Single crystals of the hexagonal triangular lattice compound AgCrSe2 have been grown by chemical vapor transport. The crystals have been carefully characterized and studied by magnetic susceptibility, magnetization, specific heat, and thermal expansion. In addition, we used Cr-electron spin resonance and neutron diffraction to probe the Cr 3d(3) magnetism microscopically. To obtain the electronic density of states, we employed x-ray absorption and resonant photoemission spectroscopy in combination with density functional theory calculations. Our studies evidence an anisotropic magnetic order below T-N = 32 K. Susceptibility data in small fields of about 1 T reveal an antiferromagnetic (AFM) type of order for H perpendicular to c, whereas for H parallel to c the data are reminiscent of a field-induced ferromagnetic (FM) structure. At low temperatures and for H perpendicular to c, the field-dependent magnetization and AC susceptibility data evidence a metamagnetic transition at H+ = 5 T, which is absent for H parallel to c. We assign this to a transition from a planar cycloidal spin structure at low fields to a planar fanlike arrangement above H+. A fully ferromagnetically polarized state is obtained above the saturation field of H-perpendicular to S = 23.7 T at 2K with a magnetization of M-s = 2.8 mu(B)/Cr. For H parallel to c, M(H) monotonically increases and saturates at the same M-s value at H-parallel to S = 25.1 T at 4.2 K. Above T-N, the magnetic susceptibility and specific heat indicate signatures of two dimensional (2D) frustration related to the presence of planar ferromagnetic and antiferromagnetic exchange interactions. We found a pronounced nearly isotropic maximum in both properties at about T* = 45 K, which is a clear fingerprint of short range correlations and emergent spin fluctuations. Calculations based on a planar 2D Heisenberg model support our experimental findings and suggest a predominant FM exchange among nearest and AFM exchange among third-nearest neighbors. Only a minor contribution might be assigned to the antisymmetric Dzyaloshinskii-Moriya interaction possibly related to the noncentrosymmetric polar space group R3m. Due to these competing interactions, the magnetism in AgCrSe2, in contrast to the oxygen-based delafossites, can be tuned by relatively small, experimentally accessible magnetic fields, allowing us to establish the complete anisotropic magnetic H-T phase diagram in detail.

Beam-driven plasma-wakefield acceleration based on external injection has the potential to significantly reduce the size of future accelerators. Stability and quality of the acceleration process substantially depends on the incoming bunch parameters. Precise control of the current profile is essential for optimising energy-transfer efficiency and preserving energy spread. At the FLASHForward facility, driver--witness bunch pairs of adjustable bunch length and separation are generated by a set of collimators in a dispersive section, which enables fs-level control of the longitudinal bunch profile. The design of the collimator apparatus and its commissioning is presented.

The FLASHForward experimental facility is a high-performance test-bed for precision plasma wakefield research, aiming to accelerate high-quality electron beams to GeV-levels in a few centimetres of ionized gas. The plasma is created by ionizing gas in a gas cell either by a high-voltage discharge or a high-intensity laser pulse. The electrons to be accelerated will either be injected internally from the plasma background or externally from the FLASH superconducting RF front end. In both cases, the wakefield will be driven by electron beams provided by the FLASH gun and linac modules operating with a 10 Hz macro-pulse structure, generating 1.25 GeV, 1 nC electron bunches at up to 3 MHz micro-pulse repetition rates. At full capacity, this FLASH bunch-train structure corresponds to 30 kW of average power, orders of magnitude higher than drivers available to other state-of-the-art LWFA and PWFA experiments. This high-power functionality means FLASHForward is the only plasma wakefield facility in the world with the immediate capability to develop, explore and benchmark high-average-power plasma wakefield research essential for next-generation facilities. The operational parameters and technical highlights of the experiment are discussed, as well as the scientific goals and high-average-power outlook. This article is part of the Theo Murphy meeting issue 'Directions in particle beam-driven plasma wakefield acceleration'.

