FREIA Report 2019/01

January 2019

DEPARTMENT OF PHYSICS AND ASTRONOMY

UPPSALA UNIVERSITY

Science Requirements and

Performance Specification
for the CompactLight
X-Ray Free-Electron Laser

Alan Mak, Peter Salén, Vitaliy Goryashko

Department of Physics and Astronomy, Uppsala University, Sweden

Jim Clarke

STFC Daresbury Laboratory, Warrington, United Kingdom

Department of
Physics and Astronomy
Uppsala University
P.O. Box 516
SE – 751 20 Uppsala Papers in the FREIA Report Series are published on internet in PDF- formats.
Sweden Download from http://uu.diva-portal.org
Science Requirements and Performance Specification for
the CompactLight X-Ray Free-Electron Laser
Alan Mak, Peter Salén, Vitaliy Goryashko
FREIA Laboratory, Uppsala University, Uppsala, Sweden

Jim Clarke
STFC Daresbury Laboratory, Warrington, United Kingdom

21 December 2018 Version 1.0

1 Introduction
CompactLight [1] is a consortium funded by the European Union through the Ho-
rizon 2020 Research and Innovation Programme under Grant Agreement No. 777431.
The 24 partner institutes are working collaboratively towards the conceptual design of
a next-generation x-ray free-electron laser (FEL). CompactLight intends to design an
x-ray FEL facility beyond today’s state of the art, using the latest concepts for bright
electron photo-injectors, high-gradient X-band structures at 12 GHz, and innovative
short-period undulators. If compared to existing facilities, the proposed facility will
(i) benefit from a lower electron beam energy, due to the enhanced undulator per-
formance, (ii) be significantly more compact, as a consequence of the lower beam en-
ergy and the high gradient of the X-band structures, (iii) have a much lower electrical
power demand and a smaller footprint. All of these enhancements will make our
design more affordable to build and operate when compared against the existing fa-
cilities. The CompactLight prime objective is to generate a compact and affordable
FEL facility design. Note that we aim to make the FEL affordable to operate as well as
to build.
The specifications of this future FEL are driven by the demands of potential users
and the associated science case. This report summarizes the findings of our inter-
actions with potential users since the start of the design study through a number of
different avenues, culminating in a dedicated CompactLight User Meeting that was
held from the 27th to the 28th of November 2018 at the European Organisation for
Nuclear Research (CERN) in Geneva, Switzerland [2]. The primary objective of the
meeting was to consult potential users on the photon characteristics required by their
current and future experiments. In addition to the dedicated User Meeting, two in-
formal meetings with potential academic and industry users have been held in the
UK, a specially developed questionnaire has been sent to over 50 FEL experts within
Europe, and CompactLight have sent representatives to the Science@FELs Conference
in Stockholm, Sweden in June 2018 and the Attosecond and FEL Science Conference
in London, UK in July 2018, to hear about the latest scientific achievements using FEL

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

facilities and to informally interact with leading researchers to gather their views on
the parameters and performance of future FELs.

