Emittance preservation in a plasma-wakefield accelerator.

Radio-frequency particle accelerators are engines of discovery, powering high-energy physics and photon science, but are also large and expensive due to their limited accelerating fields. Plasma-wakefield accelerators (PWFAs) provide orders-of-magnitude stronger fields in the charge-density wave behind a particle bunch travelling in a plasma, promising particle accelerators of greatly reduced size and cost. However, PWFAs can easily degrade the beam quality of the bunches they accelerate. Emittance, which determines how tightly beams can be focused, is a critical beam quality in for instance colliders and free-electron lasers, but is particularly prone to degradation. We demonstrate, for the first time, emittance preservation in a high-gradient and high-efficiency PWFA while simultaneously preserving charge and energy spread. This establishes that PWFAs can accelerate without degradation-an essential step toward energy boosters in photon science and multistage facilities for compact high-energy particle colliders.

Multi-GeV Electron Acceleration in Wakefields Strongly Driven by Oversized Laser Spots.

Experiments were performed on laser wakefield acceleration in the highly nonlinear regime. With laser powers P<250 TW and using an initial spot size larger than the matched spot size for guiding, we were able to accelerate electrons to energies E_{max}>2.5 GeV, in fields exceeding 500 GV m^{-1}, with more than 80 pC of charge at energies E>1 GeV. Three-dimensional particle-in-cell simulations show that using an oversized spot delays injection, avoiding beam loss as the wakefield undergoes length oscillation. This enables injected electrons to remain in the regions of highest accelerating fields and leads to a doubling of energy gain as compared to results from using half the focal length with the same laser.

In Situ Measurement of Electron Energy Evolution in a Laser-Plasma Accelerator.

We report on a novel, noninvasive method applying Thomson scattering to measure the evolution of the electron beam energy inside a laser-plasma accelerator with high spatial resolution. The determination of the local electron energy enabled the in-situ detection of the acting acceleration fields without altering the final beam state. In this Letter we demonstrate that the accelerating fields evolve from (265±119) GV/m to (9±4) GV/m in a plasma density ramp. The presented data show excellent agreement with particle-in-cell simulations. This method provides new possibilities for detecting the dynamics of plasma-based accelerators and their optimization.

Author Correction: Laser-wakefield accelerators for high-resolution X-ray imaging of complex microstructures.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Laser-wakefield accelerators for high-resolution X-ray imaging of complex microstructures.

Laser-wakefield accelerators (LWFAs) are high acceleration-gradient plasma-based particle accelerators capable of producing ultra-relativistic electron beams. Within the strong focusing fields of the wakefield, accelerated electrons undergo betatron oscillations, emitting a bright pulse of X-rays with a micrometer-scale source size that may be used for imaging applications. Non-destructive X-ray phase contrast imaging and tomography of heterogeneous materials can provide insight into their processing, structure, and performance. To demonstrate the imaging capability of X-rays from an LWFA we have examined an irregular eutectic in the aluminum-silicon (Al-Si) system. The lamellar spacing of the Al-Si eutectic microstructure is on the order of a few micrometers, thus requiring high spatial resolution. We present comparisons between the sharpness and spatial resolution in phase contrast images of this eutectic alloy obtained via X-ray phase contrast imaging at the Swiss Light Source (SLS) synchrotron and X-ray projection microscopy via an LWFA source. An upper bound on the resolving power of 2.7 ± 0.3 µm of the LWFA source in this experiment was measured. These results indicate that betatron X-rays from laser wakefield acceleration can provide an alternative to conventional synchrotron sources for high resolution imaging of eutectics and, more broadly, complex microstructures.

A spectrometer for ultrashort gamma-ray pulses with photon energies greater than 10 MeV.

We present a design for a pixelated scintillator based gamma-ray spectrometer for non-linear inverse Compton scattering experiments. By colliding a laser wakefield accelerated electron beam with a tightly focused, intense laser pulse, gamma-ray photons up to 100 MeV energies and with few femtosecond duration may be produced. To measure the energy spectrum and angular distribution, a 33 × 47 array of cesium-iodide crystals was oriented such that the 47 crystal length axis was parallel to the gamma-ray beam and the 33 crystal length axis was oriented in the vertical direction. Using an iterative deconvolution method similar to the YOGI code, modeling of the scintillator response using GEANT4 and fitting to a quantum Monte Carlo calculated photon spectrum, we are able to extract the gamma ray spectra generated by the inverse Compton interaction.

