Progress in Soft X-rays FELs

R. Bartolini

Diamond Light Source Ltdand

John Adams Institute, University of Oxford

FLS 2010SLAC, 01 March 2010

Outline• Introduction

FEL radiation properties and users’ requirements• Soft X-rays projects

layouts and performance• AP challenges

collective effects

control of the e– phase space distribution

jitter issues• FEL challenges

need for seeding

ultra short (sub-fs) pulses• Conclusions

Many projects target Soft X-rays (here 40 – 1 nm) . Soft X-rays FELs require 1-3 GeV Linacs. Hard X-rays project will also provide Soft X-rays beamlines (Swiss FEL – LCLS)

FEL radiation propertiesFELs provide peak brilliance 8 order of magnitudes larger than storage ring light sources

Average brilliance is 2-4 order of magnitude larger and radiation pulse lengths are of the order of 100s fs or less

Slicing or low charge

Transverse coherence

Users’ requirements

SASE

direct seeding - seeding + HGTemporal coherence

High repetition rates / Time structure SC/NC RF

Polarisation control

Synchronisation to external lasers VUV and THz

Ultra short pulses (<100 fs down to sub-fs)

IDs technology or novel schemes

Tunability

High peak brightness

Soft X-rays FELs

FLASH 47-6.5 nm 1 GeV SC L-band 1MHz (5Hz) SASE

FERMI 40-4 nm 1.2 GeV NC S-band 50 Hz seeded HGHG

SPARX 40-3 nm 1.5 GeV NC S-band 100 Hz SASE/seeded

Wisconsin 1 nm 2.2 GeV SC/CW L-band 1 MHz seeded HHG

LBNL 100-1 nm 2.5 GeV SC/CW L-band 1 MHz seeded

MAX-LAB 5-1 nm 3.0 GeV NC S-band 200 Hz SASE/seeded

Arc-en-Ciel 1 nm 1 GeV SC/CW L-band 10 kHz seeded HHG

Bessy-II 64-1.2 nm 2.3 Gev SC/CW L-band 1-1000 KHz seeded HGHG

NLS 20-1 nm 2.2 GeV SC/CW L-band 1-1000 kHz seeded HHG

Shanghai 10 nm 0.8-1.3 GeV NC S-band 10 Hz seeded HGHG

Swiss-FEL 10 nm 2.1 GeV NC S-band 100 Hz SASE/seeded

LCLS 4 nm 4 GeV NC S-band 120 Hz seeded

1 GeV SC L-band linac (1 nC)

5 Hz rep rate (up to 1 MHz bunch spacing)

Wavelength range: 6.8 – 47 nm

Spectral width:0.5-1 %

Pulse duration (FWHM) 10-50 fs

Power (fundamental) peak 5 GW - average 0.1 W (3000 pulses/sec)

Peak brilliance up to 51029

FLASH – operated successfully at 13.5 nm (Apr. 2006) then 6.5 nm (Oct. 2007)

Soft X-rays FELs: FLASH

73 photon science publications since 20061 Nature

2 Nature Physics

5 Nature Photonics

1 Nano Lett.

14 Phys. Rev. Lett.

9 Phys. Rev. A, B, E

9 Applied Physics Letters

6 J. of Physics B

1 Optics Letter

Courtesy R. Treusch

S-band linac 1.2 Gev (1.5 GeV) 0.8 nC, 50 Hz; FEL1 at 20 nm and FEL 2 at 4 nm

FERMI@Elettra

FEL-1: HGHG down to 20nm (design compatible with HHG seeding)

FEL2: two HGHG stages with fresh bunch technique

Injector under commissioning: beam transported up to the L0 end (95 MeV)

Courtesy E. Allaria

Wisconsin FEL (WiFEL)

• Superconducting electron gun injector • Low charge bunches (200 pC)• Seeding with High Harmonic Generation sources (< 20 fs pulse length)• Cascaded harmonic generation without “fresh bunch”

2.2 GeV CW SC L-band linac with RF separation for many high-rep-rate beamlines

Courtesy J. Bisognano

LBNL Soft X-rays project

• L-band SC CW linac – 2.5 Gev (< 1 nC)• Photon Energy: 0.25- 1.0 keV 3rd & 5th harmonics at reduced intensity• feeding an array of 10 configurable FELs, each 100+ kHz CW pulse rate• independent control of wavelength, pulse duration, polarization• Seeded, attosecond, ESASE, mode-locked, echo effect, to be tested

