Measurement of the Transverse Coherence of a VUV Free ... · The resulting images are subjected to...
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Measurement of the Transverse Coherence of a VUV Free Electron Laser R. Ischebeck, J. Feldhaus, Ch. Gerth, E. Saldin, P. Schmüser, E. Schneidmiller, K. Tiedtke, M. Tonutti, R. Treusch & M. Yurkov DESY Image Processing Double Slit Diffraction Patterns Resulting Coherence Function Evolution of Coherence along the Undulator The TTF Free Electron Laser Simulations Conclusion Coherence Free Electron Lasers (FELs) based on the self- amplification of spontaneous emission (SASE) have seen tremendous progress as radiation sources in the vacuum ultraviolet (VUV) wavelength regime. The TTF FEL at DESY has provided users with short pulses reaching gigawatt peak power. The facility is presently being extended to the soft X-ray regime and it will serve users in 2005, reaching wavelengths down to 6nm. Compared with state-of-the-art synchrotron radiation sources, FELs are characterized by a peak brilliance increased by up to eight orders of magnitude, a pulse length of 50 ... 100fs and by a high degree of transverse coherence. The investigation of these unique properties is of paramount importance for its broad range of applications. Here, a direct measurement of the transverse coherence of the TTF FEL is presented. To this effect, the diffraction pattern of a double slit has been recorded and the visibility of the interference fringes has been recorded. The experimental near field diffraction pattern is compared with a numerical model, taking into account the formation of the FEL radiation, the Fresnel diffraction and effects of the experimental set-up. In contrast to optical lasers, which achieve a full transverse coherence by spatial filtering, SASE FELs amplify the incoherent spontaneous emission in a single passage of the amplifying medium. Since this amplification favors the central radiation mode, one expects nonetheless a high degree of transverse coherence. The evolution of the coherence along the passage has been studied by recording diffraction patterns at various lengths. The transverse coherence of the TTF VUV Free Electron Laser has been measured directly with a double-slit experiment. To determine the evolution of the coherence as a function of the FEL amplification process, it has been measured as a function of the longitudinal position in the undulator. The highest transverse coherence is achieved at the end of the exponential growth regime, before the onset of saturation in the FEL process. A further increase of the effective undulator length beyond saturation degrades the coherence. For user operation of the FEL, one has to weigh up the highest peak power with the best coherence properties. The measurements described on this poster have been performed at the TTF free electron laser at DESY. This device uses a superconducting linear accelerator to produce an electron beam with a particle energy of 300MeV. During the passage of this beam through a permanent magnet undulator, electromagnetic radiation with a wavelength around 100nm is emitted. The interaction between electrons and the photon field results in the development of a micro-structure on the electron bunch. A large number of electrons emits radiation coherently and the radiation power grows exponentially with the longitudinal position in the undulator. The TTF FEL has demonstrated - the performance of superconducting cavities at high gradient, - the pulsed operation of the cavities and the compensation of beam loading of long pulse trains, - the controlled beam transport with low emittance from the gun to the end of the accelerator, - the RF regulation concept to stabilise the field amplitude and phase, - a proof-of-principle of a SASE FEL in the VUV, and finally - first experiments using this radiation The double slit diffraction patterns allow a straightforward determination of the transverse coherence, if the near field diffraction effects are taken into account. Diffraction patterns from different slit separations show the dependency of the transverse coherence on the distance. Two methods are used to determine the transverse coherence from the measurement data: 1) The fringe visibility, defined as To determine the intensities, a central part of the diffraction pattern has been projected along the direction of the slits. The resulting curve has been smoothed with a Butterworth filter, reducing the pixel noise without degrading the modulation of the observed pattern. 