Ponderomotive scattering of an electron-bunch before injection into a laser wakefield

For the purpose of laser wakefield acceleration, it turned out that also the injection of electron bunches longer than a plasma wavelength can generate accelerated femtosecond bunches with relatively low energy spread. This is of high interest because such injecting bunches can be provided, e.g., by state-of-the-art photo cathode RF guns. Here we point out that when an e-bunch is injected in the wakefield it is important to take into account the ponderomotive scattering of the injecting bunch by the laser pulse in the vacuum region located in front of the plasma. At low energies of the injected bunch this scattering results in a significant drop of the collection efficiency. Larger collection efficiency can by reached with lower intensity laser pulses and relatively high injection energies. We also estimate the minimum trapping energy for the injected electrons and the length of the trapped bunch.Comment: 12 pages, 5 figures, submitted to Phys. Rev.

Active multilayer mirrors for reflectance tuning at extreme ultraviolet (EUV) wavelengths

We propose an active multilayer mirror structure for EUV wavelengths which can be adjusted to compensate for reflectance changes. The multilayer structure tunes the reflectance via an integrated piezoelectric layer that can change its dimension due to an externally applied voltage. Here, we present design and optimization of the mirror structure for maximum reflectance tuning. In addition, we present preliminary results showing that the deposition of piezoelectric thin films with the requisite layer smoothness and crystal structure are possible. Finally, polarization switching of the smoothest piezoelectric film is presented

Ethical considerations for alcohol researchers in their relation towards policy makers

Alcohol policy research all over the world is often funded by national or local governments. Researchers involved may be confronted with several ethical questions. These questions can have quite a different character. Ethical questions may have a severe character that can be quite “clear” for the researchers involved. Miller et al. [1] for instance recently studied interference of funders, like governments or industrial and charitable organizations, in addiction research. Results show that activities occur such as censorship of research outputs, interference with the wording in reports and articles and interventions in when and how findings are released. Governments funding policy research may interfere in a way as described by Miller et al. [1]. but also less obvious ethical issues may occur: What if the research question is formulated in a “questionable” or “suggestive” way? What if policy makers deliberately ignore results of scientific research? The purpose of this contribution is to elaborate on these less obvious ethical issues, not primarily to give clear-cut answers but to raise consciousness and stimulate reflection and debate among researchers and policy makers

Electromagnetic radiation from a laser wakefield accelerator

Coherent and incoherent electromagnetic radiation emitted from a laser wakefield accelerator is calculated based on Lienard-Wiechert potentials. It is found that at wavelengths longer than the bunch length, the radiation is coherent. The coherent radiation, which typically lies in the infrared range, shows features that reveal details of the acceleration process and properties of the electron bunch, such as its duration, charge, energy, and offset with respect to the wakefield axis. The incoherent range of the spectrum, which extends to the X-ray frequency range, consists of rather broad peaks, which are caused by the acceleration.Comment: 14 pages, incl. 6 figures, submitted for publicatio

Extensive medical absenteeism among secondary school students:An observational study on their health condition from a biopsychosocial perspective

An adequate approach to reducing school absenteeism should focus on medical absenteeism as this is the most prevalent form of school absenteeism. The objective of this study is to explore the health condition of pre-vocational secondary students with extensive medical absenteeism from a biopsychosocial perspective. Data were obtained from medical assessments and Strengths and Difficulties Questionnaires (SDQs) of students with medical absence above threshold criteria (i.e. reported sick four times in 12 school weeks or more than six consecutive school days) who were referred to a youth health care physician. The results showed that the students had a mean absence rate of 14% in 12 school weeks. Of all students, 43.5% had a diagnosed disease and 81.5% had problems such as physical complaints not yet diagnosed, psychosocial problems, lifestyle problems and sleeping difficulties. Four groups could be distinguished: 13.4% with a diagnosed disease and no problem, 30.1% with a diagnosed disease and a problem, 51.5% with a problem and no diagnosed disease and 5.1% without a diagnosed disease or problem. Significantly higher scores of the Total difficulties-scale on the SDQ were found (mean 10.5; SD 5.8) in the study group, compared to a reference group (mean 9.1; SD 4.9). In conclusion, this study shows that when using the aforementioned criteria for extensive medical absenteeism to intervene with the absence, students with a mean absence rate of 14% in 12 school weeks are identified. If there was a diagnosed disease, it was accompanied by problems about twice as often. More than half of the students’ absence was caused by problems rather than a disease. The great diversity of these problems calls for a personalized approach. A broad perspective, including medical expertise, is needed to distinguish between emerging mental and physical diseases, psychosocial and lifestyle problems. Keywords: Adolescent Health, Preventive Pediatric Primary Care, School Absenteeism, Public Health, Psychosocial Problem