2 Examples of groundbreaking research with x-ray FELs

2.1 Scattering
2.1.1 Bio-imaging
The structure of biomolecules such as proteins, viruses or cells is essential for its
function. Hence, the high-resolution structure determination enabled by coherent x-
ray radiation is critical in the fields of biology and life science and permits e.g. ra-
tional drug design and the understanding of human biochemistry. A key method in
this context is “diffraction-before-destruction” in which the ultrashort duration of the
x-ray pulse is exploited for outrunning the sample radiation damage. It offers the op-
portunity to image important bio-objects that can only be formed in smaller crystals,
such as membrane proteins, or even single particles, with varying resolution. In par-
ticular, hard x-rays with wavelengths in the ångström range provide extremely high
resolution [3] while soft x-rays offer useful information of larger structures, e.g. living
cells, with high throughput. The ultrashort, intense x-ray pulses provided by FELs
additionally allow measuring the dynamics of biologically relevant molecules on their
natural, femtosecond, time scale.
Structure determination of micrometre-sized, or smaller, crystals at FELs is often
carried out using serial femtosecond crystallography. Typically a liquid jet provides a
stream of crystals that intersects the x-ray beam and the high intensity of the FEL x-ray
beam enables the collection of diffraction images of thousands of randomly oriented
crystals, which can be reconstructed into a 3D image with up to atomic resolution
[4, 5]. Major progress with respect to the sample delivery has been made recently by
increasing the speed of the liquid jet, which enables a fresh sample with every x-ray
pulse at a MHz repetition rate [6, 7]. Simultaneous detector frame-rate development
has also been carried out [8]. Another viable sample delivery method uses fixed targets
and has important advantages such as an order of magnitude increase in the probabil-
ity of hitting the sample by an x-ray pulse. This sample delivery method requires low
kHz repetition rates.
A major scientific driver of x-ray FELs is the potential of single-particle imaging
(SPI) of biological molecules at atomic resolution [9]. Although this goal is far from
being reached there have been several measurements of biological objects at lower
resolution. Moreover, SPI has tremendous potential for observing the dynamics of
biological, chemical and physical systems [10]. Further progress requires development
in sample delivery that optimizes the amount of useful collected data, specifically in
aerosol injection which is advantageous in SPI because it provides a protective layer
around the particle, thin enough to avoid excessive background scattering [11]. Recent
studies [12] demonstrate aerosol particle speeds sufficient to cope with the 4.5 MHz
intra-train repetition rate of the European XFEL.

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

2.1.2 Matter under extreme conditions

Intense laser pulses applied to solid materials can produce nanosecond or sub-
nanosecond dynamic compression into extreme pressure regimes. The material re-
sponse to the unexplored pressure and temperature conditions created by such com-
pression can be uniquely explored in diffraction experiments with 100-fs temporal
resolution that resolves the atomic motions. Moreover, the brilliance and small focus
of the x-ray FEL pulse relaxes the pulse energy requirement of the optical pump laser,
which can then also be focused to a small spot.
Studies of shock waves probed by 8-keV x-rays at FELs have measured the ultimate
compressive strength, associated with a purely elastic response, of Copper, and the
plastic flow occurring at higher strain orders [13]. Diffraction studies have also been
applied to investigations of phase transitions and melting. For example, melting of Bi
was observed after a few nanoseconds upon the release of the dynamically 8-14 GPa
induced compression [14]. At LCLS shock pressures exceeding 120 GPa were used to
demonstrate conversion of graphite to diamond and lonsdaleite phases [15] and com-
plicated structures including linear guest structures arranged as chains in channels of
the host structure have been observed. Moreover, pump-probe x-ray diffraction stud-
ies have revealed amorphous to crystalline transitions [16].
High energy-density plasmas are characterized by temperatures above 1 eV (about
11600 K) and densities higher than that of a typical solid and are relevant in e.g. planet
cores, stellar interiors, intense laser-matter interactions and fusion experiments. High-
brilliance (hard-)x-ray FELs are well adapted for such studies as they satisfy both the
requirements of generating and detecting the hot and dense plasmas deep into the
sample. It also provides the necessary spatial resolution and the temporal resolution
ranging from attosecond electron dynamics to compression processes on the nano-
second time scale. The potential of depositing mJ energies into a micrometre scale
sample in less than 100 fs enables a homogeneous high-energy density distribution
with known properties, which is formed faster than the hydrodynamic expansion. The
first experimental demonstration of such an isochoric heating process was performed
at LCLS in 2015 [17].

2.2 Spectroscopy
X-ray spectroscopy provides complementary information to imaging and diffrac-
tion measurements of the chemical and electronic properties of the system and the
techniques are ultimately combined [18]. X-ray absorption (XAS) and emission spec-
troscopy (XES) enables element specific measurements of the unoccupied and occu-
pied electronic states, respectively, while x-ray photoelectron spectroscopy (XPS) is
highly sensitive to the chemical surroundings and offers surface sensitivity. The high-
peak-brilliance FEL pulses allow new approaches to spectroscopic experiments such
as nonlinear excitations, single-shot detection and femtosecond time-resolved meas-
urements.
The potential for multiphoton excitation of atoms and molecules by FELs has been
exploited for the formation of two-site double core-hole (tsDCH) states that could be
detected using XPS [19, 20]. These states are created by the ejection of one core electron
on separate atoms and require high peak intensities in order to ionize the second atom

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

before Auger decay occurs in the first atom. These states have received attention due to
the significantly enhanced chemical sensitivity compared with single core-hole states.
XES and XAS studies carried out at x-ray FELs have been helpful in understanding
important chemical processes. Electron transfer is essential in biological systems and
for artificial light harvesting. XES and XAS measurements on an electron-harvesting
chromophore performed at SACLA demonstrated the potential of these methods for
monitoring fundamental chemical processes [21]. Moreover, resonant inelastic x-ray
scattering has proven a capable tool for investigating excited state dynamics in solu-
tion via detection of orbital interaction [22]. The potential of using XAS and XES
at FELs for understanding catalytic reactions have also been demonstrated at LCLS
where these techniques have enabled the observation of CO oxidation on a Ru surface
[23, 24].
X-ray magnetic circular dichroism (XMCD) spectra, obtained as the difference between
XAS data with opposite circular polarization, offers a way to probe the magnetic prop-
erties of materials. Recently, the high peak brilliance of the soft-x-ray FEL was ex-
ploited in a demonstration of time-resolved (tr) XMCD, which was applied to invest-
igate the element specific all-optical switching dynamics in GdFeCo with femtosecond
temporal resolution [25]. Moreover, sub-picosecond demagnetization dynamics was
studied by tr-XMCD using hard x-rays resonant with the Pt L3 edge (11.6 keV) [26].

2.3 Time-resolved experiments

2.3.1 Ultrafast magnetism
Circularly polarized optical femtosecond pulses are able to reverse magnetization
on a picosecond time scale [27]. X-ray FEL pulses offer an efficient tool for probing the
ultrafast magnetization dynamics on a femtosecond time scale with nanometre resolu-
tion. For example, soft-x-ray holography using circular polarization has demonstrated
15 nm resolution [28]. Time-resolved soft-x-ray resonant diffraction studies carried
out at LCLS detected Gd spin reversal within the first picosecond in the ferromagnetic
GdFeCo. It was explained by a nanoscale flow of angular momentum from Fe-rich to
Gd-rich regions induced by the optical pump [29]. The control of magnetic properties
via the spins opens the door to faster data storage and processing devices.

2.3.2 Strongly correlated electron systems

In strongly correlated electron systems the interaction between electrons is non-
negligible and may severely influence the character of the material. Light-induced
insulator-metal transitions (IMT) dominantly driven by electron correlation effects as
opposed to large structural changes, so-called Mott transitions, is a promising route
towards faster electronics. Time-resolved x-ray diffraction experiments at FELs fol-
lowing the structural dynamics of the THz-induced IMT in VO2 have shown that
the electronic metallization dynamics and structural phase transitions can occur on
different time scales [30]. This opens the door to efficient conductivity switching in
correlated-electron systems.
X-ray FEL pulses are also useful for studying the lattice changes associated with
light-induced superconductive phases. For example, THz pulses have been shown

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

to create superconducting properties in cuprate materials and using femtosecond x-

ray diffraction the behaviour of the lattice structure was investigated for this exotic
state. These studies revealed that the nonlinear excitation of the crystal lattice struc-
ture creates a displaced lattice geometry which cause drastic changes in the electronic
structure, and may cause destabilization of the charge-density wave order, of which
both may favour superconductivity [31].

2.3.3 Water dynamics

Water is a surprisingly complex liquid that is still far from understood. The reason
for its complexity and anomalous properties is the ability to form highly disordered
hydrogen-bonded networks. X-ray FELs permit resolving the water structural dy-
namics on a sub-100-fs time scale and atomic length scales. In a recent experiment at
LCLS [32] using 8.2 keV photon energy water structural motions was observed from
the decay of speckle contrast when tuning the pulse duration from 10 to 120 fs. They
showed that cage effects due to hydrogen bonding play an important role in the slower
dynamics of water upon cooling.

