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

2. Status of VUV/Soft X-ray FELs
Working definition:
VUV: ~200 nm – ~20 nm (5 eV – 50 eV)
Soft X-ray: ~20 nm – ~1 nm (50 eV – 1 keV)
Operating
FLASH (Germany)
- so far the only operating soft X-ray User facility
(Italy)
- call for proposals issued; 1st 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

3. 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

4. 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 2012
840 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

5. 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.

6. FLASH Radiation Parameters 2011
Wavelength range (fundamental) 4.1 – 45 nm
Average single pulse energy 10 – 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 – 1021 *
Peak Brilliance – 1031 *
* photons/s/mrad2/mm2/0.1%bw
> 150 publications on photon science, many in high impact journals
3740 hours of SASE delivery Sep – Sep. 2011
Accelerator up-time ~ 96%
SASE delivery to experiments ~ 75% (rest is tuning, set-up etc.)

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

8. FERMI@Elettra S-band linac 1.2 GeV (later 1.5 GeV)
FEL-1: HGHG down to 10nm
Status: operating nm
FEL2: two stage HGHG with fresh bunch technique, to 4 nm (1.5 GeV)
Status: install & commission in 2012 8

9. FERMI@ELETTRA Photon Beam Parameters to date
Photon energy range 19‐62 eV (20‐65 nm)
Tunability ~ 5%
Polarization LV/LO/LC/RC
Energy/pulse 20‐ 30 μJ
Estimated pulse length <150 fs
Repetition rate 10 Hz
FEL mode TEM00
FEL bandwidth 30‐90 meV n(FWHM)
FEL bandwidth ΔE/E= 6x10‐4 (rms)
Photon energy fluctuations ~1.1 meV (rms)
FEL bandwidth fluctuations ~ 3% (rms)

10. Wisconsin FEL (WiFEL) 100 200 Beam switchyard with RF separators 300
100
200
Beam switchyard
with RF separators
Experimental
Areas
300
400
500
600
700 m
Undulators
Monochromators
SRF
gun
Bunch
compressors
Superconducting L- band
electron linear accelerator
1.7 GeV
2.2 GeV
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

11. NGLS High rep. rate soft X-ray FEL facility; 2.4 GeV cw s/c linac
Up to 106 pulses per second
Seeded
Ultrashort pulses from 250 as – 250 fs
Narrow energy bandwidth to 50 meV
Adjustable photon energy from 280 eV – 1.2 keV
Polarization control
3 initial beamlines:
Seeded or self-seeded
2 color seeded
SASE or self-seeded
10 μs
5 – 150 fs
~10–100 fs
0.25 – 25fs
≤1 μs
5 – 250 fs
Up to 100 kHz
High resolution
~Time-bandwidth limited
1011 – 1012 photons/pulse
10-3 – 5x10-5 ∆w/w
Highest rep rate, MHz
High flux
photon/pulse
100 W
Up to 100 kHz
Ultra-fast
250 as pulses
Two color
108 ph/pulse
Diffractive imaging (at highest rate)
Photon correlation spectroscopy
High-resolution spectroscopy
Diffractive imaging (with harmonics)
Multidimensional X-ray spectroscopy

12. 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

13. 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.

14. 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

15. 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

16. 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.

17. 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

18. 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

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

20. Given the wavelength range and type of machine,
how to reduce costs ? …… reduce electron beam energy
BUT:
Science driven
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
Could be an interesting combination.. But reduced pulse energy ..

21. Thanks for your attention