671 research outputs found
Gigahertz repetition rate thermionic electron gun concept
We present a novel concept for the generation of gigahertz repetition rate
high brightness electron bunches. A custom design 100 kV thermionic gun
provides a continuous electron beam, with the current determined by the
filament size and temperature. A 1 GHz rectangular RF cavity deflects the beam
across a knife-edge, creating a pulsed beam. Adding a higher harmonic mode to
this cavity results in a flattened magnetic field profile which increases the
duty cycle to 30%. Finally, a compression cavity induces a negative
longitudinal velocity-time chirp in a bunch, initiating ballistic compression.
Adding a higher harmonic mode to this cavity increases the linearity of this
chirp and thus decreases the final bunch length. Charged particle simulations
show that with a 0.15 mm radius LaB6 filament held at 1760 K, this method can
create 279 fs, 3.0 pC electron bunches with a radial rms core emittance of
0.089 mm mrad at a repetition rate of 1 GHz.Comment: 12 pages, 12 figure
High quality ultrafast transmission electron microscopy using resonant microwave cavities
Ultrashort, low-emittance electron pulses can be created at a high repetition
rate by using a TM deflection cavity to sweep a continuous beam across
an aperture. These pulses can be used for time-resolved electron microscopy
with atomic spatial and temporal resolution at relatively large average
currents. In order to demonstrate this, a cavity has been inserted in a
transmission electron microscope, and picosecond pulses have been created. No
significant increase of either emittance or energy spread has been measured for
these pulses.
At a peak current of pA, the root-mean-square transverse normalized
emittance of the electron pulses is m rad in the direction parallel to the streak of the cavity, and
m rad in the perpendicular
direction for pulses with a pulse length of 1.1-1.3 ps. Under the same
conditions, the emittance of the continuous beam is
m rad.
Furthermore, for both the pulsed and the continuous beam a full width at half
maximum energy spread of eV has been measured
Morris Water Maze Learning in Two Rat Strains Increases the Expression of the Polysialylated Form of the Neural Cell Adhesion Molecule in the Dentate Gyrus But Has No Effect on Hippocampal Neurogenesis
In the current study, the authors investigated whether Morris water maze learning induces alterations in hippocampal neurogenesis or neural cell adhesion molecule (NCAM) polysialylation in the dentate gyrus. Two frequently used rat strains, Wistar and Sprague–Dawley, were trained in the spatial or the nonspatial version of the water maze. Both training paradigms did not have an effect on survival of newly formed cells that were labeled 7–9 days prior to the training or on progenitor proliferation in the subgranular zone. However, the granule cell layer of the spatially trained rats contained significantly more positive cells of the polysialylated form of the NCAM. These data demonstrate that Morris water maze learning causes plastic change in the dentate gyrus without affecting hippocampal neurogenesis.
Optimized coupling of cold atoms into a fiber using a blue-detuned hollow-beam funnel
We theoretically investigate the process of coupling cold atoms into the core
of a hollow-core photonic-crystal optical fiber using a blue-detuned
Laguerre-Gaussian beam. In contrast to the use of a red-detuned Gaussian beam
to couple the atoms, the blue-detuned hollow-beam can confine cold atoms to the
darkest regions of the beam thereby minimizing shifts in the internal states
and making the guide highly robust to heating effects. This single optical beam
is used as both a funnel and guide to maximize the number of atoms into the
fiber. In the proposed experiment, Rb atoms are loaded into a magneto-optical
trap (MOT) above a vertically-oriented optical fiber. We observe a
gravito-optical trapping effect for atoms with high orbital momentum around the
trap axis, which prevents atoms from coupling to the fiber: these atoms lack
the kinetic energy to escape the potential and are thus trapped in the laser
funnel indefinitely. We find that by reducing the dipolar force to the point at
which the trapping effect just vanishes, it is possible to optimize the
coupling of atoms into the fiber. Our simulations predict that by using a
low-power (2.5 mW) and far-detuned (300 GHz) Laguerre-Gaussian beam with a
20-{\mu}m radius core hollow-fiber it is possible to couple 11% of the atoms
from a MOT 9 mm away from the fiber. When MOT is positioned further away,
coupling efficiencies over 50% can be achieved with larger core fibers.Comment: 11 pages, 12 figures, 1 tabl
