1. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Photocathodes: Present status and future perspectives Paolo Michelato INFN – Milano LASA

2. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. photocathode main requirements  High quantum efficiency  Capability to produce high current density  Long lifetime  Short response time and no tails in the emission (ps time scale)  Capability to produce large integrated charges (some C)  Relative thin (SC cavities, RF losses)  Not too low work function (dark current, Fowler-Nordheim!)  Low thermal emittance (transverse momentum)  Capability to survive in a high electric field area OK Measured ?OK ?OK OK, months, interest in increasing it! Cs Te photocathode characteristics

3. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Cs 2 Te Cathodes production and use Substratum production Polishing Cleaning Te evaporation Insertion in the masking High temperature UHV heating (final cleaning) Cs evaporation Cathode growing Cathode qualifification: Qe vs. measure Cathode transfer Cathode loading into the gun Cathode damaging Cathode post mortem analysis

4. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Cathodes production and use: substratum Substratum production Polishing High temperature UHV heating for cleaning Roughness Optical reflectivity Chemical (impurities) Carbon content Density (Sintered vs. Arc cast) Cleaning Surface status: dust, contaminants Thermal cycle STM Surface Surface status H ion cleaning? New cleaning procedures: CO 2

5. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Cathodes production and use: evaporation Te evaporation Insertion in the masking Cs evaporation Mask – substrate distance Dep. Rate Thickness Photocurrent (Variable masking) Ellipsometry Color analysis during growing Reflectivity vs. ( ) Te deposition with different masking device Color Reflectivity Dep. Rate Thickness Photocurrent +

6. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Cathodes production and use: growth Cathode growing Cathode qualifification: Qe vs. measure Cathode transfer Photocurrent QE ( ) QE maps ( ) Photocurrent QE ( ) (CW) Color analysis Ellipsometry PEEM In situ XPS/AES Dep. Rate Thickness Photocurrent Thermal emittance Color Reflectivity XPS/AES QE vs gas exposition +

7. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Cathodes production and use: gun operation Cathode loading into the gun Cathode use and damaging Cathode post mortem analysis QE ( ) (CW and pulsed) QE map: pulsed After air exposition: Optical inspection SEM Color analysis Elipsometry PEEM In situ XPS/AES SEM In vacuum: QE ( ) (CW and pulsed) QE map: (CW and pulsed)

8. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Present and future Why we are not completely satisfied? What can be improved? What has to be ameliorated? oPreparation procedure reliability oCathode uniformity oDark current (substrate? Film? Both?) oThermal emittance control oSmall area cathode (few mm or less) o…

9. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Example: Photocathode parameters: E g and E a E g and E a measurement can be done using for instance QE vs  curves reflectivity measurements at different, etc For a model of the photoemission process  Consequences/possible results  QE and electron motion/emission  Thermal emittance control Why is interesting to know Eg and Ea?

10. 10/28 FLASH seminar –October 26, 2006 CW QE measurements: Data Analysis CW data analysis –Fitting of the spectral response Cathode #73.1 where A is a constant, E G and E A are energy gap and electron affinity. An example is given for the analysis of the CW QE data for cathode 73.1. In this case: Eg+Ea = 4.165 eV m = 1.24 (for a “fresh” Cs 2 Te cathode we tipically have Eg+Ea = 3.5eV) Laura. Monaco

11. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Photocathode production and relative diagnostics  Film color analysis during the growth: diffuse or reflected light spectroscopy  In situ Ellipsometry

12. 12/28 FLASH seminar –October 26, 2006 Production:...to photocathodes growth 5 mm Active Area during the evaporation, the plug is heated to 120°C. The dimension of the film is determined by a circular masking (the actual one is 5mm diameter) first, a thin layer of 10nm of Te is produced then Cs is evaporated at a rate of 1nm/min during the deposition, the film is illuminated with UV ( =254 nm) of a Hg-lamp to monitor the quantum efficiency. the evaporation is stopped, when the QE is at maximum. Cs 2 Te photocathode recipe: Different stoichiometric compounds form during Cs deposition till the “correct” Cs/Te ratio is reached corresponding to the QE maximum After 10min of Te deposition After 45 min of Cs deposition Laura Monaco

13. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Example: Photocathode preparation procedure oGood reproducibility, but each cathode grown is not identical with respect another one. oToo many parameters (some not easy to be controlled):  Substrate condition  Source temperature  UV light power  Vacuum  …

14. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Photocathode QE distribution

15. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Why don’t use a different production procedure? MBE: Molecular beam epitaxy (?over Mo?) Non metallic masking (resist, kapton…): Cs evaporation will be done separately. How Te grow on Mo? (S. Valeri et alt.) Doe’s Cs acts as a surfactant? Cs 2 Te is like “Snow over Alps” or “Pillars” on the Mo substrate? Mo Cs 2 Te

16. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Masking: how it works Presently a Molybdenum masking is used to limit the amplitude of the photoemissive area on the plug top and to avoid Cs contamination in the RF spring area.

17. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Present masking: troubles and limitations Masking: tolerances on the masking position. –Now: some tenths of mm, that would be critical for small area photocathode. –Can be done better but mechanics may becomes complicated –Find another way to deposit Te films with high precision position (resist?) Evaporation done in another chamber. Is it needed? New gun require very small cathodes? This will give the possibility to use different photoemissive films in the same system: film diameter depends on the Te film.

18. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Masking Masking: is it a source of contamination of the cathode surface? YES! –Particles –Scratches –Defects

19. Paolo Michelato, Workshop on High QE Photocathodes, INFN-Milano LASA, 4 – 6 October 2006. Other point to be discussed? Let us start with the workshop!