We present the synthesis and a detailed investigation of structural and magnetic properties of polycrystalline VO(HCOO)(2) center dot (H2O) by means of x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and electron-spin-resonance measurements. The compound crystallizes in an orthorhombic structure with space group Pcca. The crystal lattice features distorted VO6 octahedra connected via HCOO linkers (formate anions), forming a two-dimensional square lattice network with a bilayered structure. Analysis of magnetic susceptibility, high-field magnetization, and heat capacity data in terms of the frustrated square lattice model unambiguously establish the quasi-two-dimensional nature of the compound with nearest-neighbor interaction J(1)/k(B) similar or equal to 11.7 K and next-nearest-neighbor interaction J(2)/k(B) similar or equal to 0.02 K. A Neel antiferromagnetic ordering sets in at T-N similar or equal to 1.1 K. The ratio theta(CW)/T-N similar or equal to 10.9 reflects excellent two-dimensionality of the spin-lattice in the compound. A strong in-plane anisotropy is inferred from the linear increase of T-N with magnetic field, consistent with the structural data.

High-quality single crystals of NaYbSe2, which resembles a perfect triangular-lattice antiferromagnet without intrinsic disorder, are investigated by magnetization and specific heat, as well as the local probe techniques nuclear magnetic resonance (NMR) and electron spin resonance. The low-field measurements confirm the absence of any spin freezing or long-range magnetic order down to 50 mK, which suggests a quantum spin liquid ground (QSL) state with gapless excitations. Instability of the QSL state is observed upon applying magnetic fields. For the H perpendicular to c direction, a field-induced magnetic phase transition is observed above 2 T from the C-p(T ) data, agreeing with a clear M-x/3 plateau of M(H), which is associated with an up-up-down spin arrangement. For the H parallel to c direction, a field-induced transition could be evidenced at a much higher field range (9-21 T). The Na-23 NMR measurements provide microscopic evidence of field-induced ordering for both directions. A reentrant behavior of T-N, originating from the thermal and quantum spin fluctuations, is observed for both directions. The anisotropic exchange interactions J _ similar or equal to 4.7 K and J(z) similar or equal to 2.33 K are extracted from the modified bond-dependent XXZ model for the spin-1/2 triangular-lattice antiferromagnet. The absence of magnetic long-range order at zero fields is assigned to the effect of strong bond frustration, arising from the complex spin-orbit entangled 4f ground state. Finally, we derive the highly anisotropic magnetic phase diagram, which is discussed in comparison with the existing theoretical models for spin-1/2 triangular-lattice antiferromagnets.

We present the synthesis and a detail investigation of structural and magnetic properties of polycrystalline [VO(HCOO)_2·(H_2O)] by means of x-ray diffraction, magnetic susceptibility, high-field magnetization, heat capacity, and electron spin resonance measurements. It crystallizes in a orthorhombic structure with space group Pcca. It features distorted VO_6 octahedra connected via HCOO linker (formate anions) forming a two-dimensional square lattice network with a bilayered structure. Analysis of magnetic susceptibility, high field magnetization, and heat capacity data in terms of the frustrated square lattice model unambiguously establish quasi-two-dimensional nature of the compound with nearest neighbour interaction J_1/k_ B≃ 11.7 K and next-nearest-neighbour interaction J_2/k_ B≃ 0.02 K. It undergoes a Néel antiferromagnetic ordering at T_ N≃ 1.1 K. The ratio θ_ CW/T_ N≃ 10.9 reflects excellent two-dimensionality of the spin-lattice in the compound. A strong in-plane anisotropy is inferred from the linear increase of T_ N with magnetic field, consistent with the structural data.

A tunable plasma-based energy dechirper has been developed at FLASHForward to remove the correlated energy spread of a 681 MeV electron bunch. Through the interaction of the bunch with wakefields excited in plasma the projected energy spread was reduced from a FWHM of 1.31% to 0.33% without reducing the stability of the incoming beam. The experimental results for variable plasma density are in good agreement with analytic predictions and three-dimensional simulations. The proof-of-principle dechirping strength of 1.8  GeV/mm/m significantly exceeds those demonstrated for competing state-of-the-art techniques and may be key to future plasma wakefield-based free-electron lasers and high energy physics facilities, where large intrinsic chirps need to be removed.

