Summary of the FEL15 conference

Jürgen Pfingstner3rd September 2015

Content

1. Introduction2. Beam physics3. Photon production4. Photon science5. Poster session6. Conclusions

1. Introduction

Free electron laser conference 2015

• Daejeon: science city, 3 hours from Seoul.• 55 oral presenations and 261 posters.

• PAL-XFEL installed right now (Pohang).• Other accelerators in Busan and Daejon.

OverviewTopics:• Photon production (3 sessions)• Technology (3 sessions)• Beam physics (1 session)• Photon science (1 session)• Status reports of facilities (1session)

Tour in Thursday• Pohang: PAL-XFEL, 10 GeV, 1.1 km, 400 MDollar, ~100 people • Daejeon: KAERI, KOMAC, smaller electron and proton linacs for different

applications including mm and IR FELs.

Unexpectedly, the oral presentations are not available yet. I had to improvise for this presentation.

2. Beam Physics

Coherent synchrotron radiation (CSR)Review of CSR:• Usually PISR prop. N.• But if bunch length is in the order of the

radiation wavelength then radiation is coherent and PCSR prop. N2.

• This is a problem for FELs, since bunches are short and the CSR is not shielded fully by the vacuum walls anymore.

• Hence significant CSR can be emitted in the dipoles of the bunch compressors.

Regime 1: • Correlated energy spread along the bunch is

created. • Due to the dispersion in the bunch compressor

the projected emittance can be increased.• Simone Di Mitri presented a bunch

compressor design that cancels the effects (emittance is preserved).

Regime 2:• Also higher frequencies can be emitted by the

beam, if there are charge modulations of corresponding wave length along the bunch.

• Reason: Shot noise, maybe also due to intensity variations of the gun laser.

• This triggers the micro-bunching instability (see next slide)

Micro-bunching instability

Charge modulation in beam

Energy modulation

Longitudinal space charge

More charge modulation

Bunch compressor

(R56)

LSC induced micro-bunching instability

CSR in bunch compressor

CSR induced micro-bunching instability

• Process repeated in bunch compressors and dog legs.

• High overall gain. • Problems reported form

LCLS, LCLS2 simulations and SACLA (6 degree bend).

• At SwissFEL only 2 degree bends.

Charge

E E

Suppression of micro-bunch instability

Laser heater:• Laser in undulator adds sinusoidal

energy modulation.• Sinus is washed out in second chicane. • Introduced at low energies.• Reduced micro-instability gain

significantly, due to increase Landau damping.

Shot noise suppression: • Using the shot LSC energy modulation in

a bunch compressor to suppress shot noise for wavelength of importance.

• D. Ratner, PRL paper.

Problems due to spikes in the current profile at LCLS

• Problems at LCLS have been reported (Y. Ding et al., poster) due to the typical “horns” in the charge profile of the electron beams after bunch compression.

• CSR effects have been more pronounced and reduce the beam quality due to the high peak current.

• Also the X-ray bandwidth has been increase.

• As a remedy, the horns were clipped in the first bunch compressor. • With that the average beam current could be increase to about 4-5kA and a higher photon

power was produce (100GW).

Illustrative plot not from paper!

3. Photon production

FEL-oscillator (FELO)

• K.-J. Kim.• System with very nice properties

– Very high spectral purity – High repetition rate – High average power

• Progress on the problematic mirrors in the X-ray regime was reported.

• Plans to test scheme at LCLS2 and potentially at the EuXFEL due to high repetition rate.

Two color FEL

• Only one scheme depicted (SASE style).

• Also possible to create X-ray pulses at the same location with seeded operation.

• Possible experiments are mentioned later.

Optimal taper design

• Tapering is a topic of high interest and is used in standard operation at LCLS.

• Recently there where ideas if the LCLS photon power could be increased to 1 TW.

• Schneidmiller and Yurkov derived universal tapering law:

• A easier to use approximation has been found:

• Simulation results for both tapers are nearly the same and give very high output power.

Inverse tapering• Nowadays mainly planar undulators

in FELs (linear polarized light) • User request also circular polarised

light (helical undulator). • Cheep upgrade is a helical

afterburner undulator. • But then the linearly polarised light

has to be suppressed without destroying the bunching.

• Solution is inverse tapering (Schneidmiller):

• This has been commissioned at LCLS, with the DELTA helical undulator (H.-D. Nuhn).

