1. Development of “green” photocathode at INFN LASA Sandeep Kumar Mohanty, D. Sertore, P. Michelato, L. Monaco, G. Guerini Rocco, C. Pagani EWPAA 2019, Switzerland, September 11th, 2019 Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Contents: Introduction Process Layout R&D Experiment Layout Previous development Current development Analysis and characterization Summary & Outlook

2. INFN LASA Photocathodes at a glance: Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019  The INFN LASA group has a long standing experience in the production of cesium telluride photocathodes for high brightness photoinjectors.  So far INFN LASA have delivered 150 photocathodes. The operative lifetime in the user facilities (24/7) increased from few months to few years. LASA photocathode plugs are now a “standard”, exchangeable between the different facilities.  Our photocathodes are used regularly in the injector - RF guns of: FLASH (DESY Hamburg) PITZ (DESY Zeuthen) European XFEL FAST (Fermilab) APEX (Lawrence Berkeley National Lab.) LCLS-II (SLAC)  Stimulating from former successful development of Cesium Telluride photocathodes,now our main aim is to develop a stable recipe for multi-alkali antimonide photocathodes.  Also, we are progressing towards the assembly of a new photocathode production system which is dedicated to these types of cathodes (to develop on INFN Plugs).

3. Why Green Photocathode?  Alkali antimonide Photocathodes (K₂CsSb, Cs₃Sb,….) are the the best candidate for the high brightness electron source of the advanced electron accelerator.  Low emittance  Good lifetime  Fast response  Driven by visible light  Recent projects are aiming to CW operation to achieve MHz extraction of bunches. This poses challenging requirements to laser specifications that can hardly be achieved with UV photocathodes.  Stimulating from former successful development of Cesium Telluride photocathodes, our R&D activity has been started to develop specifically the K-Cs-Sb based multi-alkali photocathodes which are sensitive to the green light. Aim: Established a reproducible recipe to grow alkali antimonide compounds on INFN plugs and test them in the PITZ RF-Gun Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Ultra High Vacuum system

4. Process Layout Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Develop a reproducible recipe on simplified samples R&D Optimizing parameters Different substrate (Mo, Stainless steel etc.) Different substrate temp. Different thickness Different deposition techniques (sequential & co-evaporation) Material characterization (Spectral response, Reflectivity, etc. ) Surface Science AFM, XPS (Modena Group) Transfer the recipe to INFN plug In a dedicated deposition system Trial & error Production Making pancake Mo Samples

5. R&D Experiment Layout Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Preparation chamber  - metal & TOF Nd:Glass fs-laser system cathode growing chamber (base pressure 10 -11 mbar range) provided by eight NEG St707® modules from SAES Getters  -metal chamber with a Time Of Flight detector, based on a Nd:Glass fs-laser, to measure thermal emittance AES detector for contaminants investigation on sample surface Power meter lens Incident light Reflected light broadband Hg, Xe-Hg and D2 lamps (from 220 nm to 436 nm) Ar + (457 nm to 514 nm) He-Ne lasers (543 nm, 594 nm & 633 nm) Laser Driven Light Source (LDLS) with dedicated monochromators Available light sources:

6. INFN LASA K 2 CsSb Standard Recipe Procedure Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019  Substrate:  Molybdenum simplified samples High purity (99.95%), optically polishing (reflectivity > 54 % @ 543 nm w.r.t 57 % theoretical), ultrasonically cleaned Heating cycle from 20 ˚C 450 ˚C Required temp.  Coating deposition:  Sequential: Sb deposition: 5 or 10 nm K deposition : up to the Photocurrent maximum (1 nm/min) Cs deposition : up to the Photocurrent maximum (1 nm/min)  Measurements during the deposition :  Reflected power and Photocurrent (@543 nm)  Measurement after the deposition:  Spectral response (220 nm to 633 nm wavelength of light)  Reflectivity  Substrate temperature:  Vary with recipe Fig. 1:Typical photocurrent and reflected power curve during the deposition Current Optimization parameter: Substrate temperature Thickness Sb K Cs

