FEL System Research Articles

A High-Flux Compact X-ray Free-Electron Laser for Next-Generation Chip Metrology Needs

Recently, considerable work has been directed at the development of an ultracompact X-ray free-electron laser (UCXFEL) based on emerging techniques in high-field cryogenic acceleration, with attendant dramatic improvements in electron beam brightness and state-of-the-art concepts in beam dynamics, magnetic undulators, and X-ray optics. A full conceptual design of a 1 nm (1.24 keV) UCXFEL with a length and cost over an order of magnitude below current X-ray free-electron lasers (XFELs) has resulted from this effort. This instrument has been developed with an emphasis on permitting exploratory scientific research in a wide variety of fields in a university setting. Concurrently, compact FELs are being vigorously developed for use as instruments to enable next-generation chip manufacturing through use as a high-flux, few nm lithography source. This new role suggests consideration of XFELs to urgently address emerging demands in the semiconductor device sector, as identified by recent national need studies, for new radiation sources aimed at chip manufacturing. Indeed, it has been shown that one may use coherent X-rays to perform 10–20 nm class resolution surveys of macroscopic, cm scale structures such as chips, using ptychographic laminography techniques. As the XFEL is a very promising candidate for realizing such methods, we present here an analysis of the issues and likely solutions associated with extending the UCXFEL to harder X-rays (above 7 keV), much higher fluxes, and increased levels of coherence, as well as methods of applying such a source for ptychographic laminography to microelectronic device measurements. We discuss the development path to move the concept to rapid realization of a transformative XFEL-based application, outlining both FEL and metrology system challenges.

Solving the Federated Edge Learning Participation Dilemma: A Truthful and Correlated Perspective

An emerging computational paradigm, named <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">federated edge learning</i> (FEL), enables intelligent computing at the network edge with the feature of preserving data privacy for edge devices. Given their constrained resources, it becomes a great challenge to achieve high execution performance for FEL. Most of the state-of-the-arts concentrate on enhancing FEL from the perspective of system operation procedures, taking few precautions during the composition step of the FEL system. Though a few recent studies recognize the importance of FEL formation and propose server-centric device selection schemes, the impact of data sizes is largely overlooked. In this paper, we take advantage of game theory to depict the decision dilemma among edge devices regarding <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">whether to participate in FEL or not</i> given their heterogeneous sizes of local datasets. For realizing both the individual and global optimization, the server is employed to solve the participation dilemma, which requires accurate information collection for devices’ local datasets. Hence, we utilize <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mechanism design</i> to enable truthful information solicitation. With the help of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">correlated equilibrium</i> , we derive a decision making strategy for devices from the global perspective, which can achieve the long-term stability and efficacy of FEL. For scalability consideration, we optimize the computational complexity of the basic solution to the polynomial level. Lastly, extensive experiments based on both real and synthetic data are conducted to evaluate our proposed mechanisms, with experimental results demonstrating the performance advantages.

Research of coherent edge radiation generated by electron beams oscillating free-electron lasers

We have studied far-infrared coherent radiation with S-band linacs at Laboratory for Electron Beam Re-search and Application (LEBRA) at Nihon University and at Kyoto University Free Electron Laser. We have already developed a couple of terahertz-wave sources based on coherent synchrotron radiation and coherent transition radiation, which have been applied to spectroscopic research. Moreover, we developed coherent edge radiation (CER) at the downstream bending magnets in the FEL sections. Because the edge radiation has an annular shape distribution characterized by the asymmetric first-order Laguerre-Gaussian mode, the CER can be extracted from an optical cavity of the FEL system without a diffraction loss of the FEL beam. The root-mean-squared bunch length of the electron beam was evaluated by measuring the CER spectra, which was about the same level as the FEL micropulse width. Although the infrared FELs at LEBRA had a long slippage length, the FEL energy became higher as the bunch length was shorter. The CER intensity can be a guidepost enhancing the FEL power because of the existence of their correlation.

The Projected 45 T Hybrid Magnet System at the Nijmegen High Field Magnet Laboratory

The Radboud University Nijmegen has embarked on an investment program for a free-electron laser radiation source and a 45 T hybrid magnet system to enhance the capabilities of its High Field Magnet Laboratory. The new installations, together with the high-resolution and solid-state NMR, the Laser Laboratory and the NanoLab will be part of the Nijmegen Center for Advanced Spectroscopy of the Institute for Molecules and Materials (IMM-NCAS). The FEL and hybrid magnet system are planned to be operational in 2012.

Experimental Study of FEL Irradiation onto Human Enamel and Dentin

Wavelength tunable Free Electron Laser, FEL, was irradiated onto the polished surfaces of human tooth enamel and dentin using the FEL system at Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University. LEBRA-FEL system was so powerful that only 2 mJ/pulse shot resulted in an apparent pit on the surfaces. The pit showed neither marked scorch nor formation of secondary crystalline phase revealed by micro-XRD and micro-FTIR. Optimum wavelength for pit formation on the dental hard tissues was about 3.0 mm. However, the optimum wavelengths for enamel and dentin should be at 3.2 mm, 3.0 mm, respectively. The results strongly suggested that the ablation mechanism by laser irradiation for dental hard tissues might be not the simple interaction of laser beams with water, but complex phenomena, which could not be neglected of the hard tissue components and structure.

