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Downloaded by guest on September 25, 2021 www.pnas.org/cgi/doi/10.1073/pnas.2005360117 this with developments and 7. work ref. recent in soft given tunability. most is the and the approach power in of las- peak review wavelengths and limited A time, several with that however, at from region, demonstrated done X-ray been been in experi- 100 has has about work These of ing More . with 6). fields. nm the (5, 18 magnetic around 1985 pumped lasing with X-ray plasma to plasma led hot ments the the confining from also X-rays plasmas, The cases cylindrical (4). some pumping 1980s in approach: the another in possible laser made X-ray visible- an short-pulse, high-peak-power, used the pump of scientists generate development to LLNL device could that reported nuclear time been a visible that has for population It at eV power. attain available required 1 to pump about power No to pumping inversion. compared large very keV inner-shell requires 10 (3). excite lasers, to to angstroms 1 needed in levels, energy wavelength large atomic the be and of would lifetime square transition short of the laser The times Wood X-ray fs an estimated and 1 of about lifetime (LLNL) Chapline radiative Laboratory (2). the that National Townes Livermore and ini- lifetime Lawrence Schawlow out short pointed by very as levels, the tially quantum-energy of -core because excited difficult, of extremely was task (1). and materials and atomic, chemistry, , biology, electronic, to interest into of window phenomena in molecular X-ray a and processes opening interactions, , dynamical nonlinear quantum equilibrium, of from sys- studies far molecular systems nonperiodic states, and and noncrystalline atomic periodic tems, for of scale imaging duration—the length phenomena—allows pulse and femtosecond time and characteristic wavelength pulses X-ray angstrom coherent at high-intensity, generating laser A physics. S pulses transform-limited to laser X-ray way the opening capabilities. realization, current parts its beyond other research for and X-ray–based needed extend cavity optical oscillator X-ray the the the discuss of of We reaches transits. design and optical-cavity and saturates six feasibility to oscillator four the in of pass, because per full that, gain show large system very this the of simulation theo- Our and energy. study retical 8-keV medium at gain resolution spectral nitrate 48-meV copper and a of case generating the consider an we As pulses. example, oscillators transform-limited X-ray coherent, build fully to intense, possible and producing is pump, it cavity, periodic optical a crystal as Bragg a laser free- oscilla- X-ray show an an medium, We gain of range. the photon-energy study 12-keV using a to that, 5- present the we in (UV) paper, operating near-ultraviolet this tor and In visible stable, been to region. providing infrared have spectral lasers, the oscillators in optical laser only Suckewer) now, of available Szymon and Until heart Falcone pulses. Roger the by transform-limited reviewed at 2020; 23, are March review Oscillators for (sent 2020 13, May Pellegrini, Claudio by Contributed Universit Physics, Rohringer Nina ih ore LCNtoa ceeao aoaoy el ak A94025; CA Park, Menlo Laboratory, Accelerator National SLAC Source, Light Germany; 22607, Hamburg Elektronen-Synchrotron, a Halavanau oscillator Aliaksei laser X-ray inversion Population ceeao eerhDvso,SA ainlAclrtrLbrtr,MnoPr,C 94025; CA Park, Menlo Laboratory, Accelerator National SLAC Division, Research Accelerator nteao-ae ouainivrinlsrapoc,this approach, laser population-inversion atom-based the In eeoiga -a ae a enamjrga nlaser 1960, in goal in major laser a ruby been first has the laser built X-ray an Maiman developing time the ince | mlfidsotnosemission spontaneous amplified ∼ 5 tHmug abr 05,Gray and Germany; 20355, Hamburg Hamburg, at ¨ × K b,f α 10 w Bergmann Uwe , 1 10 a ieo rniinmtlcmonsa the as compounds metal transition of line nriBenediktovitch Andrei , htn e us ih3-splelength pulse 37-fs with pulse per g | -a cavity X-ray n lui Pellegrini Claudio and , c ia E iiin LCNtoa ceeao aoaoy el ak A94025; CA Park, Menlo Laboratory, Accelerator National SLAC division, FEL & Linac b let .Lutman A. Alberto , | g tnodPLEIsiue LCNtoa ceeao aoaoy el ak A94025 CA Park, Menlo Laboratory, Accelerator National SLAC Institute, PULSE Stanford e arneBree ainlLbrtr,Bree,C 94720; CA Berkeley, Laboratory, National Berkeley Lawrence a,1 ae FL uss n a euetepledrto down duration pulse the reduce can one pulses, intensity (FEL) large laser from free-electron transform-limited suffer produce and To (23–31). about limited fluctuations transform from not are (LCLS) pulses Source Coher- Linac at Light the e.g., reduce magnitude, ent of to order an harder employed by For been bandwidth energy have (17–22). for systems nanometers implemented self-seeding few successfully X-rays, a been than has larger lasers wavelengths external seed- with shot fluctuations, electron ing and from coherence community. starts longitudinal pulse improve XFEL X-ray To the the noise. in 16), (15, called mode is SASE it In as (SASE), emission neous are and bandwidth limited. energy transform large not a have How- typically (14). pulses noise ASE from ever, starting generated to were pulse (up per gain photons of values Large 2 amplified. spontaneously are and inversion hole, population core 1-s a An emitted creates (14). solutions pulse Mn recently, pump and, intense (13), Ne foils a metal in Cu (ASE) (12), the gas emission demonstrated at spontaneous been amplified power of has peak experiments capability in high This shown in the energy. been inversion of photon population because also required creating medium, have for gain They pump atomic energy effective (11). an photon very 0.1% a a of be order and to the coherence range, of longitudinal wavelength spread limited angstrom with to nanometer but time femtosecond and and coherence, duration, transverse length pulse power, atomic rev- peak the a high are at with (10), scale, today explore to to 1990s late tool the olutionary from developed and 9) (8, 1 the under Published interest.y competing no declare authors The eiwr:RF,Uiest fClfri ekly n .. rneo University. Princeton S.S., paper.y and the Berkeley; wrote C.P. California and of A.B., U.B., University A.H., R.F., A.A.L., Reviewers: A.H., U.B. tools; and and reagents/analytic data; D.C., analyzed new C.P. D.D., contributed and N.R., A.H., N.R. A.A.L., research; and designed A.B. C.P. research; and performed U.B., A.H., contributions: Author owo orsodnemyb drse.Eal [email protected] Email: addressed. be may correspondence whom To × u pis hmcldnmc,iaig pcrsoy and , imaging, quan- dynamics, measurements. in interferometric chemical areas optics, research the and tum will possibilities that oscillator laser experimental showing X-ray The eV, open reality. a 0.05 become now to about can dream 3- resolution, the high-energy a in pulses, ation radi- high-power transform-limited, coherent, fully generating ae rsnsasuyo nXryoclao ae npopula- on based oscillator of X-ray inversion an tion The of 1960s. study the a from presents physics molecu- laser paper of and dream a atomic been the time has duration femtosecond of at and wavelength, scale angstrom pulses phenomena, time coherent lar and fully length generating characteristic oscillator X-ray An Significance otXEsoeaeas nAE rsl-mlfidsponta- self-amplified or ASE, in also operate XFELs Most 1992 in proposed first (XFELs), lasers free-electron X-ray c 10 ailDePonte Daniel , 6 eeosre,and observed, were ) K α b 1 etrfrFe lcrnLsrSine Deutsches Science, Laser Electron Free for Center ursec htn ln h ieto fthe of direction the along photons fluorescence NSlicense.y PNAS K α lcrn natasto-ea compound, transition-metal a in d ail Cocco Daniele , K <1- α 1 aeeghrgo,with region, wavelength A ˚ S usso pto up of pulses ASE NSLts Articles Latest PNAS 10 e −3 , to d ia Coherent Linac f eatetof Department 10 −4 u the but , 8 y | × f6 of 1 10 7

PHYSICS X-Ray Laser Oscillator Delay Auger 1 The X-ray laser oscillator (XLO) system described here and KB mirrors shown in Fig. 1 starts with a SASE pump pulse that creates a 1-s Liquid jet core hole followed by ASE in a liquid jet. SASE pulse train To study the physics of the system, we consider, as an example, a lasing medium formed by a jet of concentrated cupric nitrate [Cu(NO3)2] solution, with 4.2 Cu per cubic nanometer. CRLs We have chosen a cupric nitrate solution as a gain medium because the copper K α1 photon energy at 8 keV is widely used Outcoupling crystal in X-ray science. Furthermore, cupric nitrate is soluble in high concentrations, does not contain any heavy elements that would reduce the gain by absorbing 8-keV photons, and is straightfor- ward to handle. It is important to note that our results for copper apply, with only minor changes, to other elements, providing a Fig. 1. XLO schematics. A train of SASE pump pulses with spacing τ passes lasing range of 5 to 12 keV. After the ASE gain in the first pass, through a semitransparent crystal and impinges