Ultrafast Imaging of Laser Driven Shock Waves using Betatron X-rays from a Laser Wakefield Accelerator.

Betatron radiation from laser wakefield accelerators is an ultrashort pulsed source of hard, synchrotron-like x-ray radiation. It emanates from a centimetre scale plasma accelerator producing GeV level electron beams. In recent years betatron radiation has been developed as a unique source capable of producing high resolution x-ray images in compact geometries. However, until now, the short pulse nature of this radiation has not been exploited. This report details the first experiment to utilize betatron radiation to image a rapidly evolving phenomenon by using it to radiograph a laser driven shock wave in a silicon target. The spatial resolution of the image is comparable to what has been achieved in similar experiments at conventional synchrotron light sources. The intrinsic temporal resolution of betatron radiation is below 100 fs, indicating that significantly faster processes could be probed in future without compromising spatial resolution. Quantitative measurements of the shock velocity and material density were made from the radiographs recorded during shock compression and were consistent with the established shock response of silicon, as determined with traditional velocimetry approaches. This suggests that future compact betatron imaging beamlines could be useful in the imaging and diagnosis of high-energy-density physics experiments.

Highly efficient angularly resolving x-ray spectrometer optimized for absorption measurements with collimated sources.

Highly collimated betatron radiation from a laser wakefield accelerator is a promising tool for spectroscopic measurements. Therefore, there is a requirement to create spectrometers suited to the unique properties of such a source. We demonstrate a spectrometer which achieves an energy resolution of <5 eV at 9 keV (E∕ΔE>1800) and is angularly resolving the x-ray emission allowing the reference and spectrum to be recorded at the same time. The single photon analysis is used to significantly reduce the background noise. Theoretical performance of various configurations of the spectrometer is calculated by a ray-tracing algorithm. The properties and performance of the spectrometer including the angular and spectral resolution are demonstrated experimentally on absorption above the K-edge of a Cu foil backlit by a laser-produced betatron radiation x-ray beam.

In Vivo T1 of Blood Measurements in Children with Sickle Cell Disease Improve Cerebral Blood Flow Quantification from Arterial Spin-Labeling MRI.

BACKGROUND AND PURPOSE: Children with sickle cell disease have low hematocrit and elevated CBF, the latter of which can be assessed with arterial spin-labeling MR imaging. Quantitative CBF values are obtained by using an estimation of the longitudinal relaxation time of blood (T1blood). Because T1blood depends on hematocrit in healthy individuals, we investigated the importance of measuring T1blood in vivo with MR imaging versus calculating it from hematocrit or assuming an adult fixed value recommended by the literature, hypothesizing that measured T1blood would be the most suited for CBF quantification in children with sickle cell disease. MATERIALS AND METHODS: Four approaches for T1blood estimation were investigated in 39 patients with sickle cell disease and subsequently used in the CBF quantification from arterial spin-labeling MR imaging. First, we used 1650 ms as recommended by the literature (T1blood-fixed); second, T1blood calculated from hematocrit measured in patients (T1blood-hematocrit); third, T1blood measured in vivo with a Look-Locker MR imaging sequence (T1blood-measured); and finally, a mean value from T1blood measured in this study in children with sickle cell disease (T1blood-sickle cell disease). Quantitative flow measurements acquired with phase-contrast MR imaging served as reference values for CBF. RESULTS: T1blood-measured (1818 ± 107 ms) was higher than the literature recommended value of 1650 ms, was significantly lower than T1blood-hematocrit (2058 ± 123 ms, P < .001), and, most interesting, did not correlate with hematocrit measurements. Use of either T1blood-measured or T1blood-sickle cell disease provided the best agreement on CBF between arterial-spin labeling and phase-contrast MR imaging reference values. CONCLUSIONS: This work advocates the use of patient-specific measured T1blood or a standardized value (1818 ms) in the quantification of CBF from arterial spin-labeling in children with SCD.

Laser-wakefield accelerators as hard x-ray sources for 3D medical imaging of human bone.

