Optimization of a charge-state analyzer for ECRIS beams

A detailed experimental and simulation study of the extraction of a 24 keV He-ion beam from an ECR ion source and the subsequent beam transport through an analyzing magnet is presented. We find that such a slow ion beam is very sensitive to space-charge forces, but also that the neutralization of the beam's space charge by secondary electrons is virtually complete for beam currents up to at least 0.5 mA. The beam emittance directly behind the extraction system is 65 pi mm mrad and is determined by the fact that the ion beam is extracted in the strong magnetic fringe field of the ion source. The relatively large emittance of the beam and its non-paraxiality lead, in combination with a relatively small magnet gap, to significant beam losses and a five-fold increase of the effective beam emittance during its transport through the analyzing magnet. The calculated beam profile and phase-space distributions in the image plane of the analyzing magnet agree well with measurements. The kinematic and magnet aberrations have been studied using the calculated second-order transfer map of the analyzing magnet, with which we can reproduce the phase-space distributions of the ion beam behind the analyzing magnet. Using the transfer map and trajectory calculations we have worked out an aberration compensation scheme based on the addition of compensating hexapole components to the main dipole field by modifying the shape of the poles. The simulations predict that by compensating the kinematic and geometric aberrations in this way and enlarging the pole gap the overall beam transport efficiency can be increased from 16 to 45%

Spatial distribution of Cherenkov radiation in periodic dielectric media

The nontrivial dispersion relation of a periodic medium affects both the spectral and the spatial distribution of Cherenkov radiation. We present a theory of the spatial distribution of Cherenkov radiation in the far-field zone inside arbitrary three- and two-dimensional dielectric media. Simple analytical expressions for the far-field are obtained in terms of the Bloch mode expansion. Numerical examples of the Cherenkov radiation in a two-dimensional photonic crystal is presented. The developed analytical theory demonstrates good agreement with numerically rigorous finite-difference time-domain calculations.Comment: 14 pages, 5 figures, Journal of Optics A (in press

Large Phase Differences between L-Cone-and M-Cone- Driven Electroretinograms in Retinitis Pigmentosa

PURPOSE. To study the dynamics and interactions of the signals originating in the long-(L-) and middle (M)-wavelength-sensitive cone pathways in patients with retinitis pigmentosa (RP). METHODS. Twenty-six patients with RP and 29 normal subjects participated in the study. Electroretinographic (ERG) responses were measured to stimuli that modulated exclusively the L-or the M-cones or the two simultaneously (both in-phase and in counter-phase) with varying ratios of Lto M-cone contrasts. S-cones were not modulated. RESULTS. The data of the normal subjects and of the patients can be described by a model in which the amplitudes and the phases of the signals originating in the L-and M-cones are vector summed. In the RP patients, there was a general reduction in ERG sensitivity. The L-cone-driven ERG response was significantly delayed, whereas the M-cone-driven ERG response was phase advanced. CONCLUSIONS. Large dynamic differences between L-and M-cone-driven ERGs can be detected in RP. As a result, the interaction between the L-and M-cone systems, when modulated simultaneously at 30 Hz, is subtractive in RP patients and additive in normal subjects. Our data show that the use of only a standard white flicker ERG might lead to a misinterpretation of the mechanisms involved in retinal disorders, because the phases of different cone-driven responses are not considered. (Invest Ophthalmol Vis Sci. 2000;41:3225-3233) T he term retinitis pigmentosa (RP) is used for a group of progressive retinal diseases representing the most frequent retinal dystrophy with a prevalence of approximately 1:4000 and a frequency of heterozygotes of approximately 1:15. 1-3 Approximately 1.5 million people are affected around the world. 5-7 It has been shown that in early cases of RP the scotopic (rod) electroretinogram (ERG) is markedly reduced, whereas the photopic (cone) flash ERG is relatively normal. 8 The conedriven responses to 30-Hz white flicker have normal or reduced amplitudes, and they are usually delayed. 4,9 -12 To date, it is unresolved how changes in the different cones or their postreceptoral pathways contribute to these delays. There have been attempts to differentiate between the involvement of the three different cone systems in RP. RP patients exhibited reduced short (S-) wavelength-sensitive cone-driven ERGs; a subset of those patients showed significantly greater loss in the S-cone-driven ERG than in the mixed long (L)-and middle (M)-cone-driven ERG, We measured ERG responses to stimuli that either selectively modulated the L-or the M-cones or modulated the two simultaneously. Extensive data on normal subjects have been published recently. 14 RP patients showed generally larger ERG thresholds for nearly all combinations of L-and M-cone modulation. Surprisingly, the L-cone-driven ERG was very much delayed, whereas the M-cone-driven ERG was phase-advanced compared with normal observers. RP patients showed smaller ERG thresholds to counter-phase modulation than to in-phase modulation of the L-and M-cones, indicative of a subtractive interaction between the L-and M-cone-driven ERGs. This is probably caused by the increase in phase difference between the L-and M-cone-driven responses. (The term "L-and Mcone-driven ERGs" is used to refer to the responses originating in the L-and the M-cones, including the subsequent postreceptoral stages. The uncertainty about the exact cellular origins of the ERG does not influence the data interpretation.) METHODS Subjects Twenty-six patients (age range, 11-59 years) with different forms of RP (7 autosomal dominant RP, 1 autosomal recessive RP, 3 Usher II syndrome, 10 simplex RP, 1 X-linked RP [X-RP], and 4 carriers of X-RP) participated in the study. The diagnosis was based on history, symmetrical bilateral involvement, the typical alterations of the pigment epithelium layer, by visual field, and Ganzfeld electroretinography according to the ISCEV standard

