IPAC2022, Bangkok, Thailand
JACoW Publishing
ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT047

PROGRESS REPORT ON POPULATION INVERSION X-RAY LASEROSCILLATOR AT LCLS
A. Halavanau, F. Fuller, R. Robles, R. Margraf, A. Aquila, M. Liang, R. Alonso-Mori,R. H. Paul, A. A. Lutman, F.-J. Decker, C. Pellegrini
SLAC National Accelerator Laboratory, Stanford University, Menlo Park CA 94025, USA
P. Manwani, N. Majernik, J. B. Rosenzweig
University of California, Los Angeles, CA, 90095, USA
N. Welke, R. Ash, U. BergmannUniversity of Wisconsin, Madison, WI, 53706, USA
Š. Krušič, M. Žitnik
Jozef Stefan Institute, Ljubljana, Slovenia
A. Benediktovitch, N. Rohringer
University of Hamburg, CFEL/DESY, Germany
Abstract

operate the oscillator. As a first step toward the realization
ofXLO we will have a two-pulse pump regenerative
We report the progress in the design and construction ofamplifier.
a population inversion x-ray laser oscillator (XLO) using
LCLS as an x-ray laser pump, being developed by a SLAC,FAST MOVING SOLID TARGET
CFEL, University of Hamburg (Germany), University ofWisconsin, Josef Stefan Institute (Slovenia) and UCLA colIn our initial work on XLO we considered using alaboration. In this proceeding, we will present the latest
high-speed
jet of a copper nitrate solution as the pump
XLO design and numerical simulations substantiated by ourpulse
target[5]. Preliminary experiments with the liquid
first experimental results. In our next experimental step
jethaveshown
that at extreme x-ray power densities theXLO will be tested on the Coherent X-ray Imaging (CXI)
liquidma-terial
undergoes through a process of violent
end-station at LCLS as a two pass Regenerative Amplifierexpansion
ontensof nanoseconds time scale [6, 7]. This
op-erating at the Copper 𝐾𝛼1 photon energy of 8048 eV.
fact
prevents
theuseof simple liquid jet systems in theWhen built, XLO will generate fully coherent transform
XLO-likeexperiments.
While an ultra-fast liquid jet islimited pulses with about 50 meV FWHM bandwidth. We
technologicallypossible,it requires a substantial
expect the XLO will pave the way for new user
development
andinstallation
effort, and is outside of theexperiments, e.g. in inelastic x-ray scattering, parametricscopeofourproject.
down conversion, quantum science, x-ray interferometry,We therefore considered using a fast spinning solidand external hard x-ray XFEL seeding.
target. There are several advantages of solid w.r.t liquid
target. First, a larger, about 10 times, density of copper
INTRODUCTION
atoms allows to reduce pump pulse requirements from
A population inversion X-ray laser oscillator, several mJ, as in the original proposal, to tens of µJ. TheXLO, pumped by an X-ray FEL like LCLS at SLAC, will reduced pulse intensity is much easier to be achieved at theprovide high intensity, transform limited pulses, XFEL, and it drastically reduces the heat load on the cavity
opening-up new experimental possibilities in the in-coupling crystal; see Ref. [1, 5]. The target thickness
exploration of atomic and molecular systems at the has been optimized in the state-of-the-art simulation
angstrom/femtosecond
spaceandtime
scale, XLO-sim toolbox that will be described in detailinterferometry, X-ray quantum science and more. We elsewhere [8]. The optimum target width was found to be
discuss in this paper several experimental and simulation 25 µm; see Fig. 1. We refer the reader to Refs. [9, 10] forsteps that the XLO collaboration is taking to realize the oscil- a detailed description of the target assembly and XFEL
lator. Our present efforts are mainly addressed to a precise induced damage.
determination of the extent of the damage done by the X-rayCu foilpump pulse to the gain medium, the development of a solid1087copper, high speed, gain medium target that would provide
10fresh copper material to the pumping pulses, repetition of106
the amplified spontaneous emission (ASE) [1–4] gain mea105
surement with and without external seeding, optical cavity
1045
1015202530
35404550z ( m)alignment using Si crystals, advanced 3D numerical simulations. In addition, we are working to generate from LCLS Figure 1: Simulated number of photons after the first pass
the train of pump pulses separated by about 30 ns needed to in the XLO.
Nphot

Nphot(z)MC2: Photon Sources and Electron Accelerators
A06: Free Electron Lasers

TUPOPT047
1107

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13th Int. Particle Acc. Conf.
ISBN: 978-3-95450-227-1

IPAC2022, Bangkok, Thailand
JACoW Publishing
ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT047

Figure 3: Top view of the first XLO experiment in theCXI hutch at LCLS.

