J.D. Silver

Highly-Ionized Atoms in Fusion Research Plasmas

Topical problems involving the interpretation of line emission from impurities or diagnostic test ions in Tokamak fusion devices are discussed briefly. The particular use of Tokamaks for precision (~ 10 ppm) wavelength spectroscopy of Δn = 0 transitions in highly ionised atoms with between 2 and 12 electrons is described in more detail. Experimental wavelengths are compared with ab initio theory and with semiempirical isoelectronic extrapolations. The Tokamak data for ions with Z ≥ 3-electrons lead to improved isoelectronic extrapolations which reveal errors in the previous data and provide transfer standards of wavelength in the XUV and VUV regions. Wavelengths of the 2-electron systems are of sufficient accuracy to be sensitive to the 2-photon exchange term in the Breit interaction as well as the 2-electron contribution to the Lamb shift. Experimental wavelengths are in agreement with recent theoretical calculations taking these contributions into account.

The Oxford electron‐beam ion trap: A device for spectroscopy of highly charged ions

An electron‐beam ion trap (EBIT) has just been completed in the Clarendon Laboratory, Oxford. The design is similar to the devices installed at the Lawrence Livermore National Laboratory. It is intended that the Oxford EBIT will be used for x‐ray and UV spectroscopy of hydrogenic and helium‐like ions, laser resonance spectroscopy of hydrogenic ions and measurements of dielectronic recombination cross sections, in order to test current understanding of simple highly charged ions.

Self correction of refractive error among young people in rural China: results of cross sectional investigation

Objective To compare outcomes between adjustable spectacles and conventional methods for refraction in young people. Design Cross sectional study. Setting Rural southern China. Participants 648 young people aged 12-18 (mean 14.9 (SD 0.98)), with uncorrected visual acuity ≤6/12 in either eye. Interventions All participants underwent self refraction without cycloplegia (paralysis of near focusing ability with topical eye drops), automated refraction without cycloplegia, and subjective refraction by an ophthalmologist with cycloplegia. Main outcome measures Uncorrected and corrected vision, improvement of vision (lines on a chart), and refractive error. Results Among the participants, 59% (384) were girls, 44% (288) wore spectacles, and 61% (393/648) had 2.00 dioptres or more of myopia in the right eye. All completed self refraction. The proportion with visual acuity ≥6/7.5 in the better eye was 5.2% (95% confidence interval 3.6% to 6.9%) for uncorrected vision, 30.2% (25.7% to 34.8%) for currently worn spectacles, 96.9% (95.5% to 98.3%) for self refraction, 98.4% (97.4% to 99.5%) for automated refraction, and 99.1% (98.3% to 99.9%) for subjective refraction (P=0.033 for self refraction v automated refraction, P=0.001 for self refraction v subjective refraction). Improvements over uncorrected vision in the better eye with self refraction and subjective refraction were within one line on the eye chart in 98% of participants. In logistic regression models, failure to achieve maximum recorded visual acuity of 6/7.5 in right eyes with self refraction was associated with greater absolute value of myopia/hyperopia (P<0.001), greater astigmatism (P=0.001), and not having previously worn spectacles (P=0.002), but not age or sex. Significant inaccuracies in power (≥1.00 dioptre) were less common in right eyes with self refraction than with automated refraction (5% v 11%, P<0.001). Conclusions Though visual acuity was slightly worse with self refraction than automated or subjective refraction, acuity was excellent in nearly all these young people with inadequately corrected refractive error at baseline. Inaccurate power was less common with self refraction than automated refraction. Self refraction could decrease the requirement for scarce trained personnel, expensive devices, and cycloplegia in children’s vision programmes in rural China.

Wavelength measurements of the satellite transitions to the n = 2 resonance lines of helium-like argon

The wavelengths of the 1s2s2p-1s22s and 1s2p2-1s22p transitions in Ar15+ have been measured using a crystal spectrometer and an electron beam ion trap. These spectral lines are satellites to the n = 2 resonance lines of He-like Ar16+ and play an important diagnostic role in high-temperature plasmas directed toward fusion research. The results of these measurements are compared with a number of different theoretical results, demonstrating the relative accuracy of these theoretical approaches.

