The DCU laser ion source

Laser ion sources are used to generate and deliver highly charged ions of various masses and energies. We present details on the design and basic parameters of the DCU laser ion source (LIS). The theoretical aspects of a high voltage (HV) linear LIS are presented and the main issues surrounding laser-plasma formation, ion extraction and modeling of beam transport in relation to the operation of a LIS are detailed. A range of laser power densities (I ∼ 108–1011 W cm−2) and fluences (F = 0.1–3.9 kJ cm−2) from a Q-switched ruby laser (full-width half-maximum pulse duration ∼ 35 ns, λ = 694 nm) were used to generate a copper plasma. In “basic operating mode,” laser generated plasma ions are electrostatically accelerated using a dc HV bias (5–18 kV). A traditional einzel electrostatic lens system is utilized to transport and collimate the extracted ion beam for detection via a Faraday cup. Peak currents of up to I ∼ 600 μA for Cu+ to Cu3+ ions were recorded. The maximum collected charge reached 94 pC (Cu2+). Hydrodynamic simulations and ion probe diagnostics were used to study the plasma plume within the extraction gap. The system measured performance and electrodynamic simulations indicated that the use of a short field-free (L = 48 mm) region results in rapid expansion of the injected ion beam in the drift tube. This severely limits the efficiency of the electrostatic lens system and consequently the sources performance. Simulations of ion beam dynamics in a “continuous einzel array” were performed and experimentally verified to counter the strong space-charge force present in the ion beam which results from plasma extraction close to the target surface. Ion beam acceleration and injection thus occur at “high pressure.” In “enhanced operating mode,” peak currents of 3.26 mA (Cu2+) were recorded. The collected currents of more highly charged ions (Cu4+–Cu6+) increased considerably in this mode of operation

The Bacterial Photosynthetic Reaction Center as a Model for Membrane Proteins

Membrane proteins participate in many fundamental cellular processes. Until recently, an understanding of the function and properties of membrane proteins was hampered by an absence of structural information at the atomic level. A landmark achievement toward understanding the structure of membrane proteins was the crystallization (1) and structure determination (2-5) the photosynthetic reaction center (RC) from the purple bacteria Rhodopseudomonas viridis, followed by that of the RC from Rhodobacter sphaeroides (6-17). The RC is an integral membrane protein-pigment complex, which carries out the initial steps of photosynthesis (reviewed in 18). RCs from the purple bacteria Rps. viridis and Rb. sphaeroides are composed of three membrane-associated protein subunits (designated L, M, and H), and the following cofactors: four bacteriochlorophylls (Bchl or B), two bacteriopheophytins (Bphe or [phi]), two quinones, and a nonheme iron. The cofactors are organized into two symmetrical branches that are approximately related by a twofold rotation axis (2, 8). A central feature of the structural organization of the RC is the presence of 11 hydrophobic [alpha]-helixes, approximately 20-30 residues long, which are believed to represent the membrane-spanning portion of the RC (3, 9). Five membrane-spanning helixes are present in both the L and M subunits, while a single helix is in the H subunit. The folding of the L and M subunits is similar, consistent with significant sequence similarity between the two chains (19-25). The L and M subunits are approximately related by the same twofold rotation axis that relates the two cofactor branches. RCs are the first membrane proteins to be described at atomic resolution; consequently they provide an important model for discussing the folding of membrane proteins. The structure demonstrates that [alpha]-helical structures may be adopted by integral membrane proteins, and provides confirmation of the utility of hydropathy plots in identifying nonpolar membrane-spanning regions from sequence data. An important distinction between the folding environments of water-soluble proteins and membrane proteins is the large difference in water concentration surrounding the proteins. As a result, hydrophobic interactions (26) play very different roles in stabilizing the tertiary structures of these two classes of proteins; this has important structural consequences. There is a striking difference in surface polarity of membrane and water-soluble proteins. However, the characteristic atomic packing and surface area appear quite similar. A computational method is described for defining the position of the RC in the membrane (10). After localization of the RC structure in the membrane, surface residues in contact with the lipid bilayer were identified. As has been found for soluble globular proteins, surface residues are less well conserved in homologous membrane proteins than the buried, interior residues. Methods based on the variability of residues between homologous proteins are described (13); they are useful (a) in defining surface helical regions of membrane and water-soluble proteins and (b) in assigning the side of these helixes that are exposed to the solvent. A unifying view of protein structure suggests that water-soluble proteins may be considered as modified membrane proteins with covalently attached polar groups that solubilize the proteins in aqueous solution

