Acceleration Gradient Research Articles

A compact dielectric grating-based charged particle bunch length diagnostic device at ARES

Dielectric gratings are already used in Dielectric Laser Acceleration due to their high damage thresholds at high acceleration gradients. When an electron bunch passes close to one of these gratings, it emits radiation, and the features of this radiation will be dependent upon the beam position relative to the grating, the bunch charge, and the bunch length. A compact high-resolution diagnostics device will be developed that consists of multiple gratings with different periods; these types of devices are required for the accurate operation of future compact accelerators which are currently undergoing development and testing. ARES linac at DESY is able to provide sub-fs electron bunches and has a range of high-resolution diagnostic devices installed, such as the PolariX Transverse Deflecting Structure, which will allow for performance verification of a new diagnostic. The electron bunches can be altered, allowing for the measurement and analysis of the emitted radiation for different bunch lengths and charges. This work will present the current progress in this area, including the presentation and discussion of simulations, and a discussion of the planned experiments at ARES.

The science case for an intermediate energy advanced and novel accelerator linear collider facility

It is widely accepted that the next lepton collider beyond a Higgs factory would require center-of-mass energy of the order of up to 15 TeV. Since, given reasonable space and cost restrictions, conventional accelerator technology reaches its limits near this energy, high-gradient advanced acceleration concepts are attractive. Advanced and novel accelerators (ANAs) are leading candidates due to their ability to produce acceleration gradients on the order of 1–100 GV/m, leading to compact acceleration facilities. However, intermediate energy facilities (IEF) are required to test the critical technology elements on the way towards multi-TeV-class collliders. Here a science case for a 20–100 GeV center-of-mass energy ANA-based lepton collider that can be a candidate for an intermediate energy facility is presented. The IEF can provide numerous opportunities for high energy physics studies including precision Quantum Chromodynamics and Beyond the Standard Model physics measurements, investigation of charged particle interactions with extreme electromagnetic fields, and exploring muon and proton beam acceleration. Possible applications of this collider include the studies of γγ and electron beam-fixed target/beamdump collider designs. Thus, the goal of the proposed IEF is to both carry out particle physics measurements in the 20-100 GeV ranges as well as to serve as an ANA demonstrator facility.

A Novel Application of High Thermal Conductivity Material on Superconducting RF Cavities: The Inner-wall Thermal Conducting Film (ITCF)

Superconducting Radiofrequency (SRF) technology has developed rapidly for the requirements of high-energy accelerators in fundamental physics research and nuclear industry applications. Thus, the SRF community endeavored to improve the performance from every aspect for niobium and thin-film cavities and made significant breakthroughs in the past decades. However, the positive feedback effect from the RF loss seriously limits the cavity’s maximum acceleration gradient and causes the high field’s unstable operation. Mitigating this effect requires increasing the cavity thermal conductivity to reduce the temperature at the heating point. Conventional technology conducts the RF-loss heat from the surface to the liquid helium outside the cavity while the inner cavity remains vacuum. This paper introduces a new strategy which coats an inner-wall thermal-conducting film (ITCF) in the cavity for the first time globally. The COMSOL simulation results show that ITCF absorbs heat from the RF surface and transmits the heat to the distance on the inner wall, generating a different heat transfer route, thereby reducing the heating-point temperature more efficiently.

Coherent nanophotonic electron accelerator.

Particle accelerators are essential tools in a variety of areas of industry, science and medicine1-4. Typically, the footprint of these machines starts at a few square metres for medical applications and reaches the size of large research centres. Acceleration of electrons with the help of laser light inside of a photonic nanostructure represents a microscopic alternative with potentially orders-of-magnitude decrease in cost and size5-16. Despite large efforts in research on dielectric laser acceleration17,18, including complex electron phase space control with optical forces19-21, noteworthy energy gains have not been shown so far. Here we demonstrate a scalable nanophotonic electron accelerator that coherently combines particle acceleration and transverse beam confinement, and accelerates and guides electrons over a considerable distance of 500 μm in a just 225-nm-wide channel. We observe a maximum coherent energy gain of 12.3 keV, equalling a substantial 43% energy increase of the initial 28.4 keV to 40.7 keV. We expect this work to lead directly to the advent of nanophotonic accelerators offering high acceleration gradients up to the GeV m-1 range utilizing high-damage-threshold dielectric materials22 at minimal size requirements14. These on-chip particle accelerators will enable transformative applications in medicine, industry, materials research and science14,23,24.

