Uncontrolled spacecraft formations on two-dimensional invariant tori

Within the class of natural motions near libration point regions quasi-periodic trajectories evolving on invariant tori are studied. Those orbits prove beneficial for relative spacecraft configurations with large distances among satellites. In this study properties of invariant tori are outlined, and non-resonant and resonant tori around the Sun/Earth libration point L1 are computed. A numerical approach to obtain the frequency base and to parametrize a torus in angular phase space is introduced. Initial states for spacecraft formations on the torus’ surface are defined. The formation naturally evolve along its surface such that the relative positions within a formation are unaltered and the relative distances and the orientation are closely bounded. An in-plane coordinate frame together with a modified torus motion is introduced and the inner and outer behaviour of the formation’s geometry is investigated

Generation of tuneable 589nm radiation as a Na guide star source using an optical parametric amplifier

We describe a 5.5W 589nm source based on a passively modelocked Nd:YVO4 laser and a multi-stage Lithium Triborate optical parametric amplifier seeded by a tuneable semiconductor laser. We show this system can produce rapidly tuneable, transform-limited pulses in near diffraction-limited beams at 589nm, useful for Na guide star applications. The attraction of this scheme is that it can be assembled from commercially available hardware and is readily scalable to high average powers

Station-keeping for quasi-periodic orbits

The shallow gravity gradient in the libration point regions enables manoeuvring at low ∆v expenses, but implicates a sensitivity to small perturbations. A variety of bounded orbits can be determined around each libration point and station-keeping is required to maintain them for multiple revolutions. In this paper, a station-keeping algorithm based on the orbital lifetime expectancy is proposed for so-called quasi-periodic solutions. The method introduced is based on the identification of a manoeuvre maximising the lifetime of an orbit within defined boundaries. The manoeuvre direction and magnitude is finally optimised with a differential evolution algorithm. The novelty of the method presented here is the identification of the downstream centre manifold by the lifetime analysis to preserve the orbit with its properties forward in time. The study shows that the manoeuvre direction is directly correlated to stability information that is provided by the Floquet modal theory. Finally, numerical calculations were carried out for trajectories around the far-side libration point in the Earth-Moon system to show the effectiveness of this station-keeping approach. The robustness is proven by the introduction of errors and the evaluation of their impact

NIHSS Scores in Ischemic Small Vessel Disease: A Study in CADASIL

Background: The National Institutes of Health Stroke Scale (NIHSS) is widely used to measure neurological deficits, evaluate the effectiveness of treatment and predict outcome in acute ischemic stroke. It has also been used to measure the residual neurological deficit at the chronic stage after ischemic events. However, the value of NIHSS in ischemic cerebral small vessel disease has not been specifically evaluated. The purpose of this study was to investigate the link between the NIHSS score and clinical severity in a large population of subjects with CADASIL (cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy), a unique model to investigate the pathophysiology and natural history of ischemic small vessel disease. Methods: Demographic and clinical data of 220 patients with one or more lacunar infarcts confirmed by MRI examination and enrolled from a prospective cohort study were analyzed. Detailed neurological examinations, including evaluation of the NIHSS and modified Rankin Scale score (mRS) for evaluating the clinical severity, were performed in all subjects. The sensitivity, specificity, positive and negative predictive values of various NIHSS thresholds to capture the absence of significant disability (mRS = 3, but only 16 (7.3%) had NIHSS >5. All but 1 subject with NIHSS >5 showed mRS >= 3. NIHSS = 3 showed a lower MMSE score than those with mRS = 3 presented either with gait disturbances or MMSE score <25. Conclusions: The present results suggest that the NIHSS cannot reflect the extent of neurological deficit and clinical severity in subjects with lacunar infarctions in the context of a chronic and diffuse small vessel disease. A specific and global neurological scale, including the assessment of cognitive and gait performances, should be developed for ischemic cerebral microangiopathy. Copyright (C) 2012 S. Karger AG, Base

Optimal manoeuvring between quasi-periodic orbits

In the past halo orbits were used for most of the spacecraft missions going to the Lagrange point regions. However, other natural motions exist near these points presenting some advantages compared to halos. Quasi-periodic motions on invariant tori are associated with frequencies and amplitudes and surround the halo and vertical Lyapunov orbits. In this paper main characteristics of quasi-periodic orbits around the far-side Lagrange point in the Earth-Moon system are discussed. Optimal manoeuvres are identified to vary properties (phases, amplitudes) of an orbit. The proposed techniques utilise the stable manifold allowing for single manoeuvre transfers. The separation of spacecraft from a periodic orbit and a rendezvous scenario are discussed with respect to future missions, that have to cope with regular vehicle traffic, rendezvous and docking activities. Fuel-optimal transfers from a halo to a quasi-periodic orbit are identified in order to separate spacecraft. A second scenario assumes two spacecraft with a given phase separation on a quasi-periodic orbit. A target orbit is defined in which the spacecraft rendezvous. Parameter studies show that phase and amplitude changes strongly depend on the time when the manoeuvre is performed

