1. Multialkali photocathode Luca Cultrera

2. Outline Motivations; State of the art; Alkali antimonides photocathodes; Growth UHV chamber; Load lock for photocathode transfer; Photocathode characterizations; Preliminary results; R&D perspectives;

3. Motivations (1) Provide experimental data to fill the voids on fundamental parameters still missed Dowell et al., NIM-A, 622, (2010) 685-697

4. Motivations (2) NEA photocathode based on GaAs activated surfaces has been demonstrated to be able to provide high brightness electron beam; Nevertheless the surface sensitivity to ion back bombardment during high average current delivery is still an issue; Multialkali photoemission is based on bulk material properties rather than surface effects: we expect longer lifetime;

5. State of the art In past years several efforts have been made to obtain a reliable photoinjector source using alkali antimonides based photocathodes. Boeing 433 MHz NC gun operating with CsK 2 Sb (typ. QE 8% at 543 nm) photocathode set the 35 mA average current (still the world record for such injector) but the lifetime of the cathode was limited to few hours due to the poor vacuum conditions. LaboratoryLos AlamosCEAWuppertalBOEING ProjectHIBAF/APEXELSASRF injectorAPLE Year1993199019891993 CathodeCsK 2 Sb Cs 3 SbCsK 2 Sb

6. Alkali antimonides photocathodes PMT industries developed reliable procedure to growth photosensitive layer based on multialkali materials with QE in visible spectra up to 43%. Alkali based photocathode could provide a few % QE at the wavelength of ERL injector prototype (520 nm) :  Cs 3 Sb  CsK 2 Sb  NaK 2 Sb

7. UHV Growth Chamber Photocathode growth chamber has been designed following several guidelines:  To be able to host a substrate/puck compatible with currently operating DC gun;  To be easily to be modified in order to host different kind of alkali metals;  To be able to be connected with a load-lock transport system;

8. UHV Growth Chamber

9. Mask Hole diameter 8 mm Si (100) high conductivity SiOx layer etched with HF CsSbK Heater inside the puck First deposition has been carried out using SAES alkali metal dispenser. Future deposition will be carried out using high capacity ALVATEC sources.

10. Vacuum Suit Transfer Cart Puck has been successfully delivered to and recovered from L0 load lock Vacuum “suitcase” with the cathode connected to the injector gun prep chamber in L0

11. Instrumentation Control System (1) Lambda Genesys UT T Power supply evaporators Lamp & Mono 842-PE 6485-gpib Power & current meter SRS 830 RGA CCG Vacuum QMB PC MOXA NI 9162 ALL DEVICES NOW CONTROLLED BY LABVIEW

12. Instrumentation Control System (2)

13. Photocathode characterizations Photoemission properties:  Quantum efficiency vs. wavelength;  Thermal emittance (EEA and DC gun);  Photoemission Uniformity; Chemical composition:  Auger vs. thickness; Chemistry at the surface:  QE vs. wavelength with controlled poisoning; Structure at the surface:  LEED;

14. Preliminary Results Few preliminary deposition test have been carried out during past year:  Cs 3 Sb  CsK 2 Sb  NaK 2 Sb So far the better results in terms of QE have been achieved with CsK 2 Sb depositions

15. Cs 3 Sb QE stable at about 0.5% Sources depleted before the right stochiometry was achieved

16. CsK 2 Sb

17. QE @ 520 nm is about 4.5%

18. CsK 2 Sb Rougness “peak-to-peak” less than 50 nm Thickness around 200 nm

19. NaK 2 Sb

20. First CsK 2 Sb cathode installed on the gun

21. R&D perspectives Three-step Model Strategy  Absorption Maximize the absorption of the layer at the wavelength of interest  Scattering Minimize photoelectrons losses due to scattering mechanism  Extraction Optimize the extraction of photoelectrons to maximize brilliance

22. Antireflective photocathode Additionally the photosensitive layer could be designed to be antireflective:  For a reflective cathode the /4 is the simplest configuration that could be used to strongly increase the absorption at a given wavelength;  Considering that the refraction index of CsK 2 Sb at 520 nm is approximately 1.9 we should be able to obtain an antireflective coating with a thickness of about 70 nm;  Measure reflectivity as function of initial Sb layer thickness;

23. Structural considerations Crystal grain size  Is well known that the film grain size will affect the final QE due to the grain boundary: The largest the crystal size the lower the traps density that could avoid electrons to reach the surface.  How we could increase the growth of larger crystal grains? Choice of a substrate that minimize lattice mismatch with targeted bialkali photocathode  Test different single crystal substrates;

24. Choice of substrate The structure of CsK 2 Sb photocathode is bcc with a lattice parameter of 8.61 Å:  Is there a simple metal or semiconductor having a structure that allows tho growth the film by heteroepitaxy?  Mo has a lattice constant of 3.14 Å 3.14 Å 8.90 Å The misfit will be: f=(8.90-8.61)/8.61=3.3% We need large area single crystal Could we use anneal and Ar sputtering?

25. AUGER, LEED and Photoemission Auger spectroscopy and LEED available in a UHV test chamber.  LEED could be used with thermal annealing and Ar ion gun to prepare the substrates for deposition.  Auger and Ar ion gun could be used to study the composition of the photocathodes as function of the thickness  Auger and photoemission in conjunction to controlled poisoning to study the reactivity towards different chemical species

26. Puck compatibility The puck compatibility with either DC gun and Electron Energy Analyzer will allow us to relate photoemission properties (Quantum Efficiency, Thermal Emittance, Spectral Response) with growth parameters; Need for an intensive parametric growth studies will require also technological challenge on pure alkali metal sources:  Pure metals evaporators;  Alkali azide thermal decomposition;

27. Storage system We have demonstrated the transfer of alkali photcathodes from the growth chamber to the gun; With the aim to perform an intensive program of R&D on photocathodes will necessitate also of a dedicated storage system.