Photo Injector Test facility at DESY in Zeuthen Acknowledgements to - PowerPoint PPT Presentation

Space-Charge Dominated Photoemission in the Photocathode RF Gun at PITZ Ye Chen and Mikhail Krasilnikov for the DESY-PITZ team The 5th Photocathode Physics for Photoinjectors (P3) Conference Santa Fe, New Mexico USA, October 15-17, 2018 Photo Injector Test facility at DESY in Zeuthen Acknowledgements to D. H. Dowell, SLAC , C. Hernandez-Garcia, J-lab , R. Ganter, PSI , C. Hessler, CERN F. Brinker, M. Dohlus, K. Floettmann, W. Hillert, S. Lederer, S. Schreiber, DESY P. Michelato, L. Monaco, C. Pagani, D. Sertore, INFN, H. Chen, Y.-Ch. Du, W.-H. Huang, Ch.-X. Tang, Q.-L. Tian, L.-X. Yan, Tsinghua University A. Arnold, J. Teichert, R. Xiang, HZDR , H. De Gersem, E. Gjonaj, T. Weiland, TEMF for kind support and useful discussions on photocathode and photoemission Page 1 / 21

Outline SCDPE: Space-Charge Dominated PhotoEmission  PITZ facility  Observation & characterization of SCDPE  Summary | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 2 / 21

The PITZ Facility DESY Zeuthen Campus nearby Berlin PITZ http://www.desy.de/ | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 3 / 21

Development, test and optimization of high brightness electron sources for SC linac driven FELs + applications The DESY-PITZ guns are in use at the European X-ray Free Electron RF Gun Laser (European-XFEL) and the  L-band (1.3 GHz) 1.6-cell copper cavity Free electron LASer in Hamburg  Ecath ≥ 60 MV/m  7 MeV/c e-beams (FLASH).  650 µs × 10 Hz  up to 45 kW av. RF power  Cs 2 Te PC (QE~5-15%)  up to 5 nC/bunch  Solenoids for emittance compensation  LLRF control for amp and phase stability <7MeV <25MeV UV PITZ Beam Line 3.0 Laser  Details: https://pitz.desy.de/ | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 4 / 21

PITZ evolution 2002-2017  Photocathode and cathode laser Highlights of the Evolution:  Increasing the brightness (decreasing the emittance)  Improving gun stability and reliability  Extending beam diagnostics  Use high brightness beam capability | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 5 / 21

Flexible Photocathode Laser pulse shaping system MBI Pulse Shaper Pulse Train Time Structure PITZ and EXFEL run bunch trains with up to 600 ( 2700 ) laser pulses Towards ultimately low emittance beams  3D ellipsoidal pulses (under development) Proof of principle demonstrated with IAP (JINR) system at PITZ (2016-2017) Comparison with simulated e - beam shapes (500pC): similarity in shape 20 Gaussian laser Ellipsoidal laser Flattop laser Comb 10 t (ps) 0 -10 @PST.Scr1 @ EMSY1 -20 -10 -5 0 5 10 x (mm) First Measurement  Details in J. Good et al., Proc. 38 th FEL Conf., WEP006 (2017) | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 6 / 21

Photocathode in the gun: Cs 2 Te  Dark current measurement  Cs 2 Te produced at DESY and INFN-LASA Visual inspection  insertable QE: 5~15% @ 257 nm Faraday cup  Up to 5 nC/single bunch possible YAG Screen  Vacuum level in the gun: ~10 -9 mbar Dark current on Screen  For nominal operation (~6.5MW × 650µs) dark current < 100µA (for different guns) QE map ~0.8m Max. dark current vs. RF power during gun conditioning ~ 1 month  ≈ 92 ~ 4 months QE measurement I F : current [A], E: field [V/m] | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 7 / 21

Operation: Working Points at European-XFEL, FLASH & PITZ 1nC  European XFEL, PITZ ~320pC  FLASH, PITZ  At European-XFEL and FLASH working points, the beam extraction at cathode strongly influenced by space- charge effects RF@~6.5MW × 650µs RF@~4.8MW × 600µs  Best emittance measured at 1.6 the working points, e.g. for Q [nC], emitted bunch charge 0.45 experiment experiment 1.4 1nC beam at European-XFEL 0.40 1.2 𝛇 𝐲,𝐨 ≈ 𝟏. 𝟖𝟑 𝐧𝐧 𝐧𝐬𝐛𝐞 0.35 1.0 0.30 𝛇 𝐳,𝐨 ≈ 𝟏. 𝟕𝟏 𝐧𝐧 𝐧𝐬𝐛𝐞 0.8 0.25 𝛇 𝐲,𝒐 𝛇 𝐳,𝐨 ≈ 0.66 mm mrad 0.20 0.6 0.15 0.4 0.10 0.2 0.05 0 0.00 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 0.02 0.04 0.06 0.08 Qbunch [nC] Laser pulse energy [µJ] Representative cathode drive laser intensity | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 8 / 21

