Low SEY Engineered Surface for Electron Cloud Mitigation Sihui Wang PhD student of Loughborough University University Supervisor: Mike D. Cropper ASTeC supervisor: Oleg B. Malyshev Reza Valizadeh

Outline Background Objective My PhD contents Examples of SEY Measurements on laser treated samples Summary 2

Background Electron cloud caused by beam-induced multipacting is a critical problem for high intensity particle accelerators. First electrons originate from ionised residual gas molecules, photoelectron emission and secondary electron emission from the vacuum chamber walls Negative impact of E-cloud: causes beam instability, beam losses, emittance growth and heat loads on cryogenic vacuum chamber, reduces a beam lifetime Electron multipacting is also a problem in RF wave guides and space related high power RF hardware. Reducing PEY and SEY in other instruments and devices is an important task! 3

My PhD project objective Reduce the Secondary Electron Yield: By Changing surface Chemistry (deposition of lower SEY material) By Engineering the surface roughness Mixture of the above By active means: Weak solenoid field along the vacuum chamber Biased electrodes Charged particle beam parameters Bunch charge and sizes Distance between bunches By passive means: Low SEY material Low SEY coating Grooved surface Special shape of vacuum chamber An antechamber allows reducing PEY 4

My PhD contents Bulk samples (Cu, Al alloys, Stainless steel, Ti, Zr, V and Hf) Coatings (Ti, Zr, V, Hf, Ti-Zr-V and Ti-Zr-V-Hf) on different substrates (Stainless steel, Si and Blacken samples) New technology Laser treated blackening samples (Cu, Al alloys and Stainless steel) Studying SEY from In my presentations, I will focus only on our recent discovery.

SEY Measurements Analysis chamber with XPS, Flood e-gun, Sample heater, Ar ion beam. IP is the primary beam current. IF is the secondary electron current including elastic and inelastic processes, measured on the Faraday cup IS is the currents on the sample 6

Laser treatments Nd:YVO4 Laser Pulse length =12 ns at Repetition Rate = 30 kHz For Aluminium Max Average Power = 20 W at  =1064 nm For Copper Max Average Power = 10 W at  = 532 nm Argon or air atmosphere Beam Raster scanned in both horizontal and vertical direction With an average laser energy fluence of just above the ablation threshold of the metal. (a) Untreated Cu (b) laser treated Cu Aluminium Stainless Steel

SEY of Cu as a function of incident electron energy Untreated Laser treated 8

SEY of SS and Al as a function of incident electron energy 9

δmax as a function of electron dose for Al, 306L SS and Cu Sample Initial After conditioning to Qmax δmax Emax (eV) Qmax (Cmm-2) Black Cu 1.12 600 0.78 3.510-3 Black SS 900 0.76 1.710-2 Black Al 1.45 2.010-2 Cu 1.90 300 1.25 200 1.010-2 SS 2.25 1.22 Al 2.55 1.34 1.510-2 10

XPS analysis of 60 µm Cu in Ar 900 800 700 600 500 400 300 200 100 Binding Energy (eV) x 10 3 2 4 6 8 10 12 14 CPS Cu2p1/2 Cu2p3/2 Cu LMM O1s Cu3p Cu3s C1s As-received After electron conditioning CuO reduced Cu increased

Laser surface treatments with different microstructure distances (a) 50 µm (b) 60 µm (c) 80 µm 12

Latest result with laser treated Copper in air: 0.58    0.80 13

XPS analysis of 50 µm Cu in Air 900 800 700 600 500 400 300 200 100 Binding Energy (eV) 5 10 15 20 25 30 35 40 CPS Cu2p3/2 Cu2p1/2 O1s Cu3s Cu3p Cu LMM As-received 50 µm Cu 103 After heating at 250C for 2 h 14

Summary Laser treatment of the metal surface is a very viable solution for reducing the SEY < 1. (a) low cost (process is carried out in an inert gas environment at atmospheric pressure) (b) no new material introduced (this is a surface re-shaping process) (c) the surface is highly reproducible (d) the surface is robust and is immune to any surface delamination (unlike thin film coating). 15

Acknowledgements People who support me with my PhD Reza Valizadeh Oleg B. Malyshev Neil Pashley Elaine A. Seddon Adrian Hannah Svetlana. A. Zotlovskaya W. Allan Guillespy Amin Abdolvand Mike D. Cropper

Thank for your attention!