Introduction The following definition of functional low vision is used in surveys to estimate the need for low vision services: corrected visual acuity in the better eye of less than 6/18 down to and including light perception from causes not amenable to treatment. However, such data for Sri Lanka is lacking. Objectives To determine the prevalence, causes and magnitude of functional low vision in a nationally representative sample of adults aged 40 years and above in Sri Lanka. Methods Distance visual acuity was measured using a LogMAR E chart. If the acuity was less than 3/60 in either eye, the ability to see hand movements, count fingers or perceive light was assessed. All underwent autorefraction followed by subjective refraction and measurement of best corrected acuity if indicated. Participants with a presenting acuity of <6/12 in the better eye were examined in details and a cause of visual loss was assigned. Analysis included those who fulfilled the definition of functional low vision. Results Among those examined, 59 participants fulfilled the definition of functional low vision: prevalence 1.02% (95% confidence interval 0.77-1.31%). The commonest causes were complications of cataract surgery (42.4%), and posterior segment conditions (30.5%) principally and age related macular degeneration (5 cases) and other retinal conditions (11 cases). Based on these estimates, 77,600 adults in Sri Lanka may benefit from low vision services. Conclusions The prevalence of functional low vision is slightly lower than the prevalence of blindness from all causes. Complications of cataract surgery are potentially avoidable causes.

Introduction Population-based data on visual impairment are required to estimate the need for services, evaluate service delivery and identify priorities that need to be addressed, as recommended in the World Health Organisation (WHO) Global Action Plan for Universal Eye Health. In the absence of national data for Sri Lanka, a nationwide survey was initiated. Objectives Determine the prevalence, distribution, magnitude and causes of blindness, visual impairment amongst adults aged >= 40 years in Sri Lanka; and to determine risk factors for blindness and visual impairment, and disability from the major causes. Methods The survey recruited a nationally representative sample of adults aged >= 40 years using proportionate- to-population size methods and >= 18 years to estimate the prevalence of disability. All nine provinces and 25 districts of the country were included. 100 participants >= 40 aged years were enumerated in each of the randomly selected 68 clusters, giving a sample of 6800 persons. An examination site was set up in each cluster with space for interviews, anthropometry and clinical examination. Results Fieldwork took place over a year from 2013 to 2014. Written informed consent was obtained before the examination. Overall response rate was 86.1%. Companion articles in this issue highlight the prevalence and causes of blindness, visual impairment and disability and the role of socio-demographic factors in the prevalence estimates. Conclusions The findings from the survey can be extrapolated to the entire country as the sample was representative of the demographic characteristics of Sri Lanka. Data provides sufficient evidence for program planners to identify priority regions for specific interventions to reduce the prevalence of blindness, visual impairment and disability.

Introduction Data on ocular morbidity is essential for planning primary and referral eye care services. Objectives Determine the prevalence and causes of self-reported ocular morbidity and eye care service utilization in Sri Lanka among adults >= 40 years of age. Methods Multistage, stratified, cluster random sampling was used to select a nationally representative sample aged >= 40 years. All participants were administered a questionnaire to assess ocular morbidity they had experienced in the previous month and the pattern of service utilization for reported eye problems. The interviews were conducted by a team of trained investigators at the clinical examination site before they underwent an eye examination. Results 5779 of those presenting for a clinical examination were interviewed among 6713 enumerated (86.1%). The prevalence of self-reported ocular morbidity was 89.9% [95% Confidence Interval (CI): 89.0-90.6%]. Near vision impairment was the commonest problem reported (72.1%; 95% CI: 70.8-73.3%) followed by distance vision impairment (36.2%; 95% CI: 34.9-37.5%). Among those reporting an eye problem, 31.4% sought treatment. 49.4% of those seeking care utilized private facilities. Financial constraints, perceptions that the condition was not serious or had insufficient impact on day-to-day tasks were the most important reasons for not accessing care. Conclusions Ocular morbidities are common in the Sri Lanka population; however utilization of services is relatively low, particularly for asymptomatic illnesses. It is important that the service planners take into account geographical and social inequalities and focus the services on underserviced areas and disadvantaged social groups.

