Infrared Single Shot Diagnostics for the Longitudinal. Profile of the Electron Bunches at FLASH. Disputation

Transcription

1 Infrared Single Shot Diagnostics for the Longitudinal Profile of the Electron Bunches at FLASH Disputation Hossein Delsim-Hashemi Tuesday 22 July 2008

2 7/23/2008 2/ 35 Introduction m eb c γ ω = + + = θ γ γ λ λ K u l + = K u l γ λ λ 1 N 2 I I N = Free electron laser

3 Introduction 7/23/2008 3/ 35 Very large number of electrons to be confined very close together Hard to achieve in sub-micrometer wavelengths Modulating a relatively long electron beam into equally spaced bunch-lets automatically inside the undulator. UV and X-ray range High gain FEL FEL Special version starting from noise: Self-Amplified Spontaneous Emission (SASE) Seeding the electron bunches by an external laser inside undulator which is tuned to the seed laser wavelength.

4 How it works? λ u K λ ph = γ 2 2 electron beam photon beam Undulator Undula tor beam dump log( radiation power ) distance Power saturation distance 7/23/2008 4/ 35

5 SASE FEL challenges 7/23/2008 5/ 35 Electron beam parameters needed: Gain Length: L g = mcγ σ r λu 2 ˆ μ0ek I 1 3 Beam transverse size σ 50μm r Peak current inside bunch Î> 1 ka

6 Bunch compression 7/23/2008 6/ 35

7 Courtesy: Martin Dohlus t [ fs 7/23/2008 7/ 35 ]

8 Has to be diagnosed. How? 7/23/2008 8/ 35 An ideal longitudinal diagnostics tool should be single shot (to monitor shot to shot fluctuations) broad-band, and with high resolution Additional requirements would be: easy to operate and maintain compact and easy to incorporate in different parts of the machine operating independent of machine parameter changes Coherent radiation diagnostics

9 Coherent Radiation 7/23/2008 9/ 35 F ( ω) = ~ ρ( t)exp( iωt) dt long bunch normalized line-charge density dun dω = N 2 F long 2 du ( ω) 1 d ω ( ω, γ, source) spectral energy density (only coherent term)

10 Transition Radiation 7/23/ / 35 Ginzburg-Frank spatial distribution for the backward transition radiation by a single electron (far-field and infinite screen). The angle against backward direction is shown by θ. 1 At = the maximum intensity appears. θ γ

11 7/23/ / 35 Transition Radiation (finite screen size) (Transition radiation of an electron bunch is described based on : TESLA report , S. Casalbuoni et al.) TR energy per frequency interval f = 1 GHz that is emitted by an electron with γ = 1000 is plotted as a function of the TR screen radius a. 2 e (lnγ + ln 2 0.5) 2 π ε c 2 0

12 7/23/ / 35 FLASH layout and infrared radiation beam-lines Terahertz and Optical SYnchrotron radiation LABoratory CTR140 TOSYLAB Circular diffraction radiator Slit diffraction radiator full screen transition radiation Off-axis screen transition radiation (kicked bunch) Bunch compressors IR Undulator

13 THz-Transport and THz-Beamline (CTR140) 7/23/ / 35

14 Transverse profile 7/23/ / 35 Horizontal pol. 50 µm wavelength

15 What type of spectrometer? Why grating spectrometers? Why not commercial grating-spectrometers? Three grating turret 7/23/ / 35

16 Reflectance gratings 7/23/ / 35

17 Efficiencies and distribution 7/23/ / 35 S pol.

18 Proof of principle experiment 7/23/ / 35

19 Staging 7/23/ / 35

20 Collecting optics 7/23/ / 35

21 7/23/ / 35

22 Next achievements 7/23/ / 35 Courtesy: B. Schmidt 1- Ring-mirror 2- Collecting cones 3- Flat mirror holders

23 Two-stage multi-channel spectrometer 7/23/ / THz-filters (remote controlled) 2- Polarizer (remote controlled) 3- Reflectance grating stage 4- Transmission grating stage 5- Reflectance gratings holder and remote controlled mover 6- Transmission gratings holder and remote controlled mover 7- Pyro-camera 8- Parabolic mirror and its linear mover

24 ACC1 phase scan 7/23/ / 35

25 ACC1 phase scan 7/23/ / 35

26 ACC1 phase scan 7/23/ / 35

27 MCP-CTR spectrum correlation 7/23/ / 35

28 GMD-CTR spectrum correlation More? 7/23/ / 35

29 Pyro-electric detector response 7/23/ / 35

30 Combined broad-band spectrum 7/23/ / 35

31 Bunch profile determination 7/23/ / 35

32 Bunch profile determination (~700 MeV) 7/23/ / 35

33 Bunch profile determination (~500 MeV) 7/23/ / 35

34 Summary and outlook Compact two-stage single-shot spectrometer has been designed, constructed and used successfully. Single shot spectroscopy shows shot to shot fluctuations. Characteristics of the longitudinal bunch profile has been determined by Fourier transform methods. Unique profile reconstruction is impossible. Different wavelength ranges of the coherent transition radiation spectra correlate or anti-correlate to the SASE intensity. Structures much shorter than the characteristics length of spike have been observed that may correspond to effects like micro-bunching instability. Lots of useful information are contained in the spectra. The entire spectrometer has to be calibrated to obtain a measured transfer function. The ongoing efforts to setup a multi-stage device composed of more compact detection units could provide a wider wavelength range coverage in a single-shot mode. 7/23/ / 35

35 Thank you very much for your attention!