Photon Diagnostics. FLASH User Workshop 08.

Transcription

1 Photon Diagnostics FLASH User Workshop 08

2 Outline What kind of diagnostic tools do user need to make efficient use of FLASH? intensity (New GMD) beam position intensity profile on the sample / focus size (wave front sensor) temporal radiation pulse profile and coherence (autocorrelator) spectral distribution (VLS, photoelectron spectrometer, etc. ) Objectives of this talk is to describe the recent developments, design concepts and prototypes

3 Photon beam diagnostics at FLASH Experimental hall Beam distribution area Gas attenuator Old diagnostics tools Intensity and beam position Spectral distribution Tunnel section Detector and aperture units 1&2)

4 Tunnel section Detector Unit F2 (Apertures, Detectors, mirror) Intensity + beam profil + diffraction (coherence) +deflection into spectrometer Detector Unit F1 (Apertures, Detectors) Intensity + beam profil + double slit (coherence) FEL Elektronen Beamline for the synchrotron radiation of the dipol magnet grading spectrometer with intensified CCD Single shot spectrum

5 BDA Beam distribution in the optical enclosure of the experimental hall beam shutter fast shutter mirror chamber 1 beam switching mirrors beam intensity and position monitor gas attenuator

6 Gas Monitor Detector: From experimental setup to reliable tools Photon Beam Intensity simple principle online absolute, calibrated pulse resolved nondestructive Photon Beam position Single photoionisation: N = N ph x n x σ x l N = number of electrons or ions N ph = number of photons n = target density σ = photoionisation cross section l = length of interaction volume In collaboration with PTB in Berlin and IOFFE institute St. Petersburg

7 Beam position monitor left electrode Ion current Moved the device by 0.5 mm right electrode I(L) / I(R) The BPM information can be used for a machine feedback in order to stabilise the beam In collaboration with PTB in Berlin and IOFFE institute St. Petersburg

8 Layout at FLASH Undulator GMD x GMD y Attenuator GMD x GMD y Mirrors Experiment GMD Intensity directly behind the undulator for operators Intensity after attenuator for experiments Intensity at / right behind experiment (mirror transmission) Position and pointing vector All values are stored in the DAQ Experiments often need additionally analogue signal The GMD is a proven tool and the service most asked-for by the experiments The detector Software architecture In collaboration with PTB in Berlin and IOFFE institute St. Petersburg

9 NEW GMD 2nd generation GMD just under commissioning Enhanced sensitivity (open electron multiplier) Robust and reliable hardware design Easy to handle like commercial detector Can be used to determine the intensity in front or behind user experiments 2nd gen. NewGMD In collaboration with PTB in Berlin and IOFFE institute St. Petersburg

10 Spectral distribution Radiation pulse length τ rad : τ rad 2π 1/2 /(Δω) FWHM M2: VLS grating Collaboration with:r. Reininger (SAS), F. Quinn (CLRC) et al.

11 Online spectrometer for single pulses Collaboration with: BESSY, R. Reininger (SAS), F. Quinn (CLRC) et al.

12 Alternative concept: i- and e- TOF spectrometer One can use the Ion and Electron TOF data to pinpoint the photon energies. Electron TOF spectrum for O 2, taken in July 2007 at FLASH by Wellhoefer et al. at a 13.9 nm photon wavelength. Ion Photoionization Ratio Spectrum for He from Bartlett et al., Physical Review A 46, 5574 (1992) Collaboration with University of Hamburg, PTB

13 Online determination of the spectral distribution using i- and e- TOF spectrometer 2 + Peak Area (A2) 1 + Peak Area (A1) signal time A2 A1 ratio ev I-TOF Photoionization Spectrum at Some Photon Energy Double-to-Single Photoionization Cross-Section Ratio Collaboration with University of Hamburg, PTB

14 Online determination of the spectral distribution using i- and e- TOF spectrometer High intensity Medium intensity Low intensity Electron TOF spectra for O2, taken in July 2007 at FLASH by Wellhoefer et al. at a 13.9 nm photon wavelength. E-TOF spectra peaks shift with different FEL pulse intensities. Collaboration with University of Hamburg, PTB

