DEPOSITION OF Pb/Nb PHOTOCATHODES Jacek Sekutowicz, Robert Nietubyć, (10.4.2) Jacek Sekutowicz, Robert Nietubyć,

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Motivation CW electron accelerator High average power FEL Lowly probable phenomena Diluted samples Special applications Industrial applications 2/19 2/15 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Eucard obligations Deliverables D.10.4.1 QE data for Pb/Nb deposited photo cathode samples (Report, M12) D.10.4.3 Cold test results for the test cavities with and without the deposited lead photo cathode (Report, M36) Milestones M10.4.1 Lead deposition on samples for photocathode development (M12) M10.4.2 Lead deposition on half cells and 1.6 cell cavities (M18) 3/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Cathodic arc Arc discharge Ions emission from a small explosive centre Rising electron emission: thermionic enhanced by the electric field and by ion impact. I2R > EXPLOSION – sudden transition to a dense plasma Ion acceleration by pressure gradient, electron collisions, coulomb field 5/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

Supersonic multiply ionised ions from the cathode Cathodic arc introduction Supersonic multiply ionised ions from the cathode Electron driven transport in the plasma channel 6/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Arc highlights Hard landing sub-plantation diffusion with kinetic energy higher than displacement energy cooling and condensation Consequences for the film regular, dense, adherent Difference to magnetron sputtering Small ion energy limited diffusion strong interaction with already deposited atoms – columns voids, defects Absence of working gas residuals 7/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Performance Optimization of deposition system transmission and deposition rate micro-droplets filtering temperature control cleanliness and vacuum Photocathodes preparation deposition processes after deposition treatment chemistry laser flashing Measurements surface diagnostics QE 4/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 IPJ implementation –general description of the system polarisation = -110 V base pressure <10-7 mbar rest gas QMS arc current = 25 A arc voltage = 17-18 V ion current = 0.75 - 2.3 mA deposition rate ≈ 0.5 nm/s deposition time < 80 min wall temperature <32 °C 8/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Pb growth – x-ray diffraction studies Dep. time ;lattice constant 3 4.95899 15 4,96449 60 4,96423 120 4,96714 5×120 4,96144 10×60 4,95896 Lattice constant does not depends on thickness Orientation distribution does not change neither Observable amount of PbO appears in roughly 10 hours of the exposition to air 10/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 8 niobium plugs coated with lead M.10.4.1 Lead deposition on samples for photocathode development Plug BNL-like Location of the heater with lead stub for plasma formation No Time [s] Nb type Distance Setup Pump 1 1800 poly 1.6 cell Straight oil 2 3 mono 4 2700 Bent dry 5 6 0.5 cell 7 8 6000 11/15 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Laser treatment and QE measurements D. 10.4.1 QE data for Pb/Nb deposited photo cathode samples QE = number of emitted photoelectrons number of incident photons Laser: 213 nm 1 min 25 Hz 0.2 mJ/mm2 per pulse 0 keV 14 7 C O Pb Pb Nb × 20 as compared to Nb Three times less than the best one (coated at the minimal distance from Pb source) Melting and re-crystallization of lead Electron induced fluorescence spectrum showed a carbon contamination of the surface. 12/15 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 We gained from the first series Conclusions and question after the first series QE = 0.002 i.e better than Nb wall, and worse than it could be We can at least double QE if the coverage is higher, film is to thin, is it possible to avoid melting when laser treatment is gentle? 13/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 30º-bent tube 30 Increased transmission to enable thicker films To improve the coating efficiency, the system has been modified by replacing the rectangular knee by the 30º‑ bent tube. That solution enabled to increase 3 times the ion current saving the lead flux free of macro-particles. Chosen angle provides the minimal bend angle for which the lead droplets, which all move rectilinearly, cannot reach the target 14/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Laser treatment After cleaning Before cleaning Gentle laser treatment : 190 nm, 30 min, 300 Hz 0.01 mJ/mm2 per pulse as compared to violent one:213 nm, 1 min, 25 Hz, 0.2 mJ/mm2 per pulse QE higher than in the first series 15/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Remaining faults EDS (with 5 keV electrons) of cathode before (red) and after (blue) laser cleaning. The carbon peak is reduced by ~30% by laser cleaning. In spite of sufficiently high ion current of 8 mA and extended deposition time, the layers thickness does not exceed 200 nm and does not rise Further cleaning at this energy density failed to further increase the QE, suggesting that the optimum case would be a thicker lead coating and somewhat more aggressive cleaning. 16/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Robert Nietubyć, Jacek Sekutowicz, EuCARD 1st Annual Meeting, Daresbury, April 7,8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Aim and means Conclusions question, and answer after the second series QE = 0.003 i.e 30 time greater than for niobium wall. We can double QE if the coverage is complete, film is to thin, is it possible to avoid melting when laser treatment is gentle? very luckily if contamination is eliminated how to icrease the thickness ? yes 17/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 Aim and means Conclusions In 2009 we gained: Reproducible processes Micro-droplets removal Optimised Laser cleaning procedure Reasonable QE in spite of significant film imperfection In 2010 we have to continue with: Contaminations removal Fatting the film 18/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010 The crew Andrzej Trembicki, Robert Nietubyć, Jan Witkowski, Bernard Kołakowski, Robert Mirowski, Mirosław Kuk, Katarzyna Nowakowska-Langier Jacek Sekutowicz TJNAL, BNL Peter Kneisel, John Smedley … 19/19 SRF-EuCARD Annual Meeting, Daresbury, April 7, 8 2010

Thanks for the attention The End Thanks for the attention