Advanced MRI in cerebral small vessel disease

Cerebral small vessel disease (cSVD) is a major cause of stroke and dementia. This review summarizes recent developments in advanced neuroimaging of cSVD with a focus on clinical and research applications. In the first section, we highlight how advanced structural imaging techniques, including diffusion magnetic resonance imaging (MRI), enable improved detection of tissue damage, including characterization of tissue appearing normal on conventional MRI. These techniques enable progression to be monitored and may be useful as surrogate endpoint in clinical trials. Quantitative MRI, including iron and myelin imaging, provides insights into tissue composition on the molecular level. In the second section, we cover how advanced MRI techniques can demonstrate functional or dynamic abnormalities of the blood vessels, which could be targeted in mechanistic research and early-stage intervention trials. Such techniques include the use of dynamic contrast enhanced MRI to measure blood–brain barrier permeability, and MRI methods to assess cerebrovascular reactivity. In the third section, we discuss how the increased spatial resolution provided by ultrahigh field MRI at 7 T allows imaging of perforating arteries, and flow velocity and pulsatility within them. The advanced MRI techniques we describe are providing novel pathophysiological insights in cSVD and allow improved quantification of disease burden and progression. They have application in clinical trials, both in assessing novel therapeutic mechanisms, and as a sensitive endpoint to assess efficacy of interventions on parenchymal tissue damage. We also discuss challenges of these advanced techniques and suggest future directions for research

WMH and long-term outcomes in ischemic stroke

Objective To investigate the relationship between baseline white matter hyperintensities (WMH) in patients with ischemic stroke and long-term risk of dementia, functional impairment, recurrent stroke, and mortality. Methods Following the Meta-analysis of Observational Studies in Epidemiology and Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines (PROSPERO protocol: CRD42018092857), we systematically searched Medline and Scopus for cohort studies of ischemic stroke patients examining whether MRI- or CT-assessed WMH at baseline are associated with dementia, functional impairment, recurrent stroke, and mortality at 3 months or later poststroke. We extracted data and evaluated study quality with the Newcastle–Ottawa scale. We pooled relative risks (RR) for the presence and severity of WMH using random-effects models. Results We included 104 studies with 71,298 ischemic stroke patients. Moderate/severe WMH at baseline were associated with increased risk of dementia (RR 2.17, 95% confidence interval [CI] 1.72–2.73), cognitive impairment (RR 2.29, 95% CI 1.48–3.54), functional impairment (RR 2.21, 95% CI 1.83–2.67), any recurrent stroke (RR 1.65, 95% CI 1.36–2.01), recurrent ischemic stroke (RR 1.90, 95% CI 1.26–2.88), all-cause mortality (RR 1.72, 95% CI 1.47–2.01), and cardiovascular mortality (RR 2.02, 95% CI 1.44–2.83). The associations followed dose-response patterns for WMH severity and were consistent for both MRI- and CT-defined WMH. The results remained stable in sensitivity analyses adjusting for age, stroke severity, and cardiovascular risk factors, in analyses of studies scoring high in quality, and in analyses adjusted for publication bias. Conclusions Presence and severity of WMH are associated with substantially increased risk of dementia, functional impairment, stroke recurrence, and mortality after ischemic stroke. WMH may aid clinical prognostication and the planning of future clinical trials

Self-similar chain conformations in polymer gels

We use molecular dynamics simulations to study the swelling of randomly end-cross-linked polymer networks in good solvent conditions. We find that the equilibrium degree of swelling saturates at Q_eq = N_e**(3/5) for mean strand lengths N_s exceeding the melt entanglement length N_e. The internal structure of the network strands in the swollen state is characterized by a new exponent nu=0.72. Our findings are in contradiction to de Gennes' c*-theorem, which predicts Q_eq proportional N_s**(4/5) and nu=0.588. We present a simple Flory argument for a self-similar structure of mutually interpenetrating network strands, which yields nu=7/10 and otherwise recovers the classical Flory-Rehner theory. In particular, Q_eq = N_e**(3/5), if N_e is used as effective strand length.Comment: 4 pages, RevTex, 3 Figure