Quantitative atomic spectroscopy for primary thermometry
Quantitative spectroscopy has been used to measure accurately the
Doppler-broadening of atomic transitions in Rb vapor. By using a
conventional platinum resistance thermometer and the Doppler thermometry
technique, we were able to determine with a relative uncertainty of
, and with a deviation of from the
expected value. Our experiment, using an effusive vapour, departs significantly
from other Doppler-broadened thermometry (DBT) techniques, which rely on weakly
absorbing molecules in a diffusive regime. In these circumstances, very
different systematic effects such as magnetic sensitivity and optical pumping
are dominant. Using the model developed recently by Stace and Luiten, we
estimate the perturbation due to optical pumping of the measured value
was less than . The effects of optical pumping on atomic and
molecular DBT experiments is mapped over a wide range of beam size and
saturation intensity, indicating possible avenues for improvement. We also
compare the line-broadening mechanisms, windows of operation and detection
limits of some recent DBT experiments
Theory and particle tracking simulations of a resonant radiofrequency deflection cavity in TM mode for ultrafast electron microscopy
We present a theoretical description of resonant radiofrequency (RF)
deflecting cavities in TM mode as dynamic optical elements for
ultrafast electron microscopy. We first derive the optical transfer matrix of
an ideal pillbox cavity and use a Courant-Snyder formalism to calculate the 6D
phase space propagation of a Gaussian electron distribution through the cavity.
We derive closed, analytic expressions for the increase in transverse emittance
and energy spread of the electron distribution. We demonstrate that for the
special case of a beam focused in the center of the cavity, the low emittance
and low energy spread of a high quality beam can be maintained, which allows
high-repetition rate, ultrafast electron microscopy with 100 fs temporal
resolution combined with the atomic resolution of a high-end TEM. This is
confirmed by charged particle tracking simulations using a realistic cavity
geometry, including fringe fields at the cavity entrance and exit apertures
Mode-interactions and polarization conversion in a crystalline microresonator
LetterAbstract not availableWenle Weng and Andre N. Luite
Beam pulsing device for use in charged-particle microscopy
A charged-particle microscope comprising: - A charged-particle source, for producing a beam of charged particles that propagates along a particle-optical axis; - A sample holder, for holding and positioning a sample; - A charged-particle lens system, for directing said beam onto a sample held on the sample holder; - A detector, for detecting radiation emanating from the sample as a result of its interaction with the beam; - A beam pulsing device, for causing the beam to repeatedly switch on and off so as to produce a pulsed beam, wherein the beam pulsing device comprises a unitary resonant cavity disposed about said particle-optical axis and having an entrance aperture and an exit aperture for the beam, which resonant cavity is embodied to simultaneously produce a first oscillatory deflection of the beam at a first frequency in a first direction and a second oscillatory deflection of the beam at a second, different frequency in a second, different direction. The resonant cavity may have an elongated (e.g. rectangular or elliptical) cross-section, with a long axis parallel to said first direction and a short axis parallel to said second direction
Insulin-like growth factor II receptors in human brain and their absence in astrogliotic plaques in multiple sclerosis
Insulin-like growth factor (IGF) II receptors were studied in human adult brain by using autoradiography with [(125)I]IGF-II. Receptors were found to be widely distributed throughout all neuronal regions. The highest densities were found in plexus choroideus, granular layer of the cerebellar cortex, gyrus dendatus and pyramidal layer of the hippocampus, striatum, and cerebral cortex. White matter was devoid of IGF-II receptors. We also examined [(125)I]IGF-II binding in six plaques of multiple sclerosis, which were characterized by a dense network of astrocytes. Ne were unable to detect IGF-II receptors in any of the astrogliotic plaques, suggesting that IGF-II receptors in human brain are not involved in astrogliosis. The regional variations in neuronal distribution of IGF-II receptors suggest involvement of IGF-II in functions associated with specific neuronal pathways. (C) 2000 Elsevier Science B.V. All rights reserved
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