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Self-seeding is one of the most promising methods to improve the longitudinal coherence and spectral purity of free-electron lasers (FELs). Measurements of the multishot-averaged, soft x-ray self-seeding spectrum at the Linac Coherent Light Source (LCLS) FEL often have a pedestal-like distribution around the seeded wavelength. In this paper, a theoretical model based on the mechanism of spectral sideband generation is developed to explain the statistical behavior of the seed and pedestal. The model is in good agreement with statistical analysis of numerical FEL simulations and experimental measurements that show that pedestal fluctuations reflect the level of long-wavelength microbunching structures in the beam, and are driven by fluctuations of the seed.

Author(s): Hemsing, E; Marcus, G; Fawley, WM; Schoenlein, RW; Coffee, R; Dakovski, G; Hastings, J; Huang, Z; Ratner, D; Raubenheimer, T; Penn, G | Abstract: We present the results from studies of soft X-ray seeding options for the LCLS-II X-ray free electron laser (FEL) at SLAC. The LCLS-II will use superconducting accelerator technology to produce X-ray pulses at up to 1 MHz repetition rate using 4 GeV electron beams. If properly seeded, these pulses will be nearly fully coherent, and highly stable in photon energy, bandwidth, and intensity, thus enabling unique experiments with intense high-resolution soft X-rays. Given the expected electron beam parameters from start to end simulations and predicted FEL performance, our studies reveal echo enabled harmonic generation (EEHG) and soft X-ray self-seeding (SXRSS) as promising and complementary seeding methods. We find that SXRSS has the advantage of simplicity and will deliver 5-35 times higher spectral brightness than EEHG in the 1-2 nm range, but lacks some of the potential for phase-stable multipulse and multicolor FEL operations enabled by external laser seeding with EEHG.

Measurements of the soft x-ray, self-seeding spectrum at the Linac Coherent Light Source (LCLS) free-electron laser generally display a pedestal-like distribution around the central seeded wavelength. This pedestal limits the spectral purity and can negatively affect some user applications not employing a post-undulator monochromator. In this paper, we investigate the detailed experimental characteristics of both the amplified seed and its accompanying pedestal using data from a number of separate LCLS shifts over the 2015-2018 time period. We find that the amplified seed shows excellent wavelength stability and an exponential growth rate whose dependence upon energy detuning is consistent with theory. The pedestal's spectral distribution and integrated strength vary strongly shot by shot, independent of electron beam energy jitter. Its shot-averaged strength relative to that of the seed grows at least linearly with $z$ and can approach values of 15% or more. The pedestal is comprised of two separate components: (1) normal self-amplified spontaneous emission (SASE) whose total strength is nominally insensitive to energy detuning and laser heater (LH) strength; (2) sideband-like emission whose strength positively correlates with that of the amplified seed and negatively with energy detuning and LH strength. We believe this latter, non-SASE component arises from comparatively long wavelength (i.e., $\ensuremath{\lambda}\ensuremath{\sim}0.3--3\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$) amplitude and phase modulations of the main seeded radiation line. Its shot to shot variability and LH sensitivity suggests an origin connected to growth of the longitudinal microbunching instability on the electron beam.

X-ray free-electron lasers (FELs), which amplify light emitted by a relativistic electron beam, are extending nonlinear optical techniques to shorter wavelengths, adding element specificity by exciting and probing electronic transitions from core levels. These techniques would benefit tremendously from having a stable FEL source, generating spectrally pure and wavelength-tunable pulses. We show that such requirements can be met by operating the FEL in the so-called echo-enabled harmonic generation (EEHG) configuration. Here, two external conventional lasers are used to precisely tailor the longitudinal phase space of the electron beam before emission of X-rays. We demonstrate high-gain EEHG lasing producing stable, intense, nearly fully coherent pulses at wavelengths as short as 5.9 nm (~211 eV) at the FERMI FEL user facility. Low sensitivity to electron-beam imperfections and observation of stable, narrow-band, coherent emission down to 2.6 nm (~474 eV) make the technique a prime candidate for generating laser-like pulses in the X-ray spectral region, opening the door to multidimensional coherent spectroscopies at short wavelengths. Echo-enabled harmonic generation in a free-electron laser enables 45th harmonic pulses from a 264 nm wavelength seed, yielding 5.9 nm wavelength coherent output.

