siegfried schreiber, desy email: [email protected] overview of the laser system running...
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Siegfried Schreiber, DESY
email: [email protected]
Overview of the laser system
Running experience
Remarks
LCLS Injector Commissioning Workshop (ICW) October 9-11, 2006
Drive-Laser Experience at FLASH
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
The Photocathode RF Gun
laser beamlaser beam
e- beame- beam
CathodeCathode
• L-band rf gun (1.3 GHz)• Pulsed 5 or 10 Hz• RF pulse length up to 900 µs• RF power 3.2 MW or
42 MV/m max gradient
• Cs2Te cathode
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Actual specs
synchronization <1 dg of RF cycle <2 ps @1.3 GHz < 1 ps rms
longitudinal and transverse size
~5 dg RF → ~10 psfield uniformity → ~mm
length 20 ps, Ø = 3 mm
charge ~1 nC per bunch
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
• L-band RF gun + requirement on e- bunches determines transverse and longitudinal shape of laser, synchronization
• Photocathode: work function, QE determines wavelength, energy • Superconducting accelerator long bunch trains
→ Laser average power in the Watt range• Suitable type of laser mode-locked solid-state system (MOPA:
synchronized oscillator + amplifiers + frequency converter to UV)
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Laser System OverviewNot a commercial system, cooperation DESY/Max-Born-Institute, Berlin
Conservative design: operationable in a ‘user’ environment
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
IR→ UVRelay imaging
telescopes
Pulse picker
Epulse= 0.3 µJ
Epulse= 6 µJ
Epulse< 0.3 mJ
Epulse< 50 µJ
Remote controlled attenuator
Double pulse generator
Beam shutter
Imaging to the
cathode
Remote controlled mirror box
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Pulse Train Oscillator (PTO)
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
• Mode-locked pulsed oscillator Diode pumped (32 W)
• Synchronized to 1.3 GHz master oscillator 1.3 GHz EO modulator with 2 AOMs (108 + 13.5 MHz) Phase stability < 300 fs rms Pulse length 12 ps (fwhh) (IR)
• Stabilized with quartz rods Thermal expansion coefficient
fused quartz = 0.59 ppm/K (Al = 24 ppm/K)
• 27 MHz pulse train Train length 2.5 ms, pulsed power 7 W
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Chain of Linear Amplifiers
• 2 diode pumped and 2 flashlamp pumped single pass amplifiers• Fully diode pumped version is running at PITZ
• Flashlamps pumped heads: cheap, powerful (pulsed, 50 kW electrical/head) current control with high power IGPT switches allows flat pulse trains energy up to 300 µJ (1 MHz), 140 µJ (3 MHz) small-signal gain = 20 extractable peak power 1.2 kW, duty cycle 2%
• Laser diodes: 32 W pulsed, 805 nm end pumped through fibers energy from 0.3 µJ to 6 µJ/pulse
5 mm
Fluorescence profile
Xe flashlampsNd:YLF rod, Ø 5 or 7 mm
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Pulse Trains
• 2 Pulse pickers, based on Pockels cell + polarizer → up to 3 MHz
• Preamplification (diodes) of 1.2 ms long train• Power amplification with variable bunch pattern
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 * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Bunch Pattern and MPS
• Change bunch pattern on user request Number of bunches Different bunch frequencies: 1 MHz, 250 kHz, 100 kHz and others
• Realized with an FPGA based controller producing the appropriate trigger for the Pockels cell
• The controller is also the interface of the machine protection system to the laser
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Lasing with long bunch trains
1 MHz
599 bunches
50 kHz
100 kHz
500 kHz
• This year: up to 600 bunches to beam dump 1 MHz, 500 kHz, 100 kHz, and 50 kHz
• Lasing with at least 450 bunches• Problems:
Toroid protection system not yet in operation, emulated by software
Improvements required for photon diagnostics Beam loading compensation was at limit for
ACC2/3 and ACC1 for 800 µs flat top Activation of beam line components due to
darkcurrent, mostly from rf gun
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Laser beamline
Compensation of path length addition in double pulse generator by telescope
• Relay imaging with spatial filtering• Hard edge aperture after diode pumped amplifiers• Aperture imaged to → amplifier heads → doubling crystals →
cathode
• Transverse profile not really flat hat
• Still noticeable pointing jitter (~10 % of spot size)
• Achieved good pointing stability with an additional iris in front of vacuum window (70 cm from cathode)
• Paid with interference fringes (20 % modulation)
• For the present FLASH running scheme: stability is more important than perfect beam shape
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
UV diagnostics/doubler
variable attenuator telescope x2
joulemeter
Movable mirrors/ splitters
prism
UV Photo-diode
λ/2 wave plate
polarizing splitter
to rf gun
Streak Camera
Double Pulse
Generator
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Virtual cathode and iris aperture
Iris
Virtual cathode (Ce:YAG crystal, CCD
camera
Mirror Box
RF gun
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-20061
2
3
1
2
3
0
0.5
1
y,mmx,mm
Transverse Laser Pulse Shape
4.0 mm
4.4
mm
on cathode (no iris) magnification ~ 5
exit BGO
1.6
mm
1.3 mm
• Transverse shape of the UV laser pulses is not TEMoo
• Intentionally closer to flat hat to avoid spatial hole burning along the pulse train
PITZ shaping (on cathode)
Exit amplifers (IR)
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
On cathode
• With additional magnification by 2 (total ~10) and remote controlled iris aperture
Nominal iris 3.0 mm diameter
Large iris (~open)
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Laser Pulse Length and Shape
Longitudinal shape is Gaussian • Measurement with a streak
camera (FESCA 200) 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)
• R&D laser at PITZ:Longitudinal flat-hat shape
Works fine in ‘lab environment’, not mature for a user facility
Present laser technology does not allow shorter rise/fall times
• New development at MBI ongoing
Time (ps)0 10 20 30 40 50
Pho
ton
dens
ity (
a.u.