Laser systems,timing & synchronization

Beam transport and switching

CW superconducting linac2.5 GeV, 13 MeV/m

Injector

Low-emittance gun, MHz bunch rate ≤ 1 nC

≤1 mm-mrad

Laser heater Bunch

compressor FELs

Courtesy J. Corlett

photoinjector

3rd harmonic cavity

BC1

BC2 BC3laser heater

accelerating modules

collimation

diagnostics

spreader

FELs

IR/THzundulators

gas filtersexperimental stations

UK New Light Source (NLS)

High brightness electron gun operating (initially) at 1 kHz

2.25 GeV SC CW linac L- band

50-200 pC

3 FELS covering the photon energy range 50 eV – 1 keV (50-300; 250-800; 430-1000)• GW power level in 20 fs pulses• laser HHG seeded for temporal coherence• cascade harmonic FEL• synchronised to conventional lasers (60 meV – 50 eV) and IR/THz sources for pump

probe experiments

NLS – recirculating linac option

High brightness electron gun operating (initially) at 1 kHz

2.25 GeV SC CW linac L- band

50-200 pC

Option with recirculating linac (10 modules instead of 18 modules)

See talk by S. Smith in ERL WG

Linac8 modules

Soft X-ray are driven by high brightness electron beam

1 – 3 GeV n 1 m

~ 1 kA / 10–4

This requires:

a low emittance gun (norm. emittance cannot be improved in the linac)

acceleration and compression through the linac keeping the low emittance

The operation of seeded FELs requires in addition

e- pulse shape control

(flat slice parameters flat gain length over ~100s fs)

careful reduction of jitter of e- beam properties

Accelerator Physics challenges

Excellent emittance has to be provided by the gun

Low rep rate (S-band)

BNL/SLAC/UCLA type - S-band photocathode gun(LCLS; FERMI@Elettra; SPARX)

Thermionic gun – Spring8

Low rep rate (L-band)

Pitz type gun - L band (FLASH, NLS Stage 1 – 1KHz)

High rep rate

VHF – band gun (LBNL)

SC RF gun (Rossendorf)

DC photocathode guns

High brightness guns

Performance of LCLS gunMEASURED SLICE EMITTANCE at 20 pC

time-slicing at 20 pCY. Ding et al., PRL 102, 254801(2009).Courtesy D. Dowell

PITZ gun (FLASH – NLS)

• 1.3 GHz cavity, coaxial RF coupler (flexible solenoid position)• Capable of high average power long electron bunch trains (SC linac)

(Photo Injector Test facility at DESY, Zeuthen site)

mrad mm 34.0%)100,5.0,1.0(

mrad mm 47.0%)100,8.0,25.0(

mmBSAnCQ

mmBSAnCQ

xy

xy

mrad mm 26.0%)90,1.0(

mrad mm 37.0%)90,25.0(

nCQ

nCQ

xy

xy

See talk by Ivanisenko in WG5

Courtesy F. Stephan

VHF – band gun (LBNL)The Berkeley normal-conducting scheme is designed for CW operation with pressures

compatible with high QE semiconductor cathodes.

K. Baptiste, et al, NIM A 599, 9 (2009)

J. Staples, F. Sannibale, S. Virostek, CBP Tech Note 366, Oct. 2006

WIPASTRA – 10k particles

VHF gun has the capability of operating in a FEL scheme

• Based on mature and reliable normal-conducting RF and mechanical technologies.

• At the VHF frequency, the cavity structure is large enough to withstand the heat load and operate in CW mode at the required gradients (gap voltage750 kV)

• Also, the long lRF allows for large apertures and thus for high vacuum conductivity.

• 187 MHz compatible with both 1.3 and 1.5 GHz super-conducting linac technologies.

