Infrared Single Shot Diagnostics for the Longitudinal Profile of the Electron Bunches at FLASH Disputation Hossein Delsim-Hashemi Tuesday 22 July 2008
7/23/2008 2/ 35 Introduction m eb c 2 3 2 γ ω = + + = 2 2 2 2 2 1 2 θ γ γ λ λ K u l + = 2 1 2 2 2 K u l γ λ λ 1 N 2 I I N = Free electron laser
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.
How it works? λ u K λ ph = 1 2 + 2γ 2 2 electron beam photon beam Undulator Undula tor beam dump log( radiation power ) distance Power saturation distance 7/23/2008 4/ 35
SASE FEL challenges 7/23/2008 5/ 35 Electron beam parameters needed: Gain Length: L g = 1 3 3 2 2mcγ σ r λu 2 ˆ μ0ek I 1 3 Beam transverse size σ 50μm r Peak current inside bunch Î> 1 ka
Bunch compression 7/23/2008 6/ 35
Courtesy: Martin Dohlus t [ fs 7/23/2008 7/ 35 ]
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
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)
Transition Radiation 7/23/2008 10 / 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
7/23/2008 11 / 35 Transition Radiation (finite screen size) (Transition radiation of an electron bunch is described based on : TESLA report 2005-15, 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
7/23/2008 12 / 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
THz-Transport and THz-Beamline (CTR140) 7/23/2008 13 / 35
Transverse profile 7/23/2008 14 / 35 Horizontal pol. 50 µm wavelength
What type of spectrometer? Why grating spectrometers? Why not commercial grating-spectrometers? Three grating turret 7/23/2008 15 / 35
Reflectance gratings 7/23/2008 16 / 35
Efficiencies and distribution 7/23/2008 17 / 35 S pol.
Proof of principle experiment 7/23/2008 18 / 35
Staging 7/23/2008 19 / 35
Collecting optics 7/23/2008 20 / 35
7/23/2008 21 / 35
Next achievements 7/23/2008 22 / 35 Courtesy: B. Schmidt 1- Ring-mirror 2- Collecting cones 3- Flat mirror holders
Two-stage multi-channel spectrometer 7/23/2008 23 / 35 1- 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
ACC1 phase scan 7/23/2008 24 / 35
ACC1 phase scan 7/23/2008 25 / 35
ACC1 phase scan 7/23/2008 26 / 35
MCP-CTR spectrum correlation 7/23/2008 27 / 35
GMD-CTR spectrum correlation More? 7/23/2008 28 / 35
Pyro-electric detector response 7/23/2008 29 / 35
Combined broad-band spectrum 7/23/2008 30 / 35
Bunch profile determination 7/23/2008 31 / 35
Bunch profile determination (~700 MeV) 7/23/2008 32 / 35
Bunch profile determination (~500 MeV) 7/23/2008 33 / 35
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. 7/23/2008 34 / 35
Thank you very much for your attention!