MHD spectroscopy through detecting toroidal Alfvén eigenmodes and Alfvén wave cascades

Toroidal Alfvén eigenmodes and a new class of modes, the Alfvén cascades, excited by energetic ions are observed in the JET tokamak. Measurements of these Alfvén instabilities are used to solve the inverse problem of identifying the plasma parameters using the dependence of the Alfