http://www.surfacetreatments.it/thinfilms The film Pb photocathodes prepared with the cathodic arc (Robert Nietubyc - 20') Speaker: Robert Nietubyc - The Andrzej Soltan Institute for Nuclear Studies | Duration: 20 min. Abstract Superconducting thin film lead photocathodes became recently a prospective approach in constructing an electron source dedicated for long pulse and high repetition rate operating accelerators. We report our recent works and achievements in construction and tests of such a source. In the branch of lead deposition, the efforts have been put into increase the transmission of the cathodic arc system which provides a satisfactorily macro-droplet removal from the plasma transit channel. A number of trial processes have been performed where polycrystalline and single-crystalline niobium plugs were coated. Next the surface characterization followed by cleaning procedure and quantum efficiency measurements using UV wavelength range of initiating laser were applied for deposited layers. Those experiments revealed an optimal procedure for an arc operation as well as for a laser cleaning of the Pb surface. In such conditions the value of QE equal to 0.0033 was found. Following the preparation phase, a 100 nm lead layer was deposited onto the back wall of 1.6-cell Tesla-like accelerating structure of electron injector. Cold rf tests were performed for the cavities with and without lead. In both cases the resonant quality higher than 1010 was observed for an accelerating gradient of 46 MVm-1

1. Thin film Pb photocathodes prepared with the cathodic arc R. Nietuby ć , J . Sekutowicz, P. Kneisel , J Smedley

2. The Goal 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 Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010

3. Capabilities I, p romissing properties of Pb Superconductor of 1 st type Critical H 8 mT@ 1.3 GHz at 2 K Critical T 7.9 K Work function 3.95 eV Pb can be used as a superconducting photocathode and deliver 10 5 1 nC bunches per second when flashed with 4 μJ pulses of 213 nm light The goal was to build an ultra high vacuum cathodic arc deposition facility dedicated to niobium coating with lead and to establish the Pb/Nb coating technology. This effort should lead us to the repetitive and reproducible lead deposition in the photo-injector cavities. Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010 Calculations of J. Smedley et al

5. Capabilities III, cathodic arc experience at SINS Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010 A laboratory has been established in the Plasma Physics Department at SINS in 2004. It participating in thin film SRF cavity programme performed in the frame of CARE. It resulted in an established procedure of three-cell Tesla-like Cu cavity coating with 2 µm Nb layer. A disadvantage was poor adhesion (< 80 bar HPWR) 3-cell cavities structure was coated For samples Nb/sapphire RRR=43

6. Proof of principle experiment Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010 Various Pb samples were prepared: Lead layer has been deposited onto Nb in the simplest geometrical arrangement. Substrate was placed in front of arcing cathode. Next it was cleaned with KrF 10 ns pulses 248 nm 0.3 mJ/mm laser pulses QE was measured magnetron sputtered electro-plated vacuum deposited arc deposited bulk lead Arc deposited layer showed the highest QE Experimental results confirm the calculations

7. Second approach - cavities Those trial depositions showed the capability of cavity treatment. In particular to match cleanliness requirements and to solve mechanical problems Pb pure Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010

9. Deposition system with knee-shaped microdroplets filter p olarisation = -110 V b ase pressure <10 -7 mbar arc current = 25 A coil current = 120 A arc v oltage = 17-18 V ion current = 2.5 mA d eposition rate ≈ 0.5 nm/s deposition tim e < 80 min w all temperature < 32 ° C Rest gases Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010

10. Optimisation of depsotion, microdroplets removal Microdroplets removal – knee-shaped filter SEM pictures of as-delivered sample No 1 (left) and sample No 5 (right). Note different scale. M.10.4.1 Lead deposition on samples for photocathode development Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010 Micro - droplets were efficiently removed. Unfortunately in cost of very low transmission Modeling a filter shape: The best was case a when applied: 200 A which gave 12 - 16 mT Verification with sample plugs dry Bent 1.6 cell poly 6000 8 dry Bent 0.5 cell mono 2700 7 dry Bent 0.5 cell mono 2700 6 dry Bent 1.6 cell poly 2700 5 dry Bent 1.6 cell poly 2700 4 oil Straight 1.6 cell mono 1800 3 oil Straight 1.6 cell poly 1800 2 oil Straight 1.6 cell poly 1800 1 Pump Setup Distance Nb type Time [s] No Location of the heater with lead stub for plasma formation

12. Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010 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 coil current = 70 A ion current = 8 up to 14 mA deposition time < 80 min no micro - droplets Optimisation of depsotion process, transmission 30 

13. Optimisation, final resuts for plug samples Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010 Gentle laser treatment : 190 nm, 30 min, 300 Hz 0.01 mJ/mm 2 per pulse as compared to:213 nm, 1 min, 25 Hz, 0.2 mJ/mm 2 per pulse QE=3.3  10 -3 Improved deposition geometry and gentle laser cleaning procedure enabled the layer to survive i n much better shape. As a result the highest QE was reached. It is however still lower than that measured in the proof of principle experiment. Still a thicker layer is needed. Carbon and oxygen contamination As deposited Pb 0.904, C 0.063 O 0.033 Cleaned Pb 0.930, C 0.045. O 0.025

14. Pb deposition onto cavities back wall of, a setup Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010 Mask: a stainless steel tube (1) with a bellow (2) and niobium cap (3) 1 2 3 1 2 3

15. DESY cavity 1, March 2010 Cold RF test performed for the coated cavities showed quality factors (Q) close to 3  10 9 at gradients on the cathodes of 29 MeVm -1 and 39 MeVm -1 . M.10.4.2 Lead deposition on half cells and 1. 6 cell cavities e ffective deposition time = 80 min (16 ×5 min arc + 30 min cooling) b ase pressure < 2  10 -7 mbar coil current = 75 A ion current = 10 mA deposition time = 80 min wall temperature < 34 °C thickness < 200 nm (roughly from EDS measurements for samples) Typical BCP acid tr ea tment and HPWR were applied. The phot o cathode wa s c hie l ded with the mask That promissing test has been interrupted by a helium leakage and could not been continued after it was mend. The cavity was sent to Świerk to prepare another Pb layer. Typical BCP acids mixture is capable to remove the old layer A final test before coating the cavity to be used in Hobicat Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010

16. HZB cavity for Hobicat, May 2010 Back wall consist of three single crystalline domains. A heluim tank is assem b led. Beam quality test are planned for this cavity in the end of 2010 Exactly the same deposition pr o cedure was repe a ted for this cavity A uniform spot o f 1 cm in diameter was obtained. Next to a deposition the cavity was filled with N 2 and sent to JLab 20 days later, when it was opened Film got orange After HPWR it got black Next it disap p eared during the standard treatment Lead nitride Pb 2 N 3 or Pb 3 N 4 In contrary to PbO does not passivate the surfac e but penetrate in depth As the layer was successfully deposited and spoilt only due to the reaction with N 2 We considered that the experiment is worth of severe work and schedule rearrangement in order to try again. The cavity was sent to Ś wierk and coated again. Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010

17. DESY cavity 2, last week coil current = 75 A , enhanced coil wrap p ing around the back wall ion current = 11 - 14 mA deposition time = 80 min Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010

18. Lattice constant does not depends on thickness Orientation distribution does not change neither Amorphous or nanocrystalline phase in the interface Observable amount of PbO appears in roughly 10 hours of the exposition to air XRD studies of Pb film growth on sapphire and Nb Reflectometry, XRD and EXAFS measurements have been performed for film of various thicknesses. Experimental results will be interpreted by ab initio calculations. Only rough results here time 2h 18h PbO Lattice constant vs thickness Two lead phases oxidation Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010 4,95896 10×60 4,96144 5×120 4,96714 120 4,96423 60 4,96449 15 4.95899 3 ;lattice constant Dep. time

19. Conclusions W e gained: Reproducible processes (4 times for 1,6-cell cavities) Micro-droplets removal Optimised l aser cleaning procedure Reasonable QE W e will continue with: Beam tests of Pb/Nb injector at Hobicat at HZB (end of 2010) Contaminations removal Fatting the film System reconstruction in order to shorten the plasma channel M . 10.4.1 Lead deposition on samples for photocathode development D.10.4.1 QE data for Pb/Nb deposited photo cathode samples M . 10.4.2 Lead deposition on the half-cell and 1.6-cell cavities D. 10.4. 3 Cold test results for the test cavities with and without the deposited lead photo cathode Robert Nietubyć, SINS Świerk, Poland 4th International Workshop on Thin Films and New Ideas f or pushing the limits of RF Superconductivity , Legnaro 2010

20. Thanks for the attention Q QE together