FCC-hh: First simulations of electron cloud build-up L. Mether, G. Iadarola, G. Rumolo FCC Design meeting

Introduction How will the increased SR in the FCC affect electron cloud build-up? Are photoelectrons numerous enough to cause problems on their own? Is the LHC type beam screen a viable option w.r.t. electron cloud? In addition to beam and machine parameters that we can control, e-cloud build-up depends crucially on: How many photoelectrons are produced, and where in the chamber Depends on SR properties as well as material properties Needs to be experimentally estimated How many secondary electrons are produced from these primaries Also depends on material and conditions in machine Needs to be experimentally estimated For the simulations here, same SEY model used as for LHC studies G. Iadarola, “Electron cloud studies for CERN particle accelerators and simulation code development”, PhD Thesis, CERN, 2014.

Outline Photoelectron production Synchrotron radiation properties Photoelectron yield and reflectivity Photoelectrons in PyECLOUD Electron cloud build-up simulations Simulation setup and scenarios Effect of photoelectron production on build-up Comparison of results for different beams and magnetic fields Conclusions & Outlook FCC Design meeting3

LHCFCCxLHCFCC Inj.xLHC E [TeV]750(7)3.3(0.5) γ (7)3500(0.5) ρ [m] (4)11300(4) N γ /p + m (2)0.003(0.1) E c [eV]444030(90)7(0.2) Synchrotron radiation properties FCC Design meeting At collision energy: roughly twice as many as in the LHC At injection energy: 10 times fewer than in the LHC 4

Experimental data on photoelectron yield The photoelectron yield and the photon reflectivity depend both on material and SR properties, so needs to be determined separately for each case For 40eV SR (LHC), several experimental studies have been made at/with CERN 1 For 4keV SR (FCC), an experimental study was made at KEK 2 For 7eV SR (FCC injection), no data was found However, since the work function of copper φ Cu ~ 4eV, expect a larger fraction of the photons to be below φ Cu Y < Y 40eV Saw-tooth CuSmooth Cu Critical SR energyEcEc 40 eV4 keV40 eV4 keV Photoelectron yieldY Photon reflectivityR γ [%] Nr of photons /p + mNγNγ Nr of electrons /p + mN pe V. Baglin et al, CERN-LHC-PROJECT-REPORT-206; V. V. Anashin et al, Nucl. Instrum.Meth. A 448 (2000) Y. Suetsugu et al, Journal of Vacuum Science & Technology A 21, 186 (2003). 5

Experimental data on reflected photons The reflected photons mainly end up scattered around the beam pipe, and produce photoelectrons originating all around the beam screen. Which is the yield? Based on studies for the LHC 1, with 40 eV SR on saw-tooth copper: Forward scattered photons have a similar spectrum as incident photons Y fw ~ Y Backwards and diffused photons have lower energies Y bd < Y Safe estimate: N rf = N γ *R γ,tot *Y For FCC we have no data on backscattered and diffused photons Saw-tooth CuSmooth Cu Critical energyEcEc 40 eV4 keV40 eV4 keV Photoelectron yieldY Forward reflectivityR γ,fw [%] Total reflectivityR γ,tot [%] Nr of diffuse e - /p + mN rf 1e-4?4e-3? 1. V. Baglin et al, CERN-LHC-PROJECT-REPORT-206; N. Mahne et al, CERN-LHC-PROJECT-REPORT

N pe,d = N pe *(1-R) Generated with uniform distribution within angle θ (=10°) from beam location Contribute to e-cloud build-up mainly in field-free regions FCC Design meeting7 Photoelectron production in PyECLOUD N pe,rf = N pe *R Generated with cos 2 distribution w.r.t. angle from initial SR impact point Main contribution to e-cloud build-up in magnetic fields Main input: Total number of photoelectrons per proton per meter: N pe = N γ *Y Fraction of photoelectrons produced by scattered photons: R Gaussian energy distributions peaked at 7eV, with rms spread 5 eV Apart from N γ, Y and R, all model assumptions as for LHC simulations Electrons from direct SR photonsElectrons from scattered photons

Simulations of e-cloud build-up Simulations of build-up in typical arc elements: Dipole, Quadrupole, Field-free region Assuming baseline beam and optics parameters Bunch patterns: 50b + 12e for 25ns spacing 250b + 60e for 5ns spacing Injection and extraction energies Approximated beam screen with appropriate ellipse, to be able to use analytical Bassetti-Erskine formula for beam field Estimated heat loads as function of SEY scanning over several combinations of Y and R FCC Design meeting8

Effect of photoelectron numbers in simulations FCC Design meeting9 Smooth Cu Saw-tooth, R tot = R fw N rf = N rf,LHC

Effect of photoelectron numbers in simulations FCC Design meeting10 Smooth Cu Saw-tooth, R tot = R fw N rf = N rf,LHC Only the number of diffuse electrons N rf is relevant There is not a very strong dependence of the threshold SEY on the nr of electrons, i.e. Y and R, since also the slope changes

Heat loads for 50 TeV, 25 ns beam in arc components Threshold SEY quite low, but higher than in LHC with (72b + 9e)*25 ns: 1.1 Heat load above threshold larger than in LHC: 1 W/m with 1.15e11 p +

Heat loads for 50 TeV, 25 ns beam in arc components Threshold SEY quite low, but higher than in LHC with (72b + 9e)*25 ns: 1.1 Heat load above threshold larger than in LHC: 1 W/m with 1.15e11 p + Threshold SEY higher than in LHC with (72b + 9e)*25 ns: 1.3. Heat load above threshold higher than LHC: < 1W/m In field-free region heat load depends only on total yield. For Y > 0.1, heat load is relatively high, without clear threshold

Heat loads for 50 TeV, 5 ns beam in arc components Threshold SEY slightly lower than for 25 ns beam: 1.2 Similar to LHC 25 ns Heat load above threshold x0.5 larger than for 25 ns Heat load below threshold lower than for 25 ns Above threshold x10 25 ns Threshold SEY lower than for 25 ns: 1.6 Heat load above threshold higher (x5)

Heat loads for 3.3 TeV, 25 ns beam in arc components Threshold as for 50 TeV 5 ns Above threshold as 50 TeV 25 ns Clearer threshold compared to 50 TeV Heat loads lower Threshold SEY similar to 50 TeV Values similar to 50 Tev with low Y

Heat loads for 3.3 TeV, 5 ns beam in arc components Similar to 25 ns case. Heat load above threshold slightly higher Clearer threshold compared to 50 TeV Heat load above threshold as for 50 TeV Threshold and heat load above threshold similar to 50 Tev 5 ns

Conclusions & Outlook Despite several uncertainties in the models and experimental data we can draw some conclusions by comparing different cases to each other Compared to the LHC with 25 ns beam, the situation seems a little bit worse Threshold SEY’s are simila, but heat loads above threshold are higher Effect of bunch pattern remains to be investigated Similarly to LHC, the component with the worst behaviour is the quadrupole Threshold SEY’s are low ( ), and heat load above threshold higher than for other components Different SEY models can predict different thresholds (see e.g. ref. on page 2) The 5ns beam is worse than the 25 ns one Threshold SEY’s lower, heat loads higher Intermediate bunch spacings could be studied Drift spaces are most sensitive to photoelectron yields Potentially disturbingly high heat loads especially at 50 TeV Reliable, independently confirmed, experimental data on photelectron yields needed So far only build up studies, effect of e-cloud on beam stability is necessary For the LHC type beam screen, the conclusion is unclear: it is not clearly excluded, but also not clearly OK FCC Design meeting16

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