Siegfried Schreiber, DESY

email: siegfried.schreiber@desy.de

The Laser SystemLaser Beam PropertiesOperational issuesOutlook

Photoinjector Laser

Machine Advisory Committee Meeting

Nov 8/9th , 2004, DESY

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY2

trains 800 µs long with up to 7200 pulses (9 MHz) @ 10 Hz

long trains of pulses with low rep rate

< 1 ps rms~2 ps @1.3 GHz~1 dg of RF cyclesynchronized

factor of ~10 overhead~1 µJ/pulse@UVCs2Te cathode

QE ~ 1…10% (UV)charge of ~1 nC per bunch required

length 20 ps, Ø = 3 mm

field uniformity ~ some mm~5 dg == ~ 10 pslongitudinal and

transverse size

TTF specs

General Design Issues of the LaserPhotocathode → the laser has to have similar transverse and longitudinal shapes as the required electron bunchesWith the given L-band RF gun, this already determines the basic properties of the laser pulsesTTF 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 * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY3

Laser System OverviewIn cooperation of DESY with Max-Born-Institute, BerlinPITZ also serves as a test bench for new developments

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

Frequency conversion to UV

Relay imaging telescopes

Pulse picker

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY4

Long Term Running Experience at TTF1

The laser has been upgraded end of 2003The basic design philosophy has been kept:

As simple as possible (avoid components which require frequent adjustments)‘Ready-to-sell’ finish and robustUse well-known technology (laser material, pump sources)Fully integration into the control system

The TTF1 laser ran for 4 years (== 36,000 h or 13•107 s)The oscillator was on during 25,000 h and made 9.2•107 trains, the amplifiers 16,000 h and 6.4•107 trains @ 1 HzTotal on-time oscillator 70%, amplifiers 50%

The major upgrade is in using diode lasers as a pump sourceUp to now 1,500 h with 2.1•107 pulse trains @ 5 Hz delivered

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY5

The Diode-pumped OscillatorSynchronized directly with 1.3 GHz from the master oscillator→ phase stability < 1 ps rms→ pulse length 12 ps fwhh27 MHz pulse train (2.5 ms) → prepared for the 9 MHz pulse pickerPumped with laser diodes→ better efficiency → increased stability in energy

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 * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY6

Chain of Linear Amplifiers2 diode pumped and 2 flashlamp pumped single pass amplifiers

Laser diodes:→ 32 W pulsed, 805 nm→ end pumped through fibers→ energy from 0.3 µJ to 6 µJ/pulse

— Inexpensive amplification, but→ Flashlamp lifetime 5•107 shots only→ Diodes give better energy stability

Flashlamps current controlled and stabilized using IGPT switches→ allows flat pulse trains→ energy up to 300 µJ (1 MHz)

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY7

Amplified Pulse Trains

Amplified laser pulse train

Time

Am

plitu

de

800 µs

Output of the laser oscillator

(27 MHz)

After amplification

(1 MHz)

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY8

Laser Beam Transport Line

Relay imaging of the beam from a hard edge aperture after the diode amplifiers onto the cathodeLast two mirrors allow remote x,y adjustments (including θ,ϕ corrections)A “virtual cathode” is used to measure the beam profile

laser

RF

gun

iris aperture

“virtual cathode” with CCD camera cathode

tunnel wall

Imaging system Vacuum window

(fused silica)

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY9

We need at least one mirror in the vacuum

To guide the laser beam onto the cathode, the last mirror is in the vacuum close to the electron beam path

We tested at TTF→ dielectric mirrors → dielectric mirrors with metallic shielding→ all metal mirrors (now new at TTF and PITZ)

laser beam

e- beam

Special care has to be taken to avoid→ wakefield effects→ charging-up of the mirror→ heating of the mirror due to absorption

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY10

The Cathode SystemCathodes: Cs2Te and KCs2Te used so far→ quantum efficiency 0.5 % (TTF1), now 5…10 %

presumably due to improved vacuum→ long lifetime ~ several months

Cathodes are produced at INFN LASA Milano and shipped to DESY in a battery pumped transport chamber

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY11

Quantum EfficiencyCharge measured for various laser energies and RF power in the gunQE is astonishing high we must have a good vacuum

PRF

Drop in QE due to vacuum conditions in the gun during operation, stabilization with a slight

increase

On-line quantum efficiency after first insertion

0.06 µJ (!) for 1 nC

cathode 37.2

QE(%) = 0.5*Q(nC)/E(µJ)

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY12

First Beam in the Gun Section March 2004

on YAG screen

Laser centered on cathode - with darkcurrent

ring from cathode edge

Scintillating cathode and darkcurrent ring used to initially center laser beam on cathode

on scintillating cathode

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY13

Bunch Charge MeasurementsWith toroids: at gun exit (T1), after acceleration to 100 MeV (T2/T3)charge jitter about 1 % rms (averaged over first 5 bunches)

toroid signals of a single bunch and of a bunch train

(30 bunches, 1 MHz)

40 ns

1.2 nC

correlation between the toroids T1, T2 and T3

T1 T1 T2

T2 T3 T3

(nC)

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY14

Phase ScansMeasure charge output of the gun as a function of phase laser/gun RF

Important and fast methode to → determine the phase of the laser in

respect to the gun RF→ to detect longitudinal problems of

the laser (satellites, phase jitter etc.)

