progress in soft x-rays fels
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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.
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