External Seeding Approaches:

S2E studies for LCLS-II

Gregg Penn, LBNL CBP

Erik Hemsing, SLAC

August 7, 2014

Why seed with an external laser?

2August 7, 2014

More timing control over x-ray pulse

• timing defined by laser seed• easy to adjust pulse duration

Shot-to-shot stability

Possibly narrower spectrum, even transform-limited

Tailored x-ray pulses

• such as frequency chirps or pulse shaping

Concerns:

• limits repetition rate, reduced x-ray energy per pulse- especially compared to self-seeding

• very large harmonic upshift from conventional lasers- commissioning may be a challenge at highest photon energies

Seeding schemes and layouts

EEHG

HGHG

3August 7, 2014

UVseeds

 

 

radiatormod1

 

 

mod2

          

   

UVseed

 

   

       

 

  

freshbunchdelay

     

mod1 rad1 mod2 rad2

quadrupoles

   

15th harmonic (160 nm) demonstrated at NLCTA

65th harmonic (4 nm) demonstrated at FERMI@Elettra

4

Common parameters for both schemes

August 7, 2014

• 4 GeV beam energy• ~ 1 kA peak current• 260 nm external lasers• final undulators

- 39 mm period, 3.4 m sections- = 15 m

• output at 1 nm- most challenging part of tuning range

Two S2E electron bunches• 100 pC

- from Paul Emma, October 2013• 300 pC

- from Lanfa Wang, April 2014

100 pC

300 pC

note: longitudinal dynamics not fully modelled

EEHG configuration: 260 nm directly to 1 nm

Compact beamline to reduce IBS

Low magnetic fields to reduce ISR

• first chicane ~9 m long, B < 0.5 T• second undulator has 0.4 m period, B < 0.4 T

Need energy spread < 3 MeV when start to radiate at 1 nm

• but large energy modulations reduce impact of IBS and ISR• pushing limits at ~2.3 MeV induced energy spread• SASE starts to compete with seeded pulse

- unless blow up energy spread everywhere

All these constraints are less severe for longer wavelengths

5August 7, 2014

EEHG seeding results from 260 nm to 1 nm

• ~ 700 MW peak power at 1nm- from ~ 1 GW laser power at 260 nm

• allows long, coherent pulses• highly sensitive to laser quality, less so to electron bunch• 300 pC bunch uses 2 extra undulator sections

Examples: better than 2 × transform limit

6August 7, 2014

0.22 eV rms

0.12 eV rms18 J9 fs rms

25 J16 fs rms

EEHG: 300 pC

7August 7, 2014

power spectrum

note SASEfrom tail

21 microJ

two seed lasers:•100 fs FWHM•50 MW and 900 MW peak power•1.5 MeV and 3 MeV modulation

2 extra undulator sections at end

longer pulse suppresses SASE

only make first laser longer:

• same output pulse length

also increase power of first laser?

• not worth the reduced power

Suppressing SASE

8August 7, 2014

1.5×109

do not rely on beam splitterfor the 2 seed pulses

HGHG configuration: 260 nm to 13 nm to 1 nm

Real estate within the bunch is at a premium

• need short pulse, short delay

Laser seed

• 20 fs to 40 fs FWHM- short enough to require extra laser power

• consider using a super-Gaussian profile ~ exp(-t4)

Fresh-bunch delay

• 25 fs to 100 fs shift of radiation relative to e-beam• dispersion weak enough that bunching from first stage

survives fresh-bunch delay

9August 7, 2014

HGHG seeding from 260 nm to 13 nm to 1 nm

• two stage fresh-bunch, pushed to high harmonics• ~ 500 MW peak power at 1 nm

- from ~ 800 MW at 260 nm

• highly sensitive to electron bunch quality

Examples: consistently poor spectrum

• performance is much better at 2 nm

10August 7, 2014

HGHG: 100 pC

11August 7, 2014

spectrumpower

used super-Gaussian profile flatter, still 20 fs FWHMmessy spectrum

HGHG: 300 pC

12August 7, 2014

spectrumpower

regular Gaussian40 fs FWHM

x-ray pulse is shortcould make longer, but spectrum will be worse

Some of the challenges for HGHG

Sensitive to incoherent energy spread

• smaller energy spread would make HGHG easier- even if peak current has to be reduced

Fresh bunch delay

• different regions of the electron beam have to co-operate• beamline sensitive to longitudinal variations in bunch

- Twiss parameters and transverse offsets- CSR has a big impact

• limits duration of x-ray pulse, little room for timing jitter- super-Gaussian profile for input laser helps

13August 7, 2014

100 pC beam properties

14August 7, 2014

Bmag=()/2 ≥ 1measure of mismatch

~0.30 micron

care about-50 fs to 30 fs

current spikes can drive SASE in EEHG

transverse offsets (not shown) of ~50 micron

300 pC beam properties

15August 7, 2014

Bmag=()/2 ≥ 1measure of mismatch

~0.43 micron

care about-200 fs to 100 fs

Summary: Tradeoffs between EEHG and HGHG

16August 7, 2014

EEHG

• allows moderate energy modulation- in practice, set by energy scattering

• good prospects for long, coherent pulses• challenging laser requirements (stability and phase control)

- will be studied further at NLCTA

• not yet tested at high harmonics, short wavelengths

HGHG with fresh bunch delay

• demonstrated good results down to ~10 nm (FERMI@Elettra)• best for short pulses

- fresh-bunch delay limits pulse duration- hard to control spectrum

• below ~ 2 nm seems to be pushing the limits

Consider other seeding schemes as well

17August 7, 2014

Alternative: staged approach to 1 nm

Start with smaller harmonic jumps initially

At 2 nm or 3 nm could switch to 1 nm near saturation

• “afterburner” configuration- only retuning of final undulators is required- peak power at 1 nm < saturation

• blow-up of energy spread is a concern• see table for EEHG, similar behavior for 3-stage HGHG

18August 7, 2014

EEHG wavelength Energy spreadat end of EEHG

Energy spread at start of 1 nm

4 nm 1.5 MeV 6 MeV

2 nm 1.8 MeV 2.5 MeV

1 nm 2.4 MeV 2.4 MeV

EEHG to 2 nm, with optional jump to 1 nm after

changes:

• 2nd laser power reduced to 400 MW (2 MeV modulation)

• first chicane, R56=11.0 mm, down from 14.4 mm

• 2nd chicane, R56=82.0 micron, up from from 53 micron

choose either 6 undulator sections tuned to 2 nm,

or 3 sections tuned to 2 nm plus 11 tuned to 1 nm

19August 7, 2014

either choice yields ~100 microJ,pulse close to transform limit

peak energy spread ~ 1.9 MeV

EEHG to 2 nm results

power at 2 nm and 1 nm spectrum at 1 nm

20August 7, 2014

transformlimited

HGHG to 1.9 nm, possible 0.9 nm afterburner

21August 7, 2014

not bad at ~ 1 nm but low pulse energy

HGHG ending at 1.9 nm

if continue to amplify 1.9 nm pulse

23 microJ pulse energy

spectrum better than at 1 nm

22August 7, 2014

Better spectrum earlier, but only ~ 4 microJ

23August 7, 2014

EEHG: 300 pC

24August 7, 2014

power spectrum

two seed lasers:•50 MW and 900 MW peak power•100 fs FWHM•1.5 MeV and 3 MeV modulation

10 microJ

note SASEfrom tail

Spectrum for longer HGHG pulse at 1 nm

25August 7, 2014

More beam comparisons

26August 7, 2014

100 pC 300 pC