A bright µm-sized source of hard synchrotron x-rays (critical energy Ecrit > 30 keV) based on the betatron oscillations of laser wakefield accelerated electrons has been developed. The potential of this source for medical imaging was demonstrated by performing micro-computed tomography of a human femoral trabecular bone sample, allowing full 3D reconstruction to a resolution below 50 µm. The use of a 1 cm long wakefield accelerator means that the length of the beamline (excluding the laser) is dominated by the x-ray imaging distances rather than the electron acceleration distances. The source possesses high peak brightness, which allows each image to be recorded with a single exposure and reduces the time required for a full tomographic scan. These properties make this an interesting laboratory source for many tomographic imaging applications.

A laser driven pulsed X-ray backscatter technique for enhanced penetrative imaging.

X-ray backscatter imaging can be used for a wide range of imaging applications, in particular for industrial inspection and portal security. Currently, the application of this imaging technique to the detection of landmines is limited due to the surrounding sand or soil strongly attenuating the 10s to 100s of keV X-rays required for backscatter imaging. Here, we introduce a new approach involving a 140 MeV short-pulse (< 100 fs) electron beam generated by laser wakefield acceleration to probe the sample, which produces Bremsstrahlung X-rays within the sample enabling greater depths to be imaged. A variety of detector and scintillator configurations are examined, with the best time response seen from an absorptive coated BaF2 scintillator with a bandpass filter to remove the slow scintillation emission components. An X-ray backscatter image of an array of different density and atomic number items is demonstrated. The use of a compact laser wakefield accelerator to generate the electron source, combined with the rapid development of more compact, efficient and higher repetition rate high power laser systems will make this system feasible for applications in the field. Content includes material subject to Dstl (c) Crown copyright (2014). Licensed under the terms of the Open Government Licence except where otherwise stated. To view this licence, visit http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3 or write to the Information Policy Team, The National Archives, Kew, London TW9 4DU, or email: psi@ nationalarchives.gsi.gov.uk.

Pituitary iron and volume imaging in healthy controls.

BACKGROUND AND PURPOSE: Patients with transfusional iron overload develop iron deposits in the pituitary gland, which are associated with volume loss and HH. The purpose of this study was to characterize R2 and volumetric data in a healthy population for diagnostic use in patients with transfusional iron overload. MATERIALS AND METHODS: One hundred healthy controls without iron overload between the ages of 2 and 48 were recruited to have MR imaging of the brain to assess their pituitary R2 and volume. Pituitary R2 was assessed with a 8-echo spin-echo sequence, and pituitary volumes, by a 3D spoiled gradient-echo sequence with 1-mm(3) resolution. A 2-component continuous piecewise linear approximation was used for creating volumetric and R2 nomograms. Equations were generated from regression relationships for convenient z-score calculation. RESULTS: Pituitary R2 rose weakly with age (r(2) = 0.19, P < .0001). Anterior and total pituitary volumes increased steadily up to 18 years of age, after which volume slightly decreased. Females had larger pituitary glands, most likely representing their larger lactotroph population. CONCLUSIONS: From these data, a clinician can calculate the z scores for R2 and pituitary volume in patients with iron overload. Normal ranges are well-differentiated from values previously associated with endocrine disease in transfusional siderosis; this finding suggests that preclinical iron overload can be recognized and appropriately treated.

Continuous remote vital sign/environment monitoring for returning soldier adjustment assessment.

A three-stage study to develop and test an unobtrusive room sensor unit and subject data management system to discover correlation between sensor-based time-series measurements of sleep quality and clinical assessments of combat veterans suffering from Post-traumatic Stress Disorder (PTSD) and mild Traumatic Brain Injury (TBI), is described. Experiments and results for testing sensitivity and robustness of the sensor unit and data management protocol are provided. The current sensitivity of remote vital sign monitoring system is below 20% and 10% for respiration and heart rates, respectively.

Quantitative analysis and modelling of hepatic iron stores using stereology and spatial statistics.