2) A fit of the intensity distribution, computed in Fresnel theory. Seven parameters are fitted: - the transverse coherence - the wavelength of the radiation - the electric field amplitude at both slits - the angles of the wave front at the two slits - a transverse displacement of the pattern. The fit algorithm is based on the reflective Newton method on trust-regions. The results of these two methods are in good agreement. They have been verified on diffraction patterns that have been simulated with numerical methods. From the measurements shown above, the transverse coherence function is determined. It is shown in the following plot as a function of slit separation. From the generic value of 1 in the origin, the coherence function drops to values below 0.7 at 1 mm separation. For distances above 2 mm, it is as low as 0.2. Systematic uncertainties of the presented analysis procedures are due to the formation of the near field diffraction pattern, the scattering in the fluorescent crystal, the limited resolution of the optical system and to noise in the CCD image. These can be quantified with the simulations, which include these effects. The formation of the FEL radiation in the undulator, the diffraction at the double slits and the propagation to the fluorescent crystal, the conversion of VUV photons to visible light and the imaging of the latter onto the CCD have been simulated. The resulting images are subjected to the same analysis routines as the measured data. The amplitude and phase of the computed electromagnetic field are precisely known. Thus, the theoretical value for the coherence function can be compared with the coherence that is determined from the images with the two analysis routines. That way, it is possible to gain information on the accuracy of the analysis. Fresnel Diffraction at a Double Slit Fachbereich Physik Institut für Experimentalphysik 0.5 1 2 3 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Slit separation / mm Coherence function The images have to be corrected for the non-linear response of the fluorescent Ce:YAG crystal and the finite resolution of the set-up. The response of the Ce:YAG crystal has been measured with the FEL: the light output has been compared to a multichannel plate detector, which has been calibrated previously. There are two main factors that limit the resolution of the measured diffraction patterns: the fluorescent crystal and the optical system of the camera. Measurements to determine their point spread function have been done. There are methods to reconstruct images that have been acquired with a limited resolution. The Lucy- Richardson algorithm is described and tested on images acquired in a laboratory set-up. The careful use of this procedure allows to improve the images. 0 2 4 6 8 10 12 0 10 20 30 40 50 60 70 80 FEL pulse beam energy / μ J Intensity of the fluorescen light / arb. units 1600 V 1650 V 1700 V 1600 V 1650 V 1700 V x / mm y / mm a) 0.05 0.1 0.15 0.2 0.25 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2 Intensity / normalised y / mm b) 0.05 0.1 0.15 0.2 0.25 0 0.05 0.1 x / mm Intensity / normalised c) -150 -100 -50 0 50 100 150 0 1 2 3 4 5 6 position / μ m Intensity / arb. units slit profile fluorescence light gauss fit σ = 22.5 μ m central visibility fitted coherence value for the averaged image σ stat. σ syst. -4 -2 0 2 4 -4 -3 -2 -1 0 1 2 3 0.5 mm -4 -2 0 2 4 -4 -3 -2 -1 0 1 2 3 1 mm -4 -2 0 2 4 -4 -3 -2 -1 0 1 2 3 2 mm -4 -2 0 2 4 -4 -3 -2 -1 0 1 2 3 3 mm x / mm y / mm -4 -2 0 2 4 -4 -3 -2 -1 0 1 2 3 -2 0 2 0 0.2 0.4 0.6 0.8 1 y / mm Visibility -2 0 2 50 100 150 200 250 300 350 400 y / mm Intensity / arb. units x / mm y / mm -4 -2 0 2 4 -4 -3 -2 -1 0 1 2 3 -2 0 2 0 0.2 0.4 0.6 0.8 1 y / mm Visibility -2 0 2 50 100 150 200 250 300 y / mm Intensity / arb. units x / mm y / mm -4 -2 0 2 4 -4 -3 -2 -1 0 1 2 3 -2 0 2 0 0.2 0.4 0.6 0.8 1 y / mm Visibility -2 0 2 50 100 150 200 y / mm Intensity / arb. units x / mm y / mm -4 -2 0 2 4 -4 -3 -2 -1 0 1 2 3 -2 0 2 0 0.2 0.4 0.6 0.8 1 y / mm Visibility -2 0 2 200 400 600 800 1000 1200 y / mm Intensity / arb. units corrected diffraction pattern fringe visibility fit to the intensity distribution The transverse coherence varies with the evolution of the FEL pulse along the undulator. The coherence is expected to increase continuously in the regime of exponential growth, because the amplification in an FEL favours the central mode TEM 00 , due to its best overlap with the electron beam. The situation is changed in the last part of the undulator, as the amplification process for the central mode begins to saturate. Other radiation modes, uncorrelated to the central mode, gain importance and a decreasing transverse coherence is expected. The total energy of the photon pulse and the energy fluctuations indicate an onset of the saturation after 11m. In the TTF FEL, it is not possible to extract the radiation from a given point in the undulator. The effective length of the undulator is varied as follows. The generation of FEL radiation depends critically on the overlap of electron and photon beam. By operating one of the horizontal steering magnets at maximum strength, the electron beam is kicked away from the ideal trajectory. Behind the selected steerer, the overlap of the electron and photon beams is too small to contribute to the FEL process. deviation due to the undulator = 10μm steerer deviation behind the activated steerer = 1mm electron trajectory electron trajectory photon trajectory quadrupoles x / mm y / mm -3 -2 -1 0 1 2 3 -3 -2 -1 0 1 2 3 -2 0 2 50 100 150 200 250 300 y / mm Intensity / arb. units -2 0 2 0 