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Future linear accelerators require klystrons with higher radio frequency (RF) to drive higher gradient accelerating structure. An x-band accelerator structure was used to accelerate electrons at the Shanghai Soft X-ray Free Electron Laser Facility (SXFEL) in Shanghai Advanced Research Institute, Chinese Academy of Sciences (SARI-CAS). A pulse transformer is a crucial device in an RF system. This study presents a highvoltage pulse transformer used for a 50 MW x-band pulsed klystron in SXFEL. Typical specifications of the pulse transformer are peak pulse voltage 420 kV, peak pulse current 300 A, 50 Hz repetition rate and 1.5 µs flattop pulse width. Design and optimization of pulse transformer are achieved by using equivalent circuit analytic methods and computational aided simulation. The relevant experiments show that this pulse transformer can meet the requirements of 50MW x-band klystron. [ABSTRACT FROM AUTHOR]

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STEP is a spherical tokamak prototype power plant that is being designed to demonstrate net electric power. The design phase involves the exploitation of plasma models to optimise fusion performance subject to satisfying various physics and engineering constraints. A modelling workflow, including integrated core plasma modelling, MHD stability analysis, SOL and pedestal modelling, coil set and free boundary equilibrium solvers, and whole plant design, has been developed to specify the design parameters and to develop viable scenarios. The integrated core plasma model JETTO is used to develop individual flat-top operating points that satisfy imposed criteria for fusion power performance within operational constraints. Key plasma parameters such as normalised beta, Greenwald density fraction, auxiliary power and radiated power have been scanned to scope the operational space and to derive a collection of candidate non-inductive flat-top points. The assumed auxiliary heating and current drive is either from electron cyclotron (EC) systems only or a combination of EC and electron Bernstein waves. At present stages of transport modelling, there is a large uncertainty in overall confinement for relevant parameter regimes. For each of the two auxiliary heating and current drive systems scenarios, two candidate flat-top points have been developed based on different confinement assumptions, totalling to four operating points. A lower confinement assumption generally suggests operating points in high-density, high auxiliary power regimes, whereas higher confinement would allow access to a broader parameter regime in density and power while maintaining target fusion power performance.

This paper presents an antenna array organized in sub-arrays and composed of densely spaced dielectric-based radiating elements. To reduce the number of active components required in the array beamforming network, each sub-array consists of a single active element, directly fed, and several passive reactively loaded elements. State-of-the-art implementations of such arrays are typically based on rectangular air-filled waveguide radiators which provide a limited bandwidth and support a single linear polarization only. By utilizing the dielectric-based radiators presented in this work, a significant increase in the operating bandwidth of the array can be achieved. In this case, the aforementioned reactive loading is implemented through short-circuited dielectric-filled waveguides. By optimizing the position of the short circuits, the radiation pattern of the sub-array can be controlled and synthesized according to a given mask. To create larger design flexibility, one can design the sub-array in the presence of the adjacent sub-arrays, achieving in this way their effective overlapping. By employing the considered design technique, a sub-array featuring flat-top radiation pattern characteristics in combination with low side-lobe levels and high gain was developed, manufactured, and thoroughly analyzed. An additional benefit of such a design choice is the possibility of supporting two orthogonal polarizations.

Structured light is desired in the micromachining of various materials. As the latest ultrashort laser sources increase the average power by increasing the repetition rate, the efficiency of the process becomes crucial in laser micromachining. The standard Gaussian beam shape, because of its slow ascending/descending slope, is not the most efficient beam, neither in precise energy deposition nor in heat management. Flat-top beams are required for precise and even energy distribution and are used in laser ablation or thin-film removal. We look at a rather abstract concept of polarization singularities and investigate them to create a vector flat-top beam. For this purpose, we devise a beam-shaping method based on the volumetric birefringent nano gratings which allow the generation of high-energy polarization singularities. By tuning the input beam waist, we have generated three most common laser micromachining beam profiles (Gaussian, Flat-top, and ring- or doughnut-shaped). The resulting high-quality and high-energy polarization singularities allow us to systematically demonstrate their use in transparent and opaque material micromachining, as well as the usual application for flat-top beams of thin-film removal. • Vector beams with polarization singularities applied in laser micro processing. • Dynamical shaping of a laser beam in focal zone using a single optical element. • Employment of geometric phase elements for high >92% transmission laser beam shaping. • Generation of flat-top beams without pre- of post-intensity peaks in the focal zone. [ABSTRACT FROM AUTHOR]

