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Laser system at PITZ. J.Bähr LCLS Injector Commissioning Workshop SLAC, October 9-11, 2006. Outline. 1. Introduction 2. Photocathode Laser 3. Laser beam-line and diagnostics 4. Practical experience 5. Outlook 6. Summary. 1. Introduction. PITZ photocathode laser

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laser system at pitz

Laser system at PITZ

J.Bähr

LCLS Injector Commissioning Workshop

SLAC, October 9-11, 2006

outline
Outline

1. Introduction

2. Photocathode Laser

3. Laser beam-line and diagnostics

4. Practical experience

5. Outlook

6. Summary

ICW Workshop SLAC J.Bähr, DESY, PITZ

1 introduction
1. Introduction
  • PITZ photocathode laser
    • Essentially sub-system of PITZ/FELs:
      • Determines essentially reachable transversal emittance
    • Demands:
      • Pulse energy: equivalent to > 1nC (O(10 μJ)):
        • For 0.5%QE and 1nC need 1μJ at cathode and ~10 μJ at laser
      • Flat-top laser beam transversal profile
      • Flat-top laser beam longitudinal profile (20 ps FWHM rise/fall time: later 2ps (now 5…6ps))
      • Stability
      • User friendly
    • Designed and realized by I.Will et al. MBI

ICW Workshop SLAC J.Bähr, DESY, PITZ

2 photocathode laser
2. Photocathode laser

Time scheme of

PITZ/FLASH laser

  • Similar to FLASH photocathode laser
    • Differences: realized at PITZ
      • Fully laser diode pumped
      • Pulse shaper: longitudinal flat top profile
  • Wavelength:262 nm

ICW Workshop SLAC J.Bähr, DESY, PITZ

2 photocathode laser1
2. Photocathode laser

ICW Workshop SLAC J.Bähr, DESY, PITZ

2 photocathode laser2
2. Photocathode laser

Micropulses have flat-top shape:

  • duration: 15…25 ps (configurable)

But:

  • edges: t = 5…6 ps
    • too long
  • fluctuation during the flat-top:s = 3…8%

 too large

Courtesy I.Will (MBI)

Streak camera record of the UV output pulses

 R&D

ICW Workshop SLAC J.Bähr, DESY, PITZ

3 laser beam line diagnostics
3. Laser Beam-line & Diagnostics
  • Allow remote adjustment of the beam
  • Include laser beam diagnostics
  • Goal:
    • Create flat-top transverse laser profile
      • Illuminate the Beam Shaping Aperture (BSA) with a laser beam (diameter larger than BSA)
      • Imaging of BSA onto photocathode
    • Best focus:

=> Maximize numberof circular interference

fringes

  • Length of beam-line 27 m

Proposed and calculated by I.Will

ICW Workshop SLAC J.Bähr, DESY, PITZ

3 laser beam line diagnostics1
3. Laser beam-line & Diagnostics
  • Remotely controlled :
  • BSA
  • 2 VirtualCathodes
  • 6 mirrors
  • Wedgeplates
  • Pinhole (not used)

Whole beam-line

Wedge plates

ICW Workshop SLAC J.Bähr, DESY, PITZ

3 laser beam line diagnostics2
3. Laser beam-line& Diagnostics
  • Virtual cathodes
    • Measure:
      • Laser beam size
      • Laser beam profile
      • Laser beam position
      • Two virtual cathodes for different pulse energy ranges
      • Camera type:

JAI M10RS

      • UV sensitivity
      • Progressive scan mode

ICW Workshop SLAC J.Bähr, DESY, PITZ

3 laser beam line diagnostics3
3. Laser beam-line & Diagnostics
  • Quadrant diode
    • Goal:
      • Measure laser beam position in the cathode plane with temporal resolution pulse to pulse (1 μs)
      • In preparation: S.Korepanov et al.
      • Mounted near to VC1, on the same movable stage

ICW Workshop SLAC J.Bähr, DESY, PITZ

3 laser beam line diagnostics4
Photomultiplier

Problem : dynamic range needed:10**4

Laser pulse energy measurement (relative) in first step

HV-power supply integrated

UV-sensitive

Type: H6780-03

Is commissioned, calibration ongoing

Pulse to pulse measurement

Pulse energy (absolute) measurement

Absolute

Device not fixed, mobile

Usable at different positions

Foreseen:Permanent absolute monitoring

3. Laser beam-line & Diagnostics

ICW Workshop SLAC J.Bähr, DESY, PITZ

3 laser beam line diagnostics5
3. Laser beam-line & Diagnostics
  • Streak camera
    • Measure longitudinal laser pulse profile
    • 2nd branch of the laser beam-line: image of crystal plane onto entrance slit of streak camera
    • Synchro-scan camera
    • C5680 (Hamamatsu)
    • UV sensitive
    • Resolution 2 ps
    • Longitudinal profile controlled by pulse shaper
    • Near future: continuous measurement and subsequent control of pulse shaper

ICW Workshop SLAC J.Bähr, DESY, PITZ

4 practical experience
4. Practical experience
  • Avoid parasitic reflections!
  • How to get laser on cathode?
    • Adjust laser beam on photo cathode:
      • 4 degrees of freedom
      • 2 conditions: keep beam on inner mirror

(use:2 diaphragms

or scintillating cathode)

Two steps:

    • Laser off; adjust diaphragms by viewing on illuminated cathode
    • Aline laser beam trough diaphragms