7. Previous development (in 2017) Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Process Parameter Sample made - 2 (K₂CsSb-1 & K₂CsSb-2) Substrate Material: i) Polished Molybdenum(sample-1) ii) Unpolished Molybdenum(sample-2) Deposition techniques: Sequential growth Procedure: (i) Sb(10 nm) K(till threshold) Cs(till threshold) (ii)Sb (5 nm) K (till threshold) Cs (till threshold) Substrate temperature: (i&ii)120 ⁰C [for deposition of Sb and Cs] and 150 ⁰C [for K] no light power density (0.125 /² ) laser driven light source power density ( ~21/² ) Summary of Results Recipe 1 & 2 View into the prep chamber Sample No Visible color Screw Q.E on the sample@514nm: KCsSb-1: 0.009% KCsSb-2 : 1.9 % From Sample -2

8. Current Development Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Process Parameter Sample number- K ₂ CsSb- 3 Substrate Material: i) Polished Molybdenum Deposition techniques: Sequential growth New addition: Measurement of reflected power Procedure: [Sb(10nm) K(till threshold) Cs(till threshold) Substrate temperature: 60 ⁰C for Sb & K 90 ⁰C for Cs (initially planned 60 ⁰C) Sample heater Summary of Results Recipe 3 Photocurrent and @60C = 543nm 10 nm Photo Current [nA] Reflected Power [mW] Deposition time [sec.] Stop K Slow chemical reaction! Q.E @514nm= 5.2% on the sample Increase the substrate temperature to 90˚C 2

9. Current Development Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Process Parameter Summary of Results Sample number- K₂CsSb- 4 Substrate Material: i) Polished Molybdenum Deposition techniques: Sequential growth New addition: Measurement of reflected power Procedure: [Sb(5 nm) K(till threshold) Cs(till threshold) Substrate temperature: 90 ⁰C Recipe-4 Sample Photo current and R Vs Time of cathode KCsSb-4 Summary:  Reflected power trends to decrease during Cs deposition ₂ 5 nm SbKCs = 543nm  QE was recorded 3.9 % at 514 nm.

10. Current Development Process Parameter Recipe-5 Sample number- KCsSb- 5 Substrate Material: i) Polished Molybdenum Deposition techniques: Sequential growth New addition: Measurement of reflected power Procedure: [Sb(10 nm) K(till threshold) Cs(till threshold) Substrate temperature: 90 ⁰C Summary of Results Photo current and R Vs Time of cathode KCsSb-5 Sb K Cs Summary: Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 A very long exposition to Cs is clearly visible without a clear maximum indicating the formation of the photocathode.  QE was recorded 4.6 % at 514 nm. 10 nm

11. Current Development Process Parameter Recipe-6 Sample number- KCsSb- 6 Substrate Material: i) Polished Molybdenum Deposition techniques: Sequential growth New addition: Measurement of reflected power Procedure: [Sb(5 nm) K(till threshold) Cs(till threshold) Substrate temperature: 90 ⁰C for Sb & Cs, K at 120 ˚C Summary of Results Photo current and R Vs Time of cathode KCsSb-6 Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Summary: Less amount of K was deposited K Sb Cs  QE was recorded 4.6 % at 514 nm.

12. Summary of all Recipe Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 CathodeSb (nm) T Sb -T K -T Cs (˚C) K (nm) Cs (nm) Q.E. (514 nm) (%) KCsSb-110120-150-120112374 9·10 ˉ ³ KCsSb-25120-150-120611581.9 KCsSb-31060-60-90*66316 (194@ 90˚C) 5.2 KCsSb-4590-90-90411063.9 KCsSb-51090-90-90813134.6 KCsSb-6590-120-90321174.6  Number of cathodes : 6  The maximum Q.E @514 nm is recorded 5.2 % for KCsSb-3 cathode.  Optimizing Parameter: Substrate temperature Thickness  The production of KCsSb with different thickness is done by changing the deposition of Sb thickness.  Thicker cathode – Sb (10 nm)  Thinner cathode – Sb ( 5 nm) Taking for further characterization! * Substrate temperature was incresed from 60 ⁰C to 90 ⁰C during Cs deposition  Estimated error of QE @ 514 nm is 1%

