1 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton Wake-fields in the Main Superconducting L-Band Linacs of the ILC Amos Dexter The University of Lancaster; Cockcroft Institute, Daresbury, UK

2 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton Overview of WP 3.1 Physics issues of mode coupling and means to ameliorate beam instabilities –Scientific excellence/fundamental physics Knowledge transfer lays in the re-designs that will be required to reduce the impact of the damaging wakefields. UK industry may gain some inroads on the fabrication of 153,000 cells. Clear potential for the UK to be a lead player in the main linacs of the international linear collider. Beam dynamics are an outstanding issues raised by the ILC R&D board. Past experience (SLAC) has shown that beam instabilities are a serious issue that can readily make the collider unusable. We must anticipate these instabilities using simulations taking into account realistic statistics on likely fabrication errors ILC main cell designs currently under R&D Potentially trapped HOM (Higher Order Mode)

3 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton Overview of WP 3.2 A) Beam-based experiments will take place on the ERLP at Daresbury and the FLASH/TTF2 (5 x 8-cavity modules) facility at DESY. Aim at understanding 1) beam-based and 2) cavity alignment based on radiation from HOM couplers. Should it prove a feasible technique all (or significant fraction) 17,000 cavities will be instrumented with these diagnostic tools. Immense value to the ILC community. B) Bench-top passive experiments: thin wire stretched through cavity takes place of beam. Will allow rapid characterisation of cavities. More economically viable. Several fundamental physics issues regarding trapped modes have to be understood. - Scientific excellence/fundamental physics. FLASH user facility at DESY Proposed wire measurement of higher order modes in L-band superconducting cavity Earlier wire measurement on X-band room temperature cavity (R.M. Jones et al, SLAC, 2005)

4 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton ILC GDE Linac Member Comments … I understand that your new study programs are very important for realization of the ILC. Especially important is to understand what will happen in mass production stage and to control the production suited for obtaining the required beam properties. To this end, we at KEK will make actual cavities and you analyse them from both theoretical approach and experimental one. The latter should include the beam study at DESY or even at KEK and the studies using low power model by applying various approach such as a wire method which you proposed. I am sure these new studies plays a very important role in L-band ILC. Hoping your proposed activities be realilzed soon, -Toshiyasu Higo, ILC R& D Board, KEK, Japan... The program you propose of both computational and experimental study of cavity wakefields is imperative for the ILC. Currently, there is little understanding of what causes the frequency variation of the cavity dipole modes and their polarization splitting. Creating a model that matches the cavity data at DESY is very important as this model can then be used to look for trapped modes in higher order dipole bands (4-8 GHz). This is one area that has not been fully explored at DESY, and yet poses a large risk for multi-bunch operation at both the XFEL and ILC. Approaching this both experimentally and theoretically, which I believe you are well suited to do, offers the best chance to resolve this issue, and I hope that this effort goes forward. -Chris Adolphsen SLAC / ILC Linac Area Leader …Your program sounds very important. Both Cornell and KEK are developing new geometries for the Alternate Design Configuration. Wakefields and HOMs for these new geometries need to be carefully studied before the Change Control Board can consider accepting a change from the Baseline Design. Both monopole and dipole modes are important.... Therefore I would encourage you strongly to pursue such studies -Hasan Pasamdee, ILC R&D Board, Cornell Univ., USA.

5 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton A TESLA module I. Shinton CI All-hands 30/03/07

6 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton 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

7 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton Alignment errors and trapped modes... Measurements Made to Date: 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.

8 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton Computational simulations to date  Model the entire region  Use Parallel super computers  Model an idealised cavity

9 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton Cascading finite element schemes: Simulation of large structures

10 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton Basic cascading techniques G.L.James, IEEE Transactions on Magnetics Vol 30, no 2, pg A.K.Hamid, Int. J. Electronics Vol 80, no 3, pg 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) Step junction cascading Double step junction cascading R.M.Jones, N. Baboi, S.G. Tantawi, Proceedings of the 2003 Particle accelerator conference, pg References

11 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton HFSS cascading simulations

12 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton Build S matrix library A generalized scattering matrix technique Derive a Globalized S matrix for each Tesla cell with respect to each module in the linac Derive unit cell S matrices including variations in geometry/ machining and alignment errors

13 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton 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.

14 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton 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.

15 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton Overall aims of the simulation effort 1.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 2.Be able to compare the realistic simulation to experiments 3.Provide a possible means by which the entire ILC structure can be accurately modelled. 4.Determine the shifts in the HOM in a real structure 5.Provide a means for evaluating determining positions of trapped modes, allowing couplers/cells in those regions to be tuned to avoid these modes.

16 Amos Dexter, on behalf of Roger M. Jones, Carl Beard, Ian Shinton