Future Light Sources Research Articles

Beam breakup simulation study for a high energy ERL

The maximum beam current that can be accelerated in an energy recovery linac (ERL) can be severely limited by the transverse multi-pass beam breakup instability (BBU), especially in future ERL light sources with multi-GeV high energy beam energy and more than 100 mA average current. In this paper, the multi-pass BBU of such a high energy ERL is studied based on the simulation of a 3-GeV ERL light source that is proposed by KEK. This work is expected to provide a reference for future high energy ERL projects.

Demonstration of inverse free-electron laser seeding in a sub-80 K, short period cryogenic undulator

Short period, high field undulators have been shown to permit operation of x-ray free-electron lasers with short gain-lengths, and at unprecedented short wavelengths. In addition, the reduced beam energy required to reach resonance with a given radiation wavelength in short period undulators could prove useful in advanced inverse free-electron laser (IFEL) seeding schemes for future light sources, such as high-gain harmonic generation and echo-enabled harmonic generation, or in IFEL acceleration. We report here the in situ beam testing of a 9 mm period length cryogenic undulator having undulator parameter near unity in the context appropriate for advanced seeding and acceleration schemes, a linear accelerator. Because of the short period length of the undulator, a 47 MeV high-brightness electron beam could be used to produce near infrared photons via the undulator radiation mechanism. The undulator radiation was observed through a filter and the spectral response of the undulator is compared to simulation. Finally, an 800 nm seed laser was introduced in order to generate an energy modulation via the IFEL mechanism. Resonance between the electron beam and the laser seed was achieved without detailed knowledge of the temperature dependent undulator magnetic field through the observation of the undulator radiation. The energy modulation (and concomitant energy spread increase) of the electron beam was observed both directly in an electron beam spectrometer and indirectly via coherent transition radiation after conversion to a density modulation in a longitudinally dispersive chicane.

Design principles for high quality electron beams via colliding pulses in laser plasma accelerators

Laser plasma based accelerators have the potential to reduce dramatically the size and cost of future particle colliders and light sources. Production of high quality beams along with reproducibility, tunability, and efficiency are required for many applications. We present design principles for two-pulse colliding laser pulse injection mechanisms, which can meet these requirements. Simulations are used to determine the best conditions for the production of high quality beams: high charge, low energy spread, and low emittance. Simulations also allow access to the internal dynamics of the interaction, providing insight regarding further improvement of the beam quality. We find that a 20 pC beam can be accelerated to 300 MeV in 4 mm with only a few percent energy spread and transverse normalized emittance close to 1 mm mrad, using a 10 TW laser. We demonstrate that this design scales according to linear theory. Control of the laser pulse mode content and subsequent evolution in the plasma channel are shown to be critical for achieving the highest beam quality.

The MAX IV storage ring project.

The MAX IV facility, currently under construction in Lund, Sweden, features two electron storage rings operated at 3 GeV and 1.5 GeV and optimized for the hard X-ray and soft X-ray/VUV spectral ranges, respectively. A 3 GeV linear accelerator serves as a full-energy injector into both rings as well as a driver for a short-pulse facility, in which undulators produce X-ray pulses as short as 100 fs. The 3 GeV ring employs a multibend achromat (MBA) lattice to achieve, in a relatively short circumference of 528 m, a bare lattice emittance of 0.33 nm rad, which reduces to 0.2 nm rad as insertion devices are added. The engineering implementation of the MBA lattice raises several technological problems. The large number of strong magnets per achromat calls for a compact design featuring small-gap combined-function magnets grouped into cells and sharing a common iron yoke. The small apertures lead to a low-conductance vacuum chamber design that relies on the chamber itself as a distributed copper absorber for the heat deposited by synchrotron radiation, while non-evaporable getter (NEG) coating provides for reduced photodesorption yields and distributed pumping. Finally, a low main frequency (100 MHz) is chosen for the RF system yielding long bunches, which are further elongated by passively operated third-harmonic Landau cavities, thus alleviating collective effects, both coherent (e.g. resistive wall instabilities) and incoherent (intrabeam scattering). In this paper, we focus on the MAX IV 3 GeV ring and present the lattice design as well as the engineering solutions to the challenges inherent to such a design. As the first realisation of a light source based on the MBA concept, the MAX IV 3 GeV ring offers an opportunity for validation of concepts that are likely to be essential ingredients of future diffraction-limited light sources.

