Ecloud meeting 22 11 2013 Roberto Cimino CERN

E-cloud meeting: 22 -11 -2013 Roberto Cimino CERN Te-VSC-SCC and Laboratori Nazionali di Frascati –INFN Discuss the status of our understanding of the SEY situation in the LHC and propose together (as well as prioritise) actions, countermeasures and studies for the next run with 25 ns beams. CERN 22 -11 -2013 R. Cimino

What we know is that e--cloud mitigation @ LHC is BASED on Scrubbing! from LHC PR 472 (Aug. 2001): “…Although the phenomenon of conditioning has been obtained reproducibly on many samples, the exact mechanism leading to this effect is not properly understood. This is of course not a comfortable situation CERN 22 -11 -2013 V. Baglin et al, LHC Project Report 472, CERN, 2001. R. Cimino

• We now know what is actually happening during scrubbing: R. Cimino et al. PRL 109 064801 (2012) ; R. Larciprete et al. PR ST: 16, 011002 (2013) Electron induced surface graphitiza sp 2 sp 3 C C Use doublet beam : 5 – 20 ns or 2. 5 – 22. 5 ns spacing Implications and issues (BI, RF, ADT) under investigation. 5 or 2. 5 ns 25 ns Important to test and push bunch population for doublet scheme (20+5 ns) in 2014 to allow faster scrubbing in the SPS and LHC (essential for 25 ns!!!) CERN 22 -11 -2013 R. Cimino

GENERAL e--CLOUD RELATED QUESTIONS : Ø Is the low energy part of the SEY spectrum still an Issue? Ø Is the RT St. of SPS – PS to be studied? Ø Is the LT Cu (LHC BS) still to be studied? Ø Is the Photo. Yield (PY) to be studied (for the upgrades? ) Ø Is the photon reflectivity R to be studied (for the upgrades? ) Ø Are other parts of the machine to be studied ? ( the insulators for the kickers for instance etc) Ø Are other materials and processes to be studied? Ø Are there new challenges for the upgrades? CERN 22 -11 -2013 R. Cimino

What we know from (many) simulations is that most of the electrons in the cloud have Low energy! R. Cimino et al. PRL 109 064801 (2012) Ø Low energy electrons, (0 e. V < E < 30 e. V), are essential ingredients for the e- cloud formation and need to be specifically studied! CERN 22 -11 -2013 R. Cimino

• Each point in d is the integral of the energy distribution of the emitted electrons R. Cimino et al Phys. Rev. Lett. 93, 14801 (2004). CERN 22 -11 -2013 R. Cimino

This has been recently questioned by Kaganovich et al. (see ecloud-12 Proceedings) based on “good old days” Russian experiments and theoretical calculations…. . DONE for clean metals! Mo I. M Bronshtein, B. S Fraiman. Secondary Electron Emission. Moscow, Russia: Atomizdat, p. 60 (1969). CERN 22 -11 -2013 Belhaj et al ecloud-12 Proceedings 7 R. Cimino

BUT…Same experimentalists looking at dirty samples (as our “as-received” Cu and SS) ……. I. M Bronshtein, B. S Fraiman. Secondary Electron Emission. Moscow, Russia: Atomizdat, p. 60 (1969). CERN 22 -11 -2013 Belhaj et al ecloud-12 Proceedings 8 R. Cimino

Calculations from Giovanni 2 Usual Cos Flat YES!!! Indeed it is very important and need more work that can be done on the SPECS System, not only for “academy” but to measure the precise shape of SEY at Low energy for Cu, SS, LTCu…. . . CERN 22 -11 -2013 R. Cimino

For sure the shape at low energy but also why in some cases SS scrub at dmax=1. 1 Pedro CP et al ecloud-12 Proceedings CERN 22 -11 -2013 R. C. Preliminary results: The chemistry is similar to Cu and Ti. N but at higher doses? !? 10 R. Cimino