• Polarization and K value can be changed via a pole adjustment.

Estimation of FEL gain length in presence of collective effects

• There are formulas from Mie and Saldin for the increase of the FEL gain length, due to general properties of the beam (emittance, energy spread, …).

• S. Di Mitri presented expressions considering not general beam properties, but as a function of collective effects:– Transverse wake fields– CSR– Micro-bunching

• These effects are written in terms of a growth of the projected emittance and converted to a gain length increase.

4. Photon Science

Experiments at different X-ray wavelengthsSoft X-rays Hard X-rays

Spectroscopy:

• Main interest.• Inelastic scattering: absorption

spectra, photoemission of electrons.

• Measuring the electronic structure of matter.

Diffraction imaging

• Only larger structures can be resolved: cell, viruses, …

Diffraction imaging:

• Scattering of X-rays on electron cloud of matter.

• Measuring the geometrical properties of matter.

• Protein imaging.• Smaller wavelength would

better resolution.

Moessbauer spectroscopy:

• Core excitation• Hyperfine structure.• 0.1A necessary.

5A 0.1A100A

Other photon beam parametersWhy monochromatic beams?• For spectroscopy.• Exact shape of the spectra contains

information about electronic structure. • If X-rays have a wide bandwidth fine

structure of the spectra is not visible (energy resolution).

• In synchrotron light sources and in SASE FELS, mono-chromators are used. But this lowers intensity.

• Seed-FEL lower X-ray bandwidth.

E [keV]

Abso

rptio

n

What repetition rates are preferred?• For HXFEL, a few 100Hz seem to be okay,

since no detectors exist. • There is a project for the EuXFEL for

detectors with MHz rate, but has to be proven.

• For SXFEL, 10-100kHz is wished for. • Reason: spectroscopy is an averaging

measurement and not shot to shot acquisition is necessary.

• Higher repetition rates are not needed due to limits in sample delivery.

Motivation for protein imaging: e.g. pharmacology

Medicaments development are nowadays still based to a good extent on trial and error.

• The action of Viagra was understood only 2003

• The drug was created for the first time in 1989.

• Tamiflu (anti-flu) was the first medicament that was specifically tailored.

• Knowledge about the atomic structure of the virus was used (Synchrotron Light Source).

• This helps to make drug research more systematic and efficient.

Example for coherent diffraction imaging in FELs

• M. Suga et al. “Native structure of photosystem II at 1.95 A resolution viewed by femtosecond X-ray pulses”, Nature Letters.

• Motivation: Photo-synthesis converts light from the sun very effective into chemical energy that triggers the conversion of CO2 to O2. If Photo-synthesis would be fully understood then it could be maybe used as an alternative source of energy.

• The involved proteins have been studied in synchrotron light sources. Problem: long measurement times could change structure of protein.

• Measurements with FEL (SACLA) are single shot! The results give slightly different results of distances between atoms.

• The mechanism is understood now better and could help to make synthetic catalysts.

Example for two color soft X-rays spectroscopy

Performed at Elettra by K. C. Prince. Intention: measure the absorption edge more precisely (spectroscopy).

Principle:• Excite electrons with the two wavelength to two different states. • The emitted photons from one state are an s-wave while the other state emits a p-wave. • Each wave itself is symmetric is asymmetric but the overlap is not! • By changing the relative phase of the two X-ray wavelengths, the spatial distribution of emitted light is

changed.

Experimental possibilities:• Very precise measurements of absorption edges are possible. • The hope is to measure Wigner times for the first time (time delay of scattering events).

5. Poster session• We had two papers:

– TUP011: Performance and tolerance studies of the X-ray production for the X-band FEL collaboration.

– TUP013: The X-band FEL collaboration.

• There was quite some interest in the X-FEL collaboration paper: mainly for general education but also two more specific questions:

– Tor Raubenheimer: Bunch compression with phase linearisation. Contact with Tessa was established.

– Zhengtang Zhao: Asked about the current status of our bunch compressor design. He also invited us to come to Shanghai.

6. Conclusions

• Installation work for three large facilities are currently on-going: European XFEL, SwissFEL and PAL-FEL.

• Many interesting talks in the areas of: beam physics, photon production, photon science and technology.

• The topics covered practical problems observed in existing facilities as well as ideas for future concepts and ideas.

• It seemed that that a good fraction of the FEL community was present at the conference.

• Very useful information gathered and sever contacts established.

Thank you for your attention!