226 research outputs found
Studies of a Terawatt X-Ray Free-Electron Laser
The possibility of constructing terawatt (TW) x-ray free-electron lasers
(FELs) has been discussed using novel superconducting helical undulators [5].
In this paper, we consider the conditions necessary for achieving powers in
excess of 1 TW in a 1.5 {\AA} FEL using simulations with the MINERVA simulation
code [7]. Steady-state simulations have been conducted using a variety of
undulator and focusing configurations. In particular, strong focusing using
FODO lattices is compared with the natural, weak focusing inherent in helical
undulators. It is found that the most important requirement to reach TW powers
is extreme transverse compression of the electron beam in a strong FODO
lattice. The importance of extreme focusing of the electron beam in the
production of TW power levels means that the undulator is not the prime driver
for a TW FEL, and simulations are also described using planar undulators that
reach near-TW power levels. In addition, TW power levels can be reached using
pure self-amplified spontaneous emission (SASE) or with novel self-seeding
configurations when such extreme focusing of the electron beam is applied.Comment: 10 pages, 12 figure
Mapping individual electromagnetic field components inside a photonic crystal
We present a method to map the absolute electromagnetic field strength inside
photonic crystals. We apply the method to map the electric field component Ez
of a two-dimensional photonic crystal slab at microwave frequencies. The slab
is placed between two mirrors to select Bloch standing waves and a
subwavelength spherical scatterer is scanned inside the resulting resonator.
The resonant Bloch frequencies shift depending on the electric field at the
position of the scatterer. To map the electric field component Ez we measure
the frequency shift in the reflection and transmission spectrum of the slab
versus the scatterer position. Very good agreement is found between
measurements and calculations without any adjustable parameters.Comment: 12 pages, 7 figure
Cluster size dependence of high-order harmonic generation
We investigate high-order harmonic generation (HHG) from noble gas clusters
in a supersonic gas jet. To identify the contribution of harmonic generation
from clusters versus that from gas monomers, we measure the high-order harmonic
output over a broad range of the total atomic number density in the jet (from
3*10^16 cm^{-3} to 3x10^18 cm{-3}) at two different reservoir temperatures (303
K and 363 K). For the firrst time in the evaluation of the harmonic yield in
such measurements, the variation of the liquid mass fraction, g, versus
pressure and temperature is taken into consideration, which we determine,
reliably and consistently, to be below 20% within our range of experimental
parameters. By comparing the measured harmonic yield from a thin jet with the
calculated corresponding yield from monomers alone, we find an increased
emission of the harmonics when the average cluster size is less than 3000.
Using g, under the assumption that the emission from monomers and clusters add
up coherently, we calculate the ratio of the average single-atom response of an
atom within a cluster to that of a monomer and find an enhancement of around 10
for very small average cluster size (~200). We do not find any dependence of
the cut-off frequency on the composition of the cluster jet. This implies that
HHG in clusters is based on electrons that return to their parent ions and not
to neighbouring ions in the cluster. To fully employ the enhanced average
single-atom response found for small average cluster sizes (~200), the nozzle
producing the cluster jet must provide a large liquid mass fraction at these
small cluster sizes for increasing the harmonic yield. Moreover, cluster jets
may allow for quasi-phase matching, as the higher mass of clusters allows for a
higher density contrast in spatially structuring the nonlinear medium.Comment: 16 pages, 6 figure
Single-shot fluctuations in waveguided high-harmonic generation
For exploring the application potential of coherent soft x-ray (SXR) and
extreme ultraviolet radiation (XUV) provided by high-harmonic generation, it is
important to characterize the central output parameters. Of specific importance
are pulse-to-pulse (shot-to-shot) fluctuations of the high-harmonic output
energy, fluctuations of the direction of the emission (pointing instabilities),
and fluctuations of the beam divergence and shape that reduce the spatial
coherence. We present the first single-shot measurements of waveguided
high-harmonic generation in a waveguided (capillary-based) geometry. Using a
capillary waveguide filled with Argon gas as the nonlinear medium, we provide
the first characterization of shot-to-shot fluctuations of the pulse energy, of
the divergence and of the beam pointing. We record the strength of these
fluctuations vs. two basic input parameters, which are the drive laser pulse
energy and the gas pressure in the capillary waveguide. In correlation
measurements between single-shot drive laser beam profiles and single-shot
high-harmonic beam profiles we prove the absence of drive laser
beam-pointing-induced fluctuations in the high-harmonic output. We attribute
the main source of high-harmonic fluctuations to ionization-induced nonlinear
mode mixing during propagation of the drive laser pulse inside the capillary
waveguide
Analysis of ultra-short bunches in free-electron lasers
Free-electron lasers (FELs) operate at wavelengths from millimeter waves through hard x-rays. At x-ray wavelengths, FELs typically rely on self-amplified spontaneous emission (SASE). Typical SASE emission contains multiple temporal 'spikes' which limit the longitudinal coherence of the optical output; hence, alternate schemes that improve on the longitudinal coherence of the SASE emission are of interest. In this paper, we consider electron bunches that are shorter than the SASE spike separation. In such cases, the spontaneously generated radiation consists of a single optical pulse with better longitudinal coherence than is found in typical SASE FELs. To investigate this regime, we use two FEL simulation codes. One (MINERVA) uses the slowly-varying envelope approximation (SVEA) which breaks down for extremely short pulses. The second (PUFFIN) is a particle-in-cell simulation code that is considered to be a more complete model of the underlying physics and which is able to simulate very short pulses. We first anchor these codes by showing that there is substantial agreement between the codes in simulation of the SPARC SASE FEL experiment at ENEA Frascati. We then compare the two codes for simulations using electron bunch lengths that are shorter than the SASE slice separation. The comparisons between the two codes for short bunch simulations elucidate the limitations of the SVEA in this regime but indicate that the SVEA can treat short bunches that are comparable to the cooperation length
A design for the generation of temporally-coherent radiation pulses in the VUV and beyond by a self-seeding high-GaIn free electron laser amplifier
A proposal for a self-seeding, tunable free-electron laser amplifier operating in the vacuum ultra-violet (VUV) region of the spectrum is presented. Full three-dimensional (3D) modelling of the free electron laser and the optical feedback system has been carried out. Simulations demonstrate the generation of near transform limited radiation pulses with peak powers in the hundreds of megawatts. Preliminary 1D simulations show that by using a similar system it may be possible to extend such operation beyond the VUV to higher photon energies
High confinement, high yield Si3N4 waveguides for nonlinear optical application
In this paper we present a novel fabrication technique for silicon nitride
(Si3N4) waveguides with a thickness of up to 900 nm, which are suitable for
nonlinear optical applications. The fabrication method is based on etching
trenches in thermally oxidized silicon and filling the trenches with Si3N4.