Associations between motor competence and physical activity levels of children with intellectual disabilities and/or autism spectrum disorder: Movement matters.

Motor competence is important for lifelong physical activity (PA). The current study aimed to examine associations between PA and motor competence. In total, 43 children aged 7-12years with intellectual disabilities and/or autism spectrum disorder completed anthropometric measures, the Bruininks-Oseretsky Test of Motor Proficiency-2, and wore a wrist accelerometer to capture total PA, moderate-to-vigorous PA (MVPA), average acceleration, and intensity gradient. No significant associations were found between PA outcomes and motor competence. Motor competence performance was commonly 'below average' or 'average'. The weakest subtests were upper limb coordination and strength. The strongest subtest was running speed and agility. Total weekly MVPA was 336.1 ± 150.3min, higher than UK recommendations of 120-180 per week for disabled children and young people. Larger scale studies are needed to better understand the relationship between PA and motor competence. Future research should also consider the influence of environmental factors on PA in this group.

Efficiently accelerated free electrons by metallic laser accelerator

Strong electron-photon interactions occurring in a dielectric laser accelerator provide the potential for development of a compact electron accelerator. Theoretically, metallic materials exhibiting notable surface plasmon-field enhancements can possibly generate a high electron acceleration capability. Here, we present a design for metallic material-based on-chip laser-driven accelerators that show a remarkable electron acceleration capability, as demonstrated in ultrafast electron microscopy investigations. Under phase-matching conditions, efficient and continuous acceleration of free electrons on a periodic nanostructure can be achieved. Importantly, an asymmetric spectral structure in which the vast majority of the electrons are in the energy-gain states has been obtained by means of a periodic bowtie-structure accelerator. Due to the presence of surface plasmon enhancement and nonlinear optical effects, the maximum acceleration gradient can reach as high as 0.335 GeV/m. This demonstrates that metallic laser accelerator could provide a way to develop compact accelerators on chip.

Study of flow separation control by bleed slot and bleed holes in a transonic compressor cascade

An extensive numerical investigation is carried out to study the effects of single-slot bleed and porous bleed on the flow separation control in a transonic compressor stator. Two single slots in different locations and three staggered rows of circular holes are placed on the suction surface. Five bleed pressure ratios are examined for each bleed scheme at the design inlet Mach number of 1.0. The numerical results show that the porous bleed performs similar to the downstream slot bleed in terms of pressure loss control when the bleed pressure ratio is below 1.0. The maximum relative reduction in total pressure loss is 73%. The stable working range of downstream slot bleed is between 0.75 and 1.0 bleed pressure ratio, which is narrower than the range of porous bleed. The bleeding acceleration and favorable pressure gradients on the suction surface are key reasons for keeping the boundary layer from separating. The position of the aft edge of the bleed configuration is a critical parameter. It determines the ultimate range of adverse pressure gradients that the blade surface boundary layer needs to withstand. The similar performance of porous bleed and slot bleed is caused by their identical aft edge position. The high shock losses caused by a single shock wave result in a narrow stable working range for single-slot bleed. Multiple shocks and side bleeding in porous bleed mitigate the influences of shock wave losses and extend its working range.

3D VELOCITY FIELDS WITH 2DH NUMERICAL STABILITY: A 3D ANALYTICAL-NUMERICAL MODULE FOR 2DH NUMERICAL MODELS

This research consists of the mathematical development of a three-dimensional (3D) analytical-numerical module that can be installed on vertically-averaged (2DH) numerical models. Designed to combine the advantages of both 3D and 2DH hydrodynamic models, it simulates a wide range of coastal natural water bodies of free surfaces and irregular geometries. Examples include estuaries, rivers, bays, lakes, and lagoons. Assuming a parabolic profile of turbulent viscosity and a predictor-corrector scheme applied to 2DH variables, a preliminary (predicted, analytical) logarithmic profile of horizontal velocity is used for the calculation of an adjusted (corrected, analytical-numerical) realistic 3D velocity field (u,v,w). Through a process of continuous synergetic interdependence, both the 3D module and the 2DH model feed on and enhance one another at every time step of a simulation in a cycle of mutual contribution that increases its accuracy. The following terms of the Navier-Stokes horizontal equations are considered in full three-dimensionality: local acceleration, non-linear advective accelerations, Coriolis acceleration, barotropic and baroclinic pressure gradients, and vertical shear stresses. All 2DH variables are allowed to vary in time and space (x,y,t) and all boundary conditions (dynamic and kinematic, bottom and surface) are analytically satisfied.