Dynamics of Femtosecond Electron Bunches

In the laser wakefield accelerator (LWFA) a short intense laser pulse, with a duration of the order of a plasma wave period, excites an unusually strong plasma wake wave (laser wakefield). Recent experiments on laser wakefield acceleration [Nature (London) 431, p.535, p.538, p.541 (2004)] demonstrated generation of ultra-short (with a duration of a few femtoseconds) relativistic electron bunches with relatively low energy spread of the order of a few percent. We have studied the dynamics of such bunches in vacuum and in laser wakefield. The results show strong bunch dynamics already on a few millimeters propagation distance in both cases. In vacuum, the bunch radius and emittance quickly grow. The latter worsens the focusability of the bunch. We found that when a femtosecond bunch is accelerated in a channel-guided laser wakefield, for realistic bunch lengths, the bunch length is approximately conserved. However, the spread in betatron frequencies leads to fast betatron phase mixing in the bunch envelope for on-axis injection. When bunch is injected in a laser wakefield off-axis, the bunch decoherence results in considerable increase in the normalized bunch emittance, and, in some cases, in increase in the energy spread, after acceleration. We also discuss a possible two-stage laser wakefield accelerator.Comment: 34 pages, incl. 11 figs. and 1 table, submitted for publicatio

Usability of volunteer brokerage websites:The why and how of user testing

Dutch volunteer centers offer online volunteer brokerage via their websites. Usability is a crucial factor for the success of this service. It determines whether or not visitors or potential volunteers stay on the website and a match can be made. In this article, user testing is applied to the websites of five volunteer centers. The results provide information on the usability of these specific websites. In addition, other volunteer centers are offered insight into the various problems of usability and a tool to test this

Plasma Citrate Levels Are Associated with an Increased Risk of Cardiovascular Mortality in Patients with Type 2 Diabetes (Zodiac-64)

Circulating citrate may represent a proxy of mitochondrial dysfunction which plays a role in the development of vascular complications in type 2 diabetes (T2D). Here, we determined the associations between plasma citrate levels and cardiovascular (CV) mortality in T2D patients. In this prospective cohort study, 601 patients were included who participated in the Zwolle Outpatient Diabetes project Integrating Available Care (ZODIAC). Plasma citrate levels were measured by nuclear magnetic resonance spectroscopy. Cox proportional hazards regression models were used to evaluate the associations between plasma citrate and the risk of CV mortality. Over a median follow-up of 11.4 years, 119 (19.8%) of the 601 patients died from a CV cause. In multivariable Cox proportional hazards regression models, adjusting for conventional risk factors, plasma citrate was associated with an increased risk of CV mortality (the hazard ratio (HR) per 1-SD increment was 1.19 (95%CI: 1.00–1.40), p = 0.048). This association was prominent in males (n = 49 with CV mortality) (HR 1.52 (95%CI: 1.14–2.03), p = 0.005), but not in females (n = 70 with CV mortality) (HR 1.11 (95%CI: 0.90–1.37), p = 0.319) (age-adjusted Pinteraction = 0.044). In conclusion, higher plasma citrate levels are associated with an increased risk of CV mortality in patients with established T2D. Future studies are warranted to unravel the potential role of citrate-related pathways in the pathogenesis of T2D-related vascular complications

Generation of stable ultra-relativistic attosecond electron bunches via the laser wakefield acceleration mechanism

In recent experiments ultra-relativistic femtosecond electron bunches were generated by a Laser Wakefield Accelerator (LWFA) in different regimes. Here we predict that even attosecond bunches can be generated by an LWFA due to the fast betatron phase mixing within a femtosecond electron bunch. The attosecond bunches are stable outside the LWFA and can propagate in vacuum many tens of centimeter without significant change in their duration. Our calculations show that evidence for the formation of attosecond bunches can be found in the spectrum of coherent betatron radiation from LWFA's