Television Viewing Does Not Have to Be Sedentary: Motivation to Participate in a TV Exercise Program

The present study explored which underlying motivations induced people to participate in a television exercise program called “The Netherlands on the Move!-television” (NOM-tv). A cross-sectional study was carried out among 1,349 viewers of NOM-tv. The respondents completed the intrinsic motivation inventory (IMI), assessing their levels of intrinsic motivation towards participating in the NOM-tv exercises. The results showed that higher levels of intrinsic motivation (i.e. enjoying the NOM-tv exercises, feeling competent to perform this activity, and willingness to put effort into the exercises) were the most important predictive factors of more frequent participation in the NOM-tv exercises. Future screen-based interventions to reduce sedentary behavior should aim especially at encouraging people's intrinsic orientations towards physical activity in an autonomy-supportive way

Family Health Climate and Adolescents’ Physical Activity and Healthy Eating: A Cross-Sectional Study with Mother-Father-Adolescent Triads

The importance of the family environment for children’s and adolescents’ health behavior has been demonstrated, the underlying mechanisms of this influence remain unclear. Therefore, the aim of the study was to investigate the relationship between family environmental and individual determinants. It was hypothesized that the Family Health Climate (FHC) is associated with adolescents’ physical activity and dietary behavior and that intrinsic motivation mediates this association. Methods Cross-sectional data were collected from 198 families (mother, father, and child) using questionnaires. Perceptions of FHC of mothers, fathers, and their children were assessed using the FHC-scales for physical activity (FHC-PA) and nutrition (FHC-NU). The adolescents also rated their intrinsic motivation for exercise and healthy eating, their physical activity and consumption of healthful food. A structural equation model was analyzed and a bootstrapping procedure was used to test direct and indirect effects. Results The FHC-PA was related to the amount of weekly physical activity and the FHC-NU to the consumption of fruit, vegetables and salad. These effects were mediated by adolescents’ intrinsic motivation; the indirect effects were significant for both behaviors. Discussion These results emphasize the importance of the FHC in shaping adolescents’ physical activity and dietary behavior. Individual motivational factors are potential mediators of family and parental influences. Considering family-level variables and their interaction with individual factors contributes to the understanding of adolescents’ health behavior