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

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

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

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

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

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

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

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?

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

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

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

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

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 (FERMI@Elettra) simulations look good at 540 eV at ~ 1 keV is challenging, definitely incoherent

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

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

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

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

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

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

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?