1,721,948 research outputs found
Analysis of the chaotic behavior of the lower hybrid wave propagation in magnetised plasma by Hamiltonian theory
No full textlicensee MDPI, Basel, Switzerland.The Hamiltonian character of the ray tracing equations describing the propagation of the Lower Hybrid Wave (LHW) in a magnetic confined plasma device (tokamak) is investigated in order to study the evolution of the parallel wave number along the propagation path. The chaotic diffusion of the “time-averaged” parallel wave number at higher values (with respect to that launched by the antenna at the plasma edge) has been evaluated, in order to find an explanation of the filling of the spectral gap (Fisch, 1987) by “Hamiltonian chaos” in the Lower Hybrid Current Drive (LHCD) experiments (Fisch, 1978). The present work shows that the increase of the parallel wave number n‖ due to toroidal effects, in the case of the typical plasma parameters of the Frascati Tokamak Upgrade (FTU) experiment, is insufficient to explain the filling of the spectral gap, and the consequent current drive and another mechanism must come into play to justify the wave absorption by Landau damping. Analytical calculations have been supplemented by a numerical algorithm based on the symplectic integration of the ray equations implemented in a ray tracing code, in order to preserve exactly the symplectic character of a Hamiltonian flow. © 2016 by the author
Reference distribution functions for magnetically confined plasmas from the minimum entropy production theorem and the MaxEnt principle, subject to the scale-invariant restrictions
No full textWe derive the expression of the reference distribution function for magnetically confined plasmas far from the thermodynamic equilibrium. The local equilibrium state is fixed by imposing the minimum entropy production theorem and the maximum entropy (MaxEnt) principle, subject to scale invariance restrictions. After a short time, the plasma reaches a state close to the local equilibrium. This state is referred to as the reference state. The aim of this Letter is to determine the reference distribution function (RDF) when the local equilibrium state is defined by the above mentioned principles. We prove that the RDF is the stationary solution of a generic family of stochastic processes corresponding to an universal Landau-type equation with white parametric noise. As an example of application, we consider a simple, fully ionized, magnetically confined plasmas, with auxiliary Ohmic heating. The free parameters are linked to the transport coefficients of the magnetically confined plasmas, by the kinetic theory. © 2013 Elsevier B.V
Family of probability distributions derived from maximal entropy principle with scale invariant restrictions
No full textUsing statistical thermodynamics, we derive a general expression of the stationary probability distribution for thermodynamic systems driven out of equilibrium by several thermodynamic forces. The local equilibrium is defined by imposing the minimum entropy production and the maximum entropy principle under the scale invariance restrictions. The obtained probability distribution presents a singularity that has immediate physical interpretation in terms of the intermittency models. The derived reference probability distribution function is interpreted as time and ensemble average of the real physical one. A generic family of stochastic processes describing noise-driven intermittency, where the stationary density distribution coincides exactly with the one resulted from entropy maximization, is presented. © 2013 American Physical Society
Analysis of the thermonuclear instability including low-power ICRH minority heating in IGNITOR
No full textThe nonlinear thermal balance equation for classical plasma in a toroidal geometry is analytically and numerically investigated including ICRH power. The determination of the equilibrium temperature and the analysis of the stability of the solution are performed by solving the energy balance equation that includes the transport relations obtained by the classical kinetic theory. An estimation of the confinement time is also provided. We show that the ICRH heating in the IGNITOR experiment, among other applications, is expected to be used to trigger the thermonuclear instability. Here a scenario is considered where IGNITOR is led to operate in a slightly sub-critical regime by adding a small fraction of 3He to the nominal 50%-50% deuterium-tritium mixture. The difference between power lost and alpha heating is compensated by additional ICRH heating, which should be able to increase the global plasma temperature via collisions between 3He minority and the background D-T ions. © 2015 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg
Study of lower hybrid wave propagation in ionized gas by Hamiltonian theory
No full textIn order to find an approximate solution to the Vlasov-Maxwell equation system describing the lower hybrid wave propagation in magnetic confined plasmas, the use of the WKB method leads to the ray tracing equations. The Hamiltonian character of the ray tracing equations is investigated analytically and numerically in order to deduce the physical properties of the wave propagating without absorption in the confined plasma. The consequences of the Hamiltonian character of the equations on the travelling wave, in particular, on the evolution of the parallel wavenumber along the propagation path have been accounted and the chaotic diffusion of the timeaveraged parallel wave-number towards higher values has been evaluated. Numerical analysis by means of a Runge-Kutta based algorithm implemented in a ray tracing code supplies the analytical considerations. A numerical tool based on the symplectic integration of the ray trajectories has been developed. © 2014 American Institute of Physics
