FCCee IR Trapped Modes Analysis Alexander Novokhatski FCCee

FCC-ee IR Trapped Modes Analysis Alexander Novokhatski FCC-ee MDI mini-workshop CERN July 17, 2017

Thanks • Thank you very much for invitation to take part in the fantastic project of the new accelerator of the future • I believe that our experience with high current operation at PEP-II can be useful for the new project • We achieved stable operation for the beam current of 3 A of the positron beam and 2 A of the electron beam 2

What is not good with HOMs and wake fields in the IR beam pipe? • Heating of the IR chamber elements when these fields are dissipating in the chamber wall. – High temperature raise will destroy good vacuum that may lead to addition background in IR or even vacuum instability • Everything starts with excitation of the so called wake fields mainly in the place where two incoming or out coming tubes are connected. Bunch field is reconstructed inside Beam direction Forward and backward excited fields will propagate to other crotches and can be reflected back. Under a resonant condition they can make a Higher Order Mode. 3

HOM spectrum and dangerous resonances of the colliding beams at the PEP-II IR Vertex bellows modes PEP-II MAC Review 01/19/20064 of 38

An unavoidable trapped mode Metal walls - + Tube connection + Beam interacts with this mode as electric field has a longitudinal component Metal walls Electric field lines 5 Surface currents have also longitudinal slope

Propagating modes • Main part of the excited fields will propagate out of IR if they do not meet other obstacles like: Shielded bellows RF seals BPMs Pumping holes 6

Beam interaction with one HOM mode Mode voltage decay Loaded time decay or filling time Loaded Q-value which includes coupling Bunch spacing Mode survives to the next bunch if and loaded Q

Coherent and incoherent excitation of trapped modes Bunch spacing Incoherent Coherent at the resonance Mode decay time (loaded) or filling time Loss power The power is twice more if the bunch spacing is equal to a mode decay time Loss factor 8

Planar model of the FCC IR Transitions to larger or smaller size m 24 m n rectio di Beam 40 mm m 24 m SR masks More accurate sizes of upstream and downstream pipes 9

HOMs spectrum Cut-off 10

IR power loss (two beams) Excitation of a cavity by electron and positron currents depends upon the difference in the arrival time and frequency of the cavity. The power may vary from 0 to 4. In average we assume to be 2. Positrons Electrons B-side BPM A good HOMs absorber in IP will solve the problem with resonant modes. 11

In this model other geometries not differ much Calculations will continue these weeks may be using CERN computers m 24 m n rectio di Beam m 24 m However additional energy losses may come from the different pipe sizes 12

Real geometry ? 13

Eleonora did a great job with 3 d calculations 14

A possible HOM absorber for IR Restraint Welded Bellows OFE Cu Prong Body Ceralloy Tile SST Water Jacket Cover SST Prong Support Glidcop Stub J-Seal Glidcop RF Fingers Inconel Spring Fingers

Some additional items • Effective beam current can be higher. It depends upon the bunch pattern – Currently ¼ of the ring is not filled. Accordantly the effective bunch current is 25% higher and energy losses will be 50% higher It will be good to have more details of the IR geometry May be give more advantage to the beam optics and not increase the chamber size, solving HOMs heating problem by installing HOMs absorbers. 16