Search Results (55)

The geographical response of western North Pacific subtropical high (SH) to environmental conditions such as the El Niño-Southern Oscillation (ENSO) and global warming has been one of the main concerns with respect to extreme events induced by tropical convections. By considering observed outgoing longwave radiation (OLR) as the strength of subtropical high, this study attempts to further understand the geographical response of SH strength to ENSO and global warming. Here, “SH strength” is defined as the inhibition of regional convections under SH environment. A meridional seesaw pattern among SH strength anomalies is found at 130°–175° E. In addition, the La Niña environment with weaker convections at lower latitudes is characterized by farther westward expansion of SH but with a weaker strength. Conversely, the El Niño environment with stronger convections at lower latitudes leads to shrunken SH but with a greater strength. The influence of the seesaw mechanism appears to be modulated by global warming. The western North Pacific subtropical high strengthens overall under warming in both the La Niña and El Niño environments. This suggests that the weakening effect by drier tropics is largely offset by anomalous highs induced by a warming atmosphere. It is most remarkable that the highest SH strengths appear in a warmer El Niño environment. The finding implies that every new El Niño environment may experience the driest atmosphere ever in the subtropics under global warming. The value of this study lies in the fact that OLR effectively illustrates how the ENSO variation and global warming bring the zonally undulating strength of boreal-summer SH. Full article

The generation of high-quality mid-infrared free-electron laser (MIR-FEL) radiation depends critically on precise control of electron beam parameters, including energy, energy spread, transverse emittance, bunch charge, and bunch length. At the PBP-CMU Electron Linac Laboratory (PCELL), effective beam diagnostics are essential for optimizing FEL performance. However, dedicated systems for direct measurement of transverse emittance and bunch length at the undulator entrance have been lacking. This paper addresses this gap by presenting the design, simulation, and analysis of diagnostic stations for accurate characterization of these parameters. A two-quadrupole emittance measurement system was developed, enabling independent control of beam-focusing in both transverse planes. An analytical model was formulated specifically for this configuration to enhance emittance reconstruction accuracy. Systematic error analysis was conducted using ASTRA beam dynamics simulations, incorporating 3D field maps from CST Studio Suite and fully including space-charge effects. Results show that transverse emittance values as low as 0.15 mm·mrad can be measured with less than 20% error when the initial RMS beam size is under 2 mm. Additionally, quadrupole misalignment effects were quantified, showing that alignment within ±0.95 mm limits systematic errors to below 33.3%. For bunch length measurements, a transition radiation (TR) station coupled with a Michelson interferometer was designed. Spectral and interferometric simulations reveal that transverse beam size and beam splitter properties significantly affect measurement accuracy. A 6% error due to transverse size was identified, while Kapton beam splitters introduced additional systematic distortions. In contrast, a 6 mm-thick silicon beam splitter enabled accurate, correction-free measurements. The finite size of the radiator was also found to suppress low-frequency components, resulting in up to 10.6% underestimation of bunch length. This work provides a practical and comprehensive diagnostic framework that accounts for multiple error sources in both transverse emittance and bunch length measurements. These findings contribute valuable insight for the beam diagnostics community and support improved control of beam quality in MIR FEL systems. Full article

The present study analyzes the hydrodynamic performance of an asymmetric offshore Oscillating Water Column device positioned in close proximity to multiple bottom standing and fully submerged breakwaters and trenches. The breakwaters and trenches are located on the leeward side of the Oscillating Water Column device. The structures are investigated in combination with a shore-fixed vertical wall. The analysis is carried out using the Boundary Element Method based on the linear potential flow theory. The results are compared with the existing analytical, numerical, and experiment results available in the literature. The effects of the various shape parameters of the submerged breakwaters/trenches and the shape parameters of the Oscillating Water Column device are investigated. The results show that the resonance effects on the efficiency performance increase as the number of breakwaters/trenches increases. The undulating bottom trench shape is effective in improving the efficiency of the Oscillating Water Column device compared to the breakwater. The efficiency bandwidth is greater in the case of a rectangular trench than in the case of a parabolic- or triangular-shaped trench. In addition, the first peak value in the efficiency curve for a lower frequency is higher in the case of a larger-draft Oscillating Water Column device front wall compared to that of the rear wall. This study demonstrates that in the long wave-length regime, a zero efficiency point is observed between two consecutive resonant peaks, whereas in the intermediate and short wave-length regimes, a trough and a zero efficiency point alternately occur between two consecutive resonance peaks. Various parameters relevant to the behavior of the Oscillating Water Column Wave Energy Converter, such as radiation susceptance, radiation conductance, hydrodynamic efficiency, and volume flux due to a scatter potential, are addressed. Full article