Comparing the influence of ‘describing findings to the examiner’ or ‘examining as in usual practice’ on the students’ performance and assessors’ judgements during physical examination skills assessment

Background: Within assessment of physical examination skills, two approaches are common: “Describing Findings” (students comment throughout); and examining as “Usual Practice” (students only report findings at the end). Despite numerous potential influences on both students’ performances and assessors’ judgements, no prior studies have investigated the influence of either approach on assessments. Methods: Two group, randomised, crossover design. Within a 2-station simulated physical examination OSCE, we manipulated whether students “described findings” or examined as “usual practice”, collecting 1/. performance scores; 2/. Students’/examiners’ cognitive load ratings; ratings of the 3/. fluency and 4/. completeness of students’ presentations and 5/. Students’ task-finishing, comparing all 5 end-points across conditions. Results: Neither students’ performance scores nor examiners’ cognitive load were influenced by experimental condition. Students reported higher cognitive load (7/9) when “describing findings” than “usual practice” (6/9, p=0.002), and were less likely to finish (4 vs 12, p=0.007). Presentation completeness was higher for “describing findings” (mean=2.40, (95CIs=2.05-2.74)) than “usual practice” (mean=1.92 (1.65-2.18),p=0.016), whilst fluency ratings showed a similar trend. Conclusions: The decision to “Describe Findings” or examine as “Usual Practice” does not appear neutral, potentially influencing students’ efficiency, recall and (by inference) learning. Institutions should explicitly select one option based on assessment goals

Locating regional health policy: Institutions, politics, and practices

Poverty reduction and health became central in the agendas of Southern regional organisations in the last two decades. Yet, little is known about how these organisations address poverty, inclusion and social inequality, and how Southern regional formations are engaging in power constellations, institutions, processes, interests and ideological positions within different spheres of governance. This article reviews academic literatures spanning global social policy, regional studies and diplomacy studies, and the state of knowledge and understanding of the ‘place’ of regional actors in health governance as a global political practice therein. It identifies theoretical and thematic points of connection between disparate literatures and how these can be bridged through research focusing on the social policies of regional organisations and regional integration processes. This framework hence locates the contributions of each of the research articles of this Special Issue of Global Social Policy on the regional dimension of health policy and diplomacy in relation to Southern Africa and South America. It also highlights the ways in which the articles bring new evidence about how social relations of welfare are being (re)made over larger scales and how regional actors may initiate new norms to improve health rights in international arenas engaging in new forms of ‘regional’ diplomacy

Perils, pitfalls and potential for the use of reporting guidelines in experimental research in medical education.

Commentary

Additivity of relative magnetic helicity in finite volumes

CONTEXT: Relative magnetic helicity is conserved by magneto-hydrodynamic evolution even in the presence of moderate resistivity. For that reason, it is often invoked as the most relevant constraint on the dynamical evolution of plasmas in complex systems, such as solar and stellar dynamos, photospheric flux emergence, solar eruptions, and relaxation processes in laboratory plasmas. However, such studies often indirectly imply that relative magnetic helicity in a given spatial domain can be algebraically split into the helicity contributions of the composing subvolumes, in other words that it is an additive quantity. A limited number of very specific applications have shown that this is not the case. AIMS: Progress in understanding the nonadditivity of relative magnetic helicity requires removal of restrictive assumptions in favor of a general formalism that can be used in both theoretical investigations and numerical applications. METHODS: We derive the analytical gauge-invariant expression for the partition of relative magnetic helicity between contiguous finite volumes, without any assumptions on either the shape of the volumes and interface, or the employed gauge. RESULTS: We prove the nonadditivity of relative magnetic helicity in finite volumes in the most general, gauge-invariant formalism, and verify this numerically. We adopt more restrictive assumptions to derive known specific approximations, which yields a unified view of the additivity issue. As an example, the case of a flux rope embedded in a potential field shows that the nonadditivity term in the partition equation is, in general, non-negligible. CONCLUSIONS: The nonadditivity of relative magnetic helicity can potentially be a serious impediment to the application of relative helicity conservation as a constraint on the complex dynamics of magnetized plasmas. The relative helicity partition formula can be applied to numerical simulations to precisely quantify the effect of nonadditivity on global helicity budgets of complex physical processes