The potential of future light sources to explore the structure and function of matter

Structural studies in general and in particular in crystallography have benefited and still do benefit dramatically from the use of synchrotron radiation. Its tuneability is mandatory for multi or single anomalous diffraction experiments, still one of the main methods for solving new crystal structures. Its tuneability is also a key for spectroscopy techniques for the determination of the local atomic environment around and the oxidation state of an absorbing atom. These techniques are powerful tools e.g. in chemistry to study reactions, and can be applied not only to crystalline matter. Low emittance storage rings of the third generation with their highly brilliant X-ray beams enable us to focus beams down to the micrometer range intense enough for the investigation of weakly scattering crystals down to the size of several micrometers. Considering these highly intense beams, if it comes to structural studies using X-ray, what are still the limitations of the most modern storage ring sources? The length of individual X-ray pulses is in the order of 100 ps, which is sufficient to trace structural changes of larger groups or the diffusion of atoms over larger atomic distances. However, fast processes as they occur e.g. during vision or photosynthesis are not accessible by these means. Also the coherent fraction of the radiation of present day storage rings in the X-ray regime is rather low (i.g. < 1 %). This limits on one hand our ability to study the structure and dynamic of non-crystalline materials by methods exploiting the coherence properties of the beam like coherent diffractive imaging and X-ray correlation spectroscopy, respectively. On the other hand the flux in an ultra small diffraction-limited focus is limited as well. Upcoming linac driven free electron laser (FEL) sources with extremely short (sub 100 fs) and intense pulse (~10^12 ph) of very high coherence circumvent some of the limitations of present day storage rings. It has been demonstrated that their individual pulses are short enough to outrun radiation damage for single pulse exposures. First structures from sub micrometer crystals using an X-ray FEL (LCLS, Stanford) have already been published. These ultra short pulses also enable time resolved studies 1000 times faster than at standard storage ring sources. Meanwhile new storage rings with more aggressive lattice designs are under construction or under consideration with significantly smaller emittances. These sources target towards the diffraction limit in the X-ray regime and will provide roughly one to two orders of magnitude higher brilliance and coherence. They will be especially suited to those experiments exploiting the coherence properties of the beams and to ultra small focal spot sizes in the several nm regime. Developments at various places are ongoing for a totally new type of X-ray source combining a linac with a storage ring. This so called energy recovery linacs (ERL) promise to provide pulses almost as short as at an FEL with brilliances and multi-user capabilities comparable to a diffraction limited storage ring. The contribution will try to give an overview of the stage of development of the various source projects and their possible impact on structural studies in future.

Femtosecond X-ray-induced explosion of C60 at extreme intensity.

Understanding molecular femtosecond dynamics under intense X-ray exposure is critical to progress in biomolecular imaging and matter under extreme conditions. Imaging viruses and proteins at an atomic spatial scale and on the time scale of atomic motion requires rigorous, quantitative understanding of dynamical effects of intense X-ray exposure. Here we present an experimental and theoretical study of C60 molecules interacting with intense X-ray pulses from a free-electron laser, revealing the influence of processes not previously reported. Our work illustrates the successful use of classical mechanics to describe all moving particles in C60, an approach that scales well to larger systems, for example, biomolecules. Comparisons of the model with experimental data on C60 ion fragmentation show excellent agreement under a variety of laser conditions. The results indicate that this modelling is applicable for X-ray interactions with any extended system, even at higher X-ray dose rates expected with future light sources.

Analysis on effects of transverse electric field in an injector cavity of compact-ERL at KEK

For a future synchrotron light source based on a linac, e.g. an X-ray free electron laser and an energy recovery linac (ERL), an injector is a key component to generate a high brightness electron beam. For the acceleration and transportation of the electron beam in the injector, the adjustment of beam orbit inside the cavity is important to avoid the deterioration of the beam quality due to the transverse electric field of it, which causes the transverse emittance growth. To adjust the beam orbit, an investigation of the electromagnetic center of the cavity is required in the beam operation. This paper shows a new method for measuring the electromagnetic center of the cavity, and describes an analytical model of emittance growth due to a combination of transverse electric field and orbit offset. The validation of the method was confirmed by the emittance measurement in the compact ERL (cERL) injector at KEK.

Direct observation of fine-scale energy banding in echo-enabled harmonic generation

We describe the experimental observation of highly nonlinear energy striations generated by two lasers in a relativistic electron beam in an echo-enabled harmonic generation (EEHG) setup. The rich energy banding structure results from strong dispersion of the sinusoidally modulated beam, and measurements of the banding spectrum enable benchmarking, optimization, and characterization of the concomitant EEHG process. Results are found to be in good agreement with theory, and suggest that the presented technique can facilitate the practical implementation of EEHG to generate intense, fully coherent light in future advanced accelerator-based light sources.

