Photocathode R&D for bERLinPro Julius Kühn High Brightness Electron Beams (HBEB) Institute for Accelerator Physics European Workshop on Photocathodes for Particle Accelerator Applications The Cockcroft Institute, Daresbury, UK 06.06.2016

Outline bERLinPro GunLab Transfer systems and vacuum suitcase Photocathode lab Cs-K-Sb photocathode preparation & characterization Summary and Outlook

model of the building

construction site 2016-05-23

3d model of

Scientific highlight in 2016 and major milestone for bERLinPro 3d model of Gunlab (gun 1.0) Scientific highlight in 2016 and major milestone for bERLinPro

cross-section of the gun modul Support posts 80 K shield Cathode transfer system #2 Helium gas return pipe Cold steerer SC Solenoid HOM absorber Warm Cold transition 2 layer magnetic shielding Cathode Insert w/ Plug Coupler with courtesy of A. Neumann (HZB)

Cold string assembly Cavity with helium vessel Gate valve Valve Cathode beam tube Coupler assembly Cathode cooler Cold part of TTF-III coupler Next steps: horizontal acceptance test Mount complete string Installation into module Module RF tests First beam this fall: with courtesy of A. Neumann (HZB)

Transfer systems and vacuum suitcase overview Cathodes must be stored and handled in < 10-10 mbar UHV Vacuum suitcase is available. Vacuum ~ 1*10-11 mbar Stability with NEG + getter pump is good for >3 months Cleaning and assembly in ISO 5 clean room conditions done! commissioning of transfer system #1 at Preparation and Analysis System (PAS) is completed! First parts for transfer system #2 have been delivered Gun Module Transfer System #2 PAS Transfer System #1 9

cross-section transfer system #2 Plug transport cart Manipulator to take the plugs off the sampe holder Gun Modul Cathode insert w/ plug Vacuum suitcase

PLUG Design For gun operation, consider: Cathode insert w/ plug in GUN 1.0 For gun operation, consider: Cooling of the thermal load from RF and drive laser: Cu has low surface resisitivity and good thermal conductivity and contact Properties as a cathode substrate: Mo is favorable for cathode QE Cu migrates into Antimony layer, results in 50% lower QE PLUG - Options : Mo-Plug: sputter cleaned and heat treatment (Mo) Mo-plug: sputter-cleaned only at the center where the cathode will be deposited  lower field emission from rim (MoOx) Cu-plug with Mo film deposited as a diffusion layer at the plug center  Mo layer deposited by e-beam evaporation Scenario Gun 1: 6 W RF losses 1.2 W laser power Scenario Gun 2: 24 W laser power

Our goals High QE CsK2Sb photocathodes Smooth substrates and films Allow high bunch charge and current 100 mA means 25 W laser power at 1% QE Smooth substrates and films Low emittance, low field emission Reproducible growth procedure & robust lifetime Necessary for operation

Photocathode Lab in operation February 2016 Bild aktualisieren!

Photocathode Lab: transfer system #1 PAS Load lock Transfer chamber Bild aktualisieren! Vacuum suitcase

We have prepared a number of photocathodes with varying results Status of the photocathode lab Photocathode Preparation and Analysis System (PAS) Several Cs-K-Sb photocathodes have been prepared and characterized Preparation chamber effusion cell for Sb, SAES Dispenser for K and Cs sequential growth and co-deposition of K and Cs e-beam evaporator for Mo bake-out for 72h at 120°C, p = 3x10-10 mbar carefully degassing of all filaments and dispensers monitor the growth process by mass spectrometer and photocurrent Surface Analysis chamber post- analysis by X-ray photoelectron spectroscopy (XPS) Momentatron for emittance measurement (M. Schmeißer, Master Thesis, 2014) Spectral response setup Spectral response measurement from 250 – 600 nm H. Kirschner, Master Thesis, ongoing Poster H. Kirschner Transfer system #1 w/ vacuum suitcase all parts were cleaned in the clean room transfer system #1 is commissioned We have prepared a number of photocathodes with varying results

Substrate characterization For cathode research: Polished polycrystalline Mo sheets, 10x10 mm roughness : ~10 nm rms, can be 1-3 nm rms Cleaning: heat to 450°C for 30 min, Ar+ sputter cleaning for 30 min or heat to 600°C for 60 min Results in very low surface contamination, confirmed by XPS AFM; Rq = 10,66 nm rms White Light Interferometer 20x; Rq = 8,30 nm work done by Grzegorz Gwalt, Frank Siewert Be aware: change in surface roughness due to heat treatment, sputter cleaning

Photocathode preparation on molybdenum sheet P004 & P006 (Aug 2015) Polished Mo substrate 10 nm Sb film evaporated at 100°C P004 und P006 ~5% QE (532nm) P005: oxygen contamination, 0.18% QE P004 : Cs0.8K1.8Sb P006 : Cs0.8K1.8Sb 17

Polished molybdenum plugs

PLUG characterization The roughness in numbers looks promising, but : Mo plug after turning: Mo plug after polishing: AFM Roughness (rms): 11,9 nm Problem: Loose grains break from sintered Mo and create new scratches Forged Mo material preferable but grain boundaries limit the ultimate roughness Single crystals can be obtained with courtesy of Sven Lederer (DESY) 19

Phtocathode preparation on a plug P007 & P008 (May 2016) Polished Mo plug 10 nm Sb film evaporated at 100°C P007 sequential growth, 1.64 % QE P008 co-deposition of Cs und K, 1.2 % QE P007 : Cs0.8K2.7Sb P008 : Cs0.7K2.2Sb 20

Why was the QE on the plug so low? Plug surface temperature 80°C when heater is set to 100°C Temperature was calibrated in preparation of the next run Thermal contact of the Plug on the sample holder has to be improved 21

Look into the preparation chamber Aperture (7mm)/ Anode K Cs Mo plug Sb QCM

Summary & Outlook Transfer systems for Gun operation are on the way Photocathodes with QE of 5% at 532 nm grown on Mo sheets by sequential growth Photocathodes have been grown by sequential growth and co-deposition on Mo-Plugs Spectral response measurement setup was commissioned roughness of Mo plug increases due heating and sputtering Plugs made from sintered Mo not ideal for polishing JUN 16 Evaluation of the co-deposition growth procedure Photocathode life time studies: Simulation of transfer to Gun 1.0 JUL 16 Setup and commissioning of transfer system #2 in the clean room AUG 16 Setup and commissioning GunLAB

Thank you for your attention! Acknowledgements Martin Schmeißer, Hans Kirschner, Thorsten Kamps, Andreas Jankowiak Axel Neumann, Michael Schuster, André Frahm (SRF Group) Markus Bürger, Kerstin Martin, Daniel Böhlick (Engineering) Grzegorz Gwalt, Frank Siewert (Nanometer Optics group) Sven Lederer (DESY) and bERLinpro project team Thank you for your attention!