Some open question remain for SPS: • Does SPS St. Scrub as Cu of (LHC): • if YES: 1) define the most efficient bunch spacing for scrubbing 2) Study if is there a way to favor and enhance such process? 3) Measure the Low energy distribution for more accurate simulation • if No: 1) Understand why (it means the chemical origin!) 2) And why some do and some do not 3) Study if is there a way to cure such discrepancy 4) finalize it and push for alternative solutions! (C coating etc) Some work that require careful SEY, Low energy SEY & XPS CERN 22 -11 -2013 R. Cimino

Does the SEY of an “as received” sample changes at LT? A. Kuzucan, H. Neupert, M. Taborelli, and H. Störi JVSTA 30, 051401 (2012); Only marginally, and decrease! CERN 22 -11 -2013 12 R. Cimino

1. Since Scrubbing is a chemical phenomenon and need energy to occur… does it occur at LT? Yes it does! At least in the lab CERN 22 -11 -2013 13 R. Cimino

1. Does the scrubbing efficiency depends on T? RC &al ECLOUD 04, Seems not significantly! In an open geometry! CERN 22 -11 -2013 14 R. Cimino

1. In a close geometry: Gas physisorb on the surface Kuzucan, et al JVSTA 30, 051401 (2012) Physisorbed gas change SEY! CERN 22 -11 -2013 15 R. Cimino

1. A closer 2. look to CO 2: Kuzucan, et al JVSTA 30, 051401 (2012) • Do 5/10 ML of CO 2 results in a dmax of 1. 7/1. 9 If are physisorbed on a scrubbed SURFACE with dmax = 1. 1? • Does an “as received” surface with physisorbed gas scrub? • If and how a gas modify scrubbing? CERN 22 -11 -2013 16 R. Cimino

SEY at cryogenic temperature • Cu can be scrubbed BUT • avoid gas condensation (H 2 O, CO 2) Fully scrubbed Cu 1. 2 d total 1. 0 Contribution of secondaries to d 0. 80 d N. Hilleret. LHC MAC December 2004 0. 60 0. 40 0. 20 Contribution of reflected electrons to d 0. 0 0 50 100 150 200 250 300 350 Primary Energy (e. V) R. Cimino , I. R. Collins, App. Surf. Sci. 235, 231 -235, (2004) N. Hilleret et. al. Chamonix 2000 CERN 22 -11 -2013 CERN-GSI Electron Cloud Workshop - 7 -8 17 R. Cimino

1. At LT we may be in presence of a thick layer of gas 2. Close geometry ===> continuous gas dosing! Scrubbed RC &al ECLOUD 04, 2 If we simulate this in the lab by scrubbing Vs. T in presence of gas dosing things dramatically changes! Still a lot to do… and XPS may be essential to understand LT SEY behavior and LT scrubbing! And this for H 2, CH 4, CO 2, H 2 O…. CERN 22 -11 -2013 R. Cimino

Repeat all those studies for a-C to validate his Vacuum behavior at LT, his SEY vs. gas absorption, its LT scrubbing behavior, its SEY Low Energy part (warm & cold) etc. May be essential for Hi. Lumi uppgrades! Need to study Ti? Or other materials? Coldex need to be back at work together with Laboratory experiments and code benchmarking! CERN 22 -11 -2013 R. Cimino

Ø NEXT 2 QUESTIONS: Are the PY and R to be studied (for the upgrades? ) (PY) Photoelectron Yield (the number of electrons created after bombardment of a single photon) (R) Photo-reflectivity (the number of photons reflected by the surface) Do depend on: material, Energy, angle, temperature, magnetic field, Conditioning etc…. Both are input parameters used in simulations of multipacting and ecloud build-ups and related instabilities But may also cause single bunch instabilities just connected to the mere existence of a certain density of e- in the accelerator chambers. Need Imputs from Giovanni 2 and may play an important role for the upgrades! CERN 22 -11 -2013 20 R. Cimino

