1,589 research outputs found
Femtosecond electron-bunch dynamics in laser wakefields and vacuum
Recent advances in laser wakefield acceleration demonstrated the generation of extremely short (with a duration of a few femtoseconds) relativistic electron bunches with relatively low (of the order of couple of percent) energy spread. In this article we study the dynamics of such bunches in drift space (vacuum) and in channel-guided laser wakefields. Analytical solutions were found for the transverse coordinate of an electron and for the bunch envelope in the wakefield in the case of arbitrary change in the energy. Our results show strong bunch dynamics already on a millimeter scale propagation distance both in plasma and in vacuum. When the bunch propagates in vacuum, its transverse sizes grow considerably; the same is observed for the normalized bunch emittance that worsens the focusability of the bunch. A scheme of two-stage laser wakefield accelerator with small drift space between the stages is proposed. It is found that fast longitudinal betatron phase mixing occurs in a femtosecond bunch when it propagates along the wakefield axis. When bunch propagates off axis, strong bunch decoherence and fast emittance degradation due to the finite bunch length was observed
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.
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
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
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
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
Association Between Serum Carnosinase Concentration and Activity and Renal Function Impairment in a Type-2 Diabetes Cohort
Introduction: Genetic studies have identified associations of carnosinase 1 (CN1) polymorphisms with diabetic kidney disease (DKD). However, CN1 levels and activities have not been assessed as diagnostic or prognostic markers of DKD in cohorts of patients with type 2 diabetes (T2D).Methods: We established high-throughput, automated CN1 activity and concentration assays using robotic systems. Using these methods, we determined baseline serum CN1 levels and activity in a T2D cohort with 970 patients with no or only mild renal impairment. The patients were followed for a mean of 1.2 years. Baseline serum CN1 concentration and activity were assessed as predictors of renal function impairment and incident albuminuria during follow up.Results: CN1 concentration was significantly associated with age, gender and estimated glomerular filtration rate (eGFR) at baseline. CN1 activity was significantly associated with glycated hemoglobin A1c (HbA1c) and eGFR. Serum CN1 at baseline was associated with eGFR decline and predicted renal function impairment and incident albuminuria during the follow-up.Discussion: Baseline serum CN1 levels were associated with presence and progression of renal function decline in a cohort of T2D patients. Confirmation in larger cohorts with longer follow-up observation periods will be required to fully establish CN1 as a biomarker of DKD
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
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