0.2 0.4 0.6 0.8 1 y / mm Visibility Response of the Ce:YAG crystal Spatial resolution of the Ce:YAG crystal Spatial resolution of the camera 0 0.2 0.4 0.6 0.8 1 Coherence (1mm) 10 -8 10 -6 10 -4 Pulse energy / J 7 8 9 10 11 12 13 14 10 20 30 40 50 60 70 Energy variation / % Undulator length / m photo- cathode RF gun photocathode laser superconducting accelerating module bunch compressor 2 bunch compressor 1 undulator beam separation energy measurement superconducting accelerating module booster cavity double slit fluorescent crystal camera -5 0 5 -0.5 0 0.5 1 x / mm Electric field amplitude of the left slit only (blue line) and of the right slit only (red line) -5 0 5 0 1 2 3 x / mm amplitude modulation (red line) and double slit diffraction pattern (blue line) -5 0 5 -1 -0.5 0 0.5 1 x / mm visibility of the double slit interference fringes, as a function of x (red line) and at the position of the maxima (circles) The intensity distribution behind a double slit can be calculated in the far field (or Fraunhofer) region by the well- known equation The visibility of the diffraction fringes is equal to the coherence C. If the far field condition is not fulfilled, Fresnel theory has to be employed. Generally, the diffraction patterns have to be computed numerically, However, at intermediate distances where , the diffraction pattern of the single slits can be computed in the far field approximation. The intensity distribution can then be computed as follows: where and AACHEN RHEINISCH-WESTFÄLISCHE TECHNISCHE HOCHSCHULE AACHEN More Information 0.5 1 2 3 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Slit separation / mm Coherence function J O IN T I N S T I T U T E F O R N U C L E A R R E S E A R C H
Transcript of Measurement of the Transverse Coherence of a VUV Free ... · The resulting images are subjected to...
Measurement of the Transverse Coherence of a VUV Free Electron LaserR. Ischebeck, J. Feldhaus, Ch. Gerth, E. Saldin, P. Schmüser, E. Schneidmiller, K. Tiedtke, M. Tonutti, R. Treusch & M. Yurkov
DESY
Image Processing
Double Slit Diffraction Patterns
Resulting Coherence Function
Evolution of Coherencealong the Undulator
The TTF Free Electron Laser
Simulations
Conclusion
CoherenceFree Electron Lasers (FELs) based on the self-amplification of spontaneous emission (SASE) have seen tremendous progress as radiation sources in the vacuum ultraviolet (VUV) wavelength regime. The TTF FEL at DESY has provided users with short pulses reaching gigawatt peak power. The facility is presently being extended to the soft X-ray regime and it will serve users in 2005, reaching wavelengths down to 6nm.Compared with state-of-the-art synchrotron radiation sources, FELs are characterized by a peak brilliance increased by up to eight orders of magnitude, a pulse length of 50 ... 100fs and by a high degree of transverse coherence. The investigation of these unique properties is of paramount importance for its broad range of applications.Here, a direct measurement of the transverse coherence of the TTF FEL is presented. To this effect, the diffraction pattern of a double slit has been
recorded and the visibility of the interference fringes has been recorded. The experimental near field diffraction pattern is compared with a numerical model, taking into account the formation of the FEL radiation, the Fresnel diffraction and effects of the experimental set-up.In contrast to optical lasers, which achieve a full transverse coherence by spatial filtering, SASE FELs amplify the incoherent spontaneous emission in a single passage of the amplifying medium. Since this amplification favors the central radiation mode, one expects nonetheless a high degree of transverse coherence.The evolution of the coherence along the passage has been studied by recording diffraction patterns at various lengths.
The transverse coherence of the TTF VUV Free Electron Laser has been measured directly with a double-slit experiment.To determine the evolution of the coherence as a function of the FEL amplification process, it has been measured as a function of the longitudinal position in the undulator.
The highest transverse coherence is achieved at the end of the exponential growth regime, before the onset of saturation in the FEL process. A further increase of the effective undulator length beyond saturation degrades the coherence.For user operation of the FEL, one has to weigh up the highest peak power with the best coherence properties.
The measurements described on this poster have been performed at the TTF free electron laser at DESY. This device uses a superconducting linear accelerator to produce an electron beam with a particle energy of 300MeV. During the passage of this beam through a permanent magnet undulator, electromagnetic radiation with a wavelength around 100nm is emitted.The interaction between electrons and the photon field results in the development of a micro-structure on the electron bunch. A large number of electrons emits radiation coherently and the radiation power grows exponentially with the longitudinal position in the undulator.
