1. HOM Studies: Beam Dynamics, Cavity-to-cavity coupling, multipacting, and field emission

2. Outline Higher Order Modes Marcel Schuh’s HOM studies Multipacting HOM coupler design decision Inter-cavity geometry Cavity-to-cavity coupling (No updates on this -- mostly theoretical work since last meeting) Blocking of field emitted electrons

3. Transverse modes are not an issue Longitudinal modes are of concern: Overlap of R/Q spectrum with charge spectrum i.e. high R/Q modes don’t matter if far from machine line Chopping adds new lines to charge spectrum But only fast schemes are of any concern Fundamental passband modes May be excited to significant levels and disrupt the beam Optimisation of transition energies can be helpful HOM Studies (A summary of Marcel Schuh’s work) M. Schuh, F. Gerigk, J. Tuckmantel, and C. P. Welsch, Phys.Rev.ST Accel.Beams 14, 051001 (2011).

4. HOM Studies (A summary of Marcel Schuh’s studies) Conclusions for SPL: Disruptive HOMs worse at lower energy HOM power limit may come from power dissipated in coupler, not the cavity walls Recommend Q ex <10 5 limit for all HOMs To allow all possible chopping patterns FPC damping of 4/5 π mode is sufficient (10 6 ) HOMs (including FPMs) should be considered when optimising transition energies M. Schuh, F. Gerigk, J. Tuckmantel, and C. P. Welsch, Phys.Rev.ST Accel.Beams 14, 051001 (2011).

5. HOM Studies (A summary of Marcel Schuh’s studies) Conclusions for ESS: No direct conclusions Study concentrated on SPL lattice, cavities, etc. The main message is the comparison of the R/Q & charge spectra Thus, no chopping leads to more relaxed HOM specs But removal of HOM couplers leads to limiting of future flexibility Tentative recommendation* is to use couplers By me, not Marcel. And I am allowed to change my mind.... M. Schuh, F. Gerigk, J. Tuckmantel, and C. P. Welsch, Phys.Rev.ST Accel.Beams 14, 051001 (2011).

6. Multipacting A resonant phenomenon Field-emitted e- impact cavity surface Secondary electron yield (SEY) > 1 Emission phase optimal for subsequent collisions/emission Let’s avoid the “SNS experience”..... Thermal detuning of HOM couplers due to MP, etc. I. E. Campisi, et al., PAC07

7. Proposed Coupler Geometries Courtesy of Rama Calaga Original design by J. Sekutowicz Rescaled from TESLA 1.3 GHz HW Glock, and the Rostock group

8. Simulate MP in both designs (Rob Ainsworth, RHUL) Combine eigensolver & particle tracker Omega3P and Track3P Emit e- every 3.6° for 1 RF cycle Emitting surfaces different for each coupler More later.... Track for further 19 cycles SEY=1 for tracking Scale by appropriate SEY during postprocessing

9. Rama’s design, 3 MV/m,1750 emitting sites

10. Rama’s Design (close-up)

11. Impact sites White: emission location | Red: Resonant location

12. Impact energy vs. gradient

13. Postprocess using typical SEY curve

14. Rostock model, 0.6 MV/m

15. Impact locations

16. Impact energy vs. gradient

17. Scaled by SEY

18. Comparison HW’s design has significant barrier Flat otherwise Questions: Normalisation? RF processing? How “hard” is the barrier? Normalised by total number of emitted particles

19. Idea? - MP suppression by ridges? (Rob Ainsworth)

20. Preliminary field emission studies (Rob Ainsworth) Eigenmode from Omega3P, tracking from Track3P

21. Summary Marcel Schuh’s study for SPL Very complete piece of work It is tentatively assumed this transfers to ESS Studies have begun... Two coupler designs studied for MP Strong MP barrier in the Rostock design Several weaker barriers in Rama’s design Field emission tracking studies started