Ellipsoidal Laser Status & Results - DESYbib-pubdb1.desy.de/record/398156/files/12. Good...pulse...

Ellipsoidal Laser Status & Results

James Good

PITZ Collaboration Meeting

14 Jun 2017

Introduction

Laser system & on-table data

Results

Redesign

Outlook

James Good | Ellipsoidal Laser Status & Results | 14.06.2017 |

Introduction

> Motivation: Further improvement of the electron beam quality by reduction of the

transverse projected beam emittance.

> Main idea: Optimization of the cathode laser pulse shape in order for to minimize the

impact of the space charge on the transverse emittance.

222

SpChRFcath

min :shapelaser cathode SpCh

cylindrical ellipsoidal

temporally

x

Fx px

x

x

Fx px

x

Minimum SC influence on beam emittance

Better longitudinal compression

Reduced beam halo

Less sensitivity to the machine settings


Photo Injector Test Facility, DESY Zeuthen

> PITZ is a testbench for photoinjector R&D

Gun quads for beam symmetrization

Plasma self-modulation experiments

Photocathode laser pulse shaping

⇒ an important tool for ↓ε


PITZ setup and original ASTRA simulation parameters

Simulation setup Three different photo cathode laser shapes have been considered in beam simulations:

Longitudinal distribution: Gaussian. Transverse distribution: radial homogeneous

Longitudinal distribution: Flat-top. Transverse distribution: radial homogeneous

Uniformly filled Ellipsoidal distribution

Bunch charge: 1 nC

Gun gradient: 60.58 MV/m corresponding to Pz~6.7 MeV/c beam momentum after the gun

CDS booster starting position: 3.1 m

CDS booster gradient: 19.76 MV/m corresponding to Pz~24 MeV/c final beam momentum

Reference point: EMSY1 (Z=5.74 m) best emittance for 3 profiles with the same bunch length

~7 MeV/c RF gun ~25 MeV/c CDS

booster

Emittance optimization screen:

5.74 m downstream the cathode


Beam overview for 3 different laser shapes (Zboo=3.1m)


Technical requirements

Parameter Value Unit Remark

wavelength 255-270 nm 4th harmonic of Nd

micropulse energy 10-12 μJ 1 nC bunch production

from Cs2Te photocathode

pulse train frequency 1 MHz Future goal: 4.5 MHz

pulse train length 0.3 ms Future goal: 0.6 ms

pulse train rep.rate 10 Hz 1,2,5 Hz as an option

micropulse rms duration 6±2 ps Quasi-ellipsoidal

distribution

transverse rms size 0.5±0.25 mm

Photocathode laser Technical requirements


> Collaborative development w/ IAP, Nihzny Novogord

> Fiber-based Er laser oscillator and Yb amplifiers

> Multi-pass diode pumped disk amplifier with Yb:KGW crystals

> Hamamatsu SLM-based spatio-temporal shapers

> Second and fourth harmonic conversion

> Scanning cross-correlators & spectrograph(s) (diagnostic)

> Installed end 2016, RF: oscillator synchronization late 2016

Simplified Ellipsoidal Laser (ELLA) setup


Current pulse shaper: Super-Gaussian

> „Generation of flat-top picosecond pulses by coherent pulse stacking in a

multicrystal birefringent filter“, Ingo Will & Guido Klemz Optics Express, Vol. 16, Issue 19, pp. 14922-14937 (2008)

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30 35

OS

S S

ign

al

(a.u

.)

Time (ps)

0

0.2

0.4

0.6

0.8

1

0 5 10 15 20 25 30 35

OS

S S

ign

al

(a.u

.)

