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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 3878-3889 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Locked (i.e., nonrotating) dynamo modes give rise to a serious edge loading problem during the operation of high current reversed field pinches. Rotating dynamo modes generally have a far more benign effect. A simple analytic model is developed in order to investigate the slowing down effect of electromagnetic torques due to eddy currents excited in the vacuum vessel on the rotation of dynamo modes in both the Madison Symmetric Torus (MST) [Fusion Technol. 19, 131 (1991)] and the Reversed Field Experiment (RFX) [Fusion Eng. Des. 25, 335 (1995)]. This model strongly suggests that vacuum vessel eddy currents are the primary cause of the observed lack of mode rotation in RFX. The eddy currents in MST are found to be too weak to cause a similar problem. The crucial difference between RFX and MST is the presence of a thin, highly resistive vacuum vessel in the former device. The MST vacuum vessel is thick and highly conducting. Various locked mode alleviation methods are discussed. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 5 (1998), S. 2340-2354 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A formalism is developed for optimizing the design of feedback coils placed around a tokamak plasma in order to control the resistive shell mode. It is found that feedback schemes for controlling the resistive shell mode fail whenever the distortion of the mode structure by the currents circulating in the feedback coils becomes too strong, in which case the mode escapes through the gaps between the coils, or through the centers of the coils. The main aim of the optimization process is to reduce this distortion by minimizing the coupling of different Fourier harmonics due to the feedback currents. It is possible to define a quantity α0 which parametrizes the strength of the coupling. Feedback fails for α0≥1. The optimization procedure consists of minimizing α0 subject to practical constraints. If there are very many evenly spaced feedback coils surrounding the plasma in the poloidal direction then the optimization can be performed analytically. Otherwise, the optimization must be performed numerically. The optimal configuration is to have many, large, overlapping coils in the poloidal direction. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 6 (1999), S. 3536-3547 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A reactor relevant reversed field pinch (RFP) must be capable of operating successfully when surrounded by a close-fitting resistive shell whose L/R time is much shorter than the pulse length. Resonant modes are largely unaffected by the shell resistivity, provided that the plasma rotation is maintained against the breaking effect of nonaxisymmetric eddy currents induced in the shell. This may require an auxiliary momentum source, such as a neutral beam injector. Nonresonant modes are largely unaffected by plasma rotation, and are expected to manifest themselves as nonrotating resistive shell modes growing on the L/R time of the shell. A general RFP equilibrium is subject to many simultaneously unstable resistive shell modes; the only viable control mechanism for such modes in a RFP reactor is active feedback. It is demonstrated than an N-fold toroidally symmetric arrangement of feedback coils, combined with a strictly linear feedback algorithm, is capable of simultaneously stabilizing all intrinsically unstable resistive shell modes over a wide range of different RFP equilibria. The number of coils in the toroidal direction N, at any given poloidal angle, must be greater than, or equal to, the range of toroidal mode numbers of the unstable resistive shell modes. However, this range is largely determined by the aspect-ratio of the device. The optimum coil configuration corresponds to one in which each feedback coil slightly overlaps its immediate neighbors in the toroidal direction. The critical current which must be driven around each feedback coils is, at most, a few percent of the equilibrium toroidal plasma current. The feedback scheme is robust to small deviations from pure N-fold toroidal symmetry or a pure linear response of the feedback circuits. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 1 (1994), S. 3337-3355 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The influence of a general static external magnetic perturbation on the stability of resistive modes in a tokamak plasma is examined. There are three main parts to this investigation. First, the vacuum perturbation is expanded as a set of well-behaved toroidal ring functions, and is, thereafter, specified by the coefficients of this expansion. Second, a dispersion relation is derived for resistive plasma instabilities in the presence of a general external perturbation, and finally, this dispersion relation is solved for the amplitudes of the tearing and twisting modes driven in the plasma by a specific perturbation. It is found that the amplitudes of driven tearing and twisting modes are negligible until a certain critical perturbation strength is exceeded. Only tearing modes are driven in low-β plasmas with εβp(very-much-less-than)1. However, twisting modes may also be driven if εβp(approximately-greater-than)1. For error-field perturbations made up of a large number of different poloidal and toroidal harmonics the critical strength to drive locked modes has a "staircase'' variation with edge-q, characterized by strong discontinuities as coupled rational surfaces enter or leave the plasma. For single harmonic perturbations, the variation with edge-q is far smoother. Both types of behavior have been observed experimentally. The critical perturbation strength is found to decrease strongly close to an ideal external kink stability boundary. This is also in agreement with experimental observations.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Physics of Plasmas 4 (1997), S. 2997-3000 
    ISSN: 1089-7674
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A feedback system, which essentially makes a resistive wall appear ideally conducting, is discussed. Such a system applied to a resistive wall surrounding a plasma will stabilize certain modes which would be unstable in the absence of the feedback system. The system discussed is similar to the "intelligent shell" by Bishop [Plasma Phys. Controlled Fusion 31, 1179 (1989)]; it utilizes a number of autonomous subsystems, each covering only a fraction of the resistive wall. A model example discussed suggests that only relatively few autonomous subsystems are needed and that the requirements of the electronics appear modest. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1365-2214
