Plasma pulsed irradiation preparation of lead coated photocathodes Robert Nietubyć on behalf of the Collaboration Narodowe Centrum Badań Jądrowych Świerk National Centre for Nuclear Studies

Workpackage: WP12 Innovative RF technologies Task: WP12.5 Photocathodes Milestone MS80 Demonstrated operation of improved deposition system M30 Report on samples characterisation NCBJ Deliverables D12.8 Optimised procedure for microdorplets flattening with an UV laser M36 Report NCBJ D12.9 Pb/Nb plug photocathodes measurements and characterization. M42 Report HZDR (+ HZB + DESY + NCBJ) Pb photocathode deposition for improved performance of SRF guns. - deposition improvement, - post-deposition treatment, - Q and QE measurements Eucard commitments Eucard2 - 2nd Annual Meeting, , Barcelona

Motivation s.c. metals TcPbLaVHg T c [K] does not exist on Earth too reactive superconductor of the 1 st type critical H 80 mT critical T 7.2 K work function 3.95 eV High average power FEL Fully sc injector Thin lead film J. Smedley low probable phenomena diluted samples special applications industrial applications 2.5 µJ at 266 nm gives 100 nC at QE = 5·10 -4,  25 W at 1 st harmonic YAG 100 kHz – ranged repetition rate 10  A of average current too low T K The goal is to build a Nb injector with the superconducting cathode made of lead for use in CW or near-CW operated high average current accelerators Calculations showed reasonable QE Experiment supported the calculation results Vacuum arc deposited film showed the best performance J. Smedley Eucard2 - 2nd Annual Meeting, , Barcelona

A number of QE, RF quality measurements as well as injector operation have been performed till now and gave with reasonable results but still rather far from what we need for the cw and long pulse operations of XFEL. Particularly the coating was still not satisfactory: uniformity thickness droplets and other roughness adhesion There is a contradiction between having a thick film and having a film without droplets because filtering reduces the plasma flow and deposition requires the time longer than the lifetime of the arc cathode Imperfections until 2014 Eucard2 - 2nd Annual Meeting, , Barcelona

straight UHV arc coating followed with rod plasma recrystalisation Filtered arc depositon 2014, two ways Eucard2 - 2nd Annual Meeting, , Barcelona

Deposition system reconstruction to find better compromise between thickness and low micro-droplets population: Post–deposition flattening: plasma ion pulses irradiation laser irradiation annealing UHV non-filtered arc UHV filtered arc Evaporation pre-deposition Nb substrate preparation polishing BCP EP Quest for film improvement Eucard2 - 2nd Annual Meeting, , Barcelona

Coating method Deposition rate C, O and N contents (wt %) EDS results Pb layers’ morphology and continuity; SEM observations Evaporation 60 nm/min Pb ≈ 90 % C ≈ 3 % O ≈ 3 % N ≈ 2 % UHV filtered arc deposition 200 nm/min Pb ≈ 93 % C ≈ 2 % O ≈ 1 % UHV non- filtered arc deposition + remelting in pulsed plasma ions 3000 nm/min.Pb ≈ 94 % C ≈ 2 % O ≈ 1 % Semi-spherical extrusions of diameters < 5 µm density <50mm -2, Spherical extrusions of diam. up to 40 µm density <50mm -2, Numerous massive craters can be flattened at a cost of layer perforation and discontinuity Results Eucard2 - 2nd Annual Meeting, , Barcelona

as deposited after 2 pulses after 6 pulses Plasma over-irradiated film Eucard2 - 2nd Annual Meeting, , Barcelona

Question: Is it possible to melt and flatten the droplets with plasma pulses 1.5 J/cm 2, 1 μs ? How thick the Pb must be to avoid perforation? Method: Time dependent measurements, 2D FEM calculation solving the heat transfer equation (FEMM) Model: Pb/Nb 10 μm Various morphological features Heat transfer calculations Eucard2 - 2nd Annual Meeting, , Barcelona

µs, pulse end 1.1 µs 1.3 µs 1.6 µs 2.0 µs 2.5 µs 3.0 µs Heat transfer movie To be continued… Eucard2 - 2nd Annual Meeting, , Barcelona

µs 7.0 µs 8.5 µs 10 µs 13 µs 19 µs 30 µs Heat transfer movie Eucard2 - 2nd Annual Meeting, , Barcelona