Surface-micromachined Electromagnets for 100μm-scale Undulators and Focusing Optics

Abstract Leveraging recent advances in microelectromechanical system (MEMS) fabrication technologies, 100 μm gap 400 μm period electromagnetic undulators and 200 μm gap quadrupole optics have been successfully fabricated. Measurement of the electromagnet impedance closely matches simulations, corresponding to 0.135 T peak field on-axis in the undulator and 1400 T/m field gradients in the quadrupole for the initially tested devices. These microfabricated undulators and quadrupoles are envisioned as insertion devices and focusing lattices for future compact light sources.

Emission property of Sn2 +-doped ZnO–P2O5 glass

Photoluminescent properties of amorphous zinc phosphate glass, containing Sn2+ as an emission center, were investigated. Varying the chemical composition changed both the excitation and emission properties. The redox reaction that occurred during melting was found to affect both valences of transition metal and Sn cations to change the emission property of the Sn2+-containing glass. We have also examined the emission property of substituted SnO–ZnO–P2O5 glass, and demonstrated that the emission color can be tailored by chemical composition to obtain blue, white, and red. It is notable that the quantum efficiency value of the present RE-free glass is comparable to that of crystal phosphor such as MgWO4. We have, therefore, emphasized that transparent inorganic glass phosphor will be one of the fascinating emitting materials adoptable to future strong LED light source.

China prepares to spend billions on science and technology

With its strong economy, the country wants to play a bigger role on the world science stage.

K16200 窒化物半導体デバイスの今後の展望(【K16200】情報・知能・精密機器部門企画,基調講演)

Market trend of GaN-based white LEDs and RF HFET is surveyed. Key points for the future white light source for general lighting and high power electron devices are discussed. Advantage of using high-quality GaN substrates grown by Na flux method on the performance of high-current density operation LEDs and also high power electron devices will be shown. Also, merit of using high-density nitrogen radical source on the growth of thick InGaN by plasma-assisted (PA-) molecular beam epitaxy (MBE) is presented.

Comprehensive Study on the Electron Transport Layer in Blue Flourescent Organic Light-Emitting Diodes

Organic light-emitting diodes (OLEDs) are essential for the development of future light sources and display technology. Here, we have investigated the effects of various electron transport layers such as Bebq2, Bepp2, BAlq, Alq3 and TPBi in blue flourescent OLEDs. It is found that properties of electron transport layer, including energy gap, hole blocking ability and electron mobility, play critical roles in determining the device performance. Particularly, it is found that the hole blocking ability is more crucial than electron mobility in terms of current efficiency. With the excellent hole blocking ability and great electron mobility, the TPBi-based blue OLED exhibits a maximum current efficiency of 17.3 cd/A and a maximum power efficiency of 18.1 lm/W. The factors for achieving highly efficient OLEDs have been discussed in detail.

Theory of high-speed nanolasers and nanoLEDs

We investigate the intrinsic high speed modulation responses of nanolasers and nanoLEDs using bulk, quantum wells (QWs), and quantum dots (QDs) based on a rigorous rate-equation model, which incorporates the optical energy confinement factor to properly account for the negative permittivity and dispersive metal plasma property. We then investigate the dependence of the bandwidth and the energy per bit on the quality factor and the normalized optical volume. We find out that the conditions for the energy per bit less than 50 fJ/bit and 10 fJ/bit are the normalized optical modal volume less than 20 and 5, respectively. In addition, with a uniform quantum dot size in a nanocavity, quantum-dot metal-cavity nanolasers exhibit the largest bandwidth among three types of active materials, and a low energy per bit. With their insensitivity to temperature, quantum-dot metal-cavity nanolasers are favorable for future high speed light sources.

Spin-Resolved Purcell Effect in a Quantum Dot Microcavity System

We demonstrate the spin selective coupling of the exciton state with cavity mode in a single quantum dot (QD)-micropillar cavity system. By tuning an external magnetic field, each spin polarized exciton state can be selectively coupled with the cavity mode due to the Zeeman effect. A significant enhancement of spontaneous emission rate of each spin state is achieved, giving rise to a tunable circular polarization degree from -90% to 93%. A four-level rate equation model is developed, and it agrees well with our experimental data. In addition, the coupling between photon mode and each exciton spin state is also achieved by varying temperature, demonstrating the full manipulation over the spin states in the QD-cavity system. Our results pave the way for the realization of future quantum light sources and the quantum information processing applications.

Ultimate storage ring based on fourth-order geometric achromats

We have designed an ``ultimate'' storage ring for a future light source that would be resided in the positron-electron-project (PEP) tunnel and achieves the diffraction-limited emittances (at 1.5 \AA{}) of 12 pm-rad in both horizontal and vertical planes with a 4.5-GeV electron beam. These emittances include the contribution of intrabeam scattering at a nominal current of 200 mA in 3300 bunches. This quality beam in conjunction with a conventional 4-m undulator in a straight section can generate synchrotron radiation having a spectral brightness above ${10}^{22}\text{ }\text{ }[\mathrm{photons}/\mathrm{s}/{\mathrm{mm}}^{2}/{\mathrm{mrad}}^{2}/0.1%\mathrm{BW}]$ at a 10 keV photon energy. The high coherence at the diffraction limit makes this design competitive with 4th generation light sources based on an energy recovery linac. In addition, the beam lifetime is several hours and the dynamic aperture is large enough to allow off-axis injection. The alignment and stability tolerances, though challenging, are achievable. A ring with all these properties is only possible because of several major advances in mitigating the effects of nonlinear resonances.

Synchrotron radiation representation in phase space

The notion of brightness is efficiently conveyed in geometric optics as density of rays in phase space. Wigner has introduced his famous distribution in quantum mechanics as a quasiprobability density of a quantum system in phase space. Naturally, the same formalism can be used to represent light including all the wave phenomena as originally done by Walther and for synchrotron radiation by Kim. It provides a natural framework for radiation propagation and optics matching by transferring the familiar ``baggage'' of accelerator physics ($\ensuremath{\beta}$ function, emittance, phase-space transforms, etc.) to synchrotron radiation. More specifically, the use of Wigner distribution formalism allows a rigorous description of partially coherent non-Gaussian sources, which is generally the case for synchrotron radiation from an undulator with a high degree of transverse coherence. This paper reviews many of the properties of the Wigner distribution starting from quantum mechanics and provides examples of how its use enables physically insightful description of partially coherent synchrotron radiation in phase space. The concepts of diffraction limit and coherence are given an exact correspondence to their quantum mechanical counterparts. In particular, it is shown that the undulator radiation on resonance by a single electron is not diffraction limited though fully coherent. An extension of how to account for practically important cases of electron beams with nearly diffraction limited emittances is presented along with a discussion of appropriate figures of merit suitable for comparing future light sources.

Light source optimization: spectral design and simulation of four-band white-light sources

The authors build on previous experience in the optimization of white light sources based on combinations of known narrow-band spectra. The authors extend these concepts by using synthetic (mathematically generated) four-band spectra to study the prospects for the spectral design of future optimal light sources. The optimization is based on achieving a trade-off of the color rendering properties against the luminous efficacy of the radiation, using the differential evolution (DE) algorithm. It is shown that this trade-off is controllable through the choice of the optimization parameters, specifically the choice of terms in the optimization fitness function. Depending on the choice of the shapes for the synthetic spectral bands, the results are comparable with those achieved in earlier work on the optimization of four-band light-emitting diode (LED) spectra. However, the range of color temperatures achieved in the present study is considerably wider than was previously found for optimum four-band LED mixtures.

Physics issues in diffraction limited storage ring design

Diffraction limited electron storage ring is considered a promising candidate for future light sources, whose main characteristics are higher brilliance, better transverse coherence and better stability. The challenge of diffraction limited storage ring design is how to achieve the ultra low beam emittance with acceptable nonlinear performance. Effective linear and nonlinear parameter optimization methods based on Artificial Intelligence were developed for the storage ring physical design. As an example of application, partial physical design of HALS (Hefei Advanced Light Source), which is a diffraction limited VUV and soft X-ray light source, was introduced. Severe emittance growth due to the Intra Beam Scattering effect, which is the main obstacle to achieve ultra low emittance, was estimated quantitatively and possible cures were discussed. It is inspiring that better performance of diffraction limited storage ring can be achieved in principle with careful parameter optimization.

Recent Developments in High Voltage Photocathode DC Gun for Accelerator

A photoinjector which can generate a high brightness, low emittance and short duration electron bunches is required for future light source accelerators and applications such as electron microscopes. A DC high voltage gun which has a semiconductor photocathode in a high DC electric field is one of the technology choices for the generation of high quality electron beam. Many techniques to the development of semiconductor based photoemission gun are introduced in briefly.