ICW Workshop SLAC J.Bähr, DESY, PITZ

5 outlook
5. Outlook
  • Program for keeping laser beam on inner mirror
  • Aspheric lenses for improved illumination of Beam Shaping Aperture (BSA): homogeneity, efficiency
  • Laser pulse energy monitoring by PM (individual pulse) and laser power meter
  • Quadrant diode commissioning
  • Continuous control of pulse shaper based on streak camera pulse profile measurement
  • Optical sampling system (I.Will et al.) for improved longitudinal pulse profile measurement
  • TV-system on laser table (monitoring and readjustment)
  • Mixing 2-channel laser system (fall/rise time 2ps)

ICW Workshop SLAC J.Bähr, DESY, PITZ

6 summary
Laser beam parameters essential for operation and optimization of FLASH and XFEL (minimization of transverse emittance)

PITZ is a test bench for photocathode laser development

PITZ laser steadily supported and upgraded by MBI

Laser beam-line essential for transverse laser beam shape control on cathode

diagnostics tools as part of laser beam-line: position, shape, intensity

6. Summary

Acknowledgement: to Ingo Will

for providing slides

ICW Workshop SLAC J.Bähr, DESY, PITZ

slide16
End

ICW Workshop SLAC J.Bähr, DESY, PITZ

completely diode pumped laser operational at pitz differences to the previously used system
completely diode-pumped laser operational at PITZ: - differences to the previously used system -

flashlamp-pumped boosters: (old system)

preamplifier has 6 passes(compared to three passes in the old system)

attenuator for tuning

the energy

of the UV output pulses

safety detector for protection

of the wavelength conversion crystals

in case of malfunction of the laser

diode-pumped boosters (new system):

ICW Workshop SLAC J.Bähr, DESY, PITZ

control program of the laser based on the standard motif environment
Control program of the laser based on the standard MOTIF environment
  • Control system programmed in C++ following ISO/ANSI standard ISO/IEC 14882-1998
  • Standard SUN C++ compiler(SUN Workshop Pro 6.02)
  • GUI written with Xt library and MOTIF
  • only C++ and ANSI C
  • additional libraries:
    • Athena Plot widget (open source)
    • Spinbox widget (open source)
  • No other expensive frameworks used.
  • Communication with DOOCS:Shared memory (POSIX standard)

ICW Workshop SLAC J.Bähr, DESY, PITZ

slide19
ICW Workshop SLAC J.Bähr, DESY, PITZ
3 laser beam line diagnostics6
3. Laser beam-line & Diagnostics
  • Remotely controlled elements
    • BSA
    • Pinhole
    • 1 switchable mirror
    • 2 cameras

16 axes: rotation or translation

Additional elements on laser diagnostics trolley

Electronics elements and electronics: MICOS (H.Henschel)

Controls: LAbView (M.Winde)

Mechanics (H.Luedecke)

    • 3(4) mirrors

ICW Workshop SLAC J.Bähr, DESY, PITZ

4 practical experience1
4. Practical experience
  • Reactions after power cut (only laser)

- of course : facility depending

    • All frequencies available for laser, master oscillator running?
    • Proper control file of laser loaded
    • Proper timing relative to rf
    • Event trigger available
    • Elements of laser beam line in proper position after zero-position run
    • Remote control of laser properly working

ICW Workshop SLAC J.Bähr, DESY, PITZ

4 practical experience2
4. Practical experience
  • How to get photo electrons ?
    • Adjust laser beam on photo cathode (4 d.o.f. and 2 conditions (inner mirror)) ; (use:2 diaphragms, scintillating cathode)
    • Obey further (trivial) conditions: rf, timing, laser intensity, cathode)
    • Choice of proper phase  phase scan

 Photo electrons!!

ICW Workshop SLAC J.Bähr, DESY, PITZ

4 laser beam line diagnostics
4. Laser beam-line: Diagnostics
  • TV-System on Laser table
    • Goal:
      • Measure:
        • Laser beam position
        • Laser beam inclination
        • Transverse laser beam profile
        • Control laser beam parameter
        • Help for re-adjustment
        • Status: in preparation, commissioning: fall 2006

ICW Workshop SLAC J.Bähr, DESY, PITZ

3 laser beam line diagnostics7
3. Laser beam-line & Diagnostics

ICW Workshop SLAC J.Bähr, DESY, PITZ

3 laser beam line
3. Laser beam-line
  • Laser beam-line at PITZ: length 27m

ICW Workshop SLAC J.Bähr, DESY, PITZ

capabilities of the laser
Capabilities of the laser

Courtesy I.Will (MBI)

Laser generates trains of picosecond UV pulses:

  • typ. duration of the pulse train: 800 ms
  • repetition rate within the train: f = 1 MHz
  • energy of the micropulsesat l = 262 nm (UV): Emicro = 20…30 mJ

Micropulses have flat-top shape:

  • duration: 15…25 ps (configurable)

But:

  • edges: t = 5…6 ps too long
  • fluctuation during the flat-top:s = 3…8% too large

Streak camera record of the UV output pulses

ICW Workshop SLAC J.Bähr, DESY, PITZ

slide27
ICW Workshop SLAC J.Bähr, DESY, PITZ
2 photocathode laser3
2. Photocathode laser

Courtesy I.Will (MBI)

ICW Workshop SLAC J.Bähr, DESY, PITZ