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine , Psychology
    Notes: Objective  To examine parent and adolescent agreement on physical, emotional, mental and social health and well-being in a representative population.Methodology  An epidemiological design was used to obtain parent–child/adolescent dyad data on comparable items and scales of a generic measure of health and well-being, the Child Health Questionnaire (parent/proxy report 50 item, self-report 80 item). Scale analysis included intraclass correlations (ICCs) to examine strength of parent–child associations and independent t-tests for differences between adolescents (with or without an illness). Where there were significant differences in scale scores, analysis of variance and two sample t-tests were used to examine the influence of social, demographic, health concern and school variables. Single items were examined for trends in response categories.Results  2096 parent–adolescent dyads (adolescent mean age of 15.1 years, males 50%, maternal parent 83.2%, biological parent 93.5%). ICCs were strong. Overall, adolescents reported poorer emotional and social health, and clinically significant differences were observed for perceptions of general health (mean difference 8.1/100), frequency and amount of body pain (5.94/100), experience of mental health (5.14/100), and impact of health on family activities (12.43/100), which widen significantly for adolescents with illness. Social, health and school enjoyment and performance significantly widened parent–child differences.Conclusions  All adolescents were much less optimistic about their health and well-being than their parents, and were only in close agreement on aspects of health and well-being they rated highly. Adolescent reports are more likely to be sensitive to pain, mental health problems, health in general and the impact of their health on family activities.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1365-2214
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine , Psychology
    Notes: Background  There is a need to measure children's ‘activity performance and participation’ as defined in the World Health Organization's International Classification of Functioning, Disability and Health for Children and Youth (WHO ICF). The aim of this review is to identify instruments that are suitable for use in postal surveys with families of children with cerebral palsy.Methods  We conducted a structured review of instruments that use child or family self-assessment of ‘activity performance and participation’. The review involved a systematic search for instruments using multiple published sources. Appraisal of the instruments used the predefined criteria of appropriateness, validity, reliability, responsiveness, precision, interpretability, acceptability and feasibility.Results  There are relatively few child or family assessed instruments appropriate for measuring children's activities and participation. Seven instruments were identified that could potentially be administered by mail. The Assessment of Life Habits for Children (LIFE-H) was the most appropriate instrument as assessed by its content but the reliability of child or family self-assessment is not known. If the LIFE-H were shown to be a reliable self-report measure then the LIFE-H would be the recommended choice. Currently, the Activities Scale for Kids and the condition-specific Lifestyle Assessment Questionnaire for cerebral palsy (LAQ-CP) provide the broadest description of what and how frequently children with cerebral palsy perform a range of activities and thereby indicate participation. The LAQ-CP also provides additional contextual information on the impact of any disability on the participation of the family unit.Conclusion  There remains much scope for developing valid and reliable self-assessed measures corresponding to the WHO ICF dimensions of activities and participation.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Science Ltd.
    Child 28 (2002), S. 0 
    ISSN: 1365-2214
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine , Psychology
    Notes: Aims To identify generic measures of health-related quality of life (HRQL) for children and adolescents developed for use within general populations. Instruments are evaluated on the basis of evidence relating to their reliability and validity.Methods Systematic literature searches were used to identify instruments, which were then assessed against predefined criteria. Information relating to instrument content, population, reliability and validity was extracted from published papers.Results Sixteen instruments were identified that had been evaluated among a general population of children or adolescents. Four instruments had reported data on both internal consistency and test–retest reliability. All except two instruments had undergone some degree of construct validation.Conclusions The evidence suggests that the Child Health Questionnaire (CHQ) has been the most extensively evaluated for younger populations but is available as a parent-completed measure only. The new version of the Child Health and Illness Profile (CHIP-CE) is particularly promising and has parallel child- and parent-completed versions for young ages. The weight of evidence suggests that versions of these two instruments are suitable for older children. The Warwick Child Health and Morbidity Profile could be used where information on morbidity and health service contacts is required. Once basic psychometric criteria are fulfilled, instruments should be chosen by assessing their content and design in the light of the prospective application.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    BJOG 76 (1969), S. 0 
    ISSN: 1471-0528
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    BJOG 81 (1974), S. 0 
    ISSN: 1471-0528
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Prostaglandin F2α (PGF2α) was assayed in samples of amniotic fluid obtained at 185 to 298 days gestation. Before 252 days (36 weeks) values below 75 pg./ml. were found in 20 of 22 samples. After 252 days but before the onset of labour 80 per cent of values exceeded 300 pg./ml., with a progressive rise with advancing gestation. During the first stage of labour a further rise in values occurred.
    Type of Medium: Electronic Resource
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