Thruster Allocation for Dynamical Positioning

Positioning a vessel at a fixed position in deep water is of great importance when working offshore. In recent years a Dynamical Positioning (DP) system was developed at Marin [2]. After the measurement of the current position and external forces (like waves, wind etc.), each thruster of the vessel is actively controlled to hold the desired location. In this paper we focus on the allocation process to determine the settings for each thruster that results in the minimal total power and thus fuel consumption. The mathematical formulation of this situation leads to a nonlinear optimization problem with equality and inequality constraints, which can be solved by applying Lagrange multipliers. We give three approaches: first of all, the full problem was solved using the MATLAB fmincon routine with the solution from the linearised problem as a starting point. This implementation, with robust handling of the situations where the thrusters are overloaded, lead to promising results: an average reduction in fuel consumption of approximately two percent. However, further analysis proved useful. A second approach changes the set of variables and so reduces the number of equations. The third and last approach solves the Lagrange equations with an iterative method on the linearized Lagrange problem

Curved planar quantum wires with Dirichlet and Neumann boundary conditions

We investigate the discrete spectrum of the Hamiltonian describing a quantum particle living in the two-dimensional curved strip. We impose the Dirichlet and Neumann boundary conditions on opposite sides of the strip. The existence of the discrete eigenvalue below the essential spectrum threshold depends on the sign of the total bending angle for the asymptotically straight strips.Comment: 7 page

Hysteresis phenomenon in deterministic traffic flows

We study phase transitions of a system of particles on the one-dimensional integer lattice moving with constant acceleration, with a collision law respecting slower particles. This simple deterministic ``particle-hopping'' traffic flow model being a straightforward generalization to the well known Nagel-Schreckenberg model covers also a more recent slow-to-start model as a special case. The model has two distinct ergodic (unmixed) phases with two critical values. When traffic density is below the lowest critical value, the steady state of the model corresponds to the ``free-flowing'' (or ``gaseous'') phase. When the density exceeds the second critical value the model produces large, persistent, well-defined traffic jams, which correspond to the ``jammed'' (or ``liquid'') phase. Between the two critical values each of these phases may take place, which can be interpreted as an ``overcooled gas'' phase when a small perturbation can change drastically gas into liquid. Mathematical analysis is accomplished in part by the exact derivation of the life-time of individual traffic jams for a given configuration of particles.Comment: 22 pages, 6 figures, corrected and improved version, to appear in the Journal of Statistical Physic

New insights into the generalized Rutherford equation for nonlinear neoclassical tearing mode growth from 2D reduced MHD simulations

Two dimensional reduced MHD simulations of neoclassical tearing mode growth and suppression by ECCD are performed. The perturbation of the bootstrap current density and the EC drive current density perturbation are assumed to be functions of the perturbed ux surfaces. In the case of ECCD, this implies that the applied power is ux surface averaged to obtain the EC driven current density distribution. The results are consistent with predictions from the generalized Rutherford equation using common expressions for and . These expressions are commonly perceived to describe only the effect on the tearing mode growth of the helical component of the respective current perturbation acting through the modi cation of Ohm’s law. Our results show that they describe in addition the effect of the poloidally averaged current density perturbation which acts through modi cation of the tearing mode stability index. Except for modulated ECCD, the largest contribution to the mode growth comes from this poloidally averaged current density perturbation

Primary radiotherapy in progressive optic nerve sheath meningiomas: a long-term follow-up study