Operation: Discrepancies of Emitted Charge in simulation vs. measurement 1nC  European XFEL, PITZ ~320pC  FLASH, PITZ simulated Charge vs. Laser Energy σ rms = 0.4mm Uniform σ rms ≈ 0.3mm C+H σ rms =0.3mm RF@~6.5MW × 650µs RF@~4.8MW × 600µs Uniform 1.6 Q [nC], emitted bunch charge 0.45 experiment experiment 1.4 0.40 1.2 0.35 1.0 0.30 0.8 0.25 0.20 0.6 Core(C)+Halo(H) model: 0.15  fits for Gaussian temporal 0.4 0.10 distribution (see backup slide) 0.2 0.05  but not for flattop case 0 0.00 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 0.02 0.04 0.06 0.08 Qbunch [nC] Laser pulse energy [µJ] NIM A 871, 97-104 (2017) Representative cathode drive laser intensity | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 9 / 21

Operation: Discrepancies of Emittance in simulation vs. measurement 1nC  European XFEL, PITZ ~320pC  FLASH, PITZ Emittance vs. Cathode Laser Spot Size 1nC RF@~6.5MW × 650µs RF@~4.8MW × 600µs 1.6 Q [nC], emitted bunch charge 0.45 experiment 250pC experiment 1.4 0.40 100pC 20pC 1.2 0.35 1.0 0.30 0.8 0.25  Lager discrepancies for stronger 0.20 0.6 space-charge dominated e-beams 0.15  Problems may (partially) 0.4 0.10 originate from photocathode 0.2 0.05 already? 0 0.00 0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0 0.02 0.04 0.06 0.08 PRST-AB 15, 100701 (2012) Qbunch [nC] Laser pulse energy [µJ] Representative cathode drive laser intensity | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 10 / 21

Modeling: Some Treatments in simulations Treatment 1  Generating more Treatment 2  Bridging beam dynamics in vacuum with Treatment 3  modeling field(RF + space- charge) effects during emission through "realistic" photoemission distribution simplified effective cathode QE well-known Schottky effect according to cathode laser (and QE map) for metals: for semiconductors: 𝚾 𝐭𝐝𝐢𝐩𝐮𝐮𝐥𝐳 𝒔 ⊥ , 𝒖 (1 − 𝑆) 𝛽(ℏ𝜕 − Φ 𝑓𝑔𝑔 ) 2 Temporal laser profile QE 𝜂 = QE∗ = 8Φ 𝑓𝑔𝑔 (𝐹 𝐺 + Φ 𝑓𝑔𝑔 ) 𝐹 𝑏 ℏ𝜕 − Φ 𝑓𝑔𝑔 ) 2 2(𝑞 0 + 1)(1 + 𝒇 𝑭 𝐒𝐆 𝒔 ⊥ , 𝒖, 𝒜 = 𝟏 ± 𝑭 𝐓𝐪𝐝𝐢 𝒔 ⊥ , 𝒖, 𝒜 = 𝟏 intensity y = 𝒇 Φ eff = 𝐹 𝑕 +𝐹 𝑏 ± Φ schottky + ⋯ Φ eff = Φ 0 ±Φ schottky + ⋯ x 𝟓𝝆𝜻 𝟏 𝐹 𝑏 : electron affinity QE Φ eff : effective cathode work function Map, f 2 Φ 0 : intrinsic work function 𝑞 0 : characteristic parameter 𝐑𝐅 𝒔 ⊥ , 𝒖, 𝐴 = 𝟏 during emission, Laser Φ schottky : Schottky term 𝑆 : reflection coefficient determined according to the RF field & Spot Map, f 1 𝐹 𝑕 : band gap ℏ𝜕 : photon energy the self-field of the beam at extraction, 𝐹 𝐺 : Fermi energy time but, the latter is NOT prior known. ∗ 𝛽 : characteristic parameter (f 1 f 2 ) (r, t) Convolution 𝜼 K. Jensen et al., J. Appl. Phys104, 044907 (2008) *D. Dowell et al., PRST-AB 12 074201 (2009) | The 5 th Photocathode Physics for Photoinjectors (P3) | Santa Fe • NM USA | Dr. Ye Chen | 15-17.10.2018 Page 11 / 21