Attosecond pulses created by high-order harmonic generation in gases often exhibit strong chromatic aberrations, arising from the broad bandwidth and wavelength-dependent nonlinear light–matter interaction. When the driving laser intensity varies spatially, as for Gaussian driving beams, the apparent source position of the harmonics differs significantly from one order to the next, thus affecting the achievable intensity and duration of the attosecond pulses when they are focused on a target. We show that these chromatic aberrations can be reduced by spatially shaping the fundamental beam to generate high-order harmonics with a driver having a flat-top profile inside the gas medium. By measuring both the intensity profile and wavefront for each harmonic in a plane, we access the extreme ultra-violet (XUV) beam properties and investigate these properties near focus. We observe that controlling chromatic aberrations by flat-top spatial shaping strongly reduces the variation of the XUV spectrum on the beam axis during propagation and, in return, the longitudinal sensitivity of both the temporal profiles and the temporal shifts of the focused attosecond pulses.

Natural convection in attic of non-conventional rooftops has received considerable attention in recent years due to its importance in thermal management of modern pitched-roof buildings. In this study, a finite-volume numerical investigation of laminar fluid dynamics and heat transfer of air within the attic of a flat-top roof structure has been predicted for bottom isothermal heating at varying pitch angle. The heat transfer between the walls results in multiple thermal plumes and multi-cellular flow structure with the number, size and strength of the counter-rotating cells reducing with increasing pitch angle. The results further show that the peculiar shape of the roof has significant effect on the fluid flow and heat transfer. Particularly, the truncated triangular architecture of the roof prevents the formation of large, dominating and upper-row cells at the midsection of the attic. At low pitch, the intensity of the vortices results in thorough mixing of air and, hence, uniform temperature distribution within the attic. The averaged Nusselt number for the hot ceiling wall which depicts the rate of convective heat transfer into the attic is in negativegradient quasilinear relationship with the roof pitch. The practical significance of the predicted results is that, due to the peculiarity of the flat-top roof structure, heat loss to the attic is minimised when the roof pitch is relatively high; particularly not less than 300 and made as low as possible if the attic is to be used for drying of food crops. [ABSTRACT FROM AUTHOR]

We present a wavelength continuously tunable all-fiber flat-top comb filter. The proposed comb filter is based on a dual-pass Mach–Zehnder interferometer (MZI), constructed with two couplers and a polarization controller (PC), along with a polarizer at the input port. While the interference is created from the optical path difference provided by the two cascaded couplers, the dual-pass structure is based on the feedback of a fiber loop mirror, formed by the second coupler with the PC inserted in the loop, which serves as a reflectivity-variable mirror. The coupling ratio of the first coupler is determined by our optimized results on characterizing a basic dual-pass MZI without a PC. By using Jones matrix formulation, we theoretically analyzed the proposed comb filter. To achieve flat-top spectra, both amplitude condition and phase condition have to be satisfied. Our simulation results revealed that by changing the state of the PC, tunable flat-top spectra can be achieved, and moreover, the flat-top spectra can be continuously adjusted within one free-spectral-range with the help of a rotatable polarizer. The theoretical prediction has been verified by our experimental results. The unique tuning property makes the proposed filter an excellent candidate for applications in dense reconfigurable wavelength division multiplexing networks. [ABSTRACT FROM AUTHOR]

Surface precision processing of Iceland spar and fused silica is shown experimentally by laserinduced microplasma action. The optimal regimes allowed to control the relief depth up to ~14 μm with the step of 50 nm are achieved and discussed. Two types of optical elements (random phase plate and spiral phase plate) are fabricated and tested in laser setups, to change initial intensity distribution (Gaussian) to flat-top and annular, consequently. Flap-top distribution characterized by low speckle modulation, i.e. high quality. The annular distribution corresponds to classical concepts. [ABSTRACT FROM AUTHOR]

To produce pulses with good flat-top quality, pulse-forming lines (PFLs) have been widely used in the field of Tesla-type pulse generators. To shorten the physical length of the PFL, a double-width PFL (DWPFL) is proposed that doubles the output pulse width while maintaining flat-top quality. A repetitively 10 GW Tesla-type long-pulse generator producing pulses with flat-top width of about 110 ns was developed with a coaxial DWPFL to produce high-current electron beams. Electron beams of about 10 GW with flat-top widths of about 110 ns were obtained on a planar vacuum diode load. With this pulse generator and a C-band high-power microwave system, microwaves of ~2.2 GW power and full-width at half-maximum of 101 ns were generated. The experiment demonstrates the feasibility and ideal output waveform quality of the DWPFL. [ABSTRACT FROM AUTHOR]