Hepatic iron overload is a common clinical problem resulting from hyperabsorption syndromes and from chronic transfusion therapy. Not only does iron loading vary between reticuloendothelial stores and hepatocytes, but iron is heterogeneously distributed within hepatocytes as well. Since the accessibility of iron particles to chelation may depend, in part, on their distribution, we sought to characterize the shape and scale of iron deposition in humans with transfusional iron overload. Toward this end, we performed a histological analysis of iron stores in liver biopsy specimens of 20 patients (1.3-57.8 mg iron/g dry tissue weight) with aid of electron and light microscopy. We estimated distributions related to variability in siderosomal size, proximity of iron centres and inter-cellular iron loading. These distributions could be well modelled by Gamma distribution functions over most of the pathologic range of iron concentrations. Thus, for a given liver iron burden, a virtual iron-overloaded liver could be created that served as a model for the true histologic appearance. Such a model may be helpful for understanding the mechanics of iron loading or in predicting response to iron removal therapy.

A simulation model to assess herd-level intervention strategies against E. coli O157.

A simulation model of a herd of grazing cattle, which has been developed to provide insight into the infection dynamics of E. coli O157 is described. The spatially explicit model enables the modelling of the infection transmission processes to be realistically addressed under field management conditions. The model is used to explore the efficacy of various potential control strategies in reducing the levels of within-herd infection. These measures include restricting the size of herds, niche engineering, improving housing hygiene and vaccination. While a vaccination strategy remains a hypothetical option, it has the potential to be particularly effective. It is likely that the most successful strategy will involve the implementation of a combination of measures.

Analysis of multivariate extreme intakes of food chemicals.

A recently published multivariate Extreme Value Theory (EVT) model is applied to the estimation of population risks associated with dietary intake of pesticides. The objective is to quantify the acute risk of pesticide intake above a threshold and relate it to the consumption of specific primary food products. As an example daily intakes of a pesticide from three foods are considered. The method models and extrapolates simultaneous intakes of pesticide, and estimates probability of exceeding unobserved large intakes. Multivariate analysis was helpful in identifying whether the avoidance of certain food combinations would reduce the likelihood of exceeding a threshold. We argue that the presented method can be an important contribution to exposure assessment studies.

Assessing the efficacy of within-animal control strategies against E. coli O157: a simulation study.

A stochastic simulation model was used to assess the efficacy of potential measures to control the levels of Escherichia coli O157 within the bovine host. The model described E. coli O157 population sizes at several sites along the bovine gut and therefore only interventions that operate at an individual animal level could be evaluated. In order to use the model to evaluate the control strategies, it was necessary to make assumptions about how each strategy affected E. coli O157 populations in vivo. The within-animal conditions under these control strategies were modelled by adjusting the growth rates of E. coli O157 at specific sites of interest in the gut, based on these assumptions. The model simulated the population dynamics of an initial dose of E. coli O157 inoculated into an animal in the presence of inhibitory probiotics or antibiotics, bactericidal antibiotics or probiotics, and following fasting. Of the control strategies considered, the use of inhibitory probiotics appeared most promising and continued development of a suitable product is to be encouraged.

A simulation model for the study of the within-animal infection dynamics of E. coli O157.

Escherichia coli 0157 can cause serious illness, even death, in humans. There is some consensus that the main reservoirs of this harmful bacterium are the rumens and intestines of cattle. Hence, a stochastic model of the bovine gut was developed to investigate the in vivo population dynamics of E. coli O157. Because bacterial numbers can reach minimal levels, a stochastic system was considered, with a birth-death process being used to represent bacterial growth and decay dynamics throughout the gastrointestinal tract. Reinfection through ingestion of bacteria present in the environment was allowed to occur and the required clustered distribution of inter-event times was implemented through the use of a random hazard doubly stochastic Poisson process. Due to the inclusion of multiple compartments, a feedback mechanism and an interest in the non-equilibrium dynamics of the process, it was not possible to obtain an analytical representation of the process and therefore, a simulation study was used to obtain results. The within-animal model can be used to explore the efficacy of control measures which act at an individual animal level.

Diagnosis of very long chain acyl-dehydrogenase deficiency from an infant's newborn screening card.

Very long chain fatty acid dehydrogenase (VLCAD) deficiency is a rare but treatable cause of cardiomyopathy, fatty liver, skeletal myopathy, pericardial effusions, ventricular arrhythmias, and sudden death. Unrecognized, VLCAD deficiency may be rapidly progressive and fatal, secondary to its cardiac involvement. Because early diagnosis improves outcome, we present a neonate with VLCAD deficiency in whom retrospective analysis of the newborn screening card revealed that a correct diagnosis could have been made by newborn screening using tandem mass spectrometry. Our patient demonstrated a classic neonatal course with transient hypoglycemia at birth, interpreted as culture-negative sepsis, followed by a quiescent period notable only for hypotonia and poor feeding. At 3 months, he presented with cardiorespiratory failure and pericardial effusions, requiring pericardiocentesis, tracheostomy, and prolonged mechanical ventilation. Plasma free-fatty acid and acylcarnitine profiles demonstrated small but significant elevations of C14:2, C14:1, C16, and C18:1 acylcarnitine species, findings consistent with a biochemical diagnosis of VLCAD deficiency. Enteral feeds were changed to Portagen formula with marked improvement in cardiac symptoms over several weeks. To confirm the biochemical diagnosis, molecular analysis was performed by analysis of genomic DNA on a blood sample of the patient. Sequencing analysis and delineation of VLCAD mutations were performed using polymerase chain reaction and genomic sequencing. The patient was heterozygous for 2 different disease-causing mutations at the VLCAD locus. The maternal mutation was a deletion of bp 842-3 in exon 8, causing a shift in the reading frame. The paternal mutation was G+1A in the consensus donor splice site after exon 1; this splice-site mutation would likely result in decreased mRNA. The likely consequence of these mutations is essentially a null phenotype. To determine whether this case could have been picked up by tandem mass spectrometry analysis at birth when the patient was asymptomatic, acylcarnitine analysis was performed on the patient's original newborn card (after obtaining parental consent, the original specimen was provided courtesy of Dr Kenneth Pass, Director, New York State Newborn Screening Program). The blood sample had been obtained at 1 week of age and stored at room temperature for 6 months and at 70 degrees C thereafter for 18 months. Electrospray tandem mass spectrometry used a LC-MS/MS API 2000 operated in ion evaporation mode with the TurboIonSpray ionization probe source. The acylcarnitine profile obtained from the patient's original newborn card was analyzed 2 years after it was obtained. In comparison with a normal control, there was a significant accumulation of long chain acylcarnitine species, with a prominent peak of tetradecenoylcarnitine (C14:1), the most characteristic metabolic marker of VLCAD deficiency. This profile would have likely been even more significant if it had been analyzed at the time of collection, yet 2 years later is sufficient to provide strong biochemical evidence of the underlying disorder. Discussion. VLCAD was first discovered in 1992, and clinical experience with VLCAD deficiency has been accumulating rapidly. Indeed, the patients originally diagnosed with long chain acyl-CoA deficiency suffer instead from VLCAD deficiency. The phenotype of VLCAD deficiency is heterogeneous, ranging from catastrophic metabolic and cardiac failure in infancy to mild hypoketotic, hypoglycemia, and exertional rhabdomyolysis in adults. This case demonstrates that VLCAD deficiency could have been detected from the patient's own neonatal heel-stick sample. Most likely, a presymptomatic diagnosis would have avoided at least part of a lengthy and intensive prediagnosis hospitalization that had an estimated cost of $400 000. Although VLCAD is relatively rare, timely and correct diagnosis leads to dramatic recovery, so that detection by newborn screening could prevent the onset of arrhythmias, heart failure, metabolic insufficiency, and death. Fatty acid oxidation defects, including VLCAD deficiency, may account for as many as 5% of sudden infant death patients. Recent instrumentation advances have made automated tandem mass spectrometry of routine neonatal heel-stick samples technically feasible. Pilot studies have demonstrated an incidence of fatty acid oxidation defects, including short chain, medium chain, and very long chain acyl-CoA dehydrogenase deficiencies, of approximately 1/12 000. As a result, cost-benefit ratios for this approach should be systematically examined.

Establishing and maintaining system linearity.

A flow cytometer system transforms a received optical signal into an electrical signal whose amplitude ideally is directly proportional to the optical signal but in practice is at least related in a linear fashion. Establishment and maintenance of system linearity are prerequisites to accurate quantitative measurements. The many potential sources of offsets and nonlinearity are found in all parts of the detection, amplification, and data acquisition systems of a flow instrument. This unit aims to help investigators evaluate performance of the hardware in flow cytometer detection systems.