1. Photocathode based Electron Sources for Particle Accelerators – Yesterday, Today and Tomorrow B.L. Militsyn STFC ASTeC, UK European Workshop on Photocathodes for Particle Accelerator Applications, 6-8 June 2016, Daresbury, UK

2. Outline Days before yesterday – discovery of the photo- effect Yesterday – first photocathode based electron sources –DC photocathode guns –RF photocathode guns Today – evolution of photoelectron sources –DC –RF (SRF) –VHF Tomorrow – what do we expect from the future photocathodes 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK

3. Days before yesterday. Discovery of the Photoeffect. The Nobel Prize in Physics 1921 was awarded to Albert Einstein "for his services to Theoretical Physics, and especially for his discovery of the law of the photoelectric effect". Philipp Eduard Anton von Lenard (1862-1947) Albert Einstein (1879-1955) Heinrich Rudolf Hertz (1847-1894) 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK

4. Yesterday. SLAC DC photocathode gun 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK Photocathode used,GaAs Maximum polarization40%

5. Yesterday. Los Alamos Photocathode RF gun 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK Operation Frequency1.3 GHz Photocathode used Cs 3 Sb Peak surface field60 MV/m Bunch charge 12 nC Peak beam current100-150 A Beam emittance32 mm · mrad

6. Yesterday. BNL RF Gun 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK Photoctahode usedCu Beam emittance3 mm · mrad

7. Today: Very general classification of the photoelectron sources Out of the gun photocathode preparation and exchange with a Load-Lock system Electron emission –Photocathodes Metal Amorphous semiconductor Crystalline semiconductors Electron acceleration –DC high voltage –Radiofrequency acceleration Normal conductive acceleration Superconductive acceleration –VHF field acceleration Normal conductive acceleration Superconductive acceleration 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK

8. Today: Modern DC gun designed for ERL Maximum voltage achieved – 500 kV Maximum design current – 10 mA Photocathode – GaAs/Sb Photocathode preparation system (not shown) integrated with the gun For protection of the ceramic insulator from field emission and scattered electrons it is made segmented B.L. Militsyn, EWPPAA, Daresbury, UK 6 June 2016

9. Today: NC RF CLARA Gun-400 H shape coupler Cathode plug and spring retaining plate. Gun Cavity RF probe* Cooling channels 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK

10. Today. Design parameters of the Gun-400 UnitsOperating mode Beam Repetition Rate100 Hz400 Hz FrequencyGHz2.9985 RF peak powerMW<10 RF average powerkW10 Gun electric fieldMV/m120100/80 Bunch chargepC20-250250 Operating modePulsed/Train pulsed Pulsed RF feedbackRequired 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK

11. Today: The LBNL VHF photocathode gun 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK Photocathode used Metal/Te/Sb

12. Today: ELBE SRF Photocathode gun 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK RF frequency1.3 GHz RF scheme3.5 cells Operation modeCW Photocathode usedCs 2 Te/Metal

13. Today: Photocathode Materials ClassMate rial QEResponse time Intrinsic Energy Wave Length GunApplication Normally conducting metals Cu, Mg 10 -5 - 10 -4 10’s fs100’s mV – 1’s VUVNC-RF, VHF Low rep rate FELs Super conducting metals Nb,Pb10 -5 - 10 -4 10’s fs100’s mV – 1’s VUVSC-RFHigh rep rate FELs Positive e - affinity Te- based Cs 2 Te0.1- 0.2 ps’s100’s mVUVNC-RF, SC-RF VHF DC High rep rate FELs Positive e - affinity Sb- based Cs 3 Sb etc 0.1- 0.2 ps’s100’s mVVisibleDC VHF, SCRF(?) NCRF(???) ERLs, High rep rate FELs Negative e - affinity semiconductor GaAs, etc 0.1- 0.35 1’s ps – 100’s ps10’s mV – 100’s mVIR- VisibleDC (XHV)Polarized sources, ERLs 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK

14. Tomorrow: general requirements for electron accelerators: Beam energy – 100’s keV – GeV’s Bunch peak current – Amps – kA’s –Bunch charge – fC’s – nC’s –Bunch duration – fs’s – ns’s Beam average current – nA’s –A’s –Bunch charge – fC’s – nC’s –Bunch repetition rate – Hz – 100’s MHz Beam energy spread – 0.1 %’s Beam emittance – sub µm’s Beam synchronisation – less than10’s of fs 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK

15. Compact Linear Accelerator for Research and Applications CLARA VELA RF Photoinjector Test Facility ELECTRONS GENERATED HERE ELECTRONS ACCELERATED AND MANIPULATED INTERACTIONS WITH LASER BEAMS FEL OUTPUT GENERATED FEL OUTPUT STUDIED Notes: 1. CLARA uses European RF frequencies 2. X-band cavity is used for phase space linearisation

16. CLARA Phase I Commissioning starts September 2016 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK

17. Tomorrow: Requirements to the photocathodes for the future accelerator High Quantum efficiency High Robustness Low intrinsic emittance Fast response time We are waiting your new ideas! 6 June 2016 B.L. Militsyn, EWPPAA, Daresbury, UK

18. Thank you for your attention!