Electromechanical actuator (EMA) systems are widely employed in missiles. Due to the influence of the nonlinearities, there is a flat-top of about 64 ms when tracking the small-angle sinusoidal signals, which significantly reduces the performance of the EMA system and even causes the missile trajectory to oscillate. Aiming to solve these problems, this paper presents a hybrid control for flat-top situations. In contrast to the traditional PID or sliding mode controllers that missiles usually use, this paper utilizes improved sliding mode control based on a novel reaching law to eliminate the flat-top during the steering of the input signal, and utilizes the PID control to replace discontinuous control and improve the performance of EMA system. In addition, boundary layer and switching function are employed to solve the high-frequency chattering problem caused by traditional sliding mode control. Experiments indicate that the hybrid control can evidently reduce the flat-top time from 64 ms to 12 ms and eliminate the trajectory limit cycle oscillation. Compared with PID controllers, the proposed controller provides better performance—less chattering, less flat-top, higher precision, and no oscillation.

This letter proposes a flat-top beampattern synthesis method in range and angle domains for frequency diverse array (FDA) based on second-order cone programming (SOCP). By dividing the array into subarrays and applying each subarray with a distinguishable frequency shift, this approach explores the separated degrees of freedom in range domain and is capable of generating a two-dimensional flat-top beampattern in range and angle domains. Basically, the flat-top beampattern synthesis in range and angle domains is formulated as an SOCP problem that can be exactly and efficiently solved with inner point method. Numerical examples are carried out to demonstrate the effectiveness of the proposed method. [ABSTRACT FROM PUBLISHER]

The pulsed high magnetic field facility funded by the Chinese National Development and Reformation Committee has been developed at the Wuhan National High Magnetic Field Center (WHMFC). Magnets of short pulse, long pulse and the combination of both with bore sizes from 12 to 34 mm have been developed and are operational for electric transport, magnetization, magneto-optics and electron spin resonance at temperatures in the range from 100 mK to 350 K. The power supplies for these magnets consist of a capacitor bank with 12 modules of 1 MJ/25 kV each and 2 modules of 0.8 MJ/25 kV each, a 100 MVA/100 MJ flywheel pulse generator and a 771 V/180 kAh battery bank. A dual-coil magnet driven by the capacitor banks has successfully generated 86.3 T field with a total pulse duration over 350 ms in a 12-mm bore. A dual-coil long pulse magnet wound from soft copper wire energized by the flywheel generator produces 50 T peak field with a 100 ms flat-top and a total 1.1 s pulse duration in a 22 mm bore. A battery bank driven long pulse magnet composed by the series connection of two nested coils generates 32 T peak field with the pulse duration of 1.5 s in a 21-mm bore. [ABSTRACT FROM PUBLISHER]

Abstract: We present a novel technique of coupling finite element method with mesh-based flat-top partition of unity method. The proposed coupling method allows us to bind any order of finite elements with flat-top partition of unity method. To verify the coupling, we test the coupling method on one- and two-dimensional boundary value problems including linear elasticity problem on a cracked domain. The coupled formulation provides a platform for stable enrichments to obtain highly accurate solution especially in the enrichment area. [Copyright &y& Elsevier]

Abstract: A novel idea to construct flat-top partition of unity functions in a closed form on a general (structured or unstructured) finite element mesh is presented. An efficient and practical construction method of a flat-top partition of unity function is important in the generalized finite element method (GFEM). Details on how to construct flat-top partition of unity functions on a provided mesh are given. The generalized finite element approximation with the use of the new flat-top partition of unity function is presented with various numerical examples that demonstrate the effectiveness of proposed flat-top partition of unity functions. [Copyright &y& Elsevier]

Abstract: We explore the nonlinear response in a multilayer system of Lorentz type single negative metamaterials by inserting a nonlinear layer into a symmetric periodic structure with alternating epsilon-negative (ENG) and mu-negative (MNG) materials. A pass band with flat-top is observed and concentrated at a fixed central frequency regardless of the change of other parameters of the multilayer system. Meanwhile the electric field distribution and bistability at the central frequency are also studied. [Copyright &y& Elsevier]

Beam emittance plays an important role in any accelerator, and it is a main parameter to judge the performance of an accelerator. Emittance optimization is an indispensable part in conditioning and operation of the facility. For a laser-driven high brightness injector, different time structure of the laser pulse has different effects on transverse emittance. In order to compare Gaussian and flat-top laser pulse, systematic simulations of 500 pC have been done. From the simulation results, one can see that flat-top pulse laser will yield smaller minimal transverse beam size and transverse beam emittance than Gaussian pulse laser. [ABSTRACT FROM AUTHOR]

