Siegfried Schreiber, DESY email: [email protected] Overview of the laser system Running...

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


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)


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


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


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


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


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


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


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


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


Virtual cathode and iris aperture

Iris

Virtual cathode (Ce:YAG crystal, CCD

camera

Mirror Box

RF gun


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)


On cathode

• With additional magnification by 2 (total ~10) and remote controlled iris aperture

Nominal iris 3.0 mm diameter

Large iris (~open)


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.

)


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


• 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


Temperature Laser Room• Temperature stability < 0.02 dgC

- if people do not access laser room

3.5 days

0.05 dgC

6 months


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


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


Cathode

• Cathode: Cs2Te film on a molybdenum plug

• RF contact with silver coated Cu-Be spring


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…


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


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


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


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


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


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

Siegfried Schreiber, DESY email: [email protected] Overview of the laser system Running...

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