Summary of the FEL15 conference Jürgen Pfingstner 3 rd September 2015

Content 1.Introduction 2.Beam physics 3.Photon production 4.Photon science 5.Poster session 6.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.

Overview Topics: 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 P ISR prop. N. But if bunch length is in the order of the radiation wavelength then radiation is coherent and P CSR prop. N 2. 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 (R 56 ) 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 wavelengths Soft 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 0.1A 100A

Other photon beam parameters Why 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] Absorption 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, kHz 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 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 CO 2 to O 2. 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!