Laser beam shaping and homogenization techniques are substantial to optimize a large number of laser-material processing applications and laser-material interaction studies. Diffractive optical elements (DOE) play important role in provision of the process-adapted laser beam shaping. In this paper, we present an approach for laser beam shaping by using innovative DOE. The circular Gaussian beam was transformed to a square flat-top intensity profile in the focal plane of the objective lens by using novel DOE from TOPAG Lasertechnik GmbH. About 95% of the laser energy was coupled in the main beam with a nearly perfect rectangular shape. Experimental results have been achieved by applying the shaped beam for laser microfabrication. The picosecond and nanosecond lasers with moderate beam quality (M²=1.5) were used in the experiment. The shaped laser beam was applied for direct laser ablation of metal film on the glass substrate, drilling in a silicon wafer, scribing of thin-film solar cells and other technical materials. This technique allows creating a rectangular-shaped flat-top intensity profile in the focal plane that shows distinct advantages in laser material processing. [ABSTRACT FROM AUTHOR]

Abstract: An efficient and accurate three-dimensional (3D) hybrid modeling, which combines a 3D beam propagation method (BPM) and the two-dimensional (2D) Kirchhoff–Huygens diffraction formula, is developed to simulate the field propagation in an arrayed waveguide grating (AWG) demultiplexer. The 2D Kirchhoff–Huygens diffraction formula is used for the simulation of the light propagation in the free propagation regions (FPRs). A 3D BPM in a polar coordinate system is used to simulate the light propagation in the transition region between the input FPR and the arrayed waveguides so that the coupling coefficients for the arrayed waveguides are calculated conveniently and accurately. For the simulation in the transition region between the arrayed waveguides and the output FPR, only the central arrayed waveguide and several adjacent ones are needed in the computational window of a standard BPM and thus the computation efficiency is improved. Finally, a flat-top AWG is designed and fabricated to verify the reliability of the present simulation method. The calculated and measured spectral responses are in a good agreement. [Copyright &y& Elsevier]

Abstract: A novel polarization-independent all-fiber flat-top comb filter based on high birefringent (Hi–Bi) fiber Sagnac interferometer is proposed for the first time to our knowledge. The numerical simulation results show that the proposed device exhibits flat-top and the extinction ratio reads about 30 dB. A good agreement between the numerical simulations and the experimental results is achieved. [Copyright &y& Elsevier]

A beam width measurement system has been developed for keV submicron ion beams of 0.1 μm or less in width assuming a round shape beam. The system enables to measure beam current change as a function of knife-edge position by cutting a beam focusing point (beam spot) with the sharp edge within a spatial resolution of 0.02 μm. The width of 30 keV order submicron H+ ion beam was estimated by fitting current change curves based on three different ion density models: uniform, flat-top and Gaussian. Among these models, the flat-top model provide the most reasonable beam width of 0.56 μm interpreting contribution of halo around the beam spot to beam width estimation. The beam width measurement system with the high spatial resolution and the data analysis based on the flat-top ion density model should contribute to accelerate developments of submicron ion beam production technologies. [Copyright &y& Elsevier]

The relationship between cascaded Mach–Zehnder interferometers (MZIs) of different forms is studied. A flat-top interleaver based on cascaded MZIs with optical delay lines of equal length and same sign is optimally designed. An optimization method which combines the genetic algorithm and a gradient method is used to find appropriate net coupling for the directional couplers. The chromatic dispersion is also discussed. [Copyright &y& Elsevier]

A heavy-ion microbeam with energy of hundreds of MeV is a significantly useful probe for research in biology and biotechnology. A single-ion hit technique using the heavy-ion microbeam is being developed at the JAERI AVF cyclotron facility for elucidation of biofunctions. For production of a microbeam with a spot size of one micro-meter in diameter, the energy spread in the beam is required to be reduced to 0.02% to minimize the effect of chromatic aberrations in the focusing lenses. The energy spread in the cyclotron beam depends on a waveform of the acceleration voltage and beam phase acceptance of the cyclotron. The typical energy spread of the cyclotron beam is around 0.1% in the ordinary acceleration mode using a sinusoidal voltage waveform. The energy spread can be reduced by superimposing a fifth-harmonic voltage waveform on the fundamental one to generate a flat-top waveform for uniform energy gain. The flat-top acceleration system has been designed for the variable-energy multi-particle AVF cyclotron with acceleration harmonic mode of 1, 2 and 3. An additional coaxial cavity has been installed to generate the fifth-harmonic voltage, coupled to the main resonator. The frequency range of the fifth harmonics, 55–110 MHz, was fully covered by the flat-top acceleration system. [Copyright &y& Elsevier]