We report the first generation of coherent, tunable, variable-polarization, soft X-ray femtosecond pulses, generated by a seeded free-electron laser (FEL) operating in the fresh bunch, two-stage harmonic upshift configuration. Characterization of the radiation proves this FEL configuration can produce single-transverse-mode, narrow-spectral-bandwidth output pulses of several tens of microjoules energy and low pulse-to-pulse wavelength jitter at final wavelengths of 10.8 nm and below. The fresh bunch configuration enhances the FEL emission at high harmonic orders by avoiding a gain depression due to the energy spread induced by the first-stage FEL interaction. Coherent signals measured down to 4.3 nm suggest this configuration is directly scalable to photon energies that will enable scientific investigations below the carbon K-edge, including access to the L-edges of many magnetic materials, with an energy per pulse unlocking the gate for experiments in the soft X-ray region with close to Fourier-transform-limited pulses. © 2013 Macmillan Publishers Limited. All rights reserved.

FERMI is the first user facility based upon an externally seeded free-electron laser (FEL) that delivers a coherent and tunable UV radiation (down to 4 nm at the fundamental) in a number of different configurations. A microbunching instability (MBI) developing in the bunch compressors and in the rest of the linac can degrade the quality of the high brightness electron beam sufficiently to reduce the FEL output intensity and spectral brightness. A laser heater installed in the low energy (100 MeV) part of the FERMI accelerator increases the local energy spread to provide Landau damping against this instability. In this paper we summarize the main results obtained with the FERMI laser heater since it commissioning in 2012. We present the measurement of the reduction of the incoherent energy spread at the linac exit induced by the heating of the electron beam at the beginning of the linac. We also discuss the positive effects of such heating upon the emission of coherent optical transition radiation and the FEL performances both in terms of intensity and spectrum. Moreover, we report about results that have been used to experimentally demonstrate that for transversely uniform heating the local energy spread augmentation is characterized by a non-Gaussian distribution that can be preserved up to the FEL undulator entrance with a significant impact on the performance of high-gain harmonic generation (HGHG) FELs, especially at soft x-ray wavelengths.

International audience; The two single-pass, externally seeded free-electron lasers (FELs) of the FERMI user facility are designed around Apple-II-type undulators that can operate at arbitrary polarization in the vacuum ultraviolet-to-soft x-ray spectral range. Furthermore, within each FEL tuning range, any output wavelength and polarization can be set in less than a minute of routine operations. We report the first demonstration of the full output polarization capabilities of FERMI FEL-1 in a campaign of experiments where the wavelength and nominal polarization are set to a series of representative values, and the polarization of the emitted intense pulses is thoroughly characterized by three independent instruments and methods, expressly developed for the task. The measured radiation polarization is consistently >90% and is not significantly spoiled by the transport optics; differing, relative transport losses for horizontal and vertical polarization become more prominent at longer wavelengths and lead to a non-negligible ellipticity for an originally circularly polarized state. The results from the different polarimeter setups validate each other, allow a cross-calibration of the instruments, and constitute a benchmark for user experiments.

FERMI, based at Elettra (Trieste, Italy) is the first free electron laser (FEL) facility operated for user experiments in seeded mode. Another unique property of FERMI, among other FEL sources, is to allow control of the polarization state of the radiation. Polarization dependence in the study of the interaction of coherent, high field, short-pulse ionizing radiation with matter, is a new frontier with potential in a wide range of research areas. The first measurement of the polarization-state of VUV light from a single-pass FEL was performed at FERMI FEL-1 operated in the 52 nm-26 nm range. Three different experimental techniques were used. The experiments were carried out at the end-station of two different beamlines to assess the impact of transport optics and provide polarization data for the end user. In this paper we summarize the results obtained from different setups. The results are consistent with each other and allow a general discussion about the viability of permanent diagnostics aimed at monitoring the polarization of FEL pulses.

We report results of a free-electron laser operating with the high gain harmonic generation configuration in the wavelength rage from 60 to 20 nm. With the recent experiments done at the FERMI free-electron laser facility in Trieste we demonstrated the capability of the used method to produce highly coherent pulses allowing to improve the performances of FELs in the EUV spectral range overcoming some of the limitations that are typical of systems based on the self amplified spontaneous emission (SASE). Taking advantage of the seeding scheme adopted for FERMI high photon energy stability and a very high longitudinal and transverse coherence have been measured. The operation of the FERMI FEL facility will allow improving the use of the FEL light for those experiments that require a small spectral bandwidth or a precise control of the photon energy.

