HOM Studies: Beam Dynamics, Cavity-to-cavity coupling, multipacting, and field emission Outline Higher Order Modes Marcel Schuhs 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 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 dont 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 Schuhs work) M. Schuh, F. Gerigk, J. Tuckmantel, and C. P. Welsch, Phys.Rev.ST Accel.Beams 14, (2011). HOM Studies (A summary of Marcel Schuhs 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 1 Emission phase optimal for subsequent collisions/emission Lets avoid the SNS experience..... Thermal detuning of HOM couplers due to MP, etc. I. E. Campisi, et al., PAC07 Proposed Coupler Geometries Courtesy of Rama Calaga Original design by J. Sekutowicz Rescaled from TESLA 1.3 GHz HW Glock, and the Rostock group 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 Ramas design, 3 MV/m,1750 emitting sites Ramas Design (close-up) Impact sites White: emission location | Red: Resonant location Impact energy vs. gradient Postprocess using typical SEY curve Rostock model, 0.6 MV/m Impact locations Impact energy vs. gradient Scaled by SEY Comparison HWs design has significant barrier Flat otherwise Questions: Normalisation? RF processing? How hard is the barrier? Normalised by total number of emitted particles Idea? - MP suppression by ridges? (Rob Ainsworth) Preliminary field emission studies (Rob Ainsworth) Eigenmode from Omega3P, tracking from Track3P Summary Marcel Schuhs 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 Ramas design Field emission tracking studies started