Introduction There is paucity of data on the epidemiology of visual impairment in Sri Lanka. Objectives Estimate the prevalence and determine causes and risk factors of visual impairment among adults aged ≥40 years in Sri Lanka. Methods Multistage, stratified, cluster random sampling was used to select a nationally representative sample aged ≥40 years. All participants underwent vision testing, autorefraction and a basic eye examination. Participants with a presenting acuity of Results 5,779 of those enumerated (6,713) were examined (response rate 86.1%). The prevalence of blindness was 1.7% (95% confidence interval [CI]: 1.3-1.99%) and low vision was 17% (95% CI: 16.0-18.0%). Cataract (66.7%) and uncorrected refractive errors (12.5%) were the commonest causes of blindness. Uncorrected refractive errors (62.4%) and cataract (24.2%) were the commonest causes of low vision. Blindness was significantly higher in older age groups (OR 132.4: 95% Cl 11.7-149.3), those residing in the North Central (OR-12.5), North (OR-12.0), North West (OR-7.3), Eastern (OR-6.7), Western (OR-5.3) and Uva provinces (OR-5.3) compared to the Southern, and in those educated up to and including secondary school (OR 2.3: 95% CI 1.5- 3.17). Gender and socio-economic status were not significant after adjusting. Conclusions The prevalence of blindness in Sri Lanka is lower than in other South Asian countries and most causes are avoidable. Access to eye care needs to improve amongst the aged, those less educated and those in provinces with higher blindness risk.

Introduction The World Health Organization estimates that 15% of the global population has a disability. Available evidence from Sri Lanka shows variable estimates of the magnitude of disability. Objectives Determine the prevalence of self-reported disability in the adult population aged ≥18 years, and associated risk factors in a nationally representative sample in Sri Lanka. Methods The Washington Group short questionnaire was used to identify persons with self-reported disability. Data were collected from responsible adults aged ≥18 years in the selected households. A four point-scale: “no difficulty”, “some difficulty”, “a lot of difficulty” and “cannot do at all” was used. Individuals screening positive for disability were administered an additional questionnaire on activity limitations, social participation and their health and financial concerns. Results Overall 41.5% (4131) [95% CI: 40.5-42.4] reported functional difficulty in at least one domain. The prevalence of disability, i.e. a lot of difficulty or cannot do at all was 3.8% (382) [95% CI: 3.5 – 4.2], while the prevalence of “some functional difficulty” was 37.6% (3749) [95% CI: 36.7-38.6].  The prevalence of disability increased with age and was higher among females, urban residents, and those with lower education and socio-economic status.  Minor degrees of functional difficulties were more common among older people, females and people with lower education. Conclusions The prevalence of disability and varying degrees of functional difficulty is high among the adult population of Sri Lanka. Evidence shows that a strategic plan is required to address the magnitude of disability and functional limitations in Sri Lanka.

Introduction Uncorrected refractive errors are the commonest cause of visual impairment globally. Despite this, the proportion of affected individuals who wear spectacles can be low, particularly in low and middle-income countries. No data were available for Sri Lanka. Objectives To estimate the prevalence of refractive errors and investigate their risk factors among adults aged 40 years and above. Another purpose was to calculate spectacle coverage and identify subgroups with low coverage. Methods Cluster random sampling was used to obtain a nationally representative sample. Presenting distance visual acuity was measured using a logMAR chart, with distance spectacle correction if usually worn. All underwent autorefraction and an optician measured best-corrected visual acuity after subjective refraction. Participants who had undergone cataract surgery were excluded. Spectacle coverage was assessed amongst participants with a visual acuity of <6/12 in the better eye due to refractive error who attended the examination site with spectacles. Results 5,779/6,713 (86.1%) enumerated adults were examined; 5,179 had refraction data. 67% had a refractive error: hyperopia 49.6%; myopia 17.4%. Refractive errors increased with age. Being aged 60 years and above and Sinhala ethnic group were independent risk factors. Spectacle coverage was 17.7% overall, being lower in females and the non-literate. Based on the findings, 1.66 million adults require spectacles for distance correction. Conclusions Refractive errors are very common in Sri Lankan adults, and there is a large unmet need for spectacles. Affordable services for refractive errors need to be scaled up, focusing on the most underserved subgroups in the population.