Melted material ranges in depth droplet10 µm * flat area 7 µm Liquid state duration at 3 µm droplet 6.5 µs flat area6.5 µs Liquid state duration at the surface droplet 13 µs flat area8 µs Boiling affects the material down to droplet 2 µm flat area 1 µm during about 1 µs *) droplet height is 5 µm Conclusions from heat transfer calculations Eucard2 - 2nd Annual Meeting, , Barcelona droplet bulk

Interpretation Material forming a droplet is hotter than the material under a flat surface during 19 µs Material under the droplet is colder than that under the flat surface down to 15 µm Plasma pulses applied to the film thicker than 10 µm can flatten the droplets and save the complete lead coverage Eucard2 - 2nd Annual Meeting, , Barcelona

1 plasma pulse of 1.8 J/cm 2 15 µm Pb underway… Deposition and plasma pulsing Too thin and thus perforated 8 µm Pb UHV arc deposition X 500 X µm Pb 1 plasma pulse of 1.5 J/cm 2 2 pulses of 1.5 J/cm 2 3 pulses of 1.5 J/cm 2 X 1000 X 2000 Distinct areas of perforations size of which did not exceed 10 µm non-coated area amounting to 3-5% of the total layer surface. 20 µm Eucard2 - 2nd Annual Meeting, , Barcelona

20 µm Eucard2 - 2nd Annual Meeting, , Barcelona

Current density from a thin layer Nb-Pb cathodes reached by different deposition methods HZDR Cathode preparation E max [MV/m] I max [nA/cm 2 ] E min [MVm] I min [nA/cm 2 ] UHV filtered arc UHV filtered arc + conditioning UHV non-filtered arc + plasma pulses Evaporated Pb layer Evaporated and coditioned Pb layer Breakdown current measurements Eucard2 - 2nd Annual Meeting, , Barcelona

Measurement setup Will be used for samples diagnostics right after deposition: Breakdown current Quantum efficiency vs hv Furnished with Xenon lamp 150 W Czerny – Turner monochromator with diffraction grating 1200 lines/mm optimised for 250 nm calibration photodiode picoampermeter 10 pA – 1 µm Diagnostic chamber in construction MEASLINE, Janusz Budzioch Eucard2 - 2nd Annual Meeting, , Barcelona

Plug with evaporated film Niobium plug with Pb evaporated on the tip Electron gun with the plug Vertical test stand at DESY D. Kostin, J. Sekutowicz movable input antenna cavity probe Evaporated layer, smooth but not adherent and not tight Eucard2 - 2nd Annual Meeting, , Barcelona

4. SRF Gun Cavity Vertical Cryostat Tests b. recent results 1.6-cell SRF-gun with Pb-coated plug and uncoated plug,T=1.8K 19 Degradation at 32 MV/m to red curve. After warm-up to 100K performance recovers to yellow curve. D. Kostin, J. Sekutowicz, Last Pb Thin Film Cathode SRF Gun Tests, April 8th, 2015 Eucard2 - 2nd Annual Meeting, , Barcelona

Performed works which have not been described here: Filtered arc coated layers were in parallel tested at HZDR for QE and at HZB for dark current. Other related activities Eucard2 - 2nd Annual Meeting, , Barcelona

Pulsed plasma treatment is a promising way Diagnostics development Further photoinjector tests with plasma melted Pb film Conclusions and prospects Eucard2 - 2nd Annual Meeting, , Barcelona

NCBJ: R. Nietubyć J. Lorkiewicz R. Mirowski M. Barlak J. Witkowski W. Kosińska W. Grabowski ………….. HZB: T. Kamps R. Barday ………….. DESY: J. Sekutowicz D. Kostin ………….. HZR: J. Teichert R. Xiang ………….. Collaboration Eucard2 - 2nd Annual Meeting, , Barcelona

Workpackage: WP12 Innovative RF technologies Task: WP12.5 Photocathodes Milestone MS80 Demonstrated operation of improved deposition system Demonstrated plasma pulse flattening of the unfiltered Pb/ Nb layer M30 Report on samples characterisation NCBJ Deliverables D12.8 Optimised procedure for microdorplets flattening with an UV laser Optimised procedure for preparation of flat clean and adherent Pb/Nb photocathode films M36 Report NCBJ D12.9 Pb/Nb plug photocathodes measurements and characterization. M42 Report HZDR (+ HZB + DESY + NCBJ) Milestone and Deliverable modifications Eucard2 - 2nd Annual Meeting, , Barcelona