On Kahan's automatic step-size control and an anadromic gradient descent iteration

Accelerating the basic gradient iteration is a critical issue in numerical optimization and deep-learning. Recently, William Kahan [Kahan, Automatic Step-Size Control for Minimization Iterations, Technical report, University of California, Berkeley CA, USA, 2019] proposed automatic step-size strategies for the gradient descent iteration without explicitly using any prior information of the Hessian; moreover, a new momentum-based gradient acceleration method, namely, an Anadromic Gradient Descent (AGD) iteration, was proposed. Besides the capability of accelerating the basic gradient descent iteration, AGD enables the iteration to return to the past iterates by just reversing steps in sign and order in the same updating formulation. Numerical performances of the automatic step-size strategies and AGD are claimed to be favourable. In this paper, for the quadratic model, through a revisit of some classical momentum-based gradient methods, we perform new analysis on the convergence and their optimal hyper-parameters. We also investigate the convergence behaviour of AGD with the optimal hyper-parameters and connect one Kahan's automatic step-size scheme with the long Barzilai–Borwein step-size. Numerical results are presented to reflect the theoretical convergence behaviours and demonstrate the practical performances of various momentum-based gradient methods.

Physical Activity Assessed by Wrist and Thigh Worn Accelerometry and Associations with Cardiometabolic Health.

Physical activity is increasingly being captured by accelerometers worn on different body locations. The aim of this study was to examine the associations between physical activity volume (average acceleration), intensity (intensity gradient) and cardiometabolic health when assessed by a thigh-worn and wrist-worn accelerometer. A sample of 659 office workers wore an Axivity AX3 on the non-dominant wrist and an activPAL3 micro on the right thigh concurrently for 24 h a day for 8 days. An average acceleration (proxy for physical activity volume) and intensity gradient (intensity distribution) were calculated from both devices using the open-source raw accelerometer processing software GGIR. Clustered cardiometabolic risk (CMR) was calculated using markers of cardiometabolic health, including waist circumference, triglycerides, HDL-cholesterol, mean arterial pressure and fasting glucose. Linear regression analysis assessed the associations between physical activity volume and intensity gradient with cardiometabolic health. Physical activity volume derived from the thigh-worn activPAL and the wrist-worn Axivity were beneficially associated with CMR and the majority of individual health markers, but associations only remained significant after adjusting for physical activity intensity in the thigh-worn activPAL. Physical activity intensity was associated with CMR score and individual health markers when derived from the wrist-worn Axivity, and these associations were independent of volume. Associations between cardiometabolic health and physical activity volume were similarly captured by the thigh-worn activPAL and the wrist-worn Axivity. However, only the wrist-worn Axivity captured aspects of the intensity distribution associated with cardiometabolic health. This may relate to the reduced range of accelerations detected by the thigh-worn activPAL.

Nb3Sn Cavities Coated by Tin Vapor Diffusion Method at Peking University

Nb3Sn-coating experiments on samples and single-cell cavities were conducted at Peking University (PKU) to understand the Nb3Sn growth process using the vapor diffusion method. The evaporation of tin and tin chloride used in the vapor diffusion process was simulated and experimentally analyzed. The results show that the nucleation process is generally uniform, and the atomic ratios of Nb/O and Sn/O were found to be 1:2 within the 10 nm surface of the nucleated samples. Three tin sources were distributed along the cavity axis to obtain a uniform grain size on the cavity surface, and a surface tin content of 25~26% was achieved. The tin segregation effect was found in long-time coatings or coatings with insufficient tin, resulting in a low Sn% and bad cavity performance. By overcoming the tin segregation problem, a Nb3Sn cavity with a 750 nm grain size was produced by 1200 °C coating for 80 min and 1150 °C annealing for 60 min. The acceleration gradient reached 17.3 MV/m without quenching and an obvious Q-slope at 4.2 K. The relationship between coating recipes and vertical test results is discussed and conclusive advice is provided in this paper.