Introduction Cataract surgical coverage and visual acuity outcomes are important population level indicators for monitoring access to and the quality of cataract surgery, allowing subgroups with poorer access to be identified. Data on these indicators are not available for Sri Lanka at national level. Objectives Determine cataract surgical coverage and the outcomes of cataract surgery in a nationally representative sample of adults aged >= 40 years. Methods Cluster random sampling with proportionate to size procedures was used. All participants were interviewed to obtain data on education level, movable assets, and the year and place of cataract surgery, if applicable. Presenting and best corrected visual acuities were measured. All participants underwent slit lamp examination, including a dilated examination of the fundus. Cataract surgical coverage was calculated at the person level vision of <3/30, <6/60 and <6/18. Outcomes of cataract surgery were categorized as good (6/18 or better), borderline (<6/18-6/60) or poor (<6/60). Results A total of 345 persons among the 5,779 participants who were examined had undergone cataract surgery in one or both eyes (486 eyes). Cataract surgical coverage, which was high overall 85.4% for vision <3/60; 79.1% for vision <6/60), was significantly higher in younger age groups (Odds Ratio [OR] 5.65, 95% confidence interval [CI] 1.42-22.52), those in urban areas (OR 2.8, 95% CI 1.01-7.74) those with higher socio-economic status (OR 6.0; 95% CI 1.96-18.4). Coverage ranged from 60% in Uva Province to 100% in Southern Province. 59.7% of eyes had good outcomes at presentation increasing to 75.1% with correction. Conclusions Cataract surgery indicators for Sri Lanka are good, being better than most other Asian countries. Services should target those living in underserved Provinces.

The test facility THAI (thermal-hydraulics, hydrogen, aerosol, and iodine) aims at addressing open questions concerning gas distribution, behaviour of hydrogen, iodine and aerosols in the containment of light water reactors during severe accidents. Main component of the facility is a 60 m(3) stainless steel vessel, 9.2 m high and 3.2 m in diameter, with exchangeable internals for multi-compartment investigations. The maximal design pressure of the vessel is 14 bar which allows H-2 combustion experiments at a severe accident relevant H-2 concentration level. The facility is approved for the use of low-level radiotracer I-123 which enables the measurement of time resolved iodine behaviour.The THAI test facility allows investigating various accident scenarios, ranging from turbulent free convection to stagnant stratified containment atmospheres and can be combined with simultaneous use of hydrogen, iodine and aerosol issues. THAI experimental research also covers investigations related to mitigation systems employed in light water reactor containments by performing experiments on, e.g. pressure suppression pool hydrodynamics, performance behaviour of passive autocatalytic recombiners, and spray interaction with hydrogen-steam-air flames in phenomenon orientated and coupled-effects experiments.The THAI experimental data have been widely used for the validation and further development of Lumped Parameter and Computational Fluid Dynamics codes with 3D capabilities, e.g. International Standard Problems ISP-47 (thermal hydraulics, gas distribution) and ISP-49 (hydrogen combustion), EU-SARNET/SARNET2 code-benchmark exercises involving THAI data on iodine/surface interactions, iodine mass transfer, passive autocatalytic recombiner performance, iodine oxide behaviour and iodine transport in multi-compartment behaviour.The present paper provides an overview of the THAI experiments related to hydrogen and fission products issues performed in the frame of national and international projects. From the comprehensive THAI experimental database, a selection of typical results is presented to illustrate the multi-functionality of the THAI facility and the broad variety of the experimental investigations. (C) 2015 Elsevier B.V. All rights reserved.