3 Science requirements on a next-generation FEL

3.1 Trends in the FEL science community
Representatives from CompactLight participated in and presented posters at two
conferences devoted to science research at FELs: (i) Science@FELs Conference in Stock-
holm, Sweden in June 2018 and (ii) Attosecond and FEL Science Conference in Lon-
don, UK in July 2018. The presented talks and informal discussions with FEL users
showed trends towards experiments demanding higher photon pulse energies, better
focus and shorter pulse durations. For instance, there is an increased interest in using
FELs to explore:
1. materials far from equilibrium such as light-induced superconductivity;

2. nonlinear x-ray optics;

3. multidimensional attosecond spectroscopy;
4. charge migration and ultrafast x-ray damage in biomolecules;

5. surface chemistry and pathways for catalysis;

6. matter under extreme conditions.

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

Figure 1: Characteristic time and energy scales of fundamental processes in atomic,

molecular, electronic, spin and lattice systems. The characteristic length scales are
indicated on the top bar. The minimum time and length resolutions for a given photon
energy are limited by the Heisenberg uncertainty. Figure adopted from Ref. [33].

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

Figure 2: Number of photons per pulse into 1% bandwidth as required by different

experimental ultrafast x-ray techniques (blue). The research areas relaying on the tech-
niques are shown in pink. The high-fluence regime enables nonlinear x-ray spectro-
scopies and single-shot imaging, potentially with atomic resolution. Figure adopted
from Ref. [33].

The trends towards shorter pulse durations and higher photon pulse energies has
been recently reviewed by a group of users from around 20 universities and national
laboratories worldwide [33]. The photon requirements for different disciplines are
graphically presented in Figs. 1 and 2. The requests for next-generation FELs collec-
ted in informal discussion at the conferences fully support the data presented in the
figures. In addition, during the discussions there were very strong requests for im-
proving the coherence and stability properties of FEL radiation pulses as well as much
better synchronization to external laser sources.

3.2 Survey on user requirements

In conjunction with the CompactLight User Meeting held at CERN, a preliminary
survey was conducted through the use of an online questionnaire. The purpose was
to gather quantitative information about the user requirements on the photon charac-
teristics. A total of 10 responses were received from potential users.
The respondents have expressed interests in experiments such as (i) pump-probe
diffraction, (ii) serial crystallography, (iii) time-resolved spectroscopy and (iv) time-
resolved scattering. In addition, each respondent has specified either one or two sets of
desired parameter values for the future x-ray FEL. These parameter values are shown
as histograms in Fig. 3.
With regard to the tunability, there is a clear demand for photon energies as low as

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

Figure 3: The results of the survey are summarized in histograms showing the users’
requirements on the (a) minimum photon energy, (b) mean photon energy, (c) max-
imum photon energy, (d) pulse energy, (e) pulse energy stability, (f) pulse duration,
(g) repetition rate, (h) transverse coherence, (i) longitudinal coherence, (j) bandwidth,
(k) focused spot size and (l) synchronization between the FEL and the external laser.

0.2 keV [see Fig. 3(a)] and as high as 20 keV [see Fig. 3(c)]. The mean photon energy of
the desired tunable range is about 4 keV [see Fig. 3(b)].
The preferable pulse energy is in the range of 3–100 µJ [see Fig. 3(d)]. Furthermore,
the demand on the stability of the pulse energy is stringent, and most respondents
want the RMS fluctuation in pulse energy to stay below 12% [see Fig. 3(e)].
Most respondents prefer a pulse duration of 10–100 fs [see Fig. 3(f)], a repetition
rate higher than 100 Hz [see Fig. 3(g)], a degree of transverse coherence higher than
70% [see Fig. 3(h)], a coherence time of 1–100 fs [see Fig. 3(i)], a bandwidth of 0.1–
1% [see Fig. 3(j)] and a microfocus of 0.1–100 µm [see Fig. 3(k)]. For pump-probe
experiments, most respondents want the synchronization between the FEL and the
external laser to be in the order of 10 fs [see Fig. 3(l)].
The questionnaire also asks the potential users to comment on any FEL feature that
would benefit their future experiments. The answers are summarized as follows:
• variable polarization (linear and elliptical);