Time (ps)


1D SLM-based pulse shaping

> Concept: Spectrally separated chirped pulse transversally modulated by

amplitude-phase mask prior to recombination



Source:

Goal:


2D SLM shaper

Faraday Isolator, polarizer and

90° beam rotation

,),( ,),(),,(),,( xiyi

inout exMeyMyxEyxE

tyxEin ,,

tyxEout ,,Cylindrical lens

Cylindrical lens

Spherical lenses 2

10~

),( xM

),( x

x

y


> Horizontal bandcut mask to define

SLM center

> Vertical edge mask to find spectral

center

ELLA spatial/spectral calibration


IR spectrographic reconstruction

> Slit-scan spectrometer

(modified IAP f600 design)

Standard Czerny-Turner layout

Motorized transverse

translation stage w/ slit

20 nm on-camera spectral dispersion


IR cross-correlation

Pulse interaction in nonlinear crystal

dttItIW213

~

)(~)(2

ttI 13

~ IW

1

2

3


Beam quality & operational issues

> Poor oscillator pointing/opto-mechanical stability

Constant slow drift (uncertainties w.r.t. Q,pos, mask)

Pockel cell contrast/leakage

> Spectral-temporal comparison

Band-masking ~1030nm removes sidelobe

> Weak shape preservation

UV beam walkoff/smearing

Pump drift

> Synchronization jitter (ΔQ, ΔP)

IR 515 UV

VC2


µTCA-based synchronization

> 1st successful trial 16th Nov 2016 (20min)

> Short-term test 20th Nov (6h)

1st Synchronized photoelectrons!

> Long-term test 25th Nov

On-table measurements

Humidity dependent

> Many thanks to:

M. Felber, T .Kozak, H. Schlarb,

G. Schlesselman, F. Tonisch, M. Pohl,

D. Melkumyan, D. Kalantaryan, G. Trowitzsch,

…, …


ELLA spectral masking (~100 pC)

~13 ps

~13 ps

unshaped

spectrally masked


ELLA Spectral masking

BSA1.2mm

100 pC

~8 ps


ELLA 3D ‘FT’ shaping (~200 pC & 500 pC)

200 pC

8 ps

500 pC

15 ps

Nominal BSA: 1.2mm, 0.5nC


ASTRA simulations for 0.5nC

Ph

oto

cath

od

e laser

Pulse shape

Temporal profile

cylindrical ellipsoid

Gaussian Flattop Homogeneous

ellipsoid

length (FWHM) 9.5 ps

Transverse Homogeneous radial Homogeneous

ellipsoid

Transverse size (rms) 0.385 mm 0.423 mm 0.353 mm

RF

gu

n

Electric field at the cathode (max) 60 MV/m

Phase (w.r.t. MMMG) -1.2° -4.2° -2.8°

Solenoid peak field -0.2247 T -0.2248 T -0.2260 T

Bo

ost

er

Electric field (max) 17.1 МV/m

Phase (w.r.t. MMMG) 0°

Ele

ctr

on

beam

(z=

5.2

77m

)

Bunch charge 0.5 nC

Beam mean momentum 21.0 MeV/c

Projected normalized emittance 0.80 mm·mrad 0.64 mm·mrad 0.35 mm·mrad

Average slice emittance 0.49 mm·mrad 0.57 mm·mrad 0.33 mm·mrad

Bunch length (rms) 1.44 mm 1.20 mm 1.34 mm

Peak current 35.4 А 39.5 А 37.8 А

Longitudinal emittance 34 mm keV 22 mm keV 12.5 mm keV


ELLA 2.0 reDesign

> Overall goal:

Design & construct a simplified, reliable,

and robust photocathode laser system

Avoid & correct original design

constraints, flaws, and errors

> Design goals:

Minimalized, simplified layout: 50%

reduction of optical elements & path

length (50m → 25m)

modularized & mechanically robust

improved thermal robustness & opto-

mechanical stability

Maximize mask usage & resolution,

minimize AOI

→ lower thermal load & LIDT risk

Uni-directional layout design

→ Independent transverse masks


> Design considerations:

Future phase masking? Transverse

masking?

Green cathodes/pulse

shaping/characterization?

Volume Bragg gratings

Asymmetric pulse envelopes?

ELLA 2.0 reDesign

SLM

SLM

expander

dove

> Pharos laser properties:

high power 20W, water-cooled, solid state

laser → energy budget up to 100uJ/pulse

variable rep. rate (1 kHz – 1 MHz)

thermo-mechanically stable & optically

flexible

→ SHG and FHG module included

→ Gaussian pulses at variable

pulse duration (0.3 – 15 ps)

Already delivered & installed

→ Synchronization/stability tests

next week(s)


µTCA-based synchronization

> 72.222MHz Oscillator

Fine tuning: 40Hz/~80V

Coarse tuning: 1.5kHz/80V

Mechanical tuning: 40kHz

> 1st successful trial 8th Jun 2017 (24h)

> Short-term test 9th – 12th Jun (~60h)

Mechanical settling pulled osc.

out of range (Δf = 2.5kHz)

> Continued tests

LEDA check next week?

uTCA phase shifter test?

