1. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Analytic Approximations of Tune Shifts and Beam Coupling Impedances for the LHC Stefania Petracca University of Sannio at Benevento and INFN

2. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Outlook - Laslett coefficients for Betatron Normal Modes Square vs Circular Pipes A Toy Twin-Liner Model Real-World Gometries: MoMs & Random Paths - Beam Coupling Impedances from Reciprocity Theorem Rounded corners and more - Leontòvich Boundary Conditions and Beyond Lossy Walls, Coated Walls, Pumping Holes, etc. …What’s it all for ?

3. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Betatron Oscillations Forces acting on beam vanishes at Nominal beam equilibrium position Linearized forces :, Incoherent regime (single particle dyn.) Coherent regime (whole beam dyn.) Nominal tune, H-V simmetry no H-V coupling under guiding field (can be relaxed), nominal orbit radius angular circulation freq.

4. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Betatron Oscillations, contd. Normal-mode Lasletts: (for the two incoherent normal modes one has always ). U is a non-diagonal tensor, Betatron oscillations ( ) =, tune-shift tensor Laslett tensor

5. Square vs. Circular Liner Francesco Ruggiero Memorial Symposium CERN, October 3 2007 a    =  / a (square),  =  / R (circle)

6. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Twin-Beam Toy Model R b a High   r yoke Several possible regimes : Incoherent-incoherent : Coherent-coherent : Mixed : Different boundary conditions to be imposed on the electric (  ) and magnetic ( A z ) potential static (=) and dynamic (  ) parts :   , high-frequency, non penetrating modes (skin depth << pipe wall thickness), low-frequency, penetrating modes (skin depth >> pipe wall thickness) …yielding different coherent and mixed dynamics. [S. Petracca, Part. Accel., 62 (1999) 241]

7. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 R b a High   r yoke Twin-Beam Toy Model, contd. The Al-frame holding the beam pipes in the LHC design will prevent the dynamic magnetic field from coupling the beams, even at the lowest frequency associated with collective beam oscillations. As a result, the two beams are dynamically uncoupled. Neglecting space-charge effects, all regimes (incoherent, coherent, & mixed) merge together in the limit as  1, yielding (both pipes):

8. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Twin-Beam Toy Model, contd. R b a ( ,  ) = local (pipe) polar coordinates R=b=2a  r = 10 4 |

9. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Twin-Beam Toy Model – One Beam R b a ( ,  ) = local (pipe) polar coordinates R=b=2a  r = 10 4 |

10. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Real-World Geometries: Numerics [S. Petracca, et al. Part. Accel. 63 (1999) 37] [S. Petracca, F. Ruggiero, et al., Proc. PAC-97, 1753] Random Paths Computes the (complex) potential only on a circle, using stochastic calculus, and then uses Cauchy integral formula for computing the Lasletts without the need of approximating derivatives with finite differences. MoM : Rounded Corners & Stadium Shape Variations Based on efficient representation of the (exact) Green’s function for rectan- gular and circular domains, allowing to shrink the unknown charge density support (and related number of unknown charge expansion coeffs) to a min.

11. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 MoMs : Lasletts vs. Round-Corner Radius (fixed off-axis distance; several angular position)

12. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 MoMs : Lasletts vs. Round-Corner Radius (fixed off-axis distance; several angular position)

13. MoMs : Hard-Cut Circle Francesco Ruggiero Memorial Symposium CERN, October 3 2007

14. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Random Paths : Cold Bore HOM Scaled cutoff wavelength of 1st HOM in the coax guide between the beam screen and the cold bore vs. (scaled) corner radius for several beam screen (scaled) sizes

15. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Outlook - Laslett coefficients for Betatron Normal Modes Square vs Circular Pipes A Toy Twin-Liner Model Real-World Gometries: MoMs & Random Paths - Beam Coupling Impedances from Reciprocity Theorem Rounded corners and more - Leontòvich Boundary Conditions and Beyond Lossy Walls, Coated Walls, Pumping Holes, etc. …What’s it all for ?

16. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 LHC Pipe Progress Design (1995) Rounded corners (manufacturing limitations) Stainless steel (lossy) pipe (pure Cu would not sustain the quenching forces due to mag- netic field penetration (& para- sitic currents) in case of failure of the cryogenics) Copper-coated surface (uncoated SS would give exces- sive parasitic losses; coating restricted to flat faces, where fields and loss would be largest ) Pumping holes (removal of gas desorbed tue to synchrotron radiation)

17. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Beam Coupling Impedances from Reciprocity Theorem Beam coupling impedances from potentials… Reciprocity… source (transverse) position field (transverse) position Potential – unperturbed pipe (known) Potential – perturbed pipe (unknown)

18. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Impedance (Leontóvich) b.c., non-perfectly conducting wall Beam Coupling Impedances from Reciprocity Theorem, contd.  S

19. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Beam Coupling Impedances from Reciprocity Theorem, contd. [S. Petracca, Part. Accel., 50 (1995) 211.] constitutive wall perturbation geometric wall perturbation ( E 0z = 0 on  S 0 )

20. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Example: Rounded Square Liner

21. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Outlook - Laslett coefficients for Betatron Normal Modes Square vs Circular Pipes A Toy Twin-Liner Model Real-World Gometries: MoMs & Random Paths - Beam Coupling Impedances from Reciprocity Theorem Rounded corners and more - Leontòvich Boundary Conditions and Beyond Lossy Walls, Coated Walls, Pumping Holes, etc. …What’s it all for ?

22. Leontóvich B.C. and Beyond Originally formulated for a planar surface bounding some highly reflecting transversely homogeneous lossy half-space… Francesco Ruggiero Memorial Symposium CERN, October 3 2007 [T. Senior, J. Volakis, Approximate Boundary Conditions in EMtics, IEE Press, 1995] [L.N. Trefethen, L. Halpern, Math. Comput. 47 (1986) 421] …but in fact much more versatile than this. Can be applied, e.g., to: -lossy stratified media (e.g., by repeated use of the TL impedance transport formula) -curved surfaces, provided e.g., local radius of curvature -inhomogeneous media, since =0

23. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Leontòvich B.C. and Beyond : Perforated Walls By solving a canonical b.v.p. we shew that perforated walls can be modeled using reproduces Gluckstern’s formula (circular pipe) [R. Gluckstern, J. Diamond, IEEE Trans. MTT-39, 274, 1991] when used in our perturbational formula for Z || ) [S. Petracca, Phys. Rev. E60, 6041, 1999] Reproduces Kurennoy’s result for Z || [S. Kurennoy, Part Accel. 50, 167, 1995], when used in our perturbative formula for Z || ). external hole polarizabilities, skin depth, cold bore TEM field, cold bore holes’ position, Axially aligned holes in a pipe surrounded by lossy co-axial shield (LHC cold bore) linear hole density = surface density of holes

24. Francesco Ruggiero Memorial Symposium CERN, October 3 2007 Leontòvich B.C. and Beyond : Perforated Walls, contd. Hole polarizabilities available for a variety of hole-shapes - see [F. de Meulenaere, J. Van Bladel, IEEE Trans., AP-25 (1977)198; also R. de Smedt, J. van Bladel, IEEE Trans., AP-28 (1980) 703] Also, see [Van Bladel, Radio Sci. 14, 319, 1979] for corrections to Bethe’s formula for polarizabilities beyond the underlying quasi-static (kD << 1) assumption (very short bunches) Corrections for hole - hole couplings also worked out [S. Petracca, Phys. Rev. E60 (1999) 6030] in the quasi- static approximation [R.E. Collin, Field Theory of Guided Waves, IEEE-McGraw-Hill, 1998]

25. Example: LHC Parasitic Loss Budget Francesco Ruggiero Memorial Symposium CERN, October 3 2007, (holes) (ohmic holes) holes) (ohmic, (ohmic) (ohmic) surface fraction not covered by holes perforated Cu strips bunch length (gaussian)

26. Example: Parasitic Loss Budget, contd., Francesco Ruggiero Memorial Symposium CERN, October 3 2007 perforated Cu strips (ohmic)

27. Example: Parasitic Loss Budget, contd., Francesco Ruggiero Memorial Symposium CERN, October 3 2007 perforated Cu strips in good agreement with measurements (Caspers, Morvillo and Ruggiero)

28. "Beauty is truth, truth beauty,“ that is all Ye know on earth, and all ye need to know. [John Keats, 1795-1821] …What’s it all for ?

29. Thanks, Francesco.