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

• pulse energy above 3 mJ;

• shorter pulse duration;
• higher stability in pulse energy and pulse duration;
• repetition rate of 1–10 kHz;
• laser-FEL synchronization better than 50 fs;
• FEL-pump FEL-probe capabilities with a large photon energy difference;
• small focused spot size;
• tunability extended to higher photon energies;
• better reliability of two-colour pulse generation.

4 Summary of Findings and CompactLight Specification

This report summarizes all of the discussions and interactions that the Compact-
Light collaboration has held with potential users of the facility during the past year.
The exploitation of FELs by numerous groups covering a diverse range of research top-
ics highlights why FELs are incredibly important engines of discovery. The diversity
also means that it is not possible to meet the current and future needs of all users with
a single facility. Indeed, there is a risk that by trying to satisfy all requirements the
facility performance would be compromised and no users would be entirely satisfied.
The CompactLight collaboration understands this issue and so has distilled all of the
user input into a coherent specification that is fully aligned with our prime strategic
objective which is to generate a compact and affordable FEL facility design.
The specification of the CompactLight FEL is summarized in the following bullet
points and in Table 1.
• Stability in all its aspects is very important to all experiments. We will bear this
in mind in all of our technical designs of systems and sub-systems.
• Seeding of the FEL enhances the output quality significantly and will be imple-
mented at all wavelengths, where feasible, and where compatible with our com-
pact and low-cost objective.

Table 1: Main parameters of the CompactLight FEL.

Parameter Unit Soft-x-ray FEL Hard-x-ray FEL
Photon energy keV 0.25 – 2.0 2.0 – 16.0
Wavelength nm 5.0 – 0.6 0.6 – 0.08
Repetition rate Hz 1000 100
Pulse duration fs 0.1 – 50 1 – 50
Polarization Variable, selectable Variable, selectable
Two-pulse delay fs ±100 ±100
Two-colour separation % 20 10
Synchronization fs <10 <10

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

• Peak brilliance is key to many experiments and we will aim to maximize this
where compatible with our compact and low-cost objective.
• Extreme synchronization between different photon sources for time-resolved pump-
probe experiments is vital and we will provide a design that can synchronize the
FEL with a conventional laser to better than 10 fs.
• Two pulses and two wavelengths are essential for many experiments. We will
develop a design that provides these capabilities in as compact and low-cost a
way as possible.
• Generating pulses as short as 100 attoseconds is desirable but may take signific-
ant extra space and cost. In this case we will relax the specification to a point
where the cost impact is negligible.
• A repetition rate of 1000 Hz for the soft-x-ray FEL will be a unique and highly
desirable feature of our facility. We recognize that this is a very challenging tar-
get for many systems and that we may have to compromise on this aspirational
target during the course of the Design Study.
• The photon pulse bandwidth will be minimized to maximize the peak brilliance
where compatible with our compact and low-cost objective.
• The facility output will cover the range between 250 eV and 16.0 keV with all
photon energies within this range being accessible from at least one of the FEL
beamlines.
• The 2 keV “boundary” between the soft-x-ray FEL and the hard-x-ray FEL is
not rigid and will be determined by the collaboration when considering all the
technical options including electron beam energies, undulator performance, and
x-ray optics capabilities.
• Tuning across photon energies will primarily be achieved by undulator scanning
rather than energy scanning to maximize the efficient operation of the facility.
We will operate the FELs at a few discrete electron beam energies, as required, to
achieve the full wavelength tuning ranges.
• The FEL output pulses will be evenly spaced in time and not provided in a burst
mode.
• To maximize the efficient use of the facility we recognize that simultaneous oper-
ation of both the soft- and hard-x-ray FELs would be beneficial. We will investig-
ate options for achieving this where compatible with our compact and low-cost
objective.
• We recognize that output pulse energies will naturally be reduced when we op-
erate at the very shortest pulse lengths, as they do at other FEL facilities. We will
ensure that our pulses remain competitive with these other facilities.
• Variable, selectable polarization is required at the sample for all photon energies.
We will investigate options for the production of such variable polarization at
different photon energies using either optical elements in the photon beamline or
elliptical undulators.