The child self-refraction study results from urban Chinese children in Guangzhou

OBJECTIVE To compare visual and refractive outcomes between self-refracting spectacles (Adaptive Eyecare, Ltd, Oxford, UK), noncycloplegic autorefraction, and cycloplegic subjective refraction. DESIGN Cross-sectional study. PARTICIPANTS Chinese school-children aged 12 to 17 years. METHODS Children with uncorrected visual acuity ≤ 6/12 in either eye underwent measurement of the logarithm of the minimum angle of resolution visual acuity, habitual correction, self-refraction without cycloplegia, autorefraction with and without cycloplegia, and subjective refraction with cycloplegia. MAIN OUTCOME MEASURES Proportion of children achieving corrected visual acuity ≥ 6/7.5 with each modality; difference in spherical equivalent refractive error between each of the modalities and cycloplegic subjective refractive error. RESULTS Among 556 eligible children of consenting parents, 554 (99.6%) completed self-refraction (mean age, 13.8 years; 59.7% girls; 54.0% currently wearing glasses). The proportion of children with visual acuity ≥ 6/7.5 in the better eye with habitual correction, self-refraction, noncycloplegic autorefraction, and cycloplegic subjective refraction were 34.8%, 92.4%, 99.5% and 99.8%, respectively (self-refraction versus cycloplegic subjective refraction, P<0.001). The mean difference between cycloplegic subjective refraction and noncycloplegic autorefraction (which was more myopic) was significant (-0.328 diopter [D]; Wilcoxon signed-rank test P<0.001), whereas cycloplegic subjective refraction and self-refraction did not differ significantly (-0.009 D; Wilcoxon signed-rank test P = 0.33). Spherical equivalent differed by ≥ 1.0 D in either direction from cycloplegic subjective refraction more frequently among right eyes for self-refraction (11.2%) than noncycloplegic autorefraction (6.0%; P = 0.002). Self-refraction power that differed by ≥ 1.0 D from cycloplegic subjective refractive error (11.2%) was significantly associated with presenting without spectacles (P = 0.011) and with greater absolute power of both spherical (P = 0.025) and cylindrical (P = 0.022) refractive error. CONCLUSIONS Self-refraction seems to be less prone to accommodative inaccuracy than noncycloplegic autorefraction, another modality appropriate for use in areas where access to eye care providers is limited. Visual results seem to be comparable. Greater cylindrical power is associated with less accurate results; the adjustable glasses used in this study cannot correct astigmatism. Further studies of the practical applications of this modality are warranted. FINANCIAL DISCLOSURE(S) Proprietary or commercial disclosure may be found after the references.

Studies of magnetic dipole transitions in highly charged argon and barium using an electron beam ion trap

Using the Oxford electron beam ion trap (EBIT), we have studied a number of magnetic-dipole fine-structure transitions in highly charged argon and barium, which lie in the visible and near-UV region of the spectrum. Our wavelength measurements, with uncertainties of between 7 and 20 ppm, are the most accurate yet reported using an EBIT as a spectroscopic source of ions, and provide a useful test of atomic structure calculations for many-electron systems. The argon transitions studied are also of astrophysical interest. Finally, we present the first studies of the polarization of a visible transition from a highly charged ion trapped in an EBIT.

Injection of various metallic elements into an electron beam ion trap: Techniques needed for systematic investigations of isoelectronic sequences

Several techniques for injecting various metallic elements into an electron beam ion trap are presented. They have been developed by making use of a metal vapor vacuum are source, a wire probe, an alkali ion source, gaseous metallic compounds, and so on. These techniques are needed to systematically investigate highly charged ions along isoelectronic sequences. Merits and demerits of these methods are discussed.Several techniques for injecting various metallic elements into an electron beam ion trap are presented. They have been developed by making use of a metal vapor vacuum are source, a wire probe, an alkali ion source, gaseous metallic compounds, and so on. These techniques are needed to systematically investigate highly charged ions along isoelectronic sequences. Merits and demerits of these methods are discussed.

A design study for a super‐EBIT/EBIS without magnetic fielda)

A super‐EBIT may be defined by its ability to ionize even hydrogenlike uranium, requiring an electron energy well above 140 keV. The first super‐EBIT has been put into operation at Livermore, operating with electron energies up to 200 keV, the second one is in progress in Tokyo, aiming at 300 keV. A distinctive advantage may be obtained by raising the electron energy from 200–300 to 500 keV, thus reducing the radiative recombination of hydrogenlike heavy ions such as U91+ and increasing their relative abundance by an order of magnitude.

Improved wavelengths for the 1s2s 3S1-1s2p 3P0,2 transitions in helium-like Si12+

Abstract The wavelengths of the 1s2s 3S1 - 1s2p 3P0,2 transitions in He-like Si12+ have been remeasured to be 87.86 ± 0.01 nm and 81.48 ± 0.01 nm. The use of Rydberg lines for the calibration of fast beam spectra is discussed.

Measurement of the ground-state Lamb shift of hydrogen-like Ti21+

Using an electron beam ion trap (EBIT) and a crystal spectrometer in the Johann geometry, the 1s Lamb shift of hydrogen-like Ti21+ has been measured; the result is L(1s) = 2.29(14) eV, in agreement with the theoretical value. The measurement was made by calibrating the Lyman-α wavelengths using the characteristic Kα wavelengths of neutral vanadium. The Kα spectrum was produced by inserting a wire probe near the electron beam of the EBIT. While the present measurement is preliminary, achieving only modest precision, it suggests that much higher precision could be achieved in the future.