Laser-heater systems have been demonstrated to be an important component for the accelerators that drive high gain free electron laser (FEL) facilities. These heater systems suppress longitudinal microbunching instabilities by inducing a small and controllable slice energy spread to the electron beam. For transversely uniform heating, the energy spread augmentation is characterized by a non-Gaussian distribution. In this Letter, we demonstrate experimentally that in addition to suppression of the microbunching instability, the laser heater-induced energy distribution can be preserved to the FEL undulator entrance, significantly impacting the performance of high-gain harmonic generation (HGHG) FELs, especially at soft x-ray wavelengths. In particular, we show that the FEL intensity has several local maxima as a function of the induced heating caused by the non-Gaussian energy distribution together with a strong enhancement of the power at high harmonics relative to that expected for an electron beam with an equivalent Gaussian energy spread at an undulator entrance. These results suggest that a single stage HGHG FEL can produce scientifically interesting power levels at harmonic numbers m≥25 and with current seed laser technology could reach output photon energies above 100 eV or greater. © Published by the American Physical Society.

We present a new diagnostic method allowing one to reconstruct certain properties of a relativistic electron beam and of a laser pulse by looking at the result of their interaction. The latter takes place within an undulator and results in energy modulation of electrons. By taking advantage of the correlation between time and electron-beam energy, measurements of the electron-beam length, current and energy spread are possible simultaneously. Moreover, the scheme allows an accurate measurement of the timing jitter between the electron beam and the external laser and also of the laser’s actual pulse length at the interaction point. Experimental demonstration of the last two measurements is reported in this paper. Finally, this diagnostic method provides an independent measure of the energy transferred from the electron beam to free-electron laser light produced when seeded electrons are injected into a long undulator. Experimental results obtained at the FERMI free-electron laser are presented after a description of the proposed technique.

FERMI is the first user facility based upon an externally seeded free-electron laser (FEL) and was designed to deliver high quality, transversely and longitudinally coherent radiation pulses in the extreme ultraviolet and soft x-ray spectral regimes. The FERMI linear accelerator includes a laser heater to control the longitudinal microbunching instability, which otherwise is expected to degrade the quality of the high brightness electron beam sufficiently to reduce the FEL output intensity and spectral brightness. In this paper, we present the results of the FERMI laser heater commissioning. For the first time, we show that optimizing the electron beam heating at an upstream location (beam energy, 100 MeV) leads to a reduction of the incoherent energy spread at the linac exit (beam energy, 1.2 GeV). We also discuss some of the positive effects of such heating upon the emission of coherent optical transition radiation and the FEL output intensity.

Exploring the dynamics of matter driven to extreme non-equilibrium states by an intense ultrashort X-ray pulse is becoming reality, thanks to the advent of free-electron laser technology that allows development of different schemes for probing the response at variable time delay with a second pulse. Here we report the generation of two-colour extreme ultraviolet pulses of controlled wavelengths, intensity and timing by seeding of high-gain harmonic generation free-electron laser with multiple independent laser pulses. The potential of this new scheme is demonstrated by the time evolution of a titanium-grating diffraction pattern, tuning the two coherent pulses to the titanium M-resonance and varying their intensities. This reveals that an intense pulse induces abrupt pattern changes on a time scale shorter than hydrodynamic expansion and ablation. This result exemplifies the essential capabilities of the jitter-free multiple-colour free-electron laser pulse sequences to study evolving states of matter with element sensitivity., Free-electron lasers are a powerful new tool for studying properties and transient states of matter. Here, the authors use a novel seed scheme for generation of two XUV laser pulses of controlled wavelength and time separation that enables access to ultrafast phenomena with elemental sensitivity.

FERMI@Elettra is the first seeded VUV/soft X-ray FEL source. It is composed of two undulatory chains: the low energy branch (FEL1) covering the wavelength range from 20 nm up to 100 nm, and the high energy branch (FEL2, employing a double stage cascade), covering the wavelength range from 4 nm up to 20 nm. At the end of 2012 FEL1 has been opened to external users while FEL2 has been turned on for the first time having demonstrated that a double cascade scheme is suitable for generating high intensity coherent FEL radiation. In this paper we will share our experience and will show our most recent results for both FERMI FEL1 and FEL2 sources. We will also present a brand new machine scheme that allows to perform two-colour pump and probe experiments as well as the first experimental results. © 2013 SPIE.