Courtesy F. Sannibale

Design and Optimisation of LINACs driving FELs

• Tracking studies to optimise the beam quality at the beginning of the undulators:

peak current, slice emittance, slice energy spread

• linac simulations include

CSR, longitudinal space charge, wake-fields in RF cavities

• Parameters used in the optimisation

Accelerating section and 3HC amplitude and phase, Bunch compressors strengths (R56)

• Validation with full start-to-end simulation Gun to FEL (time dependent)

Astra/PARMELAImpact-T

Elegant/IMPACT/CSRTrack GENESIS/GINGER

Gun A01 LH A02A39 A03 A04 A05 A06 A07 A08 BC3 A09 A10 A11 A12 A13 A14BC1 BC2SPDR FELs

Choices of number of compressors, compression ratio and compression energy may impact the overall effect of microbunching instability. Solutions adopted are machine dependent

Microbunching instability mitigation: machine design

number of BCs

Compressor type

Compression Energies

(MeV)

Compression factors

pulse length

FWHM (ps)

peak current (A)

Wi-FEL 1 BCs C 400 20 3 to 0.16 50 to 1000

FERMI 2 BCs C-C 220-600 3.5*3=10.5 95 1000

SPARX 3 BCs C(VB)-C-S 300-500-1500 70 6 to 0.08 35 to 2500

LBNL 1 BC C 250 14 0.6 1200

FLASH 2 BCs C-S 130-470 100 12 to 0.1 12 to 1200

NLS 3 BCs C-S-S 130-450-1400 2*3*12 = 72 20 to 0.25 15 to1100

XFEL 2 BCs C-C 400-2000 20*5=100 15 to 0.15 50 to 5000

Swiss XFEL 2 BCs C-C 400-2000 12*6=72 12 to 0.16 20 to 1600

LCSL 2 BCs C-C 250-4300 7.5*12=90 2 to 0.25 33 to 3000

Spring 8 3 BCs C-C-C 30-410-1400 7*10*6=420 ~40 to 0.1 10 to 4000

CSR: macroparticle approach

Macroparticle approach may suffer from numerical noise producing unphysical results

Elegant simulations showed a reasonably good agreement with LCLS data

Y. Ding,Z. Huang

1 billion particles 5 billion particles

J. Qiang et al. PRSTAB

IMPACT simulations

with1 billion+ particles

CSR: Vlasov-solver methods allow forhigh-resolution study of beam phase space

High-resolution capability ideally suited for investigation of the microbunching

instability. 1D Vlasov solver w/ impedance model

of LSC and CSR has proven quite useful for quick evaluation of lattices

Energy spread at exit of Linacvs. energy spread after laser heater

E (M

eV)

80m

22m

22m

22m

A B

DC

A B

C

D

Evolving longitudinal phase spacealong linac (FERMI)

Courtesy M. Venturini

Microbunching instability mitigation: laser heater

Courtesy Z. Huang

The wakefields in accelerating structures play an important role in the manipulation of the electron bunches and can be used to remove energy chirp.

Works nicely with S-band (LCLS, FERMI and SPARX experience)

Wakefields

Doesn’t work as nicely with low charge L-band linac (CSR in spreader actually helped)

/ = 4.110-3

/ = 2.3 10-

3

Fermi@Elettra

Residual energy chirp after compression removed by S-band cavity wakefields

NLS

Assuming a 20 fs FWHM seed laser pulse we need an electron bunch with constant slice parameters over 20 fs plus the relative time jitter between the electron bunch and the laser seed pulse.• constant slice parameters on a length of 100 fs – or longer • no residual energy chirp (or very limited)• low sensitivity to jitter

Optimisation of beam dynamics for seeding

before FEL

150 fs

The slice parameters to control are not only slice current, emittance, energy spread but also slice offset and angle and Twiss parameterModified semi-analytical expression of the Xie gain length type can be used for quick numerical optimisation of the beam dynamics

Jitter studies: the NLS case (I)

The FEL performance can be severely spoiled by jitter in the electron beam characteristics .To understand this issue one has to investigate numerically the sensitivity of the beam quality to various jitter sources with full S2E simulations including jitters source in the Gun + Linac and FEL

Gun Jitter Parameters (rms)

Solenoid Field 0.02e-3 TGun Phase 0.1 degreesGun Voltage 0.1% Charge 1%Laser spot offset 0.025 mm

Main linac cavities with split RF distirbution

RF Phase (P) 0.01 degreesRF Voltage (V) 1e-4 fractionalBunch Comp. (B) 1e-5 fractional

RF gun (P and V) 7 fsInjector (RF gun + ACC01) 11 fsLinac P + V + B combined 10 fsP + V + B + I combined 14 fs

arrival time0.005%0.005%0.003%0.006%

mean energy

Independently powering the RF cavities in all accelerating modules

and reducing the power supply jitter in the BCs to 10–5 allowed

finding a satisfactory solution for NLS

Jitter studies: the NLS case (II)

all linac: 14 fs rms 3D Xie length per slice

Independently powering the RF cavities in all accelerating modules and reducing the power supply jitter in the BCs to 10–5 allowed finding a satisfactory solution for NLS:

The 3D Xie gain length has a flat area that can accomodate the 20 fs seed laser pulse

Jitter simulations for NLS FEL3 at 1 keV cascade scheme (100 electron bunches)

Start to end simulations includes electron beam jitter in the RF gun, linac and in the seeded harmonic cascade FEL

All jitter sources from ASTRA, elegant and GENESIS coupled together

Average power at 20.6 m = 1.4 GW rms of power = 0.3 GW

Seeding improveslongitudinal coherence shorter saturation lengthstability (shot to shot power, spectrum, ...) control of pulse lengthallows synchronisation to external lasers

FEL physics challenges: need for seedingAdvantage of seeded operation vs SASE

SASE has a very spiky output: each cooperation length behaves independently: no phase relation among spikes

SASE >> 1 Seeded ~ few TFL

FEL physics challenges: seed sources (I)

Power seed requirements:

P > 100 Pshot for direct seeding

P > 100 * n2 *Pshot for HGHG

Pshot increases with decreasing wavelength. Losses during seed transport and matching have to be taken onto account.

Seed source are not available down to 1 keV. Frequency up-conversion has the be done with the FEL itself

HGHG schemes (L.H. Yu, Science, (2000))multistage HGHG (yet unproven)EEHG (yet unproven)

Seed source must be

powerful enough to dominate the shot noise power coherent (Tra & Lon)

high rep rate short pulses

tuneable stable (time jitter, pointing stability, etc)

22shot mc

21P

Conventional laser Ti:Sa and harmonics are used down to 260 nm (FERMI@Elettra)

FEL physics challenges: seed sources (II)

Tunability achieved by harmonic selection

Repetition rate:

30mJ/40fs @ 1kHz available now20mJ/40 fs @ 10kHz available in approx 3-4 years

HHG sources used at

SCSS (160 nm), SPARC (400-114 nm)

proposed at

sFLASH, NLS, LBNL, WiFEL, …

HHG sources extend down to 10 nm (124 eV)

Courtesy J. TischFor NLS 400 kW at the undulator – 1.2 MW at the seed source (100 eV)

80 40nJ

10nJ

100nJ

µJ

10µJ

100kW

MW

10MW

100MW

KrF Hanover 14mJ 500fs

Xe

Saclay : EL= 25mJ

Riken 16mJ

Ene

rgy

/ pul

se

l (nm)

Peak P

ower (50fs pulse)

Ne

Ar

Riken 130mJ

LOA 2mJ

Riken 16mJ

FEL physics challenges: harmonic cascadeOptimisation of cascaded harmonic FEL for highest power and highest contrast ratio

Conflicting requirements:

generate bunching at higher harmonics of interest

keep the induced energy spread low

Courtesy N. Thompson

u,seed n

2seed2

u,2

but

FEL physics challenges: EEHGA new method for generating harmonics based on a echo mechanism.

G. Stupakov, SLAC-PUB-13445 (2008)

Highly nonlinear phase space with significant bunching at very high harmonics

Zllaser

E E

En

Enb 3/14.0~

Bunching decreases onlywith 3rd power of harmonic as compared to

exponential decrease with HGHG

Sub-fs radiation pulses

  Slicing +wavelength

Slicing +current

Slicing + Energy chirp

Single spike

Mode-Locking

Pulse length 300 as 250 as 200 asor less 300 as 23 as

every 150 as

Photon energy 12 keV 12 keV 12 keV 12 keV 8.6keV

Photon per pulse 108 109 1010 108 108

Peak Power 5 GW 50 GW 100 GW 5 GW 5 GW

contrast poor poor good excellent good

Rep rate Laser seed Laser seed Laser seed LINAC Laser seed

synchronisation YES YES YES NO YES

• laser slicing (Zholents, Saldin, Fawley)

• mode locking (Thompson, McNeil)

• single spike (Bonifacio, Pellegrini)

• echo – based (Xiang –Huang-Stupakov)

Generation of sub-fs radiation pulses has been proposed with a variety of mechanisms

e-beam ~ 100 fs

)t(E

NLS simulations show that the electron bunch can be compressed to 1 fs FWHM and single spike FEL pulses of 450 as FWHM can be generated at 1.24 nm;

Single spike operation for sub-fs radiation pulses

When the bunch length z is smaller than 2Lc the FEL emission occurs in a single spike temporally coherent (Bonifacio et al., PRL (1994))

coopbunch L2L

l34

L rescoop 3/1

2u

2x

3A

e22

)K

1II

16]JJ[K

It requires a very aggressive compression of the electron bunch with very large compression factors (thousands). Best compression achieved at very low bunch charge (~2pC) where collective effects are negligible.