Free-Electron Laser in Hamburg (FLASH), first launched in 2005, was the first free-electron laser that provided ultrashort radiation pulses in extreme ultraviolet and soft X-ray spectral range. In 2017, it was decided to improve the existing FLASH facility within the FLASH2020+ project, which led to upgrading the existing linac with variable gap tunable undulators in the FLASH1 line and refurbishing two cryomodules to achieve a beam energy increase to 1.35 GeV. It was also a perfect opportunity to completely redesign and rebuild the radio frequency (RF) phase reference generation and distribution system. This paper presents the design and parameters of new, custom-made phase reference signal generation and distribution modules, successfully installed in FLASH. These are the main oscillator, the RF distribution module, and the frequency conversion modules. The new instrumentation presents a significant improvement in terms of RF reference signal parameters, state-of-the-art phase noise performance (an improvement in the total jitter of the 1.3 GHz RF signal from 55.9 fs to 10.7 fs in the integration range from 10 Hz to 1 MHz), module compactness (size reduction from three fully occupied rack cabinets to four 19″ modules only), and serviceability. The presented Main Oscillator system design is foreseen for easy modifications, making it suitable for applications in other accelerator facilities or hardware platforms. Full article

The main aim of this study is the concept of the magnet holder for the THz undulator utilized in the PolFEL superradiant light source. To achieve maximum flux at high K values (radiation frequencies ranging from 0.5 THz to 5 THz, and K values exceeding 3), it is necessary to use large permanent magnets with dimensions of 100 × 100 × 39.9 mm. For the above assumptions and parameters and specific requirements for magnet positioning, existing design solutions in the literature were found to be insufficient. The main challenges in the design of this holder included the following: (a) the unusually large size of the magnets, (b) requirements of wide-range calibration, and (c) large magnetic forces acting on each magnet, which can approach almost 4 kN. Taking into consideration these challenges, the prototype of the magnet holder was developed and manufactured. The paper presents the findings from both numerical and experimental studies aimed at validating the mechanical behavior and deformation of the proposed magnet holder. The measurements were conducted using two methods—traditional with the use of dial indicators and a novel approach based on the application of the Digital Image Correlation. The results from these numerical and experimental studies indicate that all specified requirements have been satisfactorily met. The study confirms the capability for accurate magnet positioning, demonstrating stable deformation of the holder under a magnetic load. Additionally, it was proved that the positioning of the magnets is both linear and repeatable, with calibration achievable within a range of at least ±0.25 mm. Full article

Existing sub-grid parameterization schemes for clear-sky direct solar radiation (SPS-CSDSR) assume that the sub-grid cells have the same atmospheric transparency. This study shows that in undulating terrain, significant errors can occur when the sub-grid is in turbid weather or partly above the cloud top. A correction factor was proposed. It can effectively eliminate errors under a cloudless sky and can reduce some errors when part of the sub-grid is above the cloud or fog top. For atmospheric models with high horizontal resolution, example test verification shows that the cast shadowless coverage method can lead to large errors. It should no longer be used based on current computing power. These improvements and the high-precision fast terrain occlusion algorithm in Part I will allow SPS-CSDSR to achieve unprecedented high accuracy. Based on the proposed daily interpolation method, the high-precision SPS-CSDSR is also feasible for regional climate simulation. The analysis pointed out that the sub-grid terrain radiative effect (STRE) is distributed over inclined surfaces with larger areas and at different heights. Existing methods of coupling STRE on one flat surface have certain physical drawbacks. This paper suggests introducing parameterization of STRE at different altitudes and improving the coupling of land–air. Full article

A superradiant FEL in the THz (3 THz) region is currently operating at Ariel University. It is based on the novel ORGAD accelerator, which is a hybrid linear RF photo-cathode 6 MeV electron gun. The hybrid term stands for its unique standing wave (SW)—traveling wave (TW) structure. The undulator generates spontaneous superradiance, which corresponds to spontaneous emission when the electron bunch duration is shorter than the radiated frequency, resulting in a much higher photon yield. However, the efficiency of this scheme is still quite low. In order to achieve higher emission (by improved efficiency), we intend to implement a new and promising radiative interaction scheme: tapering-enhanced superradiance (TES). This particular undulator design employs a tapered (amplitude) undulator in the zero-slippage condition to obtain a significantly more powerful and efficient THz radiation source. At the current stage, the scheme is designed for emission at approximately $0.5$ THz. The design and start-to-end simulations demonstrate significant enhancement of superradiant energy and extraction efficiency using this method compared to a reference uniform case. Full article

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. Full article

To resolve photons hungry for weak diffraction samples by the crystallographic method, a double-multilayer monochromator (DMM) was employed on an undulator beamline (BL17UM) at the Shanghai Synchrotron Radiation Facility (SSRF) to provide a focused sub-micron beam with high brightness for macromolecular crystallography experiments. High-quality crystallographic datasets from model protein crystal samples were collected and processed by an existing crystallographic program for structure solution and refinement. The data quality was compared with datasets from a normal silicon crystal monochromator to evaluate the bandwidth of the DMM effect on these crystallographic data. This experiment demonstrates that multilayer optics on an undulator beamline may play a valuable role in satisfying the demands of structure-related research, which requires novel methods. Full article

