Brief Introduction to (VUV/)Soft X-ray FELsBrief Introduction to (VUV/)Soft X-ray FELs

R. P. Walker

Diamond Light Source, UK

ICFA Workshop on Future Light SourcesMarch 5th-9th, 2012Thomas Jefferson National Accelerator FacilityNewport News, VA

Status of VUV/Soft X-ray FELsStatus of VUV/Soft X-ray FELs

Operating•FLASH (Germany)- so far the only operating soft X-ray User facility•FERMI@ELETTRA (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 10-100 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 2012840 MeV normal conducting S-band + C-band linacpossible 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 2011FLASH Radiation Parameters 2011Wavelength range (fundamental) 4.1 – 45 nmAverage single pulse energy 10 – 400 μJPulse duration (FWHM) 50 – 200 fsPeak power 1 – 3 GWAverage power (example for 3000 pulses/sec) ~ 300 mWSpectral width (FWHM) ~ 0.7 - 2 %Average Brilliance 1017 – 1021 *Peak Brilliance 1029 – 1031 *

* photons/s/mrad2/mm2/0.1%bw

• > 150 publications on photon science, many in high impact journals

• 3740 hours of SASE delivery Sep. 2010 – Sep. 2011• Accelerator up-time ~ 96%• SASE delivery to experiments ~ 75% (rest is tuning, set-up etc.)

FLASH-IIFLASH-II

• Construction has started

• Commissioning May 2013

• SASE: 4-60 nm

• HHG seeded: 10-40 nm (at 100 kHz)

S-band linac 1.2 GeV (later 1.5 GeV)FERMI@ElettraFERMI@Elettra

FEL-1: HGHG down to 10nm

FEL2: two stage HGHG with fresh bunch technique, to 4 nm (1.5 GeV)

Status: operating 20-65 nm

Status: install & commission in 2012

Photon energy range 19‐62 eV (20‐65 nm)Tunability ~ 5%Polarization LV/LO/LC/RCEnergy/pulse 20‐ 30 μJEstimated pulse length <150 fsRepetition rate 10 HzFEL mode TEM00FEL 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)

FERMI@ELETTRA Photon Beam Parameters FERMI@ELETTRA Photon Beam Parameters to dateto date

http://www.elettra.trieste.it/FERMI/index.php?n=Main.Parameter

0 100 200

Beam switchyardwith RF separators

ExperimentalAreas

300 400 500 600 700 m

Undulators

MonochromatorsSRFgun

Bunchcompressors

Superconducting L- bandelectron linear accelerator

1.7 GeV 2.2 GeV

Wisconsin FEL (WiFEL)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• 1011 – 1012 photons/pulse• 10-3 – 5x10-5 ∆/

• High-resolution spectroscopy

• Diffractive imaging (with harmonics)

• Up to 100 kHz• Ultra-fast• 250 as pulses• Two color• 108 ph/pulse

• Highest rep rate, MHz

• High flux• 1011 - 1012

photon/pulse• 100 W

• MultidimensionalX-ray spectroscopy

• Diffractive imaging(at highest rate)

• Photon correlationspectroscopy

10 μs

5 – 150 fs

~10–100 fs

0.25 – 25fs

≤1 μs

5 – 250 fs

NGLSNGLS

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 106 pulses per secondo Seededo Ultrashort pulses from 250 as – 250 fso Narrow energy bandwidth to 50 meVo Adjustable photon energy from 280 eV – 1.2 keVo Polarization control 3 initial beamlines:

NGLSNGLS• 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’ RequirementsUsers’ Requirements

high pulse energytransverse coherencefs pulses (or less)polarization controleasy tunabilitymultiple, simultaneous usershigh repetition rate regularly spaced pulsesTHz radiation in synchronism with FEL two-colour FEL pulseslongitudinal coherence / pulse uniformity*high degree of amplitude stability*small linewidth*precise synchronism with lasers for pump-probe expts.

Users’ RequirementsUsers’ Requirements

high pulse energytransverse coherencefs pulses (or less)polarization controleasy tunabilitymultiple, simultaneous usershigh repetition rate regularly spaced pulsesTHz radiation in synchronism with FEL two-colour FEL pulseslongitudinal 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 Especially for soft-X-ray FELs (?) .. that FELs (?) .. that waswas the the view, but now XFEL users view, but now XFEL users are starting to demand are starting to demand such propertiessuch properties

Users’ RequirementsUsers’ Requirements

high pulse energytransverse coherencefs pulses (or less)polarization controleasy tunabilitymultiple, simultaneous usershigh repetition rate regularly spaced pulses THz radiation in synchronism with FEL two-colour FEL pulseslongitudinal coherence / pulse uniformity*high degree of amplitude stability*small linewidth*precise synchronism with lasers for pump-probe expts.

SASE

Seeded* or oscillator

Users’ RequirementsUsers’ Requirements

high pulse energytransverse coherencefs pulses (or less) ……………………………….many different schemespolarization control …………… APPLE/DELTA/crossed undulator etc.easy tunability …………………………………… variable gap undulatormultiple, simultaneous users ………… electron switchyard schemeshigh repetition rate ………………………………… superconducting RFregularly spaced pulses ………………………. cw superconducting RFTHz radiation in synchronism with FEL ……………… (pre)/afterburnertwo-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 IssuesTechnical 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 IssuesTechnical 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:

21

2

2

2

Knu

n

4nN

• Shorter period undulators higher fieldnew 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 Could be an interesting combination.. But combination.. But reduced pulse energy ..reduced pulse energy ..

Thanks for your attentionThanks for your attention