> No baseline jitter or

de-sync (uTCA timeout fix?)

> Many thanks to:

T .Kozak, F. Tonisch, R. Netzel…


Summary & Outlook

> Summary

New photocathode laser

design implemented and

utilized

Significant experience gained

in 3D pulse shaping

Successful electron beam

measurements taken

Substantial technical

limitations, constraints &

issues identified

(synchronization, drift, beam

quality, etc.)

>Outlook

New core Pharos laser

system delivered & installed

Sucessful synchronization!

Momentum check next week

New linear optical beam

shaping design finalized &

components ordered

40m → 10m!

Construction begins Jul

New volume Bragg grating

experiments to produce full,

static ellipsoidal distributions


Thank you for your attention!!!


Backup slides


IR 2D reconstruction


Fiber part

D=1.5mm

Spherical Mirror 2

Spherical Mirror 1

Yb:KGW 1 2f

f

f

Yb:KGW

1

Spherical Mirror

1

Spherical Mirror 2

Yb:KGW

2 Yb:KGW 2

DESIGN OF THE AMPLIFIER

SM 1 SM 2

Yb:KGW 1

(5V passes x 2)

830 mm

Yb:KGW 2

(4V passes x 2)

Transversal

distribution

at the image plane: At the input

After the 1st pass

of the amplifier ( ̴16 m

propagation)

After the 2nd pass

of the amplifier

( ̴32 m

propagation)

Imaging longitudinal accuracy at the output < 10 mm

(22 mm needed) Astigmatism compensating

scheme

Yb:KGW

active

elements:

3% doping level

8.5x8.5x3 mm

disks

6.5% cold losses

per 1V pass

0.22%

depolarization

per 1V pass MULTIPASS

BROADBAND

Yb:KGW AMPLIFIER

FOR 3DESP LASER


10~

),( xM

),( x

x

y

Faraday Isolator, polarizer and

90° beam rotation

Spherical lenses

Cylindrical lens

Cylindrical lens

The optical scheme allows changing

amplitude and phase of spectral

components

Hamamatsu SLM

800 x 600

liquid crystal phase

modulator

λ

2

,),( ,),(),,(),,( xiyi

inout exMeyMyxEyxE



> Laser controller unit failure (fixed by IAP)

> Water cooling system issues

> All non-linear crystals replaced (incl. amplifier,

cross-correlators, & frequency conversion)

Mar: Yb:KGW amplifier crystals replaced,

frequency conversion crystals & mounts replaced

Apr: IR cross-correlator crystal & mount replaced

> Imperfect laser pointing stability

> Oscillator frequency modulation & locking

Imperfect due to piezo-eigenmodes

> Pump laser cooling circuit pressure spikes

> Holoeye SLM asynchronousity

Unforeseen technical issues

Realtime (ms) Holoeye SLM amplitude trace


Ch2 instabilities

> Significant energy jitter for maximum power Ch2 observed

Record Ch2 PD traces

Linear fit of peaks & scatter plot of fit coefficients

> Jitter reduction for reduced power

0 100 200 300 400 500 600 700 800-0.02

-0.01

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

Ch2 energy fit spread

Fit slope [/V.s-1]

Fit

off

set

[/V

]

50% Ipeak

100% Ipeak

0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4

x 10-4

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

Time [/s]

Vo

ltag

e [

/V]

CH2 photodiode trace

PD trace

peak detection

linear fit

missing

pulses


Summary of electron beam parameters for 1nC ASTRA

simulations


MBI long-Gauss (15ps) 500 pC emittance

20.01.2017 12:06

Mikhail, Y. Chen

With similar gun & laser setup

Ellipsoidal Laser Status & Results - DESYbib-pubdb1.desy.de/record/398156/files/12. Good...pulse...

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