The TTF FEL has demonstrated - the performance of superconducting cavities at high gradient,- the pulsed operation of the cavities and the compensation of beam loading of long pulse trains,- the controlled beam transport with low emittance from the gun to the end of the accelerator,- the RF regulation concept to stabilise the field amplitude and phase,- a proof-of-principle of a SASE FEL in the VUV, and finally- first experiments using this radiation
The double slit diffraction patterns allow a straightforward determination of the transverse coherence, if the near field diffraction effects are taken into account.Diffraction patterns from different slit separations show the dependency of the transverse coherence on the distance.
Two methods are used to determine the transverse coherence from the measurement data:1) The fringe visibility, defined as
To determine the intensities, a central part of the diffraction pattern has been projected along the direction of the slits.The resulting curve has been smoothed with a Butterworth filter, reducing the pixel noise without degrading the modulation of the observed pattern. 2) A fit of the intensity distribution, computed in Fresnel theory. Seven parameters are fitted:- the transverse coherence- the wavelength of the radiation- the electric field amplitude at both slits- the angles of the wave front at the two slits- a transverse displacement of the pattern.The fit algorithm is based on the reflective Newton method on trust-regions.
The results of these two methods are in good agreement. They have been verified on diffraction patterns that have been simulated with numerical methods.
From the measurements shown above, the transverse coherence function is determined. It is shown in the following plot as a function of slit separation.From the generic value of 1 in the origin, the coherence function drops to values below 0.7 at 1 mm separation. For distances above 2 mm, it is as low as 0.2.Systematic uncertainties of the presented analysis procedures are due to the formation of the near field diffraction pattern, the scattering in the fluorescent crystal, the limited resolution of the optical system and to noise in the CCD image. These can be quantified with the simulations, which include these effects.
The formation of the FEL radiation in the undulator, the diffraction at the double slits and the propagation to the fluorescent crystal, the conversion of VUV photons to visible light and the imaging of the latter onto the CCD have been simulated.The resulting images are subjected to the same analysis routines as the measured data. The amplitude and phase of the computed electromagnetic field are precisely known. Thus, the theoretical value for the coherence function can be compared with the coherence that is determined from the images with the two analysis routines.That way, it is possible to gain information on the accuracy of the analysis.
Fresnel Diffraction at a Double Slit
Fachbereich Physik
Institut für Experimentalphysik
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The images have to be corrected for the non-linear response of the fluorescent Ce:YAG crystal and the finite resolution of the set-up.The response of the Ce:YAG crystal has been measured with the FEL: the light output has been compared to a multichannel plate detector, which has been calibrated previously.
There are two main factors that limit the resolution of the measured diffraction patterns: the fluorescent crystal and the optical system of the camera. Measurements to determine their point spread function have been done. There are methods to reconstruct images that have been acquired with a limited resolution. The Lucy-Richardson algorithm is described and tested on images acquired in a laboratory set-up. The careful use of this procedure allows to improve the images.
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corrected diffraction pattern fringe visibility fit to the intensity distribution
The transverse coherence varies with the evolution of the FEL pulse along the undulator.The coherence is expected to increase continuously in the regime of exponential growth, because the amplification in an FEL favours the central mode TEM00, due to its best overlap with the electron beam. The situation is changed in the last part of the undulator, as the amplification process for the central mode begins to saturate. Other radiation modes, uncorrelated to the central mode, gain importance and a decreasing transverse coherence is expected.The total energy of the photon pulse and the energy fluctuations indicate an onset of the saturation after 11m.
In the TTF FEL, it is not possible to extract the radiation from a given point in the undulator. The effective length of the undulator is varied as follows. The generation of FEL radiation depends critically on the overlap of electron and photon beam. By operating one of the horizontal steering magnets at maximum strength, the electron beam is kicked away from the ideal trajectory. Behind the selected steerer, the overlap of the electron and photon beams is too small to contribute to the FEL process.
deviationdue to theundulator = 10µm
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activatedsteerer = 1mm
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and at the position of the maxima (circles)
The intensity distribution behind a double slit can be calculated in the far field (or Fraunhofer) region by the well-known equation
The visibility of the diffraction fringes
is equal to the coherence C.If the far field condition is not fulfilled, Fresnel theory has to be employed. Generally, the diffraction patterns have to be computed numerically, However, at intermediate distances where , the diffraction pattern of the single slits can be computed in the far field approximation.The intensity distribution can then be computed as follows:
where
and
AACHENRHEINISCH-WESTFÄLISCHE TECHNISCHE HOCHSCHULE AACHEN
More Information
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