Progress in Soft X-rays FELs

1. Progress in Soft X-raysFELs R. Bartolini Diamond Light Source Ltd and John Adams Institute, University of Oxford FLS 2010 SLAC, 01 March 2010

2. 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

3. FEL radiation properties FELs 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 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)

4. Users’ requirements High peak brightness Transverse coherence SASE Tunability Ultra short pulses (<100 fs down to sub-fs) Temporal coherence direct seeding - seeding + HG High repetition rates / Time structure SC/NC RF IDs technology or novel schemes Polarisation control Synchronisation to external lasers VUV and THz

5. Soft X-rays FELs

6. Soft X-rays FELs: FLASH FLASH – operated successfully at 13.5 nm (Apr. 2006) then 6.5 nm (Oct. 2007) Courtesy R. Treusch 73 photon science publications since 2006 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 - average0.1 W (3000 pulses/sec) Peak brilliance up to 51029 1 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

7. FERMI@Elettra S-band linac 1.2 Gev (1.5 GeV) 0.8 nC, 50 Hz; FEL1 at 20 nm and FEL 2 at 4 nm Injector under commissioning: beam transported up to the L0 end (95 MeV) FEL-1: HGHG down to 20nm (design compatible with HHG seeding) FEL2: two HGHG stages with fresh bunch technique Courtesy E. Allaria

8. Wisconsin FEL (WiFEL) 2.2 GeV CW SC L-band linac with RF separation for many high-rep-rate beamlines Courtesy J. Bisognano • 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”

9. LBNL Soft X-rays project Injector CW superconducting linac 2.5 GeV, 13 MeV/m Laser heater Bunch compressor Beam transport and switching FELs Low-emittance gun, MHz bunch rate ≤ 1 nC ≤1 mm-mrad Laser systems, timing & synchronization • 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 Courtesy J. Corlett

10. experimental stations gas filters IR/THzundulators photoinjector diagnostics 3rd harmonic cavity accelerating modules spreader laser heater collimation BC1 BC2 BC3 FELs 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

11. NLS – recirculatinglinac 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) Linac 8 modules See talk by S. Smith in ERL WG

12. Accelerator Physics challenges 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

13. High brightness guns 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

14. time-slicing at 20 pC Performance of LCLS gun MEASURED SLICE EMITTANCE at 20 pC Courtesy D. Dowell Y. Ding et al., PRL 102, 254801(2009).

15. PITZ gun (FLASH – NLS) Courtesy F. Stephan (Photo Injector Test facility at DESY, Zeuthen site) See talk by Ivanisenko in WG5 • 1.3 GHz cavity, coaxial RF coupler (flexible solenoid position) • Capable of high average power  long electron bunch trains (SC linac)

16. VHF – band gun (LBNL) The Berkeley normal-conducting scheme is designed for CW operation with pressures compatible with high QE semiconductor cathodes. Courtesy F. Sannibale WIP ASTRA – 10k particles VHF gun has the capability of operating in a FEL scheme J. Staples, F. Sannibale, S. Virostek, CBP Tech Note 366, Oct. 2006 K. Baptiste, et al, NIM A 599, 9 (2009) • 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 lRFallows for large apertures and thus for high vacuum conductivity. • Based on mature and reliable normal-conducting RF and mechanical technologies. • 187 MHz compatible with both 1.3 and 1.5 GHz super-conducting linac technologies.

17. A01 BC3 Gun BC2 SPDR FELs LH A04 A02 A09 A05 A39 A10 A06 A03 A11 A07 A12 A08 A13 BC1 A14 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/PARMELA Impact-T GENESIS/GINGER Elegant/IMPACT/CSRTrack

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

19. CSR: macroparticle approach Elegant simulations showed a reasonably good agreement with LCLS data Y. Ding, Z. Huang Macroparticle approach may suffer from numerical noise producing unphysical results 5 billion particles 1 billion particles IMPACT simulations with 1 billion+particles J. Qiang et al. PRSTAB

20. 22mm 80mm E (MeV) A B 22mm 22mm C D 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 Evolving longitudinal phase space along linac (FERMI) Energy spread at exit of Linac vs. energy spread after laser heater C A B D Courtesy M. Venturini