At the exit of the gain medium, the x-ray radiation may
possess certain degree of coherence, as it can be seen inFig. 2. However, it should be noted, that due to thesponta-neous nature of the initial XLO pulse, similar to
SASE of the electron beam, intensity fluctuations may
occur. Therefore, the purpose of Bragg crystal cavity is not
only to return the radiation but to also drastically improvethe coherence.
We performed the initial cavity design investigation
as-suming flat perfect Silicon crystals and ideal Be
compound refractive lenses (CRL). The cavity layout ispresented in Fig. 3. In brief, four flat perfect Si (4,4,4)
crystals (C1-C4) make up a cavity, recirculating the x-ray
beam in a clock-wise direction. Crystals C2 and C3 haveadjustable positions to enable cavity length tunability, from
8500 mm to 10 500 mm. The cavity radiation is refocusedwith a CRL onto the target foil. XLO cavity crystals havebeen procured from Sarton Works (Japan) in a form of 1
inch cubes (C1-C3) and 50 µm membranes (C4). We
characterized the crystal reflectivity using in-houserocking curve imaging (RCI) setup at SSRL beamline 10-2;
see Ref. [11], and compared it with the ana-lytical formulafor crystal reflectivity. We found the crystals to be very
close to perfect, as it is expected from highest grade
Silicon; see Fig. 4. The dispersive nature of the Braggreflection results in spectral-angular acceptance, that can be
described in a form of DuMond diagram; see Fig. 5. X-ray
propagation through the thin Silicon membrane (C4) is investigated using the adapted multi-slice method, recentlypresented in Ref. [12].
In addition, a significant collimation is expected throughthe CRL element.
TUPOPT047
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4246024x (mm)210401060620
rad2RC Center, XLO C40rady (mm)4202204 6x (mm)Figure 4: Rocking curve maps of the XLO crystal 1(thick Si block, left) and crystal 4 (thin membrane, right).

PLANNED EXPERIMENTAL PROGRAM
The initial demonstration of XLO will be done with Si(444) crystals, in back-scattering bow-tie geometry. TheLCLS-II HXR undulator will be tuned to provide 9 keVpump pulses, with the first 3-4 undulators will be used forseeding with 8 keV. We note that due to the Lorentziannature of the atomic lines, it is possible to seed the stimulated emission process in Copper at 𝐾𝛼1 line, even 10 eVaway from it in energy. The latter fact allows for the mostof the “seeding” SASE pulse to enhance the probability ofemission, while not being recirculated in the cavity [13].

1.040

0.8

20

Reflectivity

X-RAY CAVITY DESIGNRC Center, XLO C10, meVFigure 2: Simulated x-ray radiation profile at the exit of thecopper target: Wigner’s time-frequency distribution (top)and angular and spectral-angular distributions (bottom) after
the first pump pulse.

y (mm)

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13th Int. Particle Acc. Conf.
ISBN: 978-3-95450-227-1

0.6

00.4

200.2

40
20

100x, rad10200.0Figure 5: DuMond diagram of the ideal bow-tie x-ray Si(4,4,4) cavity as shown in Fig. 3.
MC2: Photon Sources and Electron Accelerators
A06: Free Electron Lasers

IPAC2022, Bangkok, Thailand
JACoW Publishing
ISSN: 2673-5490
doi:10.18429/JACoW-IPAC2022-TUPOPT047

In the upcoming round of XLO experiments we will first
focus on the demonstration of target replenishment and cavity performance. We will measure the target damage as afunction of pump intensity, and determine the minimum
rotation speed needed for target replenishment. After that,
we will proceed with establishing the cavity reflections. TheXLO beam will be sent onto C1, and the crystal will berocked around the theta axis to find Bragg reflection. The
reflection intensity will be optimized with a highly sensitive
PIPS diode at the C2 location, and the exercise will be repeated for C2 and C3. Finally, after the alignment of crystal
C4 will be done by placing the diode behind it and rocking it
to minimize the signal on the diode. In order to continue thecavity alignment, namely making sure the reflections stay inplane, we will introduce an intra-cavity diagnostics element,
thin Si membrane, followed by the fast photodiode. Using
this diagnostics we can ensure multiple cavity roundtrips.
After the cavity studies, we will proceed with generating two consequtive XLO pulses ∼ 30 ns apart. We will
use the LCLS multi-bunch capabilities [14]. The two- andmulti-bunch performance of the LCLS copper linac has been
recently improved by introducing two stripline-style kickers
in early in the machine [15].

SUMMARY
XLO, when built, will become an additional highly coherent hard x-ray source at LCLS, enabling many potential
new experiments. A vibrant program is underway at LCLS
to perform first cavity experiments in the near future.

ACKNOWLEDGEMENTS
This work is supported by the U.S. Department of Energy
Contract No. DE-AC02-76SF00515.

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13th Int. Particle Acc. Conf.
ISBN: 978-3-95450-227-1