Dark current at the Euro-XFEL

Transcription

1 Dark current at the Euro-XFEL Jang-Hui Han DESY, MPY Observations at PITZ and FLASH Estimation for the European XFEL Ideas to reduce dark current at the gun

2 DC at FLASH RF gun M1 M2 M3 M4 M5 M6 M7 6 undulator modules laser 1 st bunch compressor 2 nd bunch compressor collimator 4 MeV 130 MeV 350 MeV MeV 250 m bypass

3 DC at FLASH RF gun M1 M2 M3 M4 M5 M6 M7 6 undulator modules laser 1 st bunch compressor 2 nd bunch compressor collimator 4 MeV 130 MeV 350 MeV MeV 250 m bypass RF pulse lengths (flat region) Gun: 70 μs Acc. Modules: 100 μs

4 Estimation of dark current for the XFEL 5 dark current (ma) measured measurement on 9 at Nov. PITZ 2004 fit fit with extrapolation 2. 5 IFE = C1E exp( C2 / E) European XFEL gun max rf field at the cathode (MV/m)

5 Collimator Geometrical collimator (different sizes) Energy collimator (different energy distribution) RF gun M1 M2 M3 M4 M5 M6 M7 6 undulator modules laser 1 st bunch compressor 2 nd bunch compressor collimator 4 MeV 130 MeV 350 MeV MeV bypass

6 Experimental Setup at the Gun

7 Parameter summary FLASH (measure) laser XYrms ~1 mm Lt 6~7 ps Gaussian rt Ek 0.55 ev gun Ecath ~42 MV/m φemit 38 Bmax T Sol. position m ACC1 entrance 2.48 m Emax 16 MV/m ACC1 φ ~ on crest beam emittance <2 mm mrad

8 DC trajectories

9 DC trajectories (b) (c) (d) (a) The maxima of the rf field strength can be the major source of dark current

10 DC trajectories (a) z(m) (b) (c) (d) (b) (a) z(m) (c) The maxima of the rf field strength can be the major source of dark current z(m) (d) Astra simulation at 40 MV/m gradient and 300 A main solenoid current z(m)

11 DC trajectories Local maxima

12 DC trajectories Local maxima front view in the cathode chamber side view in the gun cavity

13 DC Image analysis

14 DC Image analysis

15 DC Image analysis

16 DC Image analysis without beam The green points are the electrons from the boarder of Cs 2 Te. The orange points are the electrons from the edge of Mo plug.

17 DC Image analysis with beam The green points are the electrons from the boarder of Cs 2 Te. The orange points are the electrons from the edge of Mo plug. The red points are the electron beams.

18 Field emission Vs. emission phase rf field and field emission (a.u.) ( β E sinθ ) exp[ C /( β E sinθ )] IFE = C1 max 2 rf field β field = 100 β field = 200 = 400 β field max rf phase (deg)

19 Field emission Vs. emission phase (FLASH) rf field and field emission (a.u.) kinetic energy (MeV) rf field β field = 100 β field = 200 = 400 β field rf phase (degree) rf field at cathode (MV/m) rf phase (deg)

20 Momentum distribution after gun (measurement at PITZ) measurement simulation

21 XY size of beam & dark current (FLASH)

22 Gun for PITZ and FLASH F = MHz, Q = 26754

23 Gun for PITZ and FLASH F = MHz, Q = 26754

24 Parameter summary FLASH (measure) XFEL (original) laser XYrms ~1 mm 0.44 mm Lt 6~7 ps Gaussian 20 rt 2 Ek 0.55 ev 0.55 ev gun Ecath ~42 MV/m 60 MV/m φemit Bmax T T Sol. position m m ACC1 entrance 2.48 m 3.2 m Emax 16 MV/m 21.5 MV/m ACC1 φ ~ on crest -16 beam emittance <2 mm mrad 0.7 mm mrad

25 XY size of beam & dark current (XFEL, original)

26 Momentum of beam & dark current (XFEL, original)

27 New design for the XFEL F = MHz, Q = 27621

28 Parameter summary FLASH (measure) XFEL (original) XFEL (new) laser XYrms ~1 mm 0.44 mm 0.65 mm Lt 6~7 ps Gaussian rt 2 2 Ek 0.55 ev 0.55 ev 0.55 ev gun Ecath ~42 MV/m 60 MV/m 60 MV/m φemit Bmax T T T Sol. position m m m ACC1 entrance 2.48 m 3.2 m 3.4 m Emax 16 MV/m 21.5 MV/m 13 MV/m ACC1 φ ~ on crest beam emittance <2 mm mrad 0.7 mm mrad 0.85 mm mrad

29 XY size of beam & dark current (XFEL, new design)

30 Momentum of beam & dark current (XFEL, new design)

31 Conclusion and outlook Dark current might be more serious problem at the Euro-XFEL Most of dark current at the gun is originated from the cathode area For XFEL case, higher solenoid field makes dark current over-focused quickly With enlarging the half cell length, the momentum distribution of beams and dark current can be separated. Further optimization of the cell length ratio and machine parameters are necessary. Find optimum position and size of collimators