Laser beam parameter measurements at PITZ

1
Laser beam parameter measurements at PITZ

• J.Bähr
• Workshop on Laser Pulse Shaping
• DESY,Zeuthen, November 30, 2006

2
Outline

• 1. Introduction
• 2. Photocathode Laser
• 3. Laser beam-line and diagnostics
• 5. Outlook
• 6. Summary

3
1. Introduction

• PITZ photocathode laser
• Essentially sub-system of PITZ/FELs
• Determines essentially reachable transversal
  emittance
• Demands
• Pulse energy equivalent to gt 1nC (O(10 µJ))
• For 0.5QE 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
  56ps))
• Stability
• User friendly
• Designed and realized by I.Will et al. MBI

4
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

5
2. Photocathode laser
6
2. Photocathode laser

• Micropulses have flat-top shape
• duration 1525 ps (configurable)
• But
• edges t 56 ps
• too long
• fluctuation during the flat-top s 38
• ? too large

Courtesy I.Will (MBI)
Streak camera record of the UV output pulses
? RD
7
3. Laser Beam-line 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
• gt Maximize number of circular interference
• fringes

• Allow remote adjustment of the beam
• Include laser beam diagnostics

Length of beam-line 27 m
Proposed and calculated by I.Will
8
3. Laser beam-line Diagnostics

• Laser beam-line at PITZ length 27m

9
3. Laser beam-line Diagnostics

• Remotely controlled
• BSA
• 2 VirtualCathodes
• 6 mirrors
• Wedgeplates
• Pinhole (not used)

Whole beam-line
Wedge plates
10
3. Laser beam-line Diagnostics

• Diagnostics
• Transverse position
• Virtual cathode (VC)
• Quadrant diode (QD)
• Transverse shape
• Virtual cathode (VC)
• Longitudinal Profile
• Streak camera
• Later Optical sampling system (OSS)
• Laser pulse energy
• Photomultiplier first relative , later absolute
• Manual Joule meter

11
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,triggered

12
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

Hamamatsu S4349
13
3. Laser beam-line Diagnostics

• Quadrant diode
• Commissioning ongoing
• First methodical measurements
• s 10 µm of displacement jitter in plane of
  virtual cathode for 300 shots
• Pulse train 64 pulses

Measurement Y.Ivanisenko
14
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
  (M.Krasilnikov)

15
3. Laser beam-line Diagnostics

• Photomultiplier
• Problem dynamic range needed104
• 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
• Manual use
• ?
• Usable at different positions
• Foreseen Permanent absolute monitoring ? see
  talk M.Haenel (this workshop)

16
5. Outlook

• Aspheric lenses for improved illumination of Beam
  Shaping Aperture (BSA) homogeneity, efficiency
  (2007)
• Laser pulse energy monitoring by PM (individual
  pulse), absolute laser pulse energy
  pulse-to-pulse with diode calibration ? Marc
  Haenel
• Quadrant diode commissioning (ongoing)
• Continuous control of pulse shaper based on
  streak camera pulse profile measurement
  (Beginning 2007)
• Optical sampling system (I.Will et al.) for
  improved longitudinal pulse profile measurement
  (Jan.2007)
• TV-system on laser table (monitoring and
  readjustment)
• Mixing 2-channel laser system (fall/rise time
  2ps),2007

17
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
  (including parts of diagnostics)

• Laser beam-line essential for transverse laser
  beam shape control on cathode
• diagnostics tools as part of laser beam-line
• Beam position
• Shape,profiles (longitudinal/transverse),
• Pulse en ergy
• RD on diagnostics ongoing

Acknowledgement to Ingo Will for providing
slides
18

• End

19
4. Practical experience

• How to get laser on cathode?
• Adjust laser beam on photo cathode
• 4 degrees of freedom
• 2 conditions keep beam on inner mirror
• ? (use2 diaphragms
• or scintillating cathode)
• Two steps
• Laser off adjust diaphragms by viewing on
  illuminated cathode
• Aline laser beam trough diaphragms

• Avoid parasitic reflections!

20
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)
21
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 DOOCSShared memory (POSIX
  standard)

22
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

23
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

24
4. Practical experience

• How to get photo electrons ?
• Adjust laser beam on photo cathode (4 d.o.f. and
  2 conditions (inner mirror)) (use2
  diaphragms, scintillating cathode)
• Obey further (trivial) conditions rf, timing,
  laser intensity, cathode)
• Choice of proper phase ? phase scan

? Photo electrons!!
25
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

26
3. Laser beam-line Diagnostics
27
3. Laser beam-line

• Laser beam-line at PITZ length 27m

28
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 2030 mJ

• Micropulses have flat-top shape
• duration 1525 ps (configurable)
• But
• edges t 56 ps ? too long
• fluctuation during the flat-top s 38
  ? too large

Streak camera record of the UV output pulses
29
(No Transcript)
30
2. Photocathode laser
Courtesy I.Will (MBI)