Flying focus laser Wakefield scheme for efficient acceleration

The acceleration of charged particles with a short laser pulse is a revolutionary acceleration technology, but many restrictions limit their advantages. Wave breaking and dephasing are most important of these restrictions which weaken acceleration efficiency in the laser Wakefield acceleration scheme. The purpose of this paper is to demonstrate that flying focus laser Wakefield acceleration (FF-LWFA) can confirm this effectiveness. Flying focus creates in the focal region of a hyper-chromatic lens when the chirped laser pulse passing through them. Therefore, by controlling the lens chromaticity and initial chirp characteristics, the focal point velocity can be controlled and adjusted near the speed of light. For a realistic working parameter, two-dimensional simulation results show the flying focus pulse can retain its shape and energy within a few centimeters (one order of magnitude greater than Rayleigh's length). We introduce a velocity interval around velocity of light in which dephasing and wave breaking are restricted and high acceleration gradients (about a hundred of GeV/m) are achieved in the flying focus region. Moreover, in the optimized condition for negative chirp, wakefield amplitude is increased 50 %. Under this ideal situation, 0.5 % of background electrons accelerate to 2GeV in the 5 cm, where the acceleration gradient reaches to, 40GeV/m, and about 0.2 % of them accelerate up to 10GeV.

Structure and material study of dielectric laser accelerators based on the inverse Cherenkov effect

Dielectric laser accelerators (DLAs) are considered promising candidates for on-chip particle accelerators that can achieve high acceleration gradients. This study explores various combinations of dielectric materials and accelerated structures based on the inverse Cherenkov effect. The designs utilize conventional processing methods and laser parameters currently in use. We optimize the structural model to enhance the gradient of acceleration and the electron energy gain. To achieve higher acceleration gradients and energy gains, the selection of materials and structures should be based on the initial electron energy. Furthermore, we observed that the variation of the acceleration gradient of the material is different at different initial electron energies. These findings suggest that on-chip accelerators are feasible with the help of these structures and materials.

Efficient coupling between free electrons and the supermode of a silicon slot waveguide.

Laser light can modulate the kinetic energy spectrum of free electrons and induce extremely high acceleration gradients, which are instrumental to electron microscopy and electron acceleration, respectively. We present a design scheme for a silicon photonic slot waveguide which hosts a supermode to interact with free electrons. The efficiency of this interaction relies on the coupling strength per photon along the interaction length. We predict an optimum value of 0.4266, resulting in the maximum energy gain of 28.27 keV for an optical pulse energy of only 0.22 nJ and duration 1 ps. The acceleration gradient is 1.05 GeV/m, which is lower than the maximum imposed by the damage threshold of Si waveguides. Our scheme shows how the coupling efficiency and energy gain can be maximized without maximizing the acceleration gradient. It highlights the potential of silicon photonics technology in hosting electron-photon interactions with direct applications in free-electron acceleration, radiation sources, and quantum information science.

A 24-h activity profile and adiposity among children and adolescents: Does the difference between school and weekend days matter?

Twenty-four-hour movement behaviours are gaining attention in the research community. However, no study has addressed how 24-h activity profiles vary between structured and less structured days and whether an unfavourable activity profile is associated with childhood obesity. We aimed to analyse differences between school day and weekend day 24-h activity profiles and their associations with adiposity indicators among children and adolescents. Participants were 382 children and 338 adolescents who wore wrist accelerometers for 24 hours a day for seven consecutive days. The 24-h activity profile expressed by the average acceleration (AvAcc) and intensity gradient (IG) were estimated from multi-day raw accelerometer data. Adiposity indicators included body mass index (BMI) z-score, fat mass percentage (FM%), fat mass index (FMI), and visceral adipose tissue (VAT). Multiple linear regression of activity profile metrics and adiposity indicators was performed separately for school and weekend days. Weekend days AvAcc and IG were lower compared to school days in both age groups (p <0.001 for all). Specifically, AvAcc was lower by 9.4% and 11.3% in children and adolescents, respectively. IG on weekend days was lower (more negative) by 3.4% in children and 3.1% in adolescents. Among children, on school days AvAcc and IG were negatively associated with FM%, FMI, and VAT, whilst on weekend days AvAcc was positively associated with BMI z-score, FMI, and VAT (p < 0.05 for all). Among adolescents, negative associations were found between weekend day AvAcc and IG and FM% and FMI (p < 0.05 for all), respectively. This study confirms the importance of 24-h activity profile as a potentially protective factor against excess adiposity. The variability of movement behaviours during structured and less structured days should be considered when optimizing the 24-h movement behaviours to prevent childhood obesity.

A Fast Data-Driven Iteratively Regularized Method with Convex Penalty for Solving Ill-Posed Problems

We propose a new iterative regularization method for solving inverse problems in Hilbert spaces. The iterative process of the proposed method combines classical iterative regularization format and Data-Driven approach. Data-Driven technique is based on the idea of deep learning to estimate the interior of a black box through a training set, so as to solve problems better and faster in some cases. In order to capture the special feature of solutions, convex functions are utilized to be penalty terms. Algorithmically, the two-point gradient acceleration strategy based on homotopy perturbation method is applied to the iterative scheme, which makes the method have satisfactory acceleration effect. We provide convergence analysis of the method under standard assumptions for iterative regularization methods. Finally, several numerical experiments are presented to show the effectiveness and acceleration effect of our method.

