X-ray Free-electron Laser Oscillator Research Articles

X-ray free-electron laser oscillator with nuclear-resonant cavity stabilization and quantum-optical applications

X-ray free-electron-laser oscillators with nuclear-resonant cavity stabilization hold the promise for providing X-rays with unprecedented coherence properties that will enable interesting quantum-optical and metrologial applications. Among these are atom optics with X-ray-based optical elements providing high momentum transfer, or a frequency standard far surpassing the best state-of-the-art atomic clocks.

Narrow-band GeV photons generated from an x-ray free-electron laser oscillator

We propose a scheme to generate narrow-band GeV photons, $\ensuremath{\gamma}$-rays, via Compton scattering of hard x-ray photons in an x-ray free-electron laser oscillator. Generated $\ensuremath{\gamma}$-rays show a narrow-band spectrum with a sharp peak, $\ensuremath{\sim}0.1%$ (FWHM), due to large momentum transfer from electrons to photons. The $\ensuremath{\gamma}$-ray beam has a spectral density of $\ensuremath{\sim}{10}^{2}\text{ }\text{ }\mathrm{ph}/(\mathrm{MeV}\text{ }\mathrm{s})$ with a typical set of parameters based on a 7-GeV electron beam operated at 3-MHz repetition, Such $\ensuremath{\gamma}$-rays will be a unique probe for studying hadron physics. Features of the $\ensuremath{\gamma}$-ray source, flux, spectrum, polarization, tunability and energy resolution are discussed.

X-ray comb generation from nuclear-resonance-stabilized x-ray free-electron laser oscillator for fundamental physics and precision metrology

An x-ray free-electron laser oscillator (XFELO) is a next-generation x-ray source, similar to free-electron laser oscillators at VUV and longer wavelengths but using crystals as high-reflectivity x-ray mirrors. Each output pulse from an XFELO is fully coherent with high spectral purity. The temporal coherence length can further be increased drastically, from picoseconds to microseconds or even longer, by phase-locking successive XFELO output pulses, using the narrow nuclear resonance lines of nuclei such as $^{57}\mathrm{Fe}$ as a reference. We show that the phase fluctuation due to the seismic activities is controllable and that due to spontaneous emission is small. The fluctuation of electron-bunch spacing contributes mainly to the envelope fluctuation but not to the phase fluctuation. By counting the number of standing-wave maxima formed by the output of the nuclear-resonance-stabilized (NRS) XFELO over an optically known length, the wavelength of the nuclear resonance can be accurately measured, possibly leading to a new length or frequency standard at x-ray wavelengths. A NRS-XFELO will be an ideal source for experimental x-ray quantum optics as well as other fundamental physics. The technique can be refined for other, narrower resonances such as $^{181}\mathrm{Ta}$ or $^{45}\mathrm{Sc}$.

A nanoradian x-ray beam deviation-correction device for x-ray free electron laser oscillator

An energy-recovery linac (ERL)-based x-ray free electron laser (X-FEL) combined with a low-loss x-ray optical cavity can produce a high-intensity and fully coherent x-ray beam. The idea is to let the x-ray emission beam in the cavity become the seed beam of the FEL to realize x-ray stimulated emission conditions. Because the electron bunch size in the ERL is sub-micrometer and the x-ray cavity crystals are a few tens of meters away from the electron bunch, a deviation of a few nanoradians in the beam direction can cause the seed beam miss the electron bunches. Therefore, it is necessary to develop a high-precision beam-direction correction device that is independent of the x-ray cavity tuning to ensure collision of the narrow x-ray seed beam and electron bunches. To this end, we have assembled a pair of x-ray prisms on two rotary stages. We operated the device using 1.086 Å wavelength x-rays and demonstrate that the angular resolution can reach 1 nrad within a deviation range of ±3.1 mrad.