on a liquid jet sample, cre- the K α1 signal was recirculated through the cavity and matched ating population inversion. The first pulse generates ASE of the Kα1 line, with the arrival of subsequent XFEL pulses. This created seeded which is recirculated through a cavity with a roundtrip time that matches τ, in each subsequent pass, including losses in seeding the stimulated emission in the subsequent passes. When saturation the cavity. is reached after four to eight passes, the XLO pulse is outcoupled by rapidly To analyze the XLO performance, we first considered the switching the outcoupling crystal into an off-Bragg condition. Note that in amplification process in the lasing medium. The emission from the figure, the separation between X-ray pump pulses and the cavity size is the population-inverted medium was calculated by using a not to scale. The energy diagram for population inversion (top right) and principles of ASE (middle left) and seeded stimulated emission (bottom left) correlation-function approach (49, 50). We used the XATOM are shown. software (51–54) to evaluate the photoionization cross-section of the copper atom 1-s level and the Auger decay time to be 0.0324 Mb and 0.823 fs. The evolution of atomic populations to a single coherent SASE spike, making the electron bunch was computed, in a one-dimensional (1D) model approxima- length shorter than the cooperation length (32). The use of tion, via a two-point correlation function of the atomic coher- emittance spoilers (33–35), the combination of fresh-slice, cas- ence and two-time correlation function of fields. At the initial caded amplification (36, 37), nonlinear compression schemes stage of emission, the spontaneously emitted radiation triggered (38), and recently enhanced SASE (39) have succeeded in pro- the amplification process. During the propagation through the ducing such pulses. However, in these schemes, the coherence population-inverted medium, radiation was amplified, and the is achieved by decreasing the pulse duration below 1 fs, result- contribution of became negligible com- ing in a large bandwidth and lower photon flux compared to pared to amplified stimulated emission. At this point, the field cor- self-seeded modes, as well as large shot-to-shot fluctuation. To relation function can be factorized into classical field amplitudes. drastically increase the photon flux, a double-bunch FEL scheme Corresponding equations (after neglecting contributions of spon- at LCLS has been investigated (40–42). To generate fully coher- taneous emission) became equivalent to well-known Maxwell– ent hard X-ray pulses, an X-ray FEL oscillator (XFELO) (43–48) Bloch equations. We then propagated the obtained field through has been proposed. Since the lasing medium is an electron the Bragg crystal-cavity elements and used it as a seed for the bunch in a typically 100-m-long undulator, XFELO requires a following rounds of amplification. We used the same approach few-hundred-meters-long diamond Bragg crystal cavity wrapped for solving the Maxwell–Bloch equations in the subsequent cav- around the undulator. Mechanical stability, radiation focusing, ity passes. Starting from the second pass, the spontaneous K α1 temporal-transverse beam overlap in the cavity, and crystal align- emission along the gain medium was much weaker than the stim- ment angular tolerances, of the order of 10 nrad, are important ulated emission. This approach has been used to simulate the technical issues. If these technical challenges are met, XFELO experimentally observed ASE in Ne and Mn (12, 14). could operate with a high-repetition-rate, temporally equispaced The calculation of the X-ray pulse evolution during the mul- set of long electron bunches, achieving ultra-narrow bandwidth tiple cavity transits requires choosing and optimizing a num- and very large average brightness at hard X-rays. In this paper, ber of parameters: 1) pumping-pulse power density, duration, we investigate and propose an alternative route to generate fully and focusing; and 2) lasing-medium geometry. The pumping- coherent transform-limited X-ray pulses. radiation Rayleigh length must be larger than the lasing-medium

Fig. 2. (Left) DuMond diagram of Si(444) crystal at 8,048 eV. (Center) Refocused ASE radiation at the jet location after two sets of Be lenses with state- of-the-art shape profiles. (Right) Darwin curve for Bragg reflection of 8,048-eV photons, corrected for refraction for the cases of thick (d = 5 mm) and thin (d = 50 µm) flat Si(444) crystals at room temperature.

2 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.2005360117 Halavanau et al. Downloaded by guest on September 25, 2021 Downloaded by guest on September 25, 2021 rm5 mt e ees omthtevleo SASE of value the 1/ match to separation tals size meters, lateral few and a separation to train cm 50 from ult eutn rmti prahwl mrv ayX-ray many improve will approach various (46). this match experiments for from that pulse built resulting unprecedented lines be The quality emission for parameters. can eV different experimental XLOs desired 5 using the such to by range), (3 energies 12-keV line discrete emission to the 5- limited of the is width tunability to the the compared to While stability pulses. enhanced SASE significantly monochromatized highly with were energy higher pulses pulse and one-order-of-magnitude cavity) laser provided the in and X-ray optics transform-limited Bragg resulting the by The defined the (as cavity. monochromatic in the pulses to of stabil- subsequent time ity mechanical/temperature sufficient between sufficient guaranteeing ensuring medium and train between lasing compromise the a replace is size mal later a in Design. Cavity discussed X-Ray be from will outcoupled results was pulse achieved numerical the section. was The trips, saturation cavity. round Once pho- eight the pulse. of to X-ray four number output after obtain and our the to inversion In in parameters population duration. tons of possible pulse choice pump-pulse largest the amplified the the optimized the by have emission and we defined stimulated studies, instantaneous, mostly as the is pulse, almost medium, length pump is gain posi- the photons the and the Following of through time medium. Hence, travels of lasing duration. function it the a pump-pulse fs, along speed as 1 the tion changes the about pump- than inversion time, at the shorter population decay medium XLO, population-inversion gain much like the the is system and is through light, a traveling the It of in is by pulse. defined that pulse transform-limited value ing realize the a to to for important near product be must time-bandwidth medium. duration the through pulse power pumping The uniform a obtain to length Diamond Silicon (hkl) 8,048-eV geometries. for reflection parameters Bragg crystal different diamond in and photons Silicon 1. Table aaaa tal. et Halavanau 3 031. 744748.1 27.4 23.9 4.7 37.4 8.1 9.4 45.5 23.7 37.4 6.0 65.6 5.0 75.4 9.4 13.0 14.0 48.1 9.6 39.6 70.3 76.0 59.8 31.6 45.7 12.5 331 10.3 79.3 400 68.5 311 47.5 444 533 333 o 2θ cos ) and θ B deg , h L, = h τ M − r eae oteupradlwrcrys- lower and upper the to related are rm17t 0n.Tecvt length cavity The ns. 10 to 1.7 from h L rg aiylength cavity Bragg XLO The n rg angle Bragg and , 1 2 ∆θ (L , + µrad M a 2 tan ) ∆ω i.3. Fig. meV , θ where , θ by: L ancre nhg-an(Left high-gain in curves gain XLO ∆ω/ω (L C l + = M × c K C τ = ) 10 l α h opti- The . a range can 1 −6 C ie in lines l /(1 τ fs , C − l ifato 6) eeautdtecytl’Dri reflectivity Darwin crystals’ Bragg the the evaluated on We effects crystals angle (60). symmetric azimuthal diffraction flat, additional selected any theory alleviate We diffraction to (60–62). dynamic crystals and perfect optics in Fourier of formalism the 1 of order the and systems. alignment of angle and be crystal to the estimated 1 in been tolerances have cavity position The angu- acceptance. more-than-two-times-smaller a a lar with gave bandwidth C reflection 65-meV diamond Si(444) a the the length while Using bandwidth, pulse radius. 48-meV X-ray spectral- medium the corresponding lasing defines the and reflection 2). and Bragg Fig. acceptance the and angular of 1 width (Table be Darwin bandwidth chose can The the angular We large pulse, reducing a (57–59). while laser provides it acceptance studied a as geometry, being with Bragg backscattering heating currently the or are rotation and near-zero fast employed and small includ- transmission outcoupling, a large radiation ing for with techniques status Several sat- a reflection. It reached to done has was wavelength. switching it Outcoupling chosen by cycles. when amplification pulse the several X-ray at after the 1) uration reflective outcouple Fig. in to highly right used but lower Cu was (e.g., the thin, crystals of also three angle other Bragg is pulse the the pump of at incident reflective One the highly peak. for yet keV, transparent 9 highly at and thin align- is separation temporal left) pulse-train and the angle with reflection ment Bragg Cu the the of with alignment was wavelength spectral cavity exact bowtie the The ensure 2. to Fig. tuned in graphed are properties optical Its bowtie photons amplified the large acceptance. of crystal a spread the angular matches by the overcompensated since large, smaller be much were (10 to losses gain cavity needed laser XLO the and pass, 99.7%, first about the gain the on exiting accep- Hence, Si spread energy energy medium. the and and angular above divergence crystal beam well versus of tance mrad, ratio losses the 1 cavity by order The determined reflectivity. are of good is for beam acceptance and angular of crystal length ratio aspect gain the and (55). on size depends XOP divergence code beam diffraction ASE dynamical The with diffraction calculated various optics. were and for cavity Cu crystals the the two matching for planes, these study, choices of this optimal characteristics in are The considered diamond wavelength and the silicon For Crystallog- beamlines. Femtosecond Serial raphy Free-Electron XFEL Compact European and Angstrom systems BL3 SPring-8 the Laser similar the at and at exists instrument, available setup are imaging a in X-ray Such employed (14). coherent as experiments medium, LCLS ASE lasing Mn the LCLS pump the effectively to used be a,wl ihntecpblt fpeetdycytlsupport crystal present-day of capability the within well µrad, oudrtn h ain etrso h L aiy eused we cavity, XLO the of features salient the understand To four-bounce tunable a using by recirculated were Photons can XLO, the of upstream mirrors, (KB) Kirkpatrick–Baez n o-an(Right low-gain and ) (+ − − +) rg rsa aiy(6,a hw nFg 1. Fig. in shown as (56), cavity crystal Bragg 4 .I h olwn ass h osswl be will losses the passes, following the In ). regimes. ) K α 1 msinln,aegvni al 1 Table in given are line, emission τ h rtcytl(upper crystal first The . NSLts Articles Latest PNAS ∗ 40 eeto gave reflection (400) K and µm α | 1 K peak f6 of 3 α 1

PHYSICS Fig. 4. The dependence of Nph as a function of SASE pump size σ in high-gain (Right) and low-gain (Left) regimes.

curves for given thickness and Miller indices using the XOP code pumping power and number of passes in the XLO cavity. The (55). The curves obtained were then convoluted with the spec- parameters for the simulations were determined by the LCLS tral content of the electromagnetic field. The correction to the pump power and pulse duration, by the proper- Bragg angle θB for a Gaussian photon beam is given by a sim- ties, and by the gain medium geometry. The simulations included 2 ple geometric relation sin θB ≈ sin θB0 (1 − ∆φ /2), where θB0 absorption of the 9-keV pump pulse and 8-keV Cu K α1 pho- is the Bragg angle for a nondivergent beam, and φ is the verti- tons in the cupric nitrate gain medium. For 9-keV LCLS SASE cal angle with respect to the plane perpendicular to the crystal X-ray pulses, we can assume a peak power of 30 to 50 GW with a surface. The effect of vertical divergence is of a second order for pulse-duration variable between 20 and 60 fs. Using a 60-fs pulse |∆φ| < 10−3 rad and will leave the radiation field mostly intact with 33.6-GW power, the corresponding pulse energy is 2 mJ, in φ, while selecting in θ. which is well within LCLS capabilities. The cavity crystals define The crystal reflectivity curve |R| presented in Fig. 2, Right, a horizontal angular acceptance of 31.6 µrad (Table 1). To allevi- where |R| = IH /I0, for the initial radiation intensity I0 and ate the effect of diffraction, one must satisfy d∆θ ≥ λ/4π, which diffracted intensity IH . We used a set of two compound refrac- yields d ≥ 390 nm. The gain medium can be described as a thin tive lens (CRL) assemblies of f = 52 cm and a collimator in a cylinder of length 300 µm, defined by the jet diameter, along 4-m cavity to refocus the amplified signal on the jet after four the pump-pulse direction of propagation and an effective radius reflections (Fig. 1). Fourier optics simulation results corrobo- defined by the KB focusing properties for a 9-keV X-ray pump rated this requirement (Fig. 2). After the first pass, the cavity pulse. In our calculation, we assumed a gain medium diameter efficiency is given by the crystal reflectivity and the losses in the of 800 nm, satisfying the above condition and justifying the use focusing lenses. We estimated the total efficiency in second and of a 1D simulation code. At this spot size, the Rayleigh length later passes to be about 41%. The CRL focusing optics can be is zR = 3.65 mm, i.e., much longer than the jet size. The vertical adjusted to longer/shorter cavity roundtrips, to minimize liquid angular acceptance is much larger than horizontal, and, hence, jet disturbance. Based on Mn ASE results (14), we can obtain we could optimize the system by having an elliptically shaped 8 up to almost 10 K α1 photons in the initial pass, providing pump pulse. While in principle, this solution can yield a better more than enough signal to overcome the first-pass large cavity XLO performance, for the sake of simplicity, we considered only losses. the case of a gain medium with a circular cross-section, with the While we focused on the Cu K α1 line and a Si(444) crystal radius defined by the horizontal acceptance. In addition to vary- Bragg cavity geometry that matches its wavelength, other choices ing the pump-pulse intensity, the gain can also be controlled by of the lasing medium and matching cavity configurations are pos- the focusing KB optics, albeit with slightly larger cavity losses. sible for the XLO, and switching between different lasing media Another important quantity is the number of Cu atoms in the requires only slight changes in the cavity geometry. For example, gain medium. Assuming 300-µm length and a 800-nm diame- 11 the Fe K α1 line at 6.4 keV using a Si(333) Bragg reflection with ter, there are 6.3 × 10 Cu atoms in the lasing volume of the ◦ 67.9 Bragg angle can be easily realigned to match the Zn K α1 (Cu(NO3)2) solution. This sets the upper limit for the number of line at 8.6 keV switching to the Si(444) reflection with a small XLO photons at saturation. We denote the initial pass as n = 0 ◦ realignment to a 66.3 Bragg angle. and following passes as n = 1, N , where N ≤ 7. Depending on the pump-pulse parameters, in the initial pass, the ASE process Numerical Simulations generates about 103 to 104 photons starting from fluorescence. We performed numerical simulations for cupric nitrate as a gain After seven additional passes, the total number of XLO photons medium using the 1D code (49) with different values of the in saturation reaches up to ≥ 5 · 1010, as shown in Fig. 3.

Fig. 5. (Left) Resulting time-bandwidth products. The solid black line corresponds to ∆t∆ω = 1.8 fs·eV. (Right) XLO pulse spectrum in high gain regime. Arb., arbitrary units.

4 of 6 | www.pnas.org/cgi/doi/10.1073/pnas.2005360117 Halavanau et al. Downloaded by guest on September 25, 2021 Downloaded by guest on September 25, 2021 aaaa tal. et Halavanau yielding pass, third the rapid in case, as this XFELO In early duration. as 60-fs occurred and 2-mJ saturation of pulses pump four XLO m size, medium Lasing m length, Cavity pass per Gain at Parameter parameters XFELO and XLO some of LCLS-II Comparison 2. Table w usswt aibesprto.Frtelwgi eie we regime, low-gain the For separation. using generate variable to by with (RF), pulses radiofrequency obtained linac two are to locked separation both variable lasers, (66– two with ns 200 pulses and to 10 two Two from with 68). separations demonstrated time been various has and bunches linac four Cu Performance. LCLS Jet the Liquid at bility and Linac Copper LCLS X-ray in instance, 65). for (19, experiments XLO possibilities, optics of fully experimental quantum property A the open is will 5. pulse and X-ray Fig. the in of coherence about shown high-order is as parameter to fs·eV, up degeneracy 1.8 produces to XLO low-gain equal saturated, is transform- and essentially limited is product time-bandwidth radiation saturation, XLO in XLO also 4 running Fig. of the performance. where importance best the the provides a demonstrates nm shows 800 of photons 4 diameter size XLO Fig. pump-spot when of of photons. advantage number tion of an total has number of using one dependence the sys- by configuration, increasing single-pass e.g., XLO a gradually the XLO, in While in photon in higher knob. yield, gives gain density tuning energy pump-photon the a increasing as tem, optimize size to pump SASE important is 2. Fig. it in nonlinear shown the reflectivity crystal by the determined and to is 3, Fig. similarly shape in mode pictured form, gain, The to 64. and more start 63 up supermodes builds refs. power radiation cavity con- and the these and gradually Under reduced, duration. is 60-fs gain the and ditions, 1-mJ of pulses pump eight better much and bandwidth stability. offering in while compa- reduction XFEL, order-of-magnitude is self-seeding an X-ray brightness hard XLO existing The with rable bandwidth. 48-meV in photons ubro htn (max) photons of Number tolerance, Angular FWHM fs length, Pulse MW power, Peak .S ukwr .Jel,Xrylsr at rsn,adfuture. and present, Past, laser: of X-ray Jaegle, Amplification P. Suckewer, Voorhees, S. D. 7. Keane, C. Milchberg, H. Skinner, H. C. Suckewer, S. 6. Matthews laser. X-ray L. the D. of history 5. The Hecht, lasers. J. X-ray Wood, 4. L. Chapline, G. . optical 3. and Infrared Townes, H. C. Schawlow, L. A. 2. Eds., Yano, J. Yachandra, V. Bergmann, U. 1. efis osdrdahg-anXOcs ihaSS ri of train SASE a with case XLO high-gain a considered first We half-maximum. at full-width FWHM, fe orpse nbt ih/o-ancss h resulting the cases, high-/low-gain both in passes four After process, ASE the in nonlinearity of degree high a to Due of train SASE a with case low-gain the considered then We 3 (2009). 436 column. plasma (1985). confined 1756 a in emission X-ray soft stimulated (1985). 110–113 (1958). 18. vol. 2017), UK, London, Chemistry, of Biology and Chemistry Materials, ∆t ∆ω N ph fs·eV , eosrto fasf -a amplifier. X-ray soft a of Demonstration al., et (σ ) µrad uv eoe eaieyflat. relatively becomes curve σ n a ne httegi medium gain the that infer can One . Eeg n niomn eis h oa Society Royal The Series, Environment and (Energy hs Today Phys. pi htn News Photon. Optic 10 -a reEeto aes plctosin Applications Lasers: Electron Free X-Ray pt 10 to Up 10 3 5 04 (1975). 40–48 28, × ∼270 × h osatpaeand phase constant The . 37.4 ∼10 1.8 1 10 10 −4 10 5 6 × hs Rev. Phys. 10 63 (2008). 26–33 19, utbnhcapa- Multibunch hs e.Lett. Rev. Phys. ae hs Lett. Phys. Laser 10 N hs e.Lett. Rev. Phys. ph htn.The photons. 1940–1949 112, safunc- a as . o1.5 to 1.2 ∼0.01 1753– 55, >260 ∼100 ∼4.7 10 530 4.4 411– 6, 10 10 54, 10 4 .Kroll T. 14. Yoneda H. 13. Rohringer N. 12. lasers. free-electron X-ray of Bostedt physics C. The Reiche, 11. S. Marinelli, A. Pellegrini, C. 10. .Svdk,Z un,adR conenfrvlal suggestions valuable for Schoenlein R. illustrations. with help and Marinelli, for Stewart A. Huang, G. and Marcus, research; Z. the G. throughout thank Shvyd’ko, We Y. DE-AC02-76SF00515. Contract Energy A.H. ACKNOWLEDGMENTS. and A.B. from available are performance oscillator the Availability. Data high-precision X-ray and scattering, phenomena, inelastic interferometry. multiphoton exten- imaging, optics, are The coherent source quantum linac. include X-ray copper and this LCLS of sive the applications of scientific bowtie operation jet, potential fast a multibunch a in the using optics replenishment and the medium reflection discuss lasing Bragg We the on photons. configuration, of based number with cavity saturation pulses large oscillator’s transform-limited a reach obtain and can and linewidth we medium. passes small eight that medium gain to showing lasing jet four nitrate regimes, in liquid cupric gain a a various and of XFEL in performance an pump the using periodic explore range a We photon-energy as oper- 12-keV train XLO to pulse 5- population-inversion the a in describe ating we conclusion, In Summary studied repetition future. be 120-Hz will the linac a in superconducting at the linac using operation copper Its LCLS rate. XLO the while repeti- gun, on superconducting 1-MHz operate new a would a to have up would would with and XFELO rate linac that tion superconducting Note 2. a Table by in driven 48) at be (47, XFELO LCLS-II and at keV keV 8 at 9.8 operating XLO of parameters size. projected cavity compact a to due stability more and pulses small a using when es ihvlct pt 0 /.Ti sciia,a h mini- the cav- T chosen as The critical, (69–71). time, operation transit is ity XLO for This size m/s. cavity higher-speed realistic 200 use to to time replenishing up possible mal velocity is We it with start. bunch that jets, to 10-ns noticing best the about to parameters, establish to jet only 1.7- experiments more at is perform to case, flow need therefore our hydrody- hydrodynamic the in the process on whereas separation, complicated mainly stage, focused a flow 69 The is namic ref. nanosec- scales. time jet in of different reported tens liquid at experiment of mechanisms a different order of involves the that explosion of The be the showed to onds. (69) the time mode before two-bunch travel linac provided shock-wave Cu LCLS jets be water and slow must m/s) replen- with (≤30 experiments medium be Recent arrives. gain must pulse pump fresh next medium a lasing shock and The a ished, generates wave). and (rarefaction channel wave plasma high-temperature a ates sepa- stacker pulse fixed infrared laser. with an each eight and for to lasers available pulses two linac the of using ration number the raise to plan .C Pellegrini C. regime high-gain and 9. instabilities Collective Narducci, M. L. Pellegrini, C. Bonifacio, R. 8. RF oass h uueapiain,w rvd oprsnof comparison a provide we applications, future the assess To hnteSS upipne ntegi eim tcre- it medium, gain the on impinges pump SASE the When hs e.Lett. Rev. Phys. laser. free-electron X-ray laser. free-electron X-ray (2016). 015007 Phys. Mod. Equip. Assoc. Detect. Spectrom. Accel. A Sect. Res. Phys. Methods laser. electron free 5 ps: 350 = tmltdXryeiso pcrsoyi rniinmtlcomplexes. metal transition in spectroscopy emission X-ray Stimulated al., et ia oeetlgtsuc:Tefis v years. five first The source: light coherent Linac al., et tmcinrsellsra 1.5- at laser inner-shell Atomic al., et 106(2016). 015006 88, o4n ihpwrFLo h LClinac. SLAC the on FEL power high nm 4 to 2 A al., et tal. et 323(2018). 133203 120, T tmcinrselXrylsra .6nnmte updb an by pumped nanometres 1.46 at laser X-ray inner-shell Atomic , I of Proc. Conf. 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