13. QE and Reflectivity versus thickness and substrate temperature Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Q.E. during K deposition Q.E. during Cs deposition Reflectivity during deposition Thinner Thicker 120 ⁰ C 90 ⁰ C 60 ⁰ C Substrate temperature is significant for both thin (5 nm Sb) and thick (10 nm Sb) cathodes. Higher temperature (120 °C for 5 nm and 90 ˚C for 10 nm Sb) gives higher final QE. Higher temperatures favor less amount of K deposition. Higher temperatures induce larger slopes during Cs Curve reproducible for both “5 & 10” nm Sb Related to the thickness of the material decrease along Cs deposition for “ 5 nm Sb” and increase along “10 nm Sb” photocathodes. Thinner Thicker Related to thickness

14. Reflectivity and Spectral Response Summary Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Q.E. @ all wavelengths Reflectivity @ all wavelengths Different electronic structure !  Thin cathodes (5 nm Sb) at higher wavelength (543 nm, 594 nm) behave completely opposite compare to the thick cathodes (10 nm Sb).  Q.E. behavior of both thin and thick cathodes are similar Thick Thin  Thin – K2CsSb- 4 & 6  Thick - K2CsSb- 3 & 5

15. Color comparison Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Thick Cathode Thin Cathode Blue in colorViolet in color Sb@10nm Sb@5nm Reproducible

16. Thermal treatment Thermal treatment of KCsSb-3 Heat treatment 23 ⁰C→ 120 ⁰C (30 minutes)→ 23 ⁰C Thermal treatment of KCsSb-6 Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 QE ≈ 4.7% @ 514 nm from 4.6% QE ≈ 6.5% @ 514 nm from 5.2% Excess Cs case only! Good vacuum can Maintain the Q.E.!!! Baking of unreacted Cs on the sample!!!

17. Summary & Outlook Higher temperature (not above than 140 °C) gives higher Q.E. during the deposition. Higher temperatures favour less amount of K deposition (K2CsSb-6). Higher temperatures induce larger slopes during Cs. Both color & reflectivity depends up on the thickness of the material. Thermal treatment can enhance the final Q.E. (with excess Cs only K2CsSb-3).  Next step: o We are progressing towards the assembly of a new photocathode production system dedicated to these types of cathodes. o Test these cathodes in the real environment of RF guns, in particular at the PITZ facility in DESY Zeuthen. o Investigate with different surface characterization techniques (recently communicate with Modena group). o Try with other possible variables in order to get best recipe. Sandeep Mohanty, EWPAA 2019 Conference, September 11th, 2019 Thanks to all contributors to this work and For kind attention !

18. Backup Slides Sandeep Mohanty, PITZ Collaboration Meeting, Zeuthen, May 15th, 2019 D.H. Dowell et al. / Nuclear Instruments and Methods in Physics Research A 622 (2010) 685–697

19. Development of “green” photocathode at INFN LASA Established a reproducible procedure to grow alkali antimonide compounds on INFN plugs On-going activities R&D Stabilize growth recipe and explore possibility to improve it Spectral response and reflectivity at different wavelengths also during deposition Acquisition of support for allowing co-evaporation in R&D system Production System Preparation system Some components are available: valves, manipulators, masking system (first version to be reviewed) Main chamber reviewed and tender started Vacuum pumps selection completed in order to reduce by one order of magnitude the vacuum level Definition of the illumination system under way, based on Laser Drive Light Source and monochromator. A scanning system for QE map will be added too. New Pumping System based on D2000 NexTorr Sandeep Mohanty, PITZ Collaboration Meeting, Zeuthen, May 15th, 2019