Using this technique no stress-induced cracks in the Si3N4 layer were observed
resulting in a high yield of devices on the wafer. The propagation losses of
the obtained waveguides were measured to be as low as 0.4 dB/cm at a wavelength
of around 1550 nm.Comment: 10 pages, 4 figure
Diagnostic value of a heart-type fatty acid-binding protein (H-FABP) bedside test in suspected acute coronary syndrome in primary care
AbstractBackgroundTo determine the diagnostic accuracy of a rapid heart-type fatty acid-binding protein (H-FABP) test in patients suspected of acute coronary syndrome (ACS) in primary care.MethodsGeneral practitioners included 298 patients suspected of ACS. In all patients, whether referred to hospital or not, ECG and cardiac biomarker testing was performed. ACS was determined in accordance with international guidelines. Multivariate analysis was used to determine the value of H-FABP in addition to clinical findings.ResultsMean patient age was 66years (SD 14), 52% was female and 66 patients (22%) were diagnosed with ACS. The H-FABP bedside test was performed within 24h (median 3.1, IQR 1.5 to 7.1) after symptom onset. The positive predictive value (PPV) of H-FABP was 65% (95% confidence interval (CI) 50–78). The negative predictive value (NPV) was 85% (95% CI 80–88). Sensitivity was 39% (29–51%) and specificity 94% (90–96%). Within 6h after symptom onset, the PPV was 72% (55–84) and the NPV was 83% (77–88), sensitivity 43% (31–57%) and specificity 94% (89–97%). Adding the H-FABP test to a diagnostic model for ACS led to an increase in the area under the receiver operating curve from 0.66 (95% CI 0.58–0.73) to 0.75 (95% CI 0.68–0.82).ConclusionThe H-FABP rapid test provides modest additional diagnostic certainty in primary care. It cannot be used to safely exclude rule out ACS. The test can only be used safely in patients otherwise NOT referred to hospital by the GP, as an extra precaution not to miss ACS (‘rule in’)
ATN classification and clinical progression in subjective cognitive decline
Objective: To investigate the relationship between the ATN classification system (amyloid, tau, neurodegeneration) and risk of dementia and cognitive decline in individuals with subjective cognitive decline (SCD).
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Methods: We classified 693 participants with SCD (60 ± 9 years, 41% women, Mini-Mental State Examination score 28 ± 2) from the Amsterdam Dementia Cohort and Subjective Cognitive Impairment Cohort (SCIENCe) project according to the ATN model, as determined by amyloid PET or CSF β-amyloid (A), CSF p-tau (T), and MRI-based medial temporal lobe atrophy (N). All underwent extensive neuropsychological assessment. For 342 participants, follow-up was available (3 ± 2 years). As a control population, we included 124 participants without SCD.
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Results: Fifty-six (n = 385) participants had normal Alzheimer disease (AD) biomarkers (A–T–N–), 27% (n = 186) had non-AD pathologic change (A–T–N+, A–T+N–, A–T+N+), 18% (n = 122) fell within the Alzheimer continuum (A+T–N–, A+T–N+, A+T+N–, A+T+N+). ATN profiles were unevenly distributed, with A–T+N+, A+T–N+, and A+T+N+ containing very few participants. Cox regression showed that compared to A–T–N–, participants in A+ profiles had a higher risk of dementia with a dose–response pattern for number of biomarkers affected. Linear mixed models showed participants in A+ profiles showed a steeper decline on tests addressing memory, attention, language, and executive functions. In the control group, there was no association between ATN and cognition.
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Conclusions: Among individuals presenting with SCD at a memory clinic, those with a biomarker profile A–T+N+, A+T–N–, A+T+N–, and A+T+N+ were at increased risk of dementia, and showed steeper cognitive decline compared to A–T–N– individuals. These results suggest a future where biomarker results could be used for individualized risk profiling in cognitively normal individuals presenting at a memory clinic
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