photoemission studies for the lhc - indico

Photoemission studies for the LHC


Philipp Dijkstal

12.10.2017

Many thanks to my colleaguesG. Iadarola, L. Mether, A. Romano, G. Rumolo, G. Skripka

P. Dijkstal 1 12.10.2017


Photoemission study

Modelling of photoemission

Modelling of photoelectrons in PyECLOUDI The number of photons emitted by the beam can be calculatedI Absorption of electrons and the number of photoelectrons depends on surface

propertiesI input parameter k pe st: number of photoelectrons per proton and per meterI input parameter refl frac: share of photoelectrons created by reflected photons

↪→ literature review of published measurement campaigns from 1998-2004↪→ conservative and optimistic estimate (surface conditioning by synchrotronradiation)↪→ simulations with and without photoelectrons, different distributions

P. Dijkstal 2 12.10.2017


Photoemission study

Published measurements

Emitted photons by a proton beam in the LHCPlots (Formulas in master thesis / photoemission note):

I Photon spectrum

I Power spectrum

I Number of photons higher than the Cu work function

I Share of photons above the Cu work function w.r.t. all photons

P. Dijkstal 3 12.10.2017


Photoemission study


Definition of photoemission quantities & Beam screen

Number of photons with E >WCu nγReflection rate of photons R =

nγ,reflected

nγ,incident

Photoelectron yield per incident photon Y = Nenγ,incident

Photoelectron yield per absorbed photon Y ∗ = Nenγ,absorbed

= Y1−R

I SR hits the sawtooth part of the surface, butdifferent surface properties elsewhere

I Immediately absorbed photons that generatedphotoelectrons in corner↪→ described byRi (reflectivity upon impact)Y ∗i (yield upon impact)

I Reflected photons that generatephotoelectrons elsewhere↪→ described byY ∗r (yield of reflected photons)

I Simplification:Rr := 0 (no reflections back to sawtooth area)

P. Dijkstal 4 12.10.2017


Photoemission study


Published measurement results - yields

I Baglin, Collins, Grobner: Photoelectron yield and photon reflectivity fromcandidate LHC vacuum chamber materials with implications to the vacuumchamber design (1998)

I Measurements at EPA at CERN

I Measure Y , R and calculate Y ∗ = Y1−R

I Only reflectivity in forward direction

I Measurements only valid for the employed photon spectrum

P. Dijkstal 5 12.10.2017


Photoemission study


Published measurement results - surface conditioning

I Baglin, Collins, Grobner, Grunhagel: Measurements At EPA Of Vacuum AndElectron-Cloud Related Effects (2001)

I Measurements at EPA at CERN

I Measure Y , R and calculate Y ∗ = Y1−R

I Only reflectivity in forward direction

I Measurements only valid for the employed photon spectrum

I Estimate effect of photon scrubbing

↪→ Only results from sawtooth material arepublished ↪→ Sharp decrease in yields

P. Dijkstal 6 12.10.2017


Photoemission study


Published measurement results - reflectivities

I Mahne, Baglin, Collins et al.: Photon reflectivity distributions from the LHCbeam screen and their implications on the arc beam vacuum system (2004)

I Measurements at ELETTRA in Italy

I Measure reflectivities only, but in all directions and for different photon energies

P. Dijkstal 7 12.10.2017


Photoemission study


Published measurement results - discussionI Measured photoelectron yields before (sawtooth / smooth) and after (only

sawtooth) scrubbingI Measured reflectivities from different experiment setupI Update measured photoelectron yields with more accurate reflectivitiesI Remember that Y is measured, but Y ∗ has been published

Y ∗corrected = Y ∗

old ·1− Rold

1− Rcorrected

Two sets of results:I Conservative: do not take surface conditioning into accountI Optimistic: take surface conditioning into account

k pe st = Ni + Nr = Nt =

= nγ(E >WCu) ((1− Ri )Y∗i + RiY

∗r )

= nγ(E >WCu) (Yi + RiY∗r )

refl frac =Nr

Nt=

Nr

Ni + Nr=

RiY∗r

(1− Ri )Y∗i + RiY ∗

r

I refl frac is about 20% in both the conservative and the optimistic caseI https://github.com/PyCOMPLETE/CellStudyInputPyECLOUD

P. Dijkstal 8 12.10.2017

https://github.com/PyCOMPLETE/CellStudyInputPyECLOUD


Photoemission study

Simulation results

Simulation results

I An initial distribution of electrons was used for simulations without seedingmechanism

I Uniform distribution of photoelectrons generated from reflected photons (∼20%)

↪→ Only drifts and dipoles are sensitive to the presence of photoelectrons↪→ Quadrupoles and higher order magnets are not sensitive

P. Dijkstal 9 12.10.2017


Photoemission study

Simulation results

Influence of spatial distribution

I Spatial distribution only matters in the dipolarmagnetic field

P. Dijkstal 10 12.10.2017


Photoemission study

Simulation results

Time-continuous photoemission

I In PyECLOUD, the photoelectrons are generated in parallel with the beam charge

I A time-continuous generation of photoelectrons does not change the results

P. Dijkstal 11 12.10.2017


Photoemission study

Comparison to measured heat loads


↪→ 25 ns case↪→ δmax of ∼1.2 compatible with low heat load cells↪→ δmax of ∼1.4 compatible with high head load cells (especially Arc 12)

P. Dijkstal 12 12.10.2017


Photoemission study


Thank you for your attention

P. Dijkstal 13 12.10.2017

photoemission studies for the lhc - indico

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