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CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

The target performance of CompactLight in terms of peak brilliance is shown graph-

ically in Fig. 4. The peak brilliance is expected to be comparable to the state-of-the-art
x-ray FEL facilities which are currently in operation.

10 35
European XFEL (Germany)
SACLA (Japan)
PAL XFEL (South Korea)
10 33 LCLS (USA)
Peak Brilliance [photons s-1 mm-2 mrad-2 (0.1% BW)-1]

Swiss FEL (Switzerland)

Target of CompactLight
FERMI (Italy)
10 31 FLASH (Germany)
SPring-8 (Japan)
PETRA III (Germany)
10 29 ESRF (France)
APS (USA)
NSLS II (USA)
SLS (Switzerland)
10 27 BESSY (Germany)

10 25

10 23

10 21

10 19
10 1 10 2 10 3 10 4 10 5 10 6
Photon Energy [eV]

Figure 4: Peak brilliance as a function of photon energy for a selected set of x-ray
sources. Free-electron laser facilities are shown in solid lines, and synchrotron facilities
are shown in dashed lines. This figure is adapted from Fig. 1 in Ref. [34].

Appendix A Responses to online questionnaire

As discussed in Section 3.2, a preliminary survey was conducted by means of an on-
line questionnaire in conjunction with the CompactLight User Meeting held at CERN.
The purpose was to gather quantitative information about the user requirements on
the photon characteristics. In each of the 10 responses received from potential users,
either one or two sets of desired parameter values for the CompactLight FEL were
specified. These parameter values are listed in Table 2.

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 CompactLight Collaboration

Table 2: Photon characteristics specified by potential users in the online questionnaire. Blue columns correspond to diffraction or
scattering experiments. Yellow columns correspond to spectroscopy. Orange columns correspond to interactions between x-ray and
matter. Grey columns correspond to other experiments.
Min. photon energy [keV] 4.5 0.05 0.01 3 1 0.5 0.5 0.01 1 0.2 3 3 0.7 0.5 2 0.2
Max. photon energy [keV] 12.6 7 10 20 12 10 15 0.5 9 10 16 16 10 10 20 20
Repetition rate [Hz] 120 1000 100000 1.1 100 1000 100000000 120 1000000 1000000 4500000 100 10000 1000 1000000
Pulse energy [µJ] 100 250 100 1000 2000 1 5 3000 10000 2000 5000 10 10 10 100

12
RMS pulse energy stability [%] 20 10 10 0.1 1 10 1 10 10 10 10 10 0.1 0.01 5
Microfocus [µm] 1 1 0.5 10 0.1 10 5 1 0.1 1 0.25 100 100 10 30
Degree of transverse coherence [%] 100 80 100 80 100 90 80 80 100 100 30 10
Coherence time [fs] 1 2 0.2 1 2 10 1 100 50 10
RMS bandwidth [%] 0.05 20 10 0.01 0.01 0.3 0.3 0.5 0.2 10 10
FWHM pulse duration [fs] 10 2 0.2 50 40 10 60 1 50 10 50 10 100 500
Two-pulse spectral separation [nm] 0.6 0 0 100 100
Two-pulse temporal separation [fs] 100 150 10 0 100 10000 1000
Laser-FEL sync [fs] 10 10 1 300 40 10 200 30 50 10 10 5
Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke
CompactLight Collaboration Alan Mak, Peter Salén, Vitaliy Goryashko, Jim Clarke

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