21,707 research outputs found
Cryogenic Sapphire Oscillator using a low-vibration design pulse-tube cryocooler: First results
A Cryogenic Sapphire Oscillator has been implemented at 11.2 GHz using a
low-vibration design pulse-tube cryocooler. Compared with a state-of-the-art
liquid helium cooled CSO in the same laboratory, the square root Allan variance
of their combined fractional frequency instability is for integration times s, dominated by
white frequency noise. The minimum for the two
oscillators was reached at s. Assuming equal contributions from
both CSOs, the single oscillator phase noise at 1 Hz offset from the carrier.Comment: 5 pages, 5 figures, accepted in IEEE Trans on Ultrasonics,
Ferroelectrics and Frequency Contro
Observation of noise correlated by the Hawking effect in a water tank
We measured the power spectrum and two-point correlation function for the
randomly fluctuating free surface on the downstream side of a stationary flow
with a maximum Froude number reached above a
localised obstacle. On such a flow the scattering of incident long wavelength
modes is analogous to that responsible for black hole radiation (the Hawking
effect). Our measurements of the noise show a clear correlation between pairs
of modes of opposite energies. We also measure the scattering coefficients by
applying the same analysis of correlations to waves produced by a wave maker.Comment: 11 pages, 14 figures. several points clarified; two new subsections
in the Supplemental Material on the wave equation and the links with
experiments in BEC
Ab-initio No-Core Gamow Shell Model calculations with realistic interactions
No-Core Gamow Shell Model (NCGSM) is applied for the first time to study
selected well-bound and unbound states of helium isotopes. This model is
formulated on the complex energy plane and, by using a complete Berggren
ensemble, treats bound, resonant, and scattering states on equal footing. We
use the Density Matrix Renormalization Group method to solve the many-body
Schr\"{o}dinger equation. To test the validity of our approach, we benchmarked
the NCGSM results against Faddeev and Faddeev-Yakubovsky exact calculations for
H and He nuclei. We also performed {\textit ab initio} NCGSM
calculations for the unstable nucleus He and determined the ground state
energy and decay width, starting from a realistic NLO chiral interaction.Comment: 17 pages, 14 figures. Revised version. Discussion on microscopic
overlap functions, SFs and ANCs is added. Added references. Accepted for
publication at PR
Modeling laser wakefield accelerators in a Lorentz boosted frame
Modeling of laser-plasma wakefield accelerators in an optimal frame of
reference \cite{VayPRL07} is shown to produce orders of magnitude speed-up of
calculations from first principles. Obtaining these speedups requires
mitigation of a high-frequency instability that otherwise limits effectiveness
in addition to solutions for handling data input and output in a
relativistically boosted frame of reference. The observed high-frequency
instability is mitigated using methods including an electromagnetic solver with
tunable coefficients, its extension to accomodate Perfectly Matched Layers and
Friedman's damping algorithms, as well as an efficient large bandwidth digital
filter. It is shown that choosing the frame of the wake as the frame of
reference allows for higher levels of filtering and damping than is possible in
other frames for the same accuracy. Detailed testing also revealed
serendipitously the existence of a singular time step at which the instability
level is minimized, independently of numerical dispersion, thus indicating that
the observed instability may not be due primarily to Numerical Cerenkov as has
been conjectured. The techniques developed for Cerenkov mitigation prove
nonetheless to be very efficient at controlling the instability. Using these
techniques, agreement at the percentage level is demonstrated between
simulations using different frames of reference, with speedups reaching two
orders of magnitude for a 0.1 GeV class stages. The method then allows direct
and efficient full-scale modeling of deeply depleted laser-plasma stages of 10
GeV-1 TeV for the first time, verifying the scaling of plasma accelerators to
very high energies. Over 4, 5 and 6 orders of magnitude speedup is achieved for
the modeling of 10 GeV, 100 GeV and 1 TeV class stages, respectively
CP violation conditions in N-Higgs-doublet potentials
Conditions for CP violation in the scalar potential sector of general
N-Higgs-doublet models (NHDMs) are analyzed from a group theoretical
perspective. For the simplest two-Higgs-doublet model (2HDM) potential, a
minimum set of conditions for explicit and spontaneous CP violation is
presented. The conditions can be given a clear geometrical interpretation in
terms of quantities in the adjoint representation of the basis transformation
group for the two doublets. Such conditions depend on CP-odd pseudoscalar
invariants. When the potential is CP invariant, the explicit procedure to reach
the real CP-basis and the explicit CP transformation can also be obtained. The
procedure to find the real basis and the conditions for CP violation are then
extended to general NHDM potentials. The analysis becomes more involved and
only a formal procedure to reach the real basis is found. Necessary conditions
for CP invariance can still be formulated in terms of group invariants: the
CP-odd generalized pseudoscalars. The problem can be completely solved for
three Higgs-doublets.Comment: RevTeX4 used. Minor modifications, in particular, the parameter
counting of . v3: Eqs.(28)-(31) correcte
Electromagnetic analysis and performance comparison of fully 3D-printed antennas
In this work, the possibility of directly prototyping antennas by exploiting additive manufacturing 3D-printing technology is investigated. In particular, the availability of printable filaments with interesting conductive properties allows for printing of even the antenna conductive elements. Three samples of a 2.45 GHz microstrip patch antenna have been 3D-printed by using different approaches and materials, and their performance evaluated and compared. In particular, the same dielectric substrate printed in polylactic acid (PLA) has been adopted in all cases, whilst copper tape and two different conductive filaments have been used to realize the conductive parts of the three antenna samples, respectively. Even if an expected radiation efficiency reduction has been observed for the conductive filament case, the comparative analysis clearly demonstrates that 3D-printing technology can be exploited to design working fully-printed antennas, including the conductive parts
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