Background/aims: To report the outcome of primary radiotherapy in patients with progressive optic nerve sheath meningioma (ONSM). Methods: The clinical records of all patients were reviewed in a retrospective, observational, multicentre study. Results: Thirty-four consecutive patients were included. Twenty-six women and eight men received conventional or stereotactic fractionated radiotherapy, and were followed for a median 58 (range 51–156) months. Fourteen eyes (41%) showed improved visual acuity of at least two lines on the Snellen chart. In 17 (50%) eyes, the vision stabilised, while deterioration was noted in three eyes (9%). The visual outcome was not associated with age at the time of radiotherapy (p=0.83), sex (p=0.43), visual acuity at the time of presentation (p=0.22) or type of radiotherapy (p=0.35). Optic disc swelling was associated with improved visual acuity (p<0.01) and 4/11 patients with optic atrophy also showed improvement. Long-term complications were dry eyes in five patients, cataracts in three, and mild radiation retinopathy in four. Conclusion: Primary radiotherapy for patients with ONSM is associated with long-term improvement of visual acuity and few adverse effects.Peerooz Saeed, Leo Blank, Dinesh Selva, John G. Wolbers, Peter J.C.M. Nowak, Ronald B. Geskus, Ezekiel Weis, Maarten P. Mourits, Jack Rootma

Redistribution of fast ions during sawtooth reconnection

In a tokamak-based fusion power plant, possible scenarios may include regulated sawtooth oscillations to remove thermalized helium from the core of the plasma. During a sawtooth crash, the helium ash and other impurities trapped in the core are driven by the instability to an outer region. However, in a fusion plasma, high energy ions will represent a significant population. We thus study the behaviour of these energetic particles during a sawtooth. This paper presents the modelling of the redistribution of fast ions during a sawtooth reconnection event in a tokamak plasma. Along the lines of the model for the evolution of the flux surfaces during a sawtooth collapse described in Ya.I. Kolesnichenko and Yu.V. Yakovenko 1996 Nucl. Fusion 36 159, we have built a time-dependent electromagnetic model of a sawtooth reconnection. The trajectories of the ions are described by a complete gyro-orbit integration. The fast particles were evolved from specific initial parameters (given energy and uniform spread in pitch) or distributed initially according to a slowing-down distribution created by fusion reactions. Our modelling is used to understand the main equilibrium parameters driving the motions during the collapse and to determine the evolution of the distribution function of energetic ions when different geometries of reconnection are considered.</p

Space filling by nucleation and growth in chemical vapor deposition of diamond

Phase transformations, including chemical vapor deposition (CVD) of diamond, taking place by nucleation and growth are commonly described by Avrami or Johnson-Mehl type models. In order to avoid the restrictions of such models with respect to assumptions concerning nucleation rates and growth velocities, the variation with time of nucleation and growth of diamond particles during the deposition of microwave plasma-assisted CVD was studied. The size distributions obtained from image analysis enabled us to trace back details of the nucleation and growth history. Three sources of particle formation were operating during deposition. A general growth law suitable for all particles did not exist. These observations limited the applicability of Avrami-type models to describe space filling. Computer simulation of surface coverage and particle growth was successful because one particular mode of particle formation and growth dominated surface coverage. Based on image analysis and the determination of the film growth rate, the evolution of the diamond volume fraction with time, starting from three-dimensional particle growth followed by a continuous transition to one-dimensional film growth, was describe

On the discovery of doubly-magic 48^{48}Ni

The paper reports on the first observation of doubly-magic Nickel-48 in an experimental at the SISSI/LISE3 facility of GANIL. Four Nickel-48 isotopes were identified. In addition, roughly 100 Nickel-49, 50 Iron-45, and 290 Chromium-42 isotopes were observed. This opens the possibility to search for two-proton emission from these nuclei.Comment: 4 pages, 3 figures, accepted for publication in Phys. Rev. Let

High-precision measurement of the half-life of 62^{62}Ga

The beta-decay half-life of 62Ga has been studied with high precision using on-line mass separated samples. The decay of 62Ga which is dominated by a 0+ to 0+ transition to the ground state of 62Zn yields a half-life of T_{1/2} = 116.19(4) ms. This result is more precise than any previous measurement by about a factor of four or more. The present value is in agreement with older literature values, but slightly disagrees with a recent measurement. We determine an error weighted average value of all experimental half-lives of 116.18(4) ms.Comment: 9 pages, 5 figures, accepted for publication in PR