3,146 research outputs found
Robust Non-Linear Regression Using The Dogleg Algorithm
What are the statistical and computational problems associated with robust nonlinear regression? This paper presents a number of possible approaches to these problems and develops a particular algorithm based on the work of Powell and Dennis.
Development of Rebunching Cavities at IAP
A focus of work at IAP has been the development and optimization of spiral
loaded cavities since the 1970s [A. Schempp et al, NIM 135, 409 (1976)]. These
cavities feature a high efficiency, a compact design and a big variety of
possible fields of application. They find use both as bunchers and post
accelerators to vary the final energy of the beam. In comparison to other
available designs, the advantage of these structures lies in their small size.
Furthermore they can easily be tuned to the required resonance frequency by
varying the length of the spiral. Due to the small size of the cavities the
required budget can also be kept low. Here, two slightly different types of
spiral loaded cavities, which were built for the REX-ISOLDE project at CERN and
the intensity upgrade program at GSI are being discussed.Comment: 4 pages, 9 figures PRST-AB special LINAC 2000 edition with additional
information in comparison to the 3 pages LINAC paper physics/000708
Direct Measurements of Magnetic Twist in the Solar Corona
In the present work we study evolution of magnetic helicity in the solar
corona. We compare the rate of change of a quantity related to the magnetic
helicity in the corona to the flux of magnetic helicity through the photosphere
and find that the two rates are similar. This gives observational evidence that
helicity flux across the photosphere is indeed what drives helicity changes in
solar corona during emergence.
For the purposes of estimating coronal helicity we neither assume a strictly
linear force-free field, nor attempt to construct a non-linear force-free
field. For each coronal loop evident in Extreme Ultraviolet (EUV) we find a
best-matching line of a linear force-free field and allow the twist parameter
alpha to be different for each line. This method was introduced and its
applicability was discussed in Malanushenko et. al. (2009).
The object of the study is emerging and rapidly rotating AR 9004 over about
80 hours. As a proxy for coronal helicity we use the quantity
averaged over many reconstructed lines of magnetic field. We argue that it is
approximately proportional to "flux-normalized" helicity H/Phi^2, where H is
helicity and Phi is total enclosed magnetic flux of the active region. The time
rate of change of such quantity in the corona is found to be about 0.021
rad/hr, which is compatible with the estimates for the same region obtained
using other methods Longcope et. al. (2007), who estimated the flux of
normalized helicity of about 0.016 rad/hr
Quantum-state extraction from high-Q cavities
The problem of extraction of a single-mode quantum state from a high-Q cavity
is studied for the case in which the time of preparation of the quantum state
of the cavity mode is short compared with its decay time. The temporal
evolution of the quantum state of the field escaping from the cavity is
calculated in terms of phase-space functions. A general condition is derived
under which the quantum state of the pulse built up outside the cavity is a
nearly perfect copy of the quantum state the cavity field was initially
prepared in. The results show that unwanted losses prevent the realization of a
nearly perfect extraction of nonclassical quantum states from high-Q optical
microcavities with presently available technology.Comment: RevTeX4, 9 pages with 6 figures; extended version as submitted to
Phys. Rev.
Determination of quantum-noise parameters of realistic cavities
A procedure is developed which allows one to measure all the parameters
occurring in a complete model [A.A. Semenov et al., Phys. Rev. A 74, 033803
(2006); quant-ph/0603043] of realistic leaky cavities with unwanted noise. The
method is based on the reflection of properly chosen test pulses by the cavity.Comment: 5 pages, 2 figure
Nonclassical Moments and their Measurement
Practically applicable criteria for the nonclassicality of quantum states are
formulated in terms of different types of moments. For this purpose the moments
of the creation and annihilation operators, of two quadratures, and of a
quadrature and the photon number operator turn out to be useful. It is shown
that all the required moments can be determined by homodyne correlation
measurements. An example of a nonclassical effect that is easily characterized
by our methods is amplitude-squared squeezing.Comment: 12 pages, 6 figure
Solar Magnetic Tracking. IV. The Death of Magnetic Features
The removal of magnetic flux from the quiet-sun photosphere is important for
maintaining the statistical steady-state of the magnetic field there, for
determining the magnetic flux budget of the Sun, and for estimating the rate of
energy injected into the upper solar atmosphere. Magnetic feature death is a
measurable proxy for the removal of detectable flux. We used the SWAMIS feature
tracking code to understand how nearly 20000 detected magnetic features die in
an hour-long sequence of Hinode/SOT/NFI magnetograms of a region of quiet Sun.
Of the feature deaths that remove visible magnetic flux from the photosphere,
the vast majority do so by a process that merely disperses the
previously-detected flux so that it is too small and too weak to be detected.
The behavior of the ensemble average of these dispersals is not consistent with
a model of simple planar diffusion, suggesting that the dispersal is
constrained by the evolving photospheric velocity field. We introduce the
concept of the partial lifetime of magnetic features, and show that the partial
lifetime due to Cancellation of magnetic flux, 22 h, is 3 times slower than
previous measurements of the flux turnover time. This indicates that prior
feature-based estimates of the flux replacement time may be too short, in
contrast with the tendency for this quantity to decrease as resolution and
instrumentation have improved. This suggests that dispersal of flux to smaller
scales is more important for the replacement of magnetic fields in the quiet
Sun than observed bipolar cancellation. We conclude that processes on spatial
scales smaller than those visible to Hinode dominate the processes of flux
emergence and cancellation, and therefore also the quantity of magnetic flux
that threads the photosphere.Comment: Accepted by Ap
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