A tapered multimode interferometer (MMI) is used to flatten the passband of an arrayed-waveguide grating (AWG). The tapered MMI is connected at the end of the input waveguide of the AWG. The influences of the tapering angle to the performances (such as the 3 dB passband width, the ripple, the crosstalk and the insertion loss) of the flat-top AWG are studied. An AWG which has simultaneously a good flat spectral response and a good dispersion characteristic is designed by choosing appropriately the tapering angle and the length of the tapered MMI. [Copyright &y& Elsevier]

Abstract: We construct a flat-top filter by use of a tri-layer structure with AENG and AMNG metamaterials. Transfer-matrix method is used to calculate the transmittance of our tri-layer structure, and results show that two tunneling modes emerge at some frequencies and then will merge with each other, thus a flat-top filter is realized. We also show that metamaterial loss significantly reduces the transparent light, but does not change the transmittance distribution of light. [Copyright &y& Elsevier]

A flat-top filter with adjustable, broad, and robust tunneling band is proposed, where the barrier is composed of multiple layers including a medium with adjustable permeability (MAP) and metamaterial. Using a frustrated total internal reflection structure composed of glass-MAP-metamaterial-MAP-glass, the flat-top tunneling band can be realized and shifted to other frequency bands by adjusting the permeability of MAP to satisfy permeability and incident angle match conditions. In addition, our filter has a more robust flat-top tunneling band compared with other flat-top filters when the thickness of every layer changes. At last, the effect of metamaterial loss is considered. [ABSTRACT FROM PUBLISHER]

A polarization-independent all-fiber comb filter with quasi-flat-top spectral response based on a dual-pass Mach-Zehnder (M-Z) interferometer is proposed and demonstrated. The proposed filter consists of a dual-pass M-Z interferometer with a section of high birefringence fiber (HBF) and a polarization controller (PC) in the second loop. The theoretical analysis shows that the apparatus allows to operate as a polarization-independent multiwavelength periodic filter and to gain the quasi-flat-top passband filter by adjusting the polarization state of the PC. Theoretical prediction was verified by experimental demonstration. [ABSTRACT FROM PUBLISHER]

Cyclic arrayed waveguide grating (AWG) devices with flat-top passband and uniform spectral response are proposed. Each access waveguide of the proposed cyclic AWG consists of a multimode interference region and a taper waveguide. Simulation results show that the 1-dB bandwidth of the proposed device is larger than 0.44 times the channel spacing and the corresponding nonuniformity is smaller than 1 dB. [ABSTRACT FROM PUBLISHER]

For laser users, the usual Gaussian beam (GB) may not always be the best intensity profile. Often, a preferred option is a Flat-Top (FT) beam which is generally obtained by using diffractive optics in a reshaping operation. In this case, the resulting FT profile occurs only in the focal plane of the focusing lens, and its vicinity. We consider here the generation of a shape-invariant FT laser beam resulting from the incoherent weighted (50%-50% in power) mixing of LG 00 (GB) and LG 01 (doughnut) beams. For that we consider the insertion inside the laser cavity of a diaphragm and an absorbing ring, and optimising their sizes makes possible the simultaneous oscillation on LG 00 and LG 01 modes allowing the generation of a shape-invariant laser beam. [ABSTRACT FROM AUTHOR]

Electromechanical actuator (EMA) systems are widely employed in missiles. Due to the influence of the nonlinearities, there is a flat-top of about 64 ms when tracking the small-angle sinusoidal signals, which significantly reduces the performance of the EMA system and even causes the missile trajectory to oscillate. Aiming to solve these problems, this paper presents a hybrid control for flat-top situations. In contrast to the traditional PID or sliding mode controllers that missiles usually use, this paper utilizes improved sliding mode control based on a novel reaching law to eliminate the flat-top during the steering of the input signal, and utilizes the PID control to replace discontinuous control and improve the performance of EMA system. In addition, boundary layer and switching function are employed to solve the high-frequency chattering problem caused by traditional sliding mode control. Experiments indicate that the hybrid control can evidently reduce the flat-top time from 64 ms to 12 ms and eliminate the trajectory limit cycle oscillation. Compared with PID controllers, the proposed controller provides better performance—less chattering, less flat-top, higher precision, and no oscillation. [ABSTRACT FROM AUTHOR]