Free electron lasers (FELs) are becoming more and more popular as sources of radiation for research purposes in many fields of science. They are frequently employed in second harmonic generation (SHG) studies. SHG is an important effect of nonlinear responses of matter to irradiation. It should be separated from the second harmonic of the radiation source, FELs, for correct analysis of the response. Using an analytical model for harmonic powers evolution in a single-pass FEL, we demonstrate the dependence of FEL harmonics on the key parameters of beams and undulators and show that it is possible to reduce the second FEL harmonic content if the energy spread of an electron beam is increased. For LEUTL FEL radiation in the visible range, we analytically demonstrate a reduction in the second harmonic power by an order of magnitude if electron energy spread is increased twice, which still allows efficient bunching at the fundamental wavelength. The method is valid regardless of the photon energy and, therefore, the absorption edge of the target material, and as such, it is valid also in the X-ray band. Full article

Grasslands, a vital ecosystem and component of the global carbon cycle, play a significant role in evaluating ecosystem health and monitoring the global carbon balance. In this study, based on the Carnegie–Ames–Stanford Approach (CASA) model, we estimated the Net Primary Productivity (NPP) of grasslands in northern Shaanxi from 2000 to 2020. Employing trend analysis, stability analysis, multiple regression analysis, and residual analysis, the research examined the dynamic changes of grassland NPP and its response to climatic and human factors. Key findings include: (1) Grassland NPP showed a significant increasing trend during 2000–2020, with high-coverage grasslands showing a higher rate of increase than medium and low-coverage grasslands. (2) Most grasslands (>90%) exhibited unstable growth and high NPP fluctuation. (3) While temperature, precipitation, and radiation undulate, the trends were not significant. Rainfall and radiation emerged as dominant factors affecting NPP, with temperature suppressing NPP increase to some extent. (4) Policies like returning farmland to grassland had a positive impact on grassland recovery, vegetation productivity, and regional ecosystem health. Full article

An intense narrow-band terahertz (THz) radiation source has been designed to generate a broad tuning range of radiation frequencies between 0.5 THz and 5.0 THz. The THz radiation is produced when a short-bunch electron beam propagates through an undulator. To achieve high-power peak radiation, the source requires high-brightness electron beams with low beam emittance and short bunch length. A proposed design for the photocathode RF gun used as the electron source is presented. The gun with high mode separation and high Q-factor can be achieved for producing a good beam quality. The beam dynamics of the injector have been preliminarily optimized using the software ASTRA and Elegant, investigating the impact of laser pulse shape on electron beam quality. The results of the beam dynamics studies are comprehensively discussed in this paper. Full article

The pulse duration in short-pulse schemes for Self-Amplified Spontaneous Emission Free Electron Lasers (SASE FELs) is limited by the FEL coherence time. A recently proposed concept allows to overcome the coherence time barrier and to obtain much shorter pulses. When the lasing part of an electron bunch is much shorter than the coherence time, one can suppress the radiation in the long main undulator while preserving microbunching within that short lasing slice. Then, a short radiation pulse is produced in a relatively short radiator. A possible suppression method, an excessive reverse undulator taper, is discussed and illustrated numerically in this paper. We also performed the first experimental tests of this method at the soft X-ray FEL user facility FLASH. The measured pulse duration approaches 1 fs (FWHM) at the wavelength of 5 nm. Full article

In this paper, we study spontaneous coherent undulator emission from a short dense electron bunch. A special “negative-mass” undulator ensuring stabilization of the axial size of the bunch is considered. We focus on the situation where the competition between two different transverse waveguide modes takes place, so that the undulator resonance is provided simultaneously for three waves placed on the dispersion diagram symmetrically with respect to electrons (namely, the group velocity of the higher waveguide mode coincides with the electron velocity, whereas two resonance frequencies of the lower transverse mode correspond to wave group velocities, one of which slightly exceeds the translational velocity of electrons, with the other being less than the electron velocity). We show that the higher waveguide mode can win this competition due to its proximity to group synchronization with the electron bunch. Full article

Height measurement of meter wave radar is a difficult and important problem. This paper studies the height measurement of meter wave polarimetric (MWP)-MIMO array radar under complex terrain. The traditional electrically short dipole has low radiation efficiency, and the collocated dipole vector antenna has strong mutual coupling. This paper proposes to use electrically long dipoles and separated vector antennae to solve the problems of low radiation efficiency and strong mutual coupling. In addition, different from the traditional flat terrain, the research of this paper is based on the conditions of complex undulating terrain. First, the height measurement signal model of the MWP-MIMO radar with separated electrically long dipole under the complex terrain is derived. Then, a preprocessing method of block orthogonal matching pursuit is proposed to obtain the coarse estimation of the target’s elevation. Then, under the guidance of the coarse estimation, the generalized MUSIC algorithm is used to obtain the high-precision elevation estimation of the target, and then the height measurement of the target is obtained according to the geometric relationship. Finally, the effectiveness of the proposed algorithm is proved by computer simulations. Full article