Brief Introduction to (VUV/)Soft X-ray FELs R. P. Walker Diamond Light Source, UK ICFA Workshop on Future Light Sources March 5 th -9 th, 2012 Thomas Jefferson National Accelerator Facility Newport News, VA

Status of VUV/Soft X-ray FELs Operating FLASH (Germany) - so far the only operating soft X-ray User facility (Italy) - call for proposals issued; 1 st external users later this year Planned NGLS (US): 0.28 – 1.2 keV WiFEL (US): 5 – 900 eV SPARX-FEL (Italy) : 30 eV – 2 keV JLAB VUV FEL (JLAMP) (US): 600 MeV cw recirculating linac, oscillator FEL eV Working definition: VUV: ~200 nm – ~20 nm (5 eV – 50 eV) Soft X-ray: ~20 nm – ~1 nm (50 eV – 1 keV)

Note that X-ray FEL facilities may also operate in the soft X-ray range, or contain specific soft-X-ray FELs e.g. LCLS: has operated down to 400 eV European XFEL SASE3: 4.7 nm – 0.4 nm (260 eV to 3 keV) SwissFEL “Athos” at 2.4/3.1 GeV: 7 – 0.7 nm, variable polarization, HHG seeding + single-stage HGHG (or other scheme) PAL-XFEL SXFEL1 at 3 GeV: 1-10 nm

Many (X)FEL test facilities are in the soft X-ray range Operating SCSS (Japan): 250 MeV SPARC (Italy): 200 MeV Shanghai Deep-UV FEL (SDUV): 150 MeV Construction Shanghai Soft X-ray FEL (SXFEL) approved in Feb. 2011; construction started early MeV normal conducting S-band + C-band linac possible future upgrade to 1.3 GeV for soft X-ray user facility Planned CLARA (UK) LUNEX5 (France) See separate session on test facilities

Not to mention several projects that have “fallen by the wayside..” 4GLS BESSY FEL LUX Arc-en-ciel NLS … technically not dead, just not proceeding at the moment.. Fallen by the Wayside, E.Bundy, 1886.

FLASH Radiation Parameters 2011 Wavelength range (fundamental)4.1 – 45 nm Average single pulse energy10 – 400 μJ Pulse duration (FWHM) 50 – 200 fs Peak power 1 – 3 GW Average power (example for 3000 pulses/sec)~ 300 mW Spectral width (FWHM) ~ % Average Brilliance – * Peak Brilliance – * * photons/s/mrad2/mm2/0.1%bw > 150 publications on photon science, many in high impact journals 3740 hours of SASE delivery Sep – Sep Accelerator up-time ~ 96% SASE delivery to experiments ~ 75% (rest is tuning, set-up etc.)

FLASH-II Construction has started Commissioning May 2013 SASE: 4-60 nm HHG seeded: nm (at 100 kHz)

S-band linac 1.2 GeV (later 1.5 GeV) FEL-1: HGHG down to 10nm FEL2: two stage HGHG with fresh bunch technique, to 4 nm (1.5 GeV) Status: operating nm Status: install & commission in 2012

Photon energy range19 ‐ 62 eV (20 ‐ 65 nm) Tunability ~ 5% PolarizationLV/LO/LC/RC Energy/pulse 20 ‐ 30 μJ Estimated pulse length<150 fs Repetition rate10 Hz FEL modeTEM00 FEL bandwidth 30 ‐ 90 meV n(FWHM) FEL bandwidthΔE/E= 6x10 ‐ 4 (rms) Photon energy fluctuations~1.1 meV (rms) FEL bandwidth fluctuations~ 3% (rms) Photon Beam Parameters to date

Beam switchyard with RF separators Experimental Areas m Undulators Monochromators SRF gun Bunch compressors Superconducting L- band electron linear accelerator 1.7 GeV 2.2 GeV Wisconsin FEL (WiFEL) 2.2 GeV CW SC linac with RF separation for many high-rep-rate beamlines Low charge bunches (200 pC) Seeding with High Harmonic Generation sources Cascaded harmonic generation without “fresh bunch” Development of a superconducting cw 200MHz electron gun is underway