Control of the electron-beam longitudinal-phase-space distribution is of crucial importance in a number of accelerator applications, such as linac-driven free-electron lasers, colliders and energy recovery linacs. Some longitudinal-phase-space features produced by nonlinear electron beam self- fields, such as a quadratic energy chirp introduced by geometric longitudinal wakefields in radio-frequency (rf) accelerator structures, cannot be compensated by ordinary tuning of the linac rf phases nor corrected by a single high harmonic accelerating cavity. In this Letter we report an experimental demonstration of the removal of the quadratic energy chirp by properly shaping the electron beam current at the photoinjector. Specifically, a longitudinal ramp in the current distribution at the cathode linearizes the longitudinal wakefields in the downstream linac, resulting in a flat electron current and energy distribution. We present longitudinal-phase-space measurements in this novel configuration compared to those typically obtained without longitudinal current shaping at the FERMI linac.

Energy extraction efficiency of a free electron laser (FEL) can be greatly increased using a tapered undulator and self-seeding. However, the extraction rate is limited by various effects that eventually lead to saturation of the peak intensity and power. To better understand these effects, we develop a model extending the Kroll-Morton-Rosenbluth, one-dimensional theory to include the physics of diffraction, optical guiding, and radially resolved particle trapping. The predictions of the model agree well with that of the GENESIS single-frequency numerical simulations. In particular, we discuss the evolution of the electron-radiation interaction along the tapered undulator and show that the decreasing of refractive guiding is the major cause of the efficiency reduction, particle detrapping, and then saturation of the radiation power. With this understanding, we develop a multidimensional optimization scheme based on GENESIS simulations to increase the energy extraction efficiency via an improved taper profile and variation in electron beam radius. We present optimization results for hard x-ray tapered FELs, and the dependence of the maximum extractable radiation power on various parameters of the initial electron beam, radiation field, and the undulator system. We also study the effect of the sideband growth in a tapered FEL. Such growth induces increased particle detrapping and thus decreased refractive guiding that together strongly limit the overall energy extraction efficiency.

FERMI@Elettra is a free electron-laser (FEL)-based user facility that, after two years of commissioning, started preliminary users' dedicated runs in 2011. At variance with other FEL user facilities, FERMI@Elettra has been designed to deliver improved spectral stability and longitudinal coherence. The adopted scheme, which uses an external laser to initiate the FEL process, has been demonstrated to be capable of generating FEL pulses close to the Fourier transform limit. We report on the first instance of FEL wavelength tuning, both in a narrow and in a large spectral range (fine- and coarse-tuning). We also report on two different experiments that have been performed exploiting such FEL tuning. We used fine-tuning to scan across the 1s–4p resonance in He atoms, at ≈23.74 eV (52.2 nm), detecting both UV–visible fluorescence (4p–2s, 400 nm) and EUV fluorescence (4p–1s, 52.2 nm). We used coarse-tuning to scan the M4,5 absorption edge of Ge (∼29.5 eV) in the wavelength region 30–60 nm, measured in transmission geometry with a thermopile positioned on the rear side of a Ge thin foil.

A design of a compact free‐electron laser (FEL), generating ultra‐fast, high‐peak flux, XUV pulses is presented. The FEL is driven by a high‐current, 0.5 GeV electron beam from the Lawrence Berkeley National Laboratory (LBNL) laser‐plasma accelerator, whose active acceleration length is only a few centimeters. The proposed ultra‐fast source (∼10 fs) would be intrinsically temporally synchronized to the drive laser pulse, enabling pump‐probe studies in ultra‐fast science. Owing to the high current (≳10 kA) of the laser‐plasma‐accelerated electron beams, saturated output fluxes are potentially greater than 1013 photons/pulse. Devices based both on self‐amplified spontaneous emission and high‐harmonic generated input seeds, to reduce undulator length and fluctuations, are considered.

The LOASIS Laser Wakefield Accelerator (LWFA) has achieved quasi‐mono‐energetic electron beams with energies up to 1 GeV [1]. These beams offer the potential for use with insertion devices such as wigglers and undulators to create tabletop XUV and x‐ray free‐electron laser (FEL) sources. To achieve a high quality light source producing high brightness radiation requires an electron beam with low energy spread and low emittance. Here we discuss the use of an undulator to generate XUV radiation to characterize in a single shot the electron beam energy spread and emittance with high precision.

Recently it has been pointed out that numerical simulation of some systems containing charged particles with highly relativistic directed motion can by speeded up by orders of magnitude by choice of the proper Lorentz boosted frame. A particularly good example is that of short wavelength free-electron lasers (FELs) in which a high energy (E{sub 0}{>=}250 MeV) electron beam interacts with a static magnetic undulator. In the optimal boost frame with Lorentz factor {gamma}F, the red-shifted FEL radiation and blue shifted undulator have identical wavelengths and the number of required time-steps (presuming the Courant condition applies) decreases by a factor of {gamma}{sub F}{sup 2} for fully electromagnetic simulation.We have adapted the WARP code to apply this method to several FEL problems including coherent spontaneous emission (CSE) from pre-bunched e-beams, and strong exponential gain in a single pass amplifier configuration. We discuss our results and compare with those from the 'standard' FEL simulation approach which adopts the eikonal approximation for propagation of the radiation field.

The second axis accelerator of the Dual Axis Radiographic Hydrodynamic Test (DARHT-II) facility will produce a 2-kA, 20-MeV, 2-/spl mu/s output electron beam with a design goal of less than 1000 /spl pi/ mm-mrad normalized transverse emittance. In order to meet this goal, both the beam breakup instability (BBU) and transverse "corkscrew" motion (due to chromatic phase advance) must be limited in growth. Using data from recent experimental measurements of the transverse impedance of actual DARHT-II accelerator cells by Briggs et al., we have used the LLNL BREAKUP code to predict BBU and corkscrew growth in DARHT-II. The results suggest that BBU growth should not seriously degrade the final achievable spot size at the x-ray converter, presuming the initial excitation level is of the order 100 microns or smaller. For control of corkscrew growth, a major concern is the number of "tuning" shots needed to utilize effectively the "tuning-V" algorithm. Presuming that the solenoid magnet alignment falls within spec, we believe that possibly as few as 50-100 shots will be necessary to set the dipole corrector magnet currents. We give some specific examples of tune determination for a hypothetical set of alignment errors.

We summarize the presentations and conclusions of the Working Group on FEL Physics and Numerical Modeling.

The characteristics of a beam propagating in a channel of large aperture quadrupole magnets, and an anisotropy driven instability which transfers thermal energy from the transverse to longitudinal directions, are presented as two examples of phenomena requiring fine numerical resolution to be adequately described. In the first case, adequate resolution of the thin beam‐edge sheath is found necessary to accurately simulate the particles whose orbits are driven unstable by the focusing nonlinearity. In the latter case, the tendency of the unstable mode to occur at longitudinal wavelengths comparable to beam diameter, requires adequate longitudinal resolution for the instability to be observed.

We are presently designing and constructing a high-gain, tapered wiggler 10.6 μ FEL amplifier to operate with the 50 MeV ATA e-beam. The initial experiments will be done with a constant period (λw = 8 cm), 5 m-long linear wiggler. For an input laser power of 800 MW and electron beam brightness of 2.105 A/(rad-cm)2, we hope to achieve a trapped particle fraction ˜0.5 and an energy extraction efficiency of ˜2% with a ˜10% taper in the wiggler magnetic field. This taper corresponds to decelerating the trapped particle approximately two full ponderomotive well (i.e. bucket) heights. In this talk, we will discuss the physics motivations behind our tapered wiggler design and initial experimental diagnostics.

The results of a two-dimensional simulation of a high-gain free electron laser (FEL) amplifier is presented. The simulation solves the inhomogeneous paraxial wave equation. The source term is radially resolved and is obtained by tracking the interaction of the laser field with localized macroparticles.

At the Lawrence Livermore National Laboratory, a pulsed electromagnetic wiggler has been coupled to the Experimental Test Accelerator forming the Electron Laser Facility. This is a single-pass Free-Electron Laser which, because the wiggler excitation can be varied, can operate over a wide frequency range. Efficiency of conversion of electron beam to microwave power is 7%. This new power source is being used in a collateral program for developing and testing structures suitable for a Two-Beam Accelerator. The Two-Beam Accelerator shows much promise for achieving the high average accelerating gradients, e.g., ≥250 MV/m, required in such next-generation electron accelerators as 1 TeV on 1 TeV linear colliders.

The free-electron laser simulation code FRED, developed at LLNL primarily to model single-pass free-electron laser (FEL) amplifiers driven by induction linear accelerators, is described. The main emphasis is on the modelling of optical propagation in the laser, and on the differences between the requirements for modelling rf-linac-driven vs. induction-linac-driven FEL's. Examples of optical guiding and mode cleanup are presented for a 50 pm FEL.

We develop a self-consistent, three-dimensional method for the calculation of the steady-state acceleration of nonneutral plasma at the surface of a rotating, magnetized, isolated neutron star. When the effects of surface work functions are negligible, there is a unique monotonic electrostatic potential and associated current profile. We obtain explicit solutions in the monopole and polar-cap geometries. In the latter, we neglect field-line curvature and consider only the aligned rotator. The maximum asymptotic particle energy in the polar-cap case, achieved along the central field line, is found to be E/sub max/=2.8 x 10/sup 9/ eV x (P/1 s)/sup -1/(B/10/sup 12/ gauss)/sup 1/2/(R/sub asterisk6/ cm)/sup 1/2/(Zm/m/sub e/)/sup 1/2/. This result is in good accord with previous estimates made by Michel. The total luminosity of the pulsar in the form of each charged-particle beam from a polar cap is L/sub cap/=4 x 10/sup 27/(P/1 s)/sup -3/(B/10/sup 12/ gauss)/sup 3/2/ (R/sub asterisk6/ cm)/sup 9/2/(m/Zm/sub e/)/sup 1/2/ ergs s/sup -1/. This luminosity is sufficient to explain radio pulsars if the beam energy can be converted to radio radiation with reasonably high efficiency, but is insufficient to explain pulsed ..gamma..-ray emission from the Crab and other pulsars. In these numerical estimates, we assume that the closedmore » magnetosphere extends to the light cylinder. We also investigate the rotation rate of the plasma, and show that on the open-field lines of the aligned rotator, the beam current induces a twist to the field lines such that an observer at infinity sees the plasma as nonrotating. We apply these results and point out the necessity of including the effects of field-line curvature in a realistic model.« less

A strong infrared source detected in the AFCRL sky survey is confirmed, and is identified with the binary star HD 44179, embedded in a peculiar nebula. UBVRI and broad-band photometry between 2.2 and 27 microns are combined with blue, red, and near-infrared spectra, polarimetry and spectrophotometry of the star, and a range of direct and image-tube photographs of the nebula, to suggest a composite model of the system. In this model, the infrared radiation derives from thermal emission by dust grains contained in a disklike geometry about the central object, which appears to be of spectral type B9-A0 III and which may be in pre-main-sequence evolution. Two infrared emission features are found, peaking at 8.7 and 11.3 microns, the latter corresponding to the feature seen in the spectrum of the planetary nebula NGC 7027. The complex nebular structure is discussed on the basis of photographs through narrow-band continuum and emission-line filters. The polarization data support the suggestion of a disk containing some large particles. No radio continuum emission is detected.

The paper calculates the acceleration of a nonneutral stream of charged particles from the surface of an isolated rotating magnetized neutron star in the case where the particle stream flows along the open field lines of a curved poloidal magnetic field. It is shown that very large electrostatic potential drops can occur in steady unidirectional space-charge-limited flow along the curved open field lines. The explicit incorporation of magnetic field line curvature is the essential element leading to this new result, assuming that all the charges are supplied by emission from the surface. It is emphasized that the current flow and charge density are unique if all of the assumptions are met, with the result that the accelerating electric field near the stellar surface is quite small compared to the vacuum field.

New radial-velocity data for late supergiants with maser emission, long-period variable maser stars, and similar nonmaser stars are presented. For VY CMa, 20 years of data are made use of to show that a correlation exists between the measured radial velocity and the spectral resolution. The correlation may be explained by reflection of starlight from moving dust in agreement with the line-profile calculations of van Blerkom and van Blerkom (1978). For the long-period variables, radial velocities of the K I resonance lines on 4 A/mm spectrograms strengthen the correlation between the circumstellar absorption lines and the violet edge of the thermal microwave emission.

FERMI, the seeded Free Electron Laser (FEL) located at the Elettra laboratory in Trieste, Italy, welcomed in December 2012 the first external users on the FEL-1 line. This line is based on a single stage of High Gain Harmonic Generation (HGHG), seeded by a UV laser, and covers wavelengths between 80 and 20 nm. The photon energy reached more than 300 J. The second FEL line, FEL-2, covering the lower wavelength range between 20 and 4 nm thanks to a double stage cascaded HGHG scheme, has generated its first coherent photons in October 2012. The latter is the first experimental demonstration of a seeded free electron laser configured as a two-stage cascade operating in the "fresh bunch injection” mode, where the second stage is seeded by the light produced by the first stage. This paper describes the status of the operation and user experiments with FEL-1 and reports about the progress in the commissioning of FEL-2.