21. Microbunching instability mitigation: laser heater Courtesy Z. Huang

22.  /  = 4.110-3  /  = 2.3 10-3 Wakefields 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) Fermi@Elettra Residual energy chirp after compression removed by S-band cavity wakefields Doesn’t work as nicely with low charge L-band linac (CSR in spreader actually helped) NLS

23. 150 fs Optimisation of beam dynamics for seeding • 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 The slice parameters to control are not onlyslice current, emittance, energy spread but alsoslice offset and angle and Twiss parameter Modified semi-analytical expression of the Xie gain length type can be used for quick numerical optimisation of the beam dynamics before FEL

24. 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 T Gun Phase 0.1 degrees Gun Voltage 0.1% Charge 1% Laser spot offset 0.025 mm Main linac cavities with split RF distirbution RF Phase (P) 0.01 degrees RF Voltage (V) 1e-4 fractional Bunch Comp. (B) 1e-5 fractional arrival time 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

25. all linac: 14 fs rms Jitter studies: the NLS case (II) 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 3D Xie length per slice 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

26. FEL physics challenges: need for seeding Advantage 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 Seedingimproves longitudinalcoherenceshortersaturationlength stability (shottoshotpower, spectrum, ...) controlofpulselength allowssynchronisationtoexternallasers

27. FEL physics challenges: seed sources (I) 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) 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)

28. FEL physics challenges: seed sources (II) Conventional laser Ti:Sa and harmonics are used down to 260 nm (FERMI@Elettra) 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) Tunability achieved by harmonic selection Repetition rate: 30mJ/40fs @ 1kHz available now 20mJ/40 fs @ 10kHz available in approx 3-4 years For NLS 400 kW at the undulator – 1.2 MW at the seed source (100 eV) Courtesy J. Tisch

29. FEL physics challenges: harmonic cascade Optimisation 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 but Courtesy N. Thompson

30. FEL physics challenges: EEHG A new method for generating harmonics based on a echo mechanism. Bunching decreases only with 3rd power of harmonic as compared to exponential decrease with HGHG Highly nonlinear phase space with significant bunching at very high harmonics G. Stupakov, SLAC-PUB-13445 (2008)

31. Sub-fs radiation pulses Generation of sub-fs radiation pulses has been proposed with a variety of mechanisms • laser slicing (Zholents, Saldin, Fawley) • mode locking (Thompson, McNeil) • single spike (Bonifacio, Pellegrini) • echo – based (Xiang –Huang-Stupakov) e-beam ~ 100 fs

32. Single spike operation for sub-fs radiation pulses 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; When the bunch length zis smaller than 2Lc the FEL emission occurs in a single spike temporally coherent (Bonifacio et al., PRL (1994)) 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. 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.

33. Single spike operation for sub-fs radiation pulses To operate in the single spike regime the bunch length must be shorter than 1 fs t = 470 as;  = 0.006 nm; / = 0.47% f t  0.53   1.610–3 (Lsat = 20m) 11010 ppp @ 1 keV 2.5 GW peak power Saturation in < 20 m

34. Single spike operation for sub-fs radiation pulses Gun Jitter Parameters (rms) Solenoid Field 0.02e-3 T Gun Phase 0.1 degrees Gun Voltage 0.1% Main linac cavities Phase (P) 0.01 degrees Bunch Comp. (B) 1e-5 fractional Voltage (V) 1-e4 fractional Current jitter: std = 245 A mean = 1891 A Arrival time jitter: std = 11.2 fs Electron bunch FWHM: std = 0.22 fs mean = 0.82 fs FEL power at 17 m = 2 GW rms of power = 0.9 GW 11 fs RMS 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

35. Phase shifter  Ex Ey FEL concepts: cross polarisation scheme Proposed by K-J. Kim Numerical simulations show that the maximum circular degree of polarization achievable is over 80% in SASE (LCLS parameters) Studies on seeded FEL are ongoing to assess the degree of polarisation achievable with seeded schemes Courtesy Z. Huang

36. Technologicalchallenges • 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.

37. Conclusions • 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 Thanks to many colleagues which have provided the material for this talk and thank you for your attention.