15 Online determination of the spectral distribution using i- and e- TOF spectrometer e-tofs I-TOF Gas Inlet The test chamber for the Online Spectrometer development. First tests in Oct Collaboration with University of Hamburg, PTB

16 COMPACT SPECTROMETER FOR FLASH Luca Poletto and Fabio Frasetto, CNR-INFM, Padova (Italy) The main requirements are Spectral range 1-40 nm Spectral resolution better than 0.25% Maximum acceptance angle 10 mrad High dynamic range (up to 104) Compact design Optical design The spectrometer consists of an entrance slit, a spherical grating with variableline-spacing (VLS) with flat focal plane and a detector. The entrance slit is placed at a distance of 1-3 m from the FEL focal point on the experimental chamber. Collaboration with CNR-INFM, Padova (Italy)

17 Temporal coherence and pulse duration Two Autocorrelator set-ups The FEL radiation is split and directed under grazing incidence over a set of fixed and a set of position-variable mirrors, respectively, before being recombined. R. Mitzner, H. Zacharias et al. 2008

18 Temporal coherence and pulse duration First commissioning Temporal coherence Pulse duration with nonlinear autocorrelation Fringe visibility as a function of path difference between two beams Nonresonant two-photon double ionization of He R. Mitzner, H. Zacharias et al. 2008

19 Lateral intensity- Focus determination Saturation of ion signals due to vanishing targets: Determination of FEL beam size and waist Ne + hν = 38 ev Beamline FLASH focus diameter: (26 ± 3) µm focus position N + ( N ph ) 1 e σ = N N ph A fit cross section σ is known photon number N ph is measured beam cross section A is derived A.A. Sorokin et al., Appl. Phys. Lett. 89, (2006)

20 Lateral intensity Focus determination Method based on ablation of PMMA Single-Shot Determination (but post mortem) FEL beam imprint (AFM measurement) F [a.u.] y [μm] X crossection Y crossection threshold level Reconstructed beam profile x [μm] Chalupsky et al., Opt. Express, 15, 6036 (2007)

21 Lateral intensity Focus determination/optimisation More convenient approach: Wavefront sensor Wave front sensor soft- and hardware by Imagine Optic CCD: field of view = 19.5 x 19.5 mm 1340 x 1300 pixels Hartmann plate: 51 x 51 quadratic holes tilted by 25 to prevent interference of adjacent holes camera image camera image zoom The actual beam is compared to a perfect spherical wave Courtesy Pascal Mercère, SOLEIL

22 File: himg Before adjusting ellipsoidal mirror of BL2 defocus 100mm radius 164.9µm defocus 50mm radius 87.8µm defocus 0mm radius 42.3µm Rotation: 0 Yaw: 0 PV: 110nm rms: 27 nm λ/1 defocus -50mm radius 88.9µm defocus -100mm radius 166.0µm

23 File: himg After adjustment of BL2 defocus 100mm radius 165µm defocus 50mm radius 84.1µm Rotation: Yaw: PV: 18 nm rms: 3 27 nm λ/9 defocus 0mm radius 24.1µm defocus -50mm radius 85.2µm defocus -100mm radius 166µm

24 Compact wavefront sensor of Laser Lab Göttingen Very compact design for use behind user experiments Laser drilled Hartmann plate 7µm Al-Folie 1064nm, ~100mJ 320µm hole pitch Approx. 50µm hole diameter Image on camera Calculated wavefront Collaboration with Laser Lab Göttingen

25 Schedule New GMD spring 2009 VLS summer 2009 TOF Spectrometer (first test) october 2008 Compact spectrometer spring 2009 Compact wavefront sensor ~2009 Autocorrelator (direct beamlines) ~2009

26 Thanks to all our partners: PTB, Berlin IOFFE Institute, St. Petersburg CLRC, Daresbury University of Hamburg BESSY, Berlin University of Münster CNR-INFM, Padova LASER Lab Göttingen

27 End