Search results for "Plasmas"

Analytically solvable 2×2 PT -symmetry dynamics from su(1,1)-symmetry problems

2019

A protocol for explicitly constructing the exact time-evolution operators generated by $2\ifmmode\times\else\texttimes\fi{}2$ time-dependent $PT$-symmetry Hamiltonians is reported. Its mathematical applicability is illustrated with the help of appropriate examples. The physical relevance of the proposed approach within gain-loss system scenarios, like two coupled waveguides, is discussed in detail.

Electron recombination with tungsten ions with open f-shells

2017

We calculate the electron recombination rates with target ions W$^{q+}$, $q = 18$ -- $25$, as functions of electron energy and electron temperature (i.e. the rates integrated over the Maxwellian velocity distribution). Comparison with available experimental data for W$^{18+}$, W$^{19+}$, and W$^{20+}$ is used as a test of our calculations. Our predictions for W$^{21+}$, W$^{22+}$, W$^{23+}$, W$^{24+}$, and W$^{25+}$ (where the experimental data are not available) may be used for plasma modelling in thermonuclear reactors. The results for the temperature dependent rates for each ion are fitted with the standard analytical expressions to make them easy to use. All of these ions have an open e…

3-D Shielding Analyses of the Vertical and Mid-Plane Ports in ITER

1996

A three dimensional (3-D) shielding analysis of the International Thermonuclear Experimental Reactor (ITER) has been performed with the aim of calculating the nuclear heating on the magnet system, correlating it to the existing vertical and horizontal ports. When these openings are left unshielded, more than 50 kW are calculated for the upper half of Toroidal Field Coil system and two of the Poloidal Field Coils. A simple plug, with same thickness as of the vacuum vessel can lower the heating to meet the imposed requirements. 5 refs., 6 figs., 4 tabs.

Magnetic configuration effects on the Wendelstein 7-X stellarator

2018

The two leading concepts for confining high-temperature fusion plasmas are the tokamak and the stellarator. Tokamaks are rotationally symmetric and use a large plasma current to achieve confinement, whereas stellarators are non-axisymmetric and employ three-dimensionally shaped magnetic field coils to twist the field and confine the plasma. As a result, the magnetic field of a stellarator needs to be carefully designed to minimize the collisional transport arising from poorly confined particle orbits, which would otherwise cause excessive power losses at high plasma temperatures. In addition, this type of transport leads to the appearance of a net toroidal plasma current, the so-called boot…

Pattern selection in the 2D FitzHugh–Nagumo model

2018

We construct square and target patterns solutions of the FitzHugh–Nagumo reaction–diffusion system on planar bounded domains. We study the existence and stability of stationary square and super-square patterns by performing a close to equilibrium asymptotic weakly nonlinear expansion: the emergence of these patterns is shown to occur when the bifurcation takes place through a multiplicity-two eigenvalue without resonance. The system is also shown to support the formation of axisymmetric target patterns whose amplitude equation is derived close to the bifurcation threshold. We present several numerical simulations validating the theoretical results.

Even harmonics generation of high frequency radiation in current-carrying plasmas

2005

Generation of high frequency radiation harmonics in a current-carrying plasma is studied. The physical mechanism responsible for harmonics generation is provided by electron-ion collisions. The current in the plasma is sustained by a constant electric field. It is shown that the electron distribution function anisotropy due to the static field yields generation of even harmonics. As a result, the radiation spectrum emitted by the current-carrying plasma contains both even and odd harmonics, the latter being attributed to currentless plasma. For a broad range of plasma and high frequency radiation parameters, a detailed analysis of the even harmonics properties is reported.

Hidden Oscillations in Electromechanical Systems

2016

In this paper an electromechanical system with two different types of motor is considered. It is shown that during the spin-up, the system with DC motor may experience unwanted vibration—the Sommerfeld effect. This is a well-known effect when the motor of electromechanical system gets stuck near the resonance zone instead of reaching its nominal power. The absence of this effect is demonstrated in the system with synchronous motor. Nowadays, there are many works devoted to the study of this effect in various systems. Here we discuss the Sommerfeld effect from the point of view of localization of the so-called hidden oscillations.

Mixed-mode oscillation-incrementing bifurcations and a devil’s staircase from a nonautonomous, constrained Bonhoeffer-van der Pol oscillator

2018

Engineering the Success of Quantum Walk Search Using Weighted Graphs

2016

Continuous-time quantum walks are natural tools for spatial search, where one searches for a marked vertex in a graph. Sometimes, the structure of the graph causes the walker to get trapped, such that the probability of finding the marked vertex is limited. We give an example with two linked cliques, proving that the captive probability can be liberated by increasing the weights of the links. This allows the search to succeed with probability 1 without increasing the energy scaling of the algorithm. Further increasing the weights, however, slows the runtime, so the optimal search requires weights that are neither too weak nor too strong.

Faster Quantum Walk Search on a Weighted Graph

2015

A randomly walking quantum particle evolving by Schr\"odinger's equation searches for a unique marked vertex on the "simplex of complete graphs" in time $\Theta(N^{3/4})$. In this paper, we give a weighted version of this graph that preserves vertex-transitivity, and we show that the time to search on it can be reduced to nearly $\Theta(\sqrt{N})$. To prove this, we introduce two novel extensions to degenerate perturbation theory: an adjustment that distinguishes the weights of the edges, and a method to determine how precisely the jumping rate of the quantum walk must be chosen.