Wide-Range Third Harmonic Oscillations in Free-Electron Lasers Using the Storage Ring NIJI-IV

We developed third-harmonic free-electron laser (FEL) oscillations that operated from the visible to near-infrared region. Our infrared FEL system uses the storage ring NIJI-IV, which produced the first third-harmonic storage ring FEL oscillations. By measuring the characteristics of the third-harmonic FEL oscillations, we found that behavior of the third-harmonic FEL could be explained by a one-dimensional FEL theory. We demonstrated that the higher harmonic was effective in shortening the FEL wavelength, so it will save electron-beam energy in X-ray FEL oscillators based on a cw energy-recovery linac.

Proposal for an X-Ray Free Electron Laser Oscillator with Intermediate Energy Electron Beam

Harmonic lasing of low-gain free electron laser oscillators has been experimentally demonstrated in the terahertz and infrared regions. Recently, the low-gain oscillator has been reconsidered as a promising candidate for hard x-ray free electron lasers, through the use of high reflectivity, high-resolution x-ray crystals. In this Letter, it is proposed to utilize a crystal-based cavity resonant at a higher harmonic of the undulator radiation, together with phase shifting, to enable harmonic lasing of the x-ray free electron laser oscillator, and hence allow the generation of hard x-ray radiation at a reduced electron beam energy. Results show that fully coherent free electron laser radiation with megawatt peak power, in the spectral region of 10-25 keV, can be generated with a 3.5 GeV electron beam.

An X-ray Free-Electron Laser Oscillator for Record High Spectral Purity, Brightness, and Stability

Free-electron lasers for hard X-rays can be constructed in the oscillator configuration by employing diamond crystals as X-ray mirrors. An X-ray FEL oscillator (XFELO) will produce highly stable X-ray beams of ultra-high spectral purity and high average brightness, offering scientific opportunities in areas not covered by high-gain devices. An XFELO combined with a high-gain amplifier, possibly with harmonic generation, would constitute an ultimate X-ray machine.

Operation of higher harmonic oscillations in free-electron lasers

We report for the first time the experimental achievement of a seventh-harmonic free-electron laser (FEL) oscillation. The measured FEL gains and average FEL powers for higher harmonics were identical to those calculated by a one-dimensional FEL theory. The measured linewidths of the higher-harmonic FELs were narrower than that of the fundamental FEL owing to the narrower spectral widths of the spontaneous emissions. By applying the higher-harmonic FEL oscillation to a resonator-type FEL with an advanced accelerator, an x-ray FEL oscillator can be realized at lower electron-beam energy.

Performance of the x-ray free-electron laser oscillator with crystal cavity

Simulations of the x-ray free-electron laser (FEL) oscillator are presented that include the frequency-dependent Bragg crystal reflectivity and the transverse diffraction and focusing using the two-dimensional FEL code GINGER. A review of the physics of Bragg crystal reflectors and the x-ray FEL oscillator is made, followed by a discussion of its numerical implementation in GINGER. The simulation results for a two-crystal cavity and realistic FEL parameters indicate $\ensuremath{\sim}{10}^{9}$ photons in a nearly Fourier-limited, ps pulse. Compressing the electron beam to 100 A and 100 fs results in comparable x-ray characteristics for relaxed beam emittance, energy spread, and/or undulator parameters, albeit in a larger radiation bandwidth. Finally, preliminary simulation results indicate that the four-crystal FEL cavity can be tuned in energy over a range of a few percent.

Development and applications of a two-dimensional tip-tilting stage system with nanoradian-level positioning resolution

In this paper, designs of a novel rotary weak-link stage for a vertical rotation axis and a two-dimensional tip-tilting system are presented. Applications of these new stage systems include: an advanced X-ray stereo imaging instrument for particle tracking velocimetry, an alignment stage system for hard X-ray nano-focusing Montel mirror optics, and an ultra-precision crystal manipulator for cryo-cooling optical cavities of an X-ray free-electron-laser oscillator (XFELO).

Thermomechanical optimization of the cathode design intended for the X-ray free-electron laser oscillator injector electron gun

The Advanced Photon Source at Argonne National Laboratory is developing a low-emittance thermionic gun for a proposed X-ray free-electron laser oscillator (XFEL-O) that will use a laser pulse-heated cathode. The cathode must operate at or slightly above 1500 °C for several nanoseconds and then cool down several hundred °C in approximately the same amount of time, with a 1-MHz heating–cooling cycle. A transient thermal analysis was performed to optimize the laser pulse shape needed to provide the desired temperature response of the cathode for several possible cathode materials. In addition, thermal stresses developed in the cathode during heating–cooling cycles were analysed. Both transient thermal analysis and thermal stress computations were performed using the ANSYS12 code. The computed temperature distribution and thermal stresses were utilized in the optimization of the cathode design. The results of the analysis are presented.

Nanoradian angular stabilization of x-ray optical components

An x-ray free-electron laser oscillator (XFELO) has been recently proposed [K. Kim et al., Phys. Rev. Lett. 100, 244802 (2008)]. Angular orientation and position in space of Bragg mirrors of the XFELO optical cavity must be continuously adjusted to compensate for the instabilities and maximize the output intensity. An angular stability of about 10 nrad (rms) is required [K. Kim and Y. Shvyd'ko, Phys. Rev. ST Accel. Beams 12, 030703 (2009)]. To approach this goal, a feedback loop based on a null-detection principle was designed and used for stabilization of a high-energy-resolution x-ray monochromator (DeltaE/E approximately 4 x 10(-8), E=23.7 keV) and a high-heat-load monochromator. Angular stability of about 13 nrad (rms) has been demonstrated for x-ray optical elements of the monochromators.

Mode growth and competition in the x-ray free-electron laser oscillator start-up from noise

We describe the radiation properties of an x-ray free-electron laser (FEL) oscillator, beginning with its start-up from noise through saturation. We first decompose the initially chaotic undulator radiation into the growing longitudinal modes of the composite system consisting of the electron beam, the undulator, and the Bragg mirror resonator cavity. Because the radiation initially comprises several modes whose growth rates are comparable, we find that only after many oscillator passes is the output pulse dominantly characterized by the lowest-order Gaussian mode. We verify our analytic results with a novel, reduced one-dimensional FEL code (derived in the text), and with two-dimensional FEL simulations. Understanding the full longitudinal structure during the initial amplification will be critical in assessing the tolerances on the electron beam, undulator, and optical cavity required for robust operation.

Tunable optical cavity for an x-ray free-electron-laser oscillator

An x-ray free-electron laser oscillator proposed recently for hard x rays [K. Kim, Y. Shvyd'ko, and S. Reiche, Phys. Rev. Lett. 100, 244802 (2008)] can be made tunable by using an x-ray cavity composed of four crystals, instead of two. The tunability of x-ray energy will significantly enhance the usefulness of an x-ray free-electron laser oscillator. We present a detailed analysis of the four-crystal optical cavity and choice of crystals for several applications: inelastic x-ray scattering, nuclear resonant scattering, bulk-sensitive hard x-ray photoemission spectroscopy, other high-energy-resolution ($\ensuremath{\lesssim}1\text{ }\text{ }\mathrm{meV}$) spectroscopic probes, and for imaging with hard x rays at near-atomic resolution ($\ensuremath{\simeq}1\text{ }\text{ }\mathrm{nm}$).

A Proposal for an X-Ray Free-Electron Laser Oscillator with an Energy-Recovery Linac

We show that a free-electron laser oscillator generating x rays with wavelengths of about 1 A is feasible using ultralow emittance electron beams of a multi-GeV energy-recovery linac, combined with a low-loss crystal cavity. The device will produce x-ray pulses with 10{9} photons at a repetition rate of 1-100 MHz. The pulses are temporarily and transversely coherent, with a rms bandwidth of about 2 meV, and rms pulse length of about 1 ps.