departure away from the idealised geometry, cause by incorrect alignment or manufacture of a series of cells. I. Shinton CI All-hands 30/03/07

  5. Computational simulations to date • Model the entire region • Use Parallel super computers • Model an idealised cavity I. Shinton CI All-hands 30/03/07

  6. Cascading finite element schemes:Simulation of large structures I. Shinton CI All-hands 30/03/07

  7. Basic cascading techniques Step junction cascading All cascading techniques originate from the generalized scattering matrix technique: R.Mitra and S.W.Lee 'Analytical techniques in the theory of guided waves', Macmillan Comp, New York (1971) Double step junction cascading References R.M.Jones, N. Baboi, S.G. Tantawi, Proceedings of the 2003 Particle accelerator conference, pg 1270-1272 G.L.James, IEEE Transactions on Magnetics Vol 30, no 2, pg 1059-1066 A.K.Hamid, Int. J. Electronics Vol 80, no 3, pg 471-477 I. Shinton CI All-hands 30/03/07

  8. HFSS cascading simulations I. Shinton CI All-hands 30/03/07

  9. A generalized scattering matrix technique Build S matrix library Derive unit cell S matrices including variations in geometry/ machining and alignment errors Derive a Globalized S matrix for each Tesla cell with respect to each module in the linac I. Shinton CI All-hands 30/03/07

  10. Reasons for using HFSS • HFSS uses an FEM rather than FD • FEM is more accurate than FD. If the amount of individual error is not minimised in the unit cell used in the cascading technique, then cascaded errors will grow... • FEM has adaptive meshing • FEM convergence can readily be proved • HFSS uses edge elements – so the prospect of spurious solutions is eliminated. I. Shinton CI All-hands 30/03/07

  11. Things to keep in mind when cascading.... • Enough modes must be modelled in the chosen unit cell • A very accurate mesh should be used • When applying a frequency sweep, to obtain or gauge the resonant frequencies and shifts in these frequencies, both a fine enough choice in sweep step and mesh used in the sweep must be implemented; otherwise degeneracy may result.... • Calibration lines or a very refined mesh must be used to prevent degeneracy. I. Shinton CI All-hands 30/03/07

  12. Things to keep in mind when cascading.... I. Shinton CI All-hands 30/03/07

  13. Overall aims of the simulation effort • Provide a realistic simulation including machining and alignment errors, to which the effect of the passage of the beam and wake field effects can be determined • Be able to compare the realistic simulation to experiments • Provide a possible means by which the entire ILC structure can be accurately modelled. • Determine the shifts in the HOM in a real structure • Provide a means for evaluating determining positions of trapped modes, allowing couplers/cells in those regions to be tuned to avoid these modes. I. Shinton CI All-hands 30/03/07

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