Observation of SASE in LEBRA FEL system

A large enhancement of spontaneous undulator radiation has been observed during FEL lasing experiments at LEBRA. The enhancement has been observed only with the detector for the infrared fundamental radiation. The detector output signal showed spikes during the electron beam pulse, yet no apparent enhancement was observed with a CCD camera monitoring the visible harmonic radiations. An enhancement factor greater than 10 has been obtained with a 2.4 m long undulator with a completely detuned FEL optical cavity length and depends strongly on the parameters of the linac RF system. This implies that the SASE operation is possible even with a conventional electron beam by achieving suitable bunch compression.

Upgrade of the UVSOR storage ring FEL

The UVSOR electron storage ring has been shutdown since April till August 2003 for upgradation of its performance. This upgrade includes high brightness lattice, longer straight sections for in-vacuum undulators and high-quality vacuum system, and it can increase the brightness of synchrotron radiation from undulators with reduced beam emittance. The improved quality of the electron beam is also of great advantage to the storage ring FEL. To further improve the performance of the FEL and deliver shorter wavelength laser to the user, we are considering a project, which can upgrade the FEL system. One of the solutions is to construct a new in-vacuum optical klystron type undulator in the new long straight section, which is optimized for lasing in the deep UV and VUV region.

Study of partial-waveguide rf-linac FELs for intense THz-pulse generation

In this paper, we present a time-domain analysis of a short pulse partial-waveguide FEL oscillator employing toroidal mirrors and a hole outcoupling. The use of toroidal mirrors with optimized radius of curvatures helps to reduce cavity losses arising from the mismatch of the free space propagating optical field into a waveguided one. We introduce semi-analytical expressions for the calculation of the scattering matrix elements describing the loss and mode coupling mechanism as well as the amount of the extracted power from the cavity. The formulation is implemented in a time-domain FEL code based on modal expansion approach. The described model is applied to a partial-waveguide FEL system and simulation results are compared with measurements.

Observation of SASE in LEBRA FEL system

Observation of SASE in LEBRA FEL system

Upgrade of the UVSOR storage ring FEL

The UVSOR electron storage ring has been shutdown since April till August 2003 for upgradation of its performance. This upgrade includes high brightness lattice, longer straight sections for in-vacuum undulators and high-quality vacuum system, and it can increase the brightness of synchrotron radiation from undulators with reduced beam emittance . The improved quality of the electron beam is also of great advantage to the storage ring FEL. To further improve the performance of the FEL and deliver shorter wavelength laser to the user, we are considering a project, which can upgrade the FEL system. One of the solutions is to construct a new in-vacuum optical klystron type undulator in the new long straight section, which is optimized for lasing in the deep UV and VUV region.

Results of the VISA SASE FEL experiment at 840 nm

VISA (Visible to Infrared SASE Amplifier) is a high-gain self-amplified spontaneous emission FEL, which achieved saturation at 840nm within a single-pass 4-m undulator. A gain length shorter than 18cm has been obtained, yielding the gain of 2×108 at saturation. The FEL performance, including spectral, angular, and statistical properties of SASE radiation, has been characterized for different electron beam conditions. The results are compared to 3-D SASE FEL theory and start-to-end numerical simulations of the entire injector, transport, and FEL system. Detailed agreement between simulations and experimental results is obtained over the wide range of the electron beam parameters.

Characteristics of the fundamental FEL and the higher harmonic generation at LEBRA

The FEL system of Laboratory for Electron Beam Research and Application (LEBRA) at Nihon University has provided relatively high FEL gain around 10% at the wavelength of 1.5μm. The numerical estimation of the FEL gain suggests that LEBRA-FEL system has a sufficient performance on the electron beam bunching.In addition, the amplification of the third harmonics accompanying the intense fundamental FEL has been observed and the spectra of which have been measured. The experimental result indicates that the phenomenon is a kind of nonlinear harmonic generation.

First lasing of LEBRA FEL at Nihon University at a wavelength of [formula omitted

The FEL system covering wavelengths from 800 nm to 5 μm has been developed at Laboratory for Electron Beam Research and Application (LEBRA) in Nihon University. The system consists of a conventional 125 MeV S-band linac and a planar undulator with the alternative field of 50 periods. The first lasing of the system was achieved at the wavelength of 1.5 μm using dielectric mirrors. Although the saturation of the FEL power has not been observed yet, amplification of the spontaneous radiation power by about 10 8 times has been obtained with the measurement using the InSb detector. The absolute power has been estimated to be a few mJ/macropulse. An intense visible light has been observed frequently depending on the intensity of the fundamental FEL. The measured time structure of the phenomenon suggests that this is the radiation related with the formation of the microbunches in the electron beam.

Status of the Institute of Free-Electron Laser, Osaka University

Institute of Free-Electron Laser is one of the FEL supplying facilities for 2000 hours in a year. The FEL system consists of two RF-linacs and five wigglers, and covers the wavelength range from 0.28 to 100 µm. In the operational period from April 2001 to March 2002, the mid-infrared light of 5 to 20 µm was supplied for over 40 cooperative research programs on fields of semiconductor, bio-medical and environmental chemistry. An effort is still carrying on to improve the property of the light.

Simulation studies of the proposed far-infrared amplifier at ISIR, Osaka University

Computer simulation studies are currently underway for the proposed FEL amplifier experiment to be carried out using the existing 38 MeV, L-band linac based FIR FEL system at the Institute of Scientific and Industrial Research (ISIR), Osaka University. This paper describes the time-dependent and time-independent simulation results using Genesis code. Dependence of FEL amplifier characteristics are investigated such as the power spectrum and profile, frequency spectrum and FEL bunching parameter on parameters like the beam current, input power and interaction length. The operating current regime and input seed laser powers required for the optimum amplification are given.

FEL system for gamma–gamma collider at TESLA

The present paper contains the results of optimization of the free electron laser for the gamma–gamma collider at TESLA. A superconducting linear accelerator, similar to the TTF (TESLA Test Facility) accelerator, produces a driving electron beam for the FEL. The MOPA FEL scheme is studied when the radiation from a master oscillator is amplified in the FEL amplifier with tapered undulator. The FEL produces a radiation of TW level with a wavelength of 1 μm . Optimization of the FEL amplifier is performed with a three-dimensional, time-dependent simulation code FAST.

Lasing at 150 μm wavelength and measurement of the characteristics of the free-electron laser at ISIR, Osaka University

We are conducting FEL experiments using the infrared FEL system based on the L-band electron linac at ISIR, Osaka University, to extend the wavelength region toward the longer-wavelength side and to study FEL dynamics in the long wavelength region. The FEL wavelength was extended to 150 μm, which is the longest wavelength so far obtained with FELs based on RF linacs. We observed a phenomenon that FEL oscillation was obtained at two different optical cavity lengths but not in between them.

Basic study of self-amplified spontaneous emission in the infrared region with the L-band linac at Osaka University

We are conducting experiments of self-amplified spontaneous emission (SASE) in the far-infrared region at ISIR, Osaka University by using a high brightness single-bunch beam from the L-band linac. As the first step, characteristics of the SASE-FEL are estimated using the one-dimensional analytic formulas and the measured characteristics of the electron beam. The calculated FEL parameter is 0.02 and the gain length is 0.14 m at an electron energy of 13 MeV and K=1.472. Recently, we observed a strong optical signal radiated from a single-bunch beam passing through the wiggler for the existing infrared FEL system.

Improvement of the beam quality by chromaticity correction for wavelength shortening in the NIJI-IV FEL

Electron-beam qualities improved by chromaticity correction in the storage ring NIJI-IV were investigated at the beam energy of 309 MeV. Sextupole–quadrupole–sextupole (SQS) magnets, which were installed in all of the short-straight sections in NIJI-IV, perfectly corrected a horizontal and a vertical chromaticity. This improvement suppressed a head–tail instability, so that higher beam current (∼30 mA) and higher peak-electron density (∼6×10 16 m −3) were available for FEL experiments. The maximum FEL gain was estimated to be about 2.5% at a wavelength of 240 nm. The lasing of an FEL around 300 nm was achieved in March 1998, and the lasing of an FEL at around 240 nm was successfully observed in May 1998. The shortest wavelength of FELs with the NIJI-IV FEL system was 228 nm.

Numerical simulation to UV coherent harmonic radiation on Hefei storage-ring FEL

It is a prospective technical route to generate UV coherent radiation using external high-power laser to pre-bunch ultra-relativistic electron beam provided by storage ring and then amplifying its harmonic components with the super-radiation method. In this paper, based on the primary parameters of Hefei Storage Ring (NSRL) FEL system, firstly we made the simulation to the electron beam energy modulation process in an optical klystron, the simulation parameters include: γ=1000, the relativistic factor of the electron beam; the wavelength and power of the injected external laser are, λ s=0.353 μm and 100 MW; the period number and wavelength of the undulators are N u=20, and λ u=7.2 cm (we have the same structure for the modulator and radiator); and the normalized amplitude of the magnetic field is K=4.2. An effective energy modulation can be achieved when the rms energy spread for the electron beam is less than 10 −4, i.e., σ γ / γ⩽10 −4. Then density modulation for the energy-modulated electron beam in the dispersive section is also simulated. Under the above-mentioned conditions when the dispersive parameter N d≅76, a good density modulation results can be obtained. Finally, we also give the harmonic super radiation output power components P 3, P 5, P 7 in the radiator section.