1. Seeding in the presence of microbunching
Gregg Penn, LBNL CBP
July 29, 2015

2. Sensitivity of seeding schemes to microbunching
Vary laser heater to select different microbunching levels
currently using 300 pC bunch
will also explore 100 pC
Output photon energies of 540 eV and 1.24 keV
Look at EEHG, HGHG, self-seeding (R=15000, 2% effic)
laser heater at 12 keV
laser heater at 6 keV
also using 9 keV

3. Seeding schemes and layouts: allows for 1.24 keV out
EEHG HGHG Self-seed
mod1
mod2
radiator UV
seeds
9 m
mod1
rad1
rad2
mod2 UV
seed
fresh
bunch
delay
monochromator

4. EEHG seeding results from 260 nm to 1 nm
Get a long, coherent pulse
~ 400 MW peak power at 1nm
from ~ 1 GW laser power at 260 nm, 400 fs FWHM
75 fs and 40 meV FWHM: ~ 2 × transform limit
weakest LH setting, bigger pedestal and ½ peak brightness

5. EEHG seeding results from 257 nm to 2.3 nm (540 eV)
Showing highest LH setting (running 6 keV case now)
EEHG seeding results from 257 nm to 2.3 nm (540 eV)
Outputs 4 GW at 540 eV
from 400 MW at ~260 nm
112th harmonic
starts with >4% bunching
125 fs and 20 meV FWHM:
~ 1.5 × transform limit

6. HGHG seeding from 260 nm to 13 nm to 1 nm
Basically, does not work
may get a short but incoherent pulse
large induced energy spreads
competes with SASE from current spikes
Results for best quality beam
with LH at 6 keV, the seeded pulse is lost
seeded part

7. HGHG seeding from 257 nm to 18.4 nm to 2.3 nm
Short run (in progress)
Narrow pulse, ~5 fs FWHM
Not bad time-bandwidth product
but essentially a single spike

8. Self-seeding at 1.24 keV
significant numerical noise monochromator close to center chicane set to 1mm looks like did not reach saturation
is this okay?

9. Self-seeding at 1.24 keV: spectrum
some SASE or other noise is showing through
before
monochromator
final
spectrum

10. Moving the monochromator upstream
Slightly more pedestal
could be SASE or wakefields
Not much difference otherwise
at 45 m
at 54 m

11. Microbunching has a significant impact on spectrum
Half the peak brightness, worse signal to noise
when laser heater does not sufficiently damp microbunching

12. Comments on self-seeding
For these runs, did not re-randomize particle phases
modeled propagation through chicane
pessimistic simulation, very susceptible to numerical noise
Optics model not optimized – no transverse focusing
radiation diffracts across chicane
in practice, will be re-imaged to roughly same spot size
Overall impact
pessimistic signal-to-noise ratio
monochromator positioned further upstream than needed

13. Summary: seeding schemes and microbunching
EEHG
good output power, control over pulse length
somewhat sensitive to microbunching and wakefields
Self-Seeding
whole core of bunch radiates (unless chirp beam)
still working to quantify background noise
2-stage HGHG
only for short pulses?
demonstrated good results down to ~4 nm
simulations look good at 540 eV
at ~ 1 keV is challenging, definitely incoherent

15. Long drift is before chicane, should be okay
EEHG at FERMI-2
Basic layout:
Long drift is before chicane, should be okay
Some impact of betatron motion and geometric emittance?
Sources of radial dependence of energy modulation
laser waist is ~ 3x e-beam size
somewhat long modulators, self-modulation
delay line
up to 1mm R56
chicane,
undulators
‘off’
mod1
mod2
rad2 UV
seeds
chicane 2
both fixes require
more laser power

16. First try at laser parameters
260 nm seeding, targeting 65 nm bunching
1st energy modulation ~ 1 MeV, 32 MW peak power
2nd energy modulation ~ 2.4 MeV, 265 MW peak power
required to keep R56 of 1st chicane <= 1 mm
laser waist ~ 290 micron
Rms energy spread after EEHG stage is 1.9 MeV
too high, debunching happens very fast

17. Ideal bunching generated through EEHG
Complex bunching parameter, ignoring issues like scatter where combines laser phases, m and p are any integers which give the desired bunching wavenumber

18. Achieving bunching at 4 nm with R1<=1 mm
Usually expect only one combination of m,p to be significant
m=0 is like HGHG, typical EEHG choice is m=1
High ratio of energy modulation to energy spread can give multiple contributions
they can either interfere or add to each other
more erratic spectrum of modulated current
For m=2, modulating by 0.9 MeV, 2.4 MeV seems optimal
normally, bunching always better with increased hM1
here, other effects seem to suppress the extra terms
self-modulation, laser profile, beam emittance

19. idealized results bunching spectrum Simulation results, m=2
250 keV energy spread
100 keV energy spread

20. Want energy spread < 1.5 MeV to get significant power
A few options:
find a way to increase R1
first chicane and undulators too weak
kick the beam to generate dispersion?
use m=3
smaller beta=10 m (could go as low as 8 m?)
higher peak current
Try keeping R1=1 mm, use m=3 (p=68) and beta=10 m
modulate by 0.9 MeV and 1.6 MeV
final energy spread ~ 1.3 MeV
hope to get 4% bunching

21. First serious try at a seeding study
m=3, final rms energy spread 1.3 MeV
only get 2% bunching
debunches immediately
get ~ 1.5 MW same as when tried m=2
For EEHG, microbunching in phase space is highest at extremes of energy distribution
very little bunching at central energy
is it even worse for m>1?
rms energy spread is too optimistic a criterion
To get gain, need to reduce modulation
 bigger R56 or longer wavelength (or both)
aim for only 500 keV energy spread?