262 research outputs found
Cavity-enhanced photoionization of an ultracold rubidium beam for application in focused ion beams
A two-step photoionization strategy of an ultracold rubidium beam for
application in a focused ion beam instrument is analyzed and implemented. In
this strategy the atomic beam is partly selected with an aperture after which
the transmitted atoms are ionized in the overlap of a tightly cylindrically
focused excitation laser beam and an ionization laser beam whose power is
enhanced in a build-up cavity. The advantage of this strategy, as compared to
without the use of a build-up cavity, is that higher ionization degrees can be
reached at higher currents. Optical Bloch equations including the
photoionization process are used to calculate what ionization degree and
ionization position distribution can be reached. Furthermore, the ionization
strategy is tested on an ultracold beam of Rb atoms. The beam current is
measured as a function of the excitation and ionization laser beam intensity
and the selection aperture size. Although details are different, the global
trends of the measurements agree well with the calculation. With a selection
aperture diameter of 52 m, a current of pA is
measured, which according to calculations is 63% of the current equivalent of
the transmitted atomic flux. Taking into account the ionization degree the ion
beam peak reduced brightness is estimated at A/(msreV).Comment: 13 pages, 9 figure
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
Direct magneto-optical compression of an effusive atomic beam for high-resolution focused ion beam application
An atomic rubidium beam formed in a 70 mm long two-dimensional
magneto-optical trap (2D MOT), directly loaded from a collimated Knudsen
source, is analyzed using laser-induced fluorescence. The longitudinal velocity
distribution, the transverse temperature and the flux of the atomic beam are
reported. The equivalent transverse reduced brightness of an ion beam with
similar properties as the atomic beam is calculated because the beam is
developed to be photoionized and applied in a focused ion beam. In a single
two-dimensional magneto-optical trapping step an equivalent transverse reduced
brightness of A/(m sr eV) was
achieved with a beam flux equivalent to nA. The
temperature of the beam is further reduced with an optical molasses after the
2D MOT. This increased the equivalent brightness to A/(m sr eV). For currents below 10 pA, for which disorder-induced
heating can be suppressed, this number is also a good estimate of the ion beam
brightness that can be expected. Such an ion beam brightness would be a six
times improvement over the liquid metal ion source and could improve the
resolution in focused ion beam nanofabrication.Comment: 10 pages, 8 figures, 1 tabl
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
Direct Observation of Sub-Poissonian Temporal Statistics in a Continuous Free Electron Beam with Sub-picosecond Resolution
Recently, interest in anti-bunched electron beams has surged because of their
potential in free electron quantum optics and electron microscopy. However, the
limited temporal resolution of available detectors poses a challenge in
observing the statistics of such beams. We propose a novel method to measure
the arrival time statistics of continuous electron beams with sub-ps
resolution. We developed a 2D streak camera based on the combination of
RF-cavity-based electron beam deflection and fast single electron imaging. We
observed Poissonian statistics within time bins from 100~ns to 1~ns and
increasingly pronounced sub-Poissonian statistics as the time bin decreases
from 2~ps to 340~fs
Design and characterization of dielectric filled TM microwave cavities for ultrafast electron microscopy
Microwave cavities oscillating in the TM mode can be used as dynamic
electron-optical elements inside an electron microscope. By filling the cavity
with a dielectric material it becomes more compact and power efficient,
facilitating the implementation in an electron microscope. However, the
incorporation of the dielectric material makes the manufacturing process more
difficult. Presented here are the steps taken to characterize the dielectric
material, and to reproducibly fabricate dielectric filled cavities. Also
presented are two versions with improved capabilities. The first, called a
dual-mode cavity, is designed to support two modes simultaneously. The second
has been optimized for low power consumption. With this optimized cavity a
magnetic field strength of 2.84 0.07 mT was generated at an input power
of 14.2 0.2 W. Due to the low input powers and small dimensions, these
dielectric cavities are ideal as electron-optical elements for electron
microscopy setups
Measurement of the temperature of an ultracold ion source using time-dependent electric fields
We report on a measurement of the characteristic temperature of an ultracold
rubidium ion source, in which a cloud of laser-cooled atoms is converted to
ions by photo-ionization. Extracted ion pulses are focused on a detector with a
pulsed-field technique. The resulting experimental spot sizes are compared to
particle-tracking simulations, from which a source temperature
mK and the corresponding transversal reduced emittance m rad are determined. We find that this result is
likely limited by space charge forces even though the average number of ions
per bunch is 0.022.Comment: 8 pages, 11 figure
Dual mode microwave deflection cavities for ultrafast electron microscopy
This paper presents the experimental realization of an ultrafast electron
microscope operating at a repetition rate of 75 MHz based on a single compact
resonant microwave cavity operating in dual mode. This elliptical cavity
supports two orthogonal TM modes with different resonance frequencies
that are driven independently. The microwave signals used to drive the two
cavity modes are generated from higher harmonics of the same Ti:Sapphire laser
oscillator. Therefore the modes are accurately phase-locked, resulting in
periodic transverse deflection of electrons described by a Lissajous pattern.
By sending the periodically deflected beam through an aperture, ultrashort
electron pulses are created at a repetition rate of 75 MHz. Electron pulses
with fs pulse duration are created with only W
of microwave input power; with normalized rms emittances of
pm rad and pm rad for
a peak current of nA. This corresponds to an rms normalized
peak brightness of A/m sr V, equal
to previous measurements for the continuous beam. In addition, the FWHM energy
spread of eV is also unaffected by the dual mode
cavity. This allows for ultrafast pump-probe experiments at the same spatial
resolution of the original TEM in which a 75 MHz Ti:Sapphire oscillator can be
used for exciting the sample. Moreover, the dual mode cavity can be used as a
streak camera or time-of-flight EELS detector with a dynamic range
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