The minimum pulse length is limited by Lcoop and hence the minimum number of optical cycles is ~1/20, e.g.

with = 10–3 we have about 50 optic cycles, i.e. 150 as at 1 nm but 1.5 fs at 10 nm.

High gain FEL operation at 1 keV has the potential to generate sub-fs coherent pulses.

Single spike operation for sub-fs radiation pulses

0 5 10 15 20 25 30 35 4010

-5

10-4

10-3

10-2

10-1

100

101

distance along undulator (m)

peak

pow

er (G

W)

t = 470 as;

l = 0.006 nm; l/l = 0.47%

f t 0.53 1.610–3 (Lsat = 20m)

11010 ppp @ 1 keV

2.5 GW peak power

Saturation in < 20 m

To operate in the single spike regime the bunch length must be shorter than 1 fs

-8 -6 -4 -2 0 2 4 6 80

200

400

600

800

1000

1200

1400

1600

1800

2000

time (fs)

curre

nt (A

)

-2 -1.5 -1 -0.5 0 0.5 1 1.5 20

0.5

1

1.5

2

2.5

3

time (fs)

pow

er (G

W)

1.23 1.235 1.24 1.245 1.25 1.255 1.260

2

4

6

8

10

12

14

16x 10

4

wavelength (nm)

pow

er (a

rb.)

Single spike operation for sub-fs radiation pulses

Jitter effects are very strong. Tighter tolerances on the RF stability are required COTR can be used to timestamp the arrival time of the bunch and photon pulse

Gun Jitter Parameters (rms)Solenoid Field 0.02e-3 TGun Phase 0.1 degreesGun Voltage 0.1%

Main linac cavitiesPhase (P) 0.01 degreesBunch Comp. (B) 1e-5 fractionalVoltage (V) 1-e4 fractional

Current jitter:std = 245 Amean = 1891 A Arrival time jitter:std = 11.2 fs Electron bunch FWHM:std = 0.22 fsmean = 0.82 fs

FEL power at 17 m = 2 GW

rms of power = 0.9 GW

-30 -20 -10 0 10 20 300

2

4

6

8

10

12

14

16

18

arrival time (fs)

frequ

ency

Single Spike Arrival Time Jitter

-25 -20 -15 -10 -5 0 5 10 15 20 250

1

2

3

4

5

6

7

time (fs)

pow

er (G

W)

11 fs RMS

-20 -10 0 10 20 30 400

500

1000

1500

2000

2500

arrival time (fs)

curre

nt (A

)

FEL concepts: cross polarisation scheme

Ex Ey

Phase shifter

Proposed by K-J. Kim

Studies on seeded FEL are ongoing to assess the degree of polarisation achievable with seeded schemes

Numerical simulations show that the maximum circular degree of polarization achievable is over 80% in SASE (LCLS parameters)

Courtesy Z. Huang

• Insertion devices: Minimum gap and tunability requirements defines the energy of the linac. Development of new undulators beyond Apple-II (shorter periods, higher fields, wakefield control)

• SC RF: Optimise performance and reduce cost (gradient choices 13-15 MV/m for LBNL, NLS, BESSY)

• Diagnostics: New diagnostics for ultra short bunches, arrival time, low charge but also dealing with COTR

• Timing and synchronisation: sub 10-fs resolution over 100s m and long term stability

• Stability and feedbacks: positions (sub m over large frequency range), energy, charge, …

• Laser systems: for seeding: short wavelength reach, repetition rate - for photocathode gun: pulse shaping.

Technological challenges

Users’ requirements pose difficult challenges for FEL design and operation

• High repetition rate requires SC technology – crucial cost driver

• Temporal coherence require seeding and challenging frequency up-conversion schemes

The methods and solutions developed show that these challenges can be met.

Experimental tests of seeding in the coming future will confirm the extent of seeding capabilities to cover the whole Soft X-ray spectrum down to 1 nm

Conclusions

Thanks to many colleagues which have provided the material for this talkand

thank you for your attention.