We use a fit to the expected shape to chose the right phase:→ reproducibility = +/- 1.3 dg (rms)

It gives an estimate of the laser pulse length from the derivative of the rising edge

Cha

rge

(nC

)A

mpl

itude

Time (ps)

Phase (rad)

σ = 5 ps

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY15

Control of the Oscillator LengthThe stability of the oscillator length is crucial to reduce phase drifts – mainly caused by temperature driftsA slow feedback system corrects the oscillator length and keeps the phase stable

In this experiment, the oscillator length has been changed (voltage on the Piezo driver) while the feedback was off

phas

e la

ser/g

un (d

g)

PTO Piezo Voltage (V)

δϕ/δT = 1.75 dg/K (w/o feedback)

–2.8 K/V

PTO Piezo Voltage (V)

Tem

pera

ture

nea

r PTO

(d

gC)

response of the feedback on changes of the temperature

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY16

Phase Laser/RF Gun with FeedbackWith feedback, the relative phase laser/RF gun is stable→ sensitivity on temperature drifts is much reduced

Temperature near PTO (dgC)

phas

e la

ser/R

F gu

n (d

g)

PTO Piezo Voltage (V)

phas

e la

ser/R

F gu

n (d

g)

3 dgC

2 dg

of R

FFor this experiment, the temperature has been changed by switching off/on the air conditioning. While the temperature was floating, the relative phase laser/RF gun has been determined by the phase scan tool (accuracy ~1-2 dg)

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY17

3 mm

Transverse Bunch Size and Shape

at laser exit:→ gaussian shape → slightly ellipticalσx= 0.18 mmσy= 0.23 mmat cathode:→ partially flat hatσx= 0.9 mmσy= 1.0 mm(PITZ 0.6 mm)

exit of laser

1.6

mm

4.0 mm

4.4

mm

on cathode

1.3 mm

0.6 mm

Transverse shape of the UV laser beam → measured with a Ce:YAG crystal and a CCD camera

Shown are single pixel slices

Vertical slices

Horizontal slices

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY18

Laser Pulse Length and Shape

UV single laser pulse, measured with a streak camera (FESCA 200)Average over 50 measurements gives σL= 4.4 ± 0.1 ps (as expected)Longitudinal shape is gaussian

Time (ps)0 10 20 30 40 50

σL= 4.4 ± 0.1 ps

Note:a 20 ps long flat hat profile is required when the 3rd

harmonic cavity is available

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY19

1

2

3

12

3

0

0.5

1

y,mmx,mm1

2

3

12

3

0

0.5

1

y,mmx,mm

σx = 0.51 mmσy = 0.62 mm

σx = 0.57 mmσy = 0.58 mm

Laser Profiles at PITZin 2003 now:longitudinal

transverse

longitudinal

transverse

-200

20406080

100120

0 10 20 30 40 50 60

ampl

itude

Measured (UV, 262 nm)Flat Top Fit

FWHM = 19-24 psrise/fall = 7-9 ps

Time (ps)

-200

20406080

100120

0 10 20 30 40 50 60Time (ps)

Measured (523 nm)Flat Top Fit FWHM = 18-23 ps

rise/fall = 5-7 ps

ampl

itude

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY20

Beam Based AlignmentPrecise alignment of the laser spot onto the center of the accelerating field→ position of electron spot on a screen must not move if the field is changed

22

23

24

25

26

27

28

29

30

31

31 31.2 31.4 31.6 31.8 32

X (mm)

Y (m

m)

before alignmentafter laser alignment step 1after laser alignment step 2

Cavity axisX = 31.4mmY = 26.0mm

Beam position on screen ds gun

20

21

22

23

24

25

26

27

28

29

30

30 31 32 33 34 35 36 37 38 39 40X (mm)

Y (m

m)

Before solenoid alignment

After solenoid alignment

I = 0 A

I = 400 A

I = 0 A I = 400 A

Similar alignment procedure with solenoids → position of electron spot on a screen must

not move if the solenoid current I is changed

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY21

Reliability and other Issues

Frequent problems with the laser system during the last run:• Frequent problems with interrupts in VME crate controlling the laser

system → has been solved now• Pump diodes power supply overload needs manual reset (in average

once per month) → remote control in preparation• Interface machine protection system to laser is late → temporal solution

installed • Short lifetime of CCD cameras in the tunnel • Transverse laser beam spot not satisfactory yet

→ tuning finished up to exit of laser• Replacement of flashlamps required in average every 100 days (5 Hz)

After the installation of an independent chiller, no cooling water problem so far (the most frequent problem at TTF1)

Siegfried Schreiber, DESY * Machine Advisory Committee Meeting, Nov 9th, 2004, DESY22

SummaryLaser upgrade to diode pumped oscillator and diode pump pre-amplifiers gives a better stabilityNo major problems beside normal maintenance up to nowSmooth start-up, first beam 16-Mar-2004Up to now 1,500 h with 2.1•107 pulse trains @ 5 Hz deliveredLaser pulse

• has a longitudinal gaussian shape, σl = 4.4±0.1 ps• transverse neither gaussian nor flat

σx= 0.74 mm, σy= 0.65 mm - requires improvement• Phase jitter < 0.5 ps measured at MBI, we are not yet able to verify this with

beam (~1 ps resolution obtained so far)Mirror inside the vacuum has been replaced by an all metal Al mirrorFurther plans:→ improve transverse beam profile→ longitudinal flat hat with pulse stacker is possible if required→ replace all flashlamp with laser diodes→ install backup laser system