Laser beam parameter measurements at PITZ

Laser beam parameter measurements at PITZ

J.Bähr

Workshop on Laser Pulse Shaping

DESY,Zeuthen, November 30, 2006

30/11/2006 Workshop on Laser Pulse Shaping, Zeuthen J.Bähr, DESY, PITZ

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Outline

1. Introduction2. Photocathode Laser3. Laser beam-line and diagnostics5. Outlook 6. Summary


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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


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2. Photocathode laser

Similar to FLASH photocathode laser Differences: realized at

PITZ Fully laser diode

pumped Pulse shaper:

longitudinal flat top profile

Wavelength: 262 nm

Time scheme of PITZ/FLASH laser


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mainpulse picker

attenuatorgrating

pulse shaper2-stage

preamplifiermodelocked

oscillator2-stage double-

pass preamplifier

integratedauxiliary

pulse picker

wavelengthconversion

IR -> UV

diode-pumpedbooster

(2 stages)

outputpulses

photo-diode

#1

photo-diode

#2

protectionsystem


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Micropulses have flat-top shape: duration: 15…25 ps

(configurable)

But: edges: = 5…6 ps

too long

fluctuation during the flat-top: = 3…8%

too large

Streak camera record of the UV output pulses

Courtesy I.Will (MBI)

R&D


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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:=> Maximize number of circular interferencefringes

Proposed and calculated by I.Will

Allow remote adjustment of the beam

Include laser beam diagnostics

Length of beam-line 27 m


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3. Laser beam-line& Diagnostics

Laser beam-line at PITZ: length 27m


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3. Laser beam-line & Diagnostics

Whole beam-line

Wedge plates

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


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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


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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


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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


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Quadrant diode Commissioning ongoing First methodical

measurements: σ = 10 µm of

displacement jitter in plane of virtual cathode for 300 shots

Pulse train: 64 pulses Measurement Y.Ivanisenko


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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)


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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 Manual use

Usable at different positions

Foreseen: Permanent absolute monitoring see talk M.Haenel (this workshop)


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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


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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

R&D on diagnostics ongoing

Acknowledgement: to Ingo Will for providing slides


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End


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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

(use:2 diaphragms

or scintillating cathode)

Two steps:

1. Laser off; adjust diaphragms by viewing on illuminated cathode

2. Aline laser beam trough diaphragms

•Avoid parasitic reflections!


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

gratingpulse shaper

3-stagepreamplifier

modelockedoscillator

auxiliarypulse picker

mainpulse picker


IR -> UV

flashlamp-pumpedbooster

(2 stages)

outputpulses

photo-diode

#1

photo-diode

#2

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

mainpulse picker

attenuatorgrating

pulse shaper2-stage



pass preamplifier

integratedauxiliary

pulse picker


IR -> UV

diode-pumpedbooster

(2 stages)

outputpulses

photo-diode

#1

photo-diode

#2

protectionsystem

diode-pumped boosters (new system):


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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)


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Remotely controlled elements BSA Pinhole 1 switchable mirror 2 cameras

16 axes: rotation or translationAdditional elements on laser diagnostics trolleyElectronics elements and electronics: MICOS (H.Henschel)Controls: LAbView (M.Winde)Mechanics (H.Luedecke) 3(4) mirrors


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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


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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!!


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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


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3. Laser beam-line & Diagnostics Properties of laser beam

Property Measurement Tuning Pulse number oscilloscope Laser control Pulse intensity a) power meter Attenuator on laser

table b) PM relative

monitoring

Transverse profile a) virtual cathode VC1, VC2 (for

diff. pulse energy)

b) TV system on laser table

Inclination of converter crystals

Laser beam inclination on laser

table

TV system on laser table

Mirror on laser table

Laser beam diameter VC1, VC2 Beam shaping aperture

Beam on virtual cathodes

VC1, VC2 x,y drive

Longitudinal profile Streak camera Pulse shaper Laser beam position Quadrant diode M4, wedge plates Intensity envelope of

pulse train oscilloscope Laser control


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3. Laser beam-line

Laser beam-line at PITZ: length 27m


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Capabilities of the laserLaser generates trains of picosecond UV pulses:

typ. duration of the pulse train: 800 s

repetition rate within the train: f = 1 MHz

energy of the micropulsesat = 262 nm (UV): Emicro = 20…30 J

Micropulses have flat-top shape:

duration: 15…25 ps (configurable)

But: edges: = 5…6 ps too long fluctuation during the flat-top:

= 3…8% too large

Streak camera record of the UV output pulses



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mainpulse picker

attenuatorgrating

pulse shaper2-stage



pass preamplifier

integratedauxiliary

pulse picker


IR -> UV

diode-pumpedbooster

(2 stages)

outputpulses

photo-diode

#1

photo-diode

#2

protectionsystem


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Laser beam parameter measurements at PITZ

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