)
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Vacuum mirror
• Best would be to have high quality dielectric mirror
• However, darkcurrent may charge it up
• Charging and discharging effects beam orbit and destroys the mirror
→ after some tries with Al shielding we have now a good quality solid aluminum mirror (R=90%)
→ no long time experience with long pulse trains yet (heating due to absorption, ablation)
TTF1 dielectric mirror with
discharge traces
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
• Laser is fully integrated into the control system – say almost, still pieces missing (rf phases, BBO, diodes)
• Controlled by a Sparc cpu running Solaris in a VME crate, mixed standard (timing, ADCs) and special equipment (ns delays, …)
Server running real time operations and controls Server handing over parameters to controls Some other servers for specific tasks
• Interlocks controlled by an SPS system with an interface to controls (cpu, cooling water flow, temperatures, door contacts etc)
SPS controlled special mode to allow laser in tunnel while having access
• Operators have easy control of the laser with options of detailed access to parameters
Easy: number of laser pulses, bunch frequency, attenuator Expert: timing, pump parameters, feedback parameters etc
Controls
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Temperature Laser Room• Temperature stability < 0.02 dgC
- if people do not access laser room
3.5 days
0.05 dgC
6 months
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Charge Stability
Stability = 1.4% rms
• Problem: to maintain a stability < 2 % rms fine tuning of phase matching angle of the frequency conversion crystal green to UV (BBO) often required
• If not tuned frequently, stability drops to 5 % rms
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
RF Gun llrf with SIMCON
• Amplitude and phase regulation by calculating the vector sum of forward and reflected power
• FPGA based controller in operation since Dec 2005
• Phase stability of 0.14 dg of 1.3 GHz or 300 fs achieved1 dg of 1.3 GHz = 2.1 ps
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Cathode
• Cathode: Cs2Te film on a molybdenum plug
• RF contact with silver coated Cu-Be spring
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
QE maps
78.178.1 73.1
• QE map: scan with small laser spot (200 µm) over the cathode (step size 300 µm)
• useful to diagnose status of cathode uniformity • Example of a non-uniform cathode → We need to consider the
uniformity of both, the laser beam profile and the cathode QE• May need a feedback based on beam profiles…
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Alignment of Laser
• Initial alignment of laser beam onto the cathode using a scintillator mounted into the plug
• Fine adjustment with Beam based alignment (electron beam position as a
function of rf phase for low charge, solenoids off)→ time consuming (2 shifts) and difficult, but yields center in respect to rf, precise
Scan laser over cathode→ scan is fast, but assumes that cathode film is centered in respect to the rf, correction less accurate
• Virtual cathode helps to recover alignment and to monitor movements and pointing
• However, for high QE cathodes we need an intensified and gated camera to have a permanent monitoring
darkcurrent ring may also be used for rough alignment
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Quantum Efficiency
• Example of a QE measurement:charge at rf gun exit as a function of laser pulse energy
• QE = 4.8 % in this case
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Cathode lifetime
0
0.5
1
1.5
2
2.5
3
3.5
4
13-Mar-06 19-Mar-06 25-Mar-06 31-Mar-06 6-Apr-06
date
QE (
%)
73.1
mean 73.1
CW_73.1
End of lifetime QE < 0.5%
End of lifetime QE < 0.5%
• Lifetime 6 months, early 2006 we observed shorter lifetimes of 2 months
• Problem for the laser: large change in output required
• Variable attenuator useful in order not to change laser operational parameters of the laser too much
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Long Term Running
• TTF phase 1: 12/1998 to 12/2002→ 4 years of running (36,000 h or 13•107 s)
oscillator 25,000 h or 9.2•107 shots, amplifiers 16,000 h or 6.4•107 trains @ 1 Hz
Total on-time oscillator 70%, amplifiers 50%
• TTF phase 2: upgrade end of 2003: diode pumped oscillator and preamplifiers
• FLASH: Running since 3/2004 with 2 and now 5 Hz about 3•108 shots/trains with 20 pulses av. delivered for
beam Total on-time == always,
even mostly during the maintenance days (4 %), off during maintenance weeks (~6 per year)
• Backup system, fully diode pumped soon
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Maintenance
• All components need maintenance and have to be replaced from time to time
Flashlamps: life time 1 to 5 107 shots (1 month = 1.3 107) Pockels cell drivers: roughly once per year Flashlamp power supplies: once per 3 years Laser crystals: once per 5 years LBO/BBO: once per 5 years Mirrors: no change required yet Control hardware: frequent changes, cpu, ADCs, other
boards Diode lasers: no exchange yet (20,000 h now) Cooling water/equipment: often Component checks once per week, occasional adjustments
of timing and other parameters
Siegfried Schreiber, DESY * LCLS Injector Commissioning Workshop (ICW) * 9/11-Okt-2006
Conclusive remarks
• Good experience with the present laser system Advantage of collaboration with one laser institute: permanent support
and ongoing development to your needs, no overhead Risk: no quick alternative if cooperation ceases However, similar argument holds for companies in case of complex laser
systems
• Design philosophy As simple as possible (avoid components which require frequent
adjustments) ‘Ready-to-sell’ finish and robust Use well-known technology (laser material, pump sources) Fully integration into the control system as a must
• Capability of long running in accelerator environment is proven• Laser room and laser development are separated:
laser is part of the machine (access only for maintenance/repair)• Diagnostics as complete as possible to assist operators