Toward a Laser-Driven Traveling-Wave Linac on a Chip

Restricted by pulse dispersion, dephasing, and bunch deflection, it has been a challenging task to realize high-gain particle acceleration and bunch focusing by using an ultrashort-pulse laser to drive an integrable acceleration structure, which is necessary to develop an on-chip particle accelerator for practical applications. Here we propose a laser-driven traveling-wave linac (linear accelerator), which uses the cascade reflection and refraction of an ultrashort laser pulse on a microscale dielectric structure to achieve long-range laser-particle interaction, avoiding waveguide dispersion, and enhancing sustainable acceleration gradient. It is scalable and extensible, and can realize full-course particle acceleration by using a single laser source via the inverse Cherenkov effect. With this accelerator scheme, we further propose a dynamic synchronization and focusing method, which not only counteracts dephasing but also restrains the bunch spread and deflection caused by space-charge and emittance effects, realizing stable bunch transport and acceleration in a tiny-size bunch channel without resorting to the need for external focusing equipment. This accelerator scheme paves the way toward a high-efficiency laser-driven all-dielectric on-chip particle accelerator for practical applications.

Analysis of the beam loading effect in a compact linac with long bunch tails

Beam loading effect is an important issue in all kinds of accelerating structures, especially in a high current case, which might result in a significant difference between the actual output energy and the designed energy. In this paper, we perform simulations and experimental studies on the beam loading effect in an S-band traveling wave (TW) linac with a maximum acceleration gradient of 19.4 MV/m for an electron bunch larger than 200 pC. The TW linac is designed to drive a terahertz free electron laser (THz-FEL) and can produce beam energies varying from 8 ∼ 14 MeV. The main challenge of the beam loading analysis come from the long bunch tail produced by the thermionic cathode gun. The long bunch tail causes considerable beam loss in the linac and brings great difficulties in the beam commissioning, especially in the beam energy tuning. So, the macro-particle tracking (MPT) algorithm is proposed to accurately analyze the beam loading effect of the long tail beams. The MPT algorithm uses energy conservation in each cell of the TW linac to take into account the particle loss and phase shift during the acceleration process. Based on the THz-FEL test facility, beam energies were measured under different loading cases. The experimental results confirmed that the new algorithm could predict the beam energy with a relative error less than 1.4%, while that of the traditional method is 4% ∼ 6%.

An ultrahigh-vacuum S-band photocathode radio-frequency electron gun

High gradient photocathode radio-frequency guns have enabled a broad range of modern accelerator-based scientific instruments. To further enhance the capabilities of these instruments by generating brighter beams, it is highly desirable to the synergies this type of electron sources with advanced photocathodes. However, various photocathodes with low intrinsic emittance and high quantum efficiency require ultrahigh vacuum condition which is far beyond available in existing high-gradient RF guns. Therefore, the physics, performance and feasibility of utilizing these advanced photocathodes under high gradient essentially remain unexplored. In this paper, we report on the concept and comprehensive physical design of a novel S-band photocathode gun that is compatible with advanced photocathodes. The new design features a more than ten-fold improvement in the vacuum level, supported by detailed vacuum modeling. This new gun will enable studies of advanced photocathodes under high acceleration gradient, and eventually be capable of delivering significantly brighter electron beams.

Multistage Positron Acceleration by an Electron Beam-Driven Strong Terahertz Radiation

Laser–plasma accelerators (LPAs) have been demonstrated as one of the candidates for traditional accelerators and have attracted increasing attention due to their compact size, high acceleration gradients, low cost, etc. However, LPAs for positrons still face many challenges, such as the beam divergence controlling, large energy spread, and complicated plasma backgrounds. Here, we propose a possible multistage positron acceleration scheme for high energy positron beam acceleration and propagation. It is driven by the strong coherent THz radiation generated when an injected electron ring beam passes through one or more solid targets. Multidimensional particle-in-cell simulations demonstrated that each acceleration stage is able to provide nearly 200 MeV energy gain for the positrons. Meanwhile, the positron beam energy spread can be controlled within 2%, and the beam emittance can be maintained during the beam acceleration and propagation. This may attract one’s interests in potential experiments on both large laser facilities and a traditional accelerator together with a laser system.