50,773 research outputs found
Breakdown of adiabatic invariance in spherical tokamaks
Thermal ions in spherical tokamaks have two adiabatic invariants: the
magnetic moment and the longitudinal invariant. For hot ions, variations in
magnetic-field strength over a gyro period can become sufficiently large to
cause breakdown of the adiabatic invariance. The magnetic moment is more
sensitive to perturbations than the longitudinal invariant and there exists an
intermediate regime, super-adiabaticity, where the longitudinal invariant
remains adiabatic, but the magnetic moment does not. The motion of
super-adiabatic ions remains integrable and confinement is thus preserved.
However, above a threshold energy, the longitudinal invariant becomes
non-adiabatic too, and confinement is lost as the motion becomes chaotic. We
predict beam ions in present-day spherical tokamaks to be super-adiabatic but
fusion alphas in proposed burning-plasma spherical tokamaks to be
non-adiabatic.Comment: 6 pages, 8 figure
Similarity-Aware Spectral Sparsification by Edge Filtering
In recent years, spectral graph sparsification techniques that can compute
ultra-sparse graph proxies have been extensively studied for accelerating
various numerical and graph-related applications. Prior nearly-linear-time
spectral sparsification methods first extract low-stretch spanning tree from
the original graph to form the backbone of the sparsifier, and then recover
small portions of spectrally-critical off-tree edges to the spanning tree to
significantly improve the approximation quality. However, it is not clear how
many off-tree edges should be recovered for achieving a desired spectral
similarity level within the sparsifier. Motivated by recent graph signal
processing techniques, this paper proposes a similarity-aware spectral graph
sparsification framework that leverages efficient spectral off-tree edge
embedding and filtering schemes to construct spectral sparsifiers with
guaranteed spectral similarity (relative condition number) level. An iterative
graph densification scheme is introduced to facilitate efficient and effective
filtering of off-tree edges for highly ill-conditioned problems. The proposed
method has been validated using various kinds of graphs obtained from public
domain sparse matrix collections relevant to VLSI CAD, finite element analysis,
as well as social and data networks frequently studied in many machine learning
and data mining applications
Variational calculations for resonance oscillations of inhomogeneous plasmas
The electrostatic resonance properties of an inhomogeneous plasma column are reported by application of the Rayleigh-Ritz method. A description of the rf equation of motion and pressure term that expresses the system of equations in Euler-Lagrange form is presented. The Rayleigh-Ritz procedure is applied to the corresponding Lagrangian to obtain approximate resonance frequencies and eigenfunctions. An appropriate set of trial coordinate functions is defined, which leads to frequency and eigenfunction estimates
The Temporal and Spectral Characteristics of "Fast Rise and Exponential Decay" Gamma-Ray Burst Pulses
In this paper we have analyzed the temporal and spectral behavior of 52 Fast
Rise and Exponential Decay (FRED) pulses in 48 long-duration gamma-ray bursts
(GRBs) observed by the CGRO/BATSE, using a pulse model with two shape
parameters and the Band model with three shape parameters, respectively. It is
found that these FRED pulses are distinguished both temporally and spectrally
from those in long-lag pulses. Different from these long-lag pulses only one
parameter pair indicates an evident correlation among the five parameters,
which suggests that at least 4 parameters are needed to model burst
temporal and spectral behavior. In addition, our studies reveal that these FRED
pulses have correlated properties: (i) long-duration pulses have harder spectra
and are less luminous than short-duration pulses; (ii) the more asymmetric the
pulses are the steeper the evolutionary curves of the peak energy () in
the spectrum within pulse decay phase are. Our statistical
results give some constrains on the current GRB models.Comment: 18 pages, 7 figures, accepted for publication in the Astrophysical
Journa
Establishing the Relative Merits of Interior and Spoke-Type Permanent-Magnet Machines With Ferrite or NdFeB Through Systematic Design Optimization
In this paper, a multiobjective design optimization method combining design-of-experiments techniques and differential-evolution algorithms is presented. The method was implemented and utilized in order to provide practical engineering insights for the optimal design of interior and spoke-type permanent-magnet machines. Two combinations with 12 slots and 8 poles and 12 slots and 10 poles, respectively, have been studied in conjunction with rare-earth neodymium-iron-boron (NdFeB) and ferrites. As part of the optimization process, a computationally efficient finite-element electromagnetic analysis was employed for estimating the performance of thousands of candidate designs. Three optimization objectives were concurrently considered for minimum total material cost, power losses, and torque ripple, respectively. Independent variables were considered for both the stator and rotor geometries. A discussion based on a systematic comparison is included, showing, among other things and despite common misconception, that comparable cost versus loss Paretos can be achieved with any of the rotor topologies studied
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Monovision LASIK in emmetropic presbyopic patients.
BackgroundTo evaluate the efficacy and patient satisfaction of laser in situ keratomileusis (LASIK) monovision correction in presbyopic emmetropic patients.MethodsA retrospective review of 294 patients who underwent LASIK for monovision was conducted. All patients had preoperative uncorrected distance visual acuity in each eye of 20/25 or better in both eyes and underwent primary LASIK treatment in one eye with a near target; 82 patients underwent surgery in the distant eye for hypermetropia. Patients completed a patient-reported-outcome questionnaire at their one-month postoperative visit. Analysis was performed on a per patient basis with a logistic regression model.ResultsPatients achieved a postoperative mean spherical equivalent of -0.05 diopters (D) in the distant eye and -1.92 D in the near eye. Prior to surgery, 64.7% (n=178) of patients reported they were satisfied or very satisfied with their vision; postoperatively, this increased to 85.4% (n=251). The greatest predictor of dissatisfaction after surgery was severe patient-reported visual phenomena (glare, halos, starbursts, ghosting) (odds ratio 1.18, P=0.001).ConclusionsLASIK monovision for presbyopic patients with low refractive error and good preoperative uncorrected distance visual acuity is both safe and effective with high patient satisfaction. Patients who were dissatisfied in the postoperative period tended to be those with postoperative visual symptoms
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