We performed preliminary measurement and we could continue measuring photon reflectivity using the reflectometer at BESSY II in Berlin. ECLOUD-12 proceeding and to be submitted to Phys. Rev. Special Topics SOFT X-RAY REFLECTIVITY: FROM QUASI-PERFECT MIRRORS TO ACCELERATOR WALLS F. Schäfers, Institute for Nanometre Optics and Technology, HZB BESSY-II, Berlin, Germany R. Cimino, LNF / INFN, Frascati, Italy Abstract Reflection of light from surfaces is a very common, but complex phenomenon not only in science and technology, but in every day life. The underlying basic optical principles have been developed within the last 5 centuries using visible light available from the sun or other laboratory light sources. X-rays were detected in 1895, and the full potential of soft- and hard-x ray radiation for material analysis and characterisation is available only since to the advent of synchrotron radiation sources some 50 years ago. On the other hand high-energy machines and accelerator-based light sources suffer from serious performance drop or limitation due to the interaction of the light with the accelerator walls, thus producing photoelectrons which in turn interact with the accelerated beam. Thus the suitable choice of accelerator materials and its surface coating which determines its x-ray optical behaviour is of utmost importance to achieve ultimate emittance performance. Basic principles and examples on reflectivity are given here. CERN 22 -11 -2013 R. Cimino

Photon energy: 20 - Optic beamline @ Bessy 2 1600 e. V - a low grating (150 l/mm) - is used for 20 -150 e. V a high grating (1228. 1 l/mm) is used for 130 -1600 e. V Samples: aluminum, copper, and stainless steal Spot size: 0. 25 mm in vertical, 1. 1 mm in horizontal F. Schäfers and R. Cimino ECLOUD-12 proceeding and to be submitted to Phys. Rev. Special Topics CERN 22 -11 -2013 R. Cimino

Reflectometer at BESSY II F. Schäfers and R. Cimino ECLOUD-12 proceeding and to be submitted to Phys. Rev. Special Topics CERN 22 -11 -2013 R. Cimino

Samples and Sample holder • During the preliminary beam period, we measured different samples: - as example here we show CU from LHC beam screen • The samples are isolated from the sample holder by Kapton to also measure the photo yield. 120 mm 48 mm F. Schäfers and R. Cimino ECLOUD-12 proceeding and to be submitted to Phys. Rev. Special Topics CERN 22 -11 -2013 R. Cimino

Reflectivity from LHC Cu F. Schäfers and R. Cimino ECLOUD-12 proceeding and to be submitted to Phys. Rev. Special Topics CERN 22 -11 -2013 R. Cimino

Photo Yield F. Schäfers and R. Cimino ECLOUD-12 proceeding and to be submitted to Phys. Rev. Special Topics CERN 22 -11 -2013 R. Cimino

Beam scrubbing effect with photons (how it compares with e- scrubbing)? And what about photons scrubbing at LT? V. Baglin et al, Chamonix 2001, LEP performance 5, 10 (2001). R. Cimino et al Phys. Rew. AB-ST 2 063201 (1999) Some more fun, maybe not high priority! CERN 22 -11 -2013 R. Cimino

NEXT 3 QUESTIONS: Ø Are other parts of the machine to be studied ? ( the insulators for the kickers for instance etc) Ø Are other materials and processes to be studied? Ø Are there new challenges for the upgrades? Yes, But (maybe) not yet high priority: the story will continue…… CERN 22 -11 -2013 28 R. Cimino

What is planned in the TE-VAC-SCC Ø Coldex: “le Retour” (mainly to test the a-C vacuum behavior) Ø Upgrade SEY in the m-metal SPECS system (in few months) to investigate: • the Low energy part of SEY spectrum (of the various materials!!!) • Refine SEY-XPS-Scrubbing Analysis at RT Ø Transport to Bld. 101 and restart the LT-SEY system (next month) • continue measure SEY on LT absorbate • upgrade it for LT SEY-Scrubbing in presence of physisorbed gas (to be decided: if OK ready in 4 to 6 months) • Dote the SPECS System with a LT manipulator for measuring Low energy part of SEY spectrum &SEY-XPS-Scrubbing Analysis at RT&LT (to be decided: if OK ready in 9 to 12 months) Need coordination, continuous support (€), synergic efforts between different departments, collaboration with external Labs (LNF, BESSYII, Trieste, Wien, Madrid. etc) and well focused and dedicated (more? ) people. CERN 22 -11 -2013 29 R. Cimino