Post on 25-Dec-2015
Siegfried Schreiber, DESY
email: siegfried.schreiber@desy.de
• The TTF Laser System• Laser Material Properties• Conclusion?
Issues on Longitudinal Photoinjector Laser Pulse Shape
XFEL Beam Dynamics
27-Feb-2006, DESY
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY2
TTF specs
synchronized ~1 dg of RF cycle ~2 ps @1.3 GHz < 1 ps rms
longitudinal and transverse size
~5 dg == ~ 10 psfield uniformity ~ some mm
length 20 ps, Ø = 3 mm
charge of ~1 nC per bunch required
Cs2Te cathode QE ~ 1…10% (UV)
~1 µJ/pulse@UVfactor of ~10 overhead
long trains of pulses with low rep rate
trains 800 µs long with up to 7200 pulses (9 MHz) @ 10 Hz
General Design Issues of the Laser
• Photocathode the laser has to have similar transverse and longitudinal shapes as the required electron bunches
• With the given L-band RF gun, this already determines the basic properties of the laser pulses
• TTF as a sc accelerator has long bunch trains
• Suitable type of laser mode-locked solid-state system (synchronized oscillator + amplifiers + frequency converter to UV)
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY3
Laser System Overview
In cooperation of DESY and Max-Born-Institute, Berlin, I. Will et al., NIM A541 (2005) 467, S. Schreiber et al., NIM A445 (2000)
fround trip= 27 MHz
Modulators (AOM EOM AOM) 108 MHz 1.3 GHz 13.5 MHz
Faraday isolator
Piezo tuning of cavity length
Stabilized by quartz tubes Fiber-coupled
pump diodes
Pulse picker Pockels cell
Faraday isolator
Fast current control
Fast current control
Diode-pumped Nd:YLF Oscillator
Diode pumped Nd:YLF amplifiers
LBO BBO
Flashlamp pumped Nd:YLF amplifiers
Conversion to UVRelay imaging
telescopes
Pulse picker
Epulse= 0.3 µJ
Epulse= 6 µJ
Epulse< 0.3 mJ Epulse< 50 µJ
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY4
Pulse Train Oscillator (PTO)• Mode-locked pulsed oscillator
diode pumped (32 W)
• Synchronized to 1.3 GHz from the master oscillator, stabilized with quartz rods 1.3 GHz EO modulator with two AOM phase stability 0.2 ps rms pulse length 12 ps fwhh
• 27 MHz pulse train length 2.5 ms, pulsed power 7 W pulse picker up to 3 MHz
fround trip= 27 MHz
Modulators (AOM EOM AOM) 108 MHz 1.3 GHz 13.5 MHz Piezo tuning of
cavity length
Stabilized by quartz tubes Fiber-coupled
pump diodes
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY5
Chain of Linear Amplifiers• 2 diode pumped and 2 flashlamp pumped single pass amplifiers
• Fully diode pumped version is being tested now at PITZ, DESY Zeuthen
• Flashlamps: cheap, powerful (pulsed, 50 kW electrical/head) current control with IGPT switches allows flat pulse trains energy up to 300 µJ (1 MHz), 140 µJ (3 MHz)
• Laser diodes: 32 W pulsed, 805 nm end pumped through fibers energy from 0.3 µJ to 6 µJ/pulse
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY6
Pulse Trains
• Amplified laser pulse train - up to now 3 MHz possible, 9 MHz in preparation
Time
Am
plit
ud
e
800 µs
Output of the laser oscillator (27 MHz)
After amplification (1 MHz)
Electron beam pulse train (30
bunches, 1 MHz)
1.2 nC
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY7
Beam Arrival or Phase Stability
• Phase stability measured with electro-optic decoding technique: arrival time fluctuations 200 fs rms
• After acceleration to 450 MeV, dominant contributions: jitter of beam energy (magnetic chicane bunch compressors) and phase of laser in respect to linac rf
shotsA
rriv
al
Tim
e (
ps
)
-1.8
-
1.4
-1.0
-
0.6
-0.2
0 100 200 300
1 p
s
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY8
Time Bandwidth Limit
• From the uncertainty principle we have a limit in the time bandwidth product
Δt * Δv > K, K in the order of 1 (gaussian laser pulses = 0.44, cw mode-locked 0.36)
• Δt is the fwhh length of the pulse, Δv the fwhh gain bandwidth of the laserline at the frequency v
• For TTF, choice of laser crystal not only by bandwidth arguments, others like emission cross section, fluorescence lifetime, diode-pumping capability have been more important
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY9
Principle Scheme of a Laser Transition
• Example of a 4 level system, typical for high gain solid state lasers
Pump levelsUpper laser level,
metastable
Ene
rgy
Δv
Laser transition
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY10
Examples for laser materials in question
Laser material Bandwidth (GHz) Shortest pulse length
Nd:YAG 150 2.9 ps
Nd:YLF 350 1.3 ps
Nd:KGW 720 (2.73 nm) 0.6 ps
Ti:Sapphire dλ=400nm 2.5 fs, 15 fs achieved
Shortest pulse length obtainable using K=0.4
Siegfried Schreiber, DESY * XFEL Beam Dynamics 27-Feb-2006, DESY11
Laser Pulse Length and Shape
• VUV-FEL:Longitudinal shape is Gaussian
• Average over 50 gives L= 4.4 ± 0.1 ps (at 262 nm)
-20
0
20
40
60
80
100
120
0 10 20 30 40 50 60
amp
litu
de
Measured (UV, 262 nm)
Flat Top FitFWHM = 19-24 psrise/fall = 7-9 ps
Time (ps)
• PITZ:Longitudinal flat-hat shape
• Works fine in ‘lab environment’, not yet mature for the VUV-FEL
Time (ps)0 10 20 30 40 50