Distribution function for plasma with RF heating from quasilinear Fokker-Planck equation
No full textAuxiliary plasma heating by radio-frequency waves is a usual procedure in the modern tokamaks. In the case of 3He minority heating is analysed the equilib-rium distribution function for minority species and the factor modification of the Maxwellian form is plotted for specific parameters values as function of radial coordinate and normalized velocity. The energetic minority tail develops with the heating
Ray-tracing WKB analysis of Whistler waves in non-uniform magnetic fields applied to space thrusters
No full textRadiofrequency magnetized cylindrical plasma sources are proposed for the development of space thrusters, whose thrust efficiency and specific impulse depend on the power coupled into the plasma. At this stage of research, emphasis has been on the absorption of Whistler wave energy by non-uniform plasmas but not much on the role played by the magneto-static confinement field, considered uniform, constant and aligned with the axis of the source. We present RAYWh (RAY-tracing Whistler), a three-dimensional (3D) ray-tracing solver for electromagnetic propagation and power deposition in cylindrical plasma sources for space plasma thrusters, where actual magnetic confinement configurations along with plasma density profiles are included. The propagation and absorption of Whistler waves are investigated by solving the 3D Maxwell-Vlasov model equations by a Wentzel-Kramers-Brillouin (WKB) asymptotic expansion. The reduced set of equations for the wave phase and for the square amplitude of the electric field is solved numerically by means of a modified Runge-Kutta algorithm. Unexpected cut-offs, resonances, radial reflections, mode conversions and power deposition profile of the excited waves are found, when realistic confinement magnetic fields are considered. An analysis of the influence of axial wavenumbers and the axial length of the system on the power deposition is presented. © 2014 IOP Publishing Ltd
Asymptotic analysis of the whistler waves propagation in space plasma thrusters
No full textWe present a three-dimensional Ray-Tracing solver, called RAYWh (RAY-tracing Whistler), for the electromagnetic propagation and power deposition in plasma sources for space thrusters, where high density plasmas (ranging from 1017 to 1019 particles/m3) are confined by general magnetic configurations with magnitude below < 0.15T. The 3D Maxwell-Vlasov equations are solved by means of a WKB asymptotic expansion, to investigate the propagation and absorption of whistler waves (excitation frequency is 13.56 MHz) under the influence of general confinement magnetic field, and axisymmetric realistic density profiles. The reduced set of the WKB equations for the wave phase and for the square amplitude of the electric field are solved numerically by means of Hamming's modified predictor-corrector method; the verification of WKB hypothesis are monitored during the simulation. The Ray Tracing approach is employed - for the first time - in the analysis of plasma sources for space plasma thrusters. A direct comparison between common helicon sources with axial, constant and uniform confinement magnetic field, and plasma sources with actual confinement magnetic field lines revealed a propagative picture with unconventional mode conversions, cut-offs and resonances inside the source that affect the power deposition
On the interaction of ion Bernstein waves with alpha particles
No full textAlpha channelling is a possible mechanism to transfer the energy of the fusion generated alpha particles directly to the thermal ion population through a wave propagating in the plasma. The interaction of ion Bernstein waves with a population of alpha particles is investigated to determine the conditions that maximize the absorption of the wave by the thermal ions and minimize the absorption by the thermal electrons under reactor relevant conditions. The ray trajectories equations are analytically and numerically solved beyond the mode conversion layer and the optimal conditions for alpha channelling are determined
Tritium Minority Heating with Mode Conversion of Fast Waves
No full textA new ion heating scenario in tokamak plasmas, based on cyclotron damping of ion Bernstein waves (IBW) by Tritium minority at the first cyclotron harmonic, is discussed in a theoretical framework. The IBW are coupled by mode conversion of fast magnetosonic waves in a D(H) plasma. The mode conversion layer is located near the center of the plasma column provided the D/H density ratio approaches unity, and close to the resonant layer of the Tritium minority. A specific scenario for the JET tokamak, has been analyzed by the code TORIC. As a result, efficient heating of the Tritium minority is expected. © 2008 American Institute of Physics
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