Up to 100 kHz High resolution ~Time-bandwidth limited – photons/pulse – 5x10 -5 ∆  /  High-resolution spectroscopy Diffractive imaging (with harmonics) Up to 100 kHz Ultra-fast 250 as pulses Two color 10 8 ph/pulse Highest rep rate, MHz High flux photon/pulse 100 W Multidimensional X-ray spectroscopy Diffractive imaging (at highest rate) Photon correlation spectroscopy 10 μs 5 – 150 fs ~10–100 fs 0.25 – 25fs ≤1 μs 5 – 250 fs NGLS Seeded or self-seeded 2 color seeded SASE or self-seeded High rep. rate soft X-ray FEL facility; 2.4 GeV cw s/c linac o Up to 10 6 pulses per second o Seeded o Ultrashort pulses from 250 as – 250 fs o Narrow energy bandwidth to 50 meV o Adjustable photon energy from 280 eV – 1.2 keV o Polarization control 3 initial beamlines:

NGLS LBNL submitted a proposal to DoE in December 2010 DOE approved CD-0: “Mission Need” for a Next Generation Light Source in April 2011 Currently no DOE budget to pursue a Project LBNL is –Performing Accelerator and Detector R&D –Performing feasibility studies which will inform a Conceptual Design

Users’ Requirements high pulse energy transverse coherence fs pulses (or less) polarization control easy tunability multiple, simultaneous users high repetition rate regularly spaced pulses THz radiation in synchronism with FEL two-colour FEL pulses longitudinal coherence / pulse uniformity* high degree of amplitude stability* small linewidth* precise synchronism with lasers for pump-probe expts.

Users’ Requirements high pulse energy transverse coherence fs pulses (or less) polarization control easy tunability multiple, simultaneous users high repetition rate regularly spaced pulses THz radiation in synchronism with FEL two-colour FEL pulses longitudinal coherence / pulse uniformity* high degree of amplitude stability* small linewidth* precise synchronism with lasers for pump-probe expts. Most requirements not specific to soft X-rays … Especially for soft-X-ray FELs (?).. that was the view, but now XFEL users are starting to demand such properties

Users’ Requirements high pulse energy transverse coherence fs pulses (or less) polarization control easy tunability multiple, simultaneous users high repetition rate regularly spaced pulses THz radiation in synchronism with FEL two-colour FEL pulses longitudinal coherence / pulse uniformity* high degree of amplitude stability* small linewidth* precise synchronism with lasers for pump-probe expts. SASE Seeded * or oscillator

Users’ Requirements high pulse energy transverse coherence fs pulses (or less) ……………………………….many different schemes polarization control …………… APPLE/DELTA/crossed undulator etc. easy tunability …………………………………… variable gap undulator multiple, simultaneous users ………… electron switchyard schemes high repetition rate ………………………………… superconducting RF regularly spaced pulses ………………………. cw superconducting RF THz radiation in synchronism with FEL ……………… (pre)/afterburner two-colour FEL pulses ………………………………..… various schemes longitudinal coherence / pulse uniformity* high degree of amplitude stability* small linewidth* precise synchronism with lasers for pump-probe expts.

Technical Issues Seeding: - laser sources for shorter wavelength and higher rep. rate - seeding / modulation / harmonic generation schemes - modelling thereof Electron beam optimisation: Bunch compression – how many? Seeded FELs in particular require: - e- pulse shape control: flat slice parameters  flat gain length over length of seed pulse + timing jitter; even more for “fresh- bunch” schemes - timing jitter reduction due to linac phase and voltage fluctuations  careful optimisation of gun + linac parameters to meet these requirements

Technical Issues Electron sources -development of a low emittance, high rep rate injector, is still an an area of active R&D (see session on Electron Sources) Electron beam switching schemes for multiple FELs -magnetic or RF separation ? tolerances … Diagnostics (electron & photon) for low charge, short pulses

In addition, a big issue is COST particularly for high repetition rate FELs, SCRF technology, especially cw SCRF, is very expensive: “Cost” “Rep. rate” cw SCRF (NLS, NGLS, WiFEL) pulsed SCRF (FLASH, EXFEL) ~ 1 kHz NCRF ~ 100 Hz NCRF (LCLS, SACLA, PAL XFEL, SwissFEL etc.) Science driven currently no demand, but is there a “niche” ?

Given the wavelength range and type of machine, how to reduce costs ? …… reduce electron beam energy BUT: Shorter period undulators  higher field new designs, materials …SCUs ?  lower gap – what really is the limit ?? Higher harmonics - schemes to enhance harmonic output (e.g. phase jumps) - seeding and harmonic generation schemes Ultra-low emittance guns - “conventional” guns at low charge - novel electron sources Science driven Could be an interesting combination.. But reduced pulse energy..

Thanks for your attention