Locally enriched hydrogen as in stratification may contribute to early containment failure in the course of severe nuclear reactor accidents. During accident sequences steam might accumulate as well to stratifications which can directly influence the distribution and ignitability of hydrogen mixtures in containments. An international code benchmark including Computational Fluid Dynamics (CFD) and Lumped Parameter (LP) codes was conducted in the frame of the German THAI program. Basis for the benchmark was experiment TH24.3 which investigates the dissolution of a steam layer subject to natural convection in the steam-air atmosphere of the THAI vessel. The test provides validation data for the development of CFD and LP models to simulate the atmosphere in the containment of a nuclear reactor installation. In test TH24.3 saturated steam is injected into the upper third of the vessel forming a stratification layer which is then mixed by a superposed thermal convection. In this paper the simulation benchmark will be evaluated in addition to the general discussion about the experimental transient of test TH24.3. Concerning the steam stratification build-up and dilution of the stratification, the numerical programs showed very different results during the blind evaluation phase, but improved noticeable during open simulation phase. (C) 2015 Elsevier B.V. All rights reserved.

We present single-shot measurements of the longitudinal bunch profile from a laser-wakefield accelerator with sub-fs resolution, based on detection of coherent transition radiation in a broad spectral range. A previously developed phase retrieval algorithm enables reconstruction of the bunch profile without prior assumptions about its shape. In this study, a variable-length gas target is used to explore the dynamics of bunch evolution. Our results show that once the laser energy is depleted the time structure of the generated electron beam changes from a single bunch to a double bunch structure, well suited for driver-witness type experiments.

Hintergrund/Einleitung: Die Harnstoffproduktionsrate (HPR) ist eine etablierte Methode zum Nachweis einer Katabolie, die jedoch mit großem methodischem Aufwand verbunden ist. Eine einfachere, schnell durchführbare Methode wäre deshalb wünschenswert. Da eine pathologisch erhöhte HPR mit einem erhöhten Protein-(Muskel-)abbau assoziiert ist, stellten wir uns die Frage, ob eine Muskelfunktionsbestimmung eine Katabolie erfassen könnte.

This paper reports on a systematic investigation of the ultrashort pulse laser driven acceleration of protons from thin targets of finite size, so-called reduced mass targets (RMTs). Reproducible series of targets, manufactured with lithographic techniques, and varying in size, thickness, and mounting geometry, were irradiated with ultrashort (30 fs) laser pulses of intensities of about 8 × 1020 W cm−2. A robust maximum energy enhancement of almost a factor of two was found when comparing gold RMTs to reference irradiations of plain gold foils of the same thickness. Furthermore, a change of the thickness of these targets has less influence on the measured maximum proton energy when compared to standard foils, which, based on detailed particle-in-cell simulations, can be explained by the influence of the RMT geometry on the electron sheath. The performance gain was, however, restricted to lateral target sizes of greater than 50 µm, which can be attributed to edge and mounting structure influences.

ABSTRACTSupport structures are one of the main cost drivers offshore, and since other offshore rules apply with regards to design and site conditions, different methodologies have to be developed to mitigate the loads on the support structure and therefore to reduce the associated component costs. Detailed onshore studies and field tests have already shown that advanced control algorithms can be an effective way to reduce fatigue and extreme loading on a wind turbine. Still, the overall cost‐effectiveness for large offshore wind farms has not been studied in detail yet. In the scope of this paper, studies of new controller concepts are integrated within the design process of offshore support structures in order to stretch the applicability of monopiles to larger water depths or simply to reduce the structural weight and associated costs.A reference design of the support structure is made for a site in the Dutch North Sea. The focus is on reducing the dominant hydrodynamic excitation on the support structure. The implemented load mitigation concept leads to significant reductions in loading, allowing considerable material savings and therefore a more cost‐effective design. The presented approach allows material reductions of more than 9% for the studied support structure. Undesired side effects, such as increased wear of turbine components, are unlikely, as other system loadings and characteristics remain within an acceptable range. Even if some of the rotor–nacelle assembly loads are slightly increased by the applied controller, the increases are low and probably still within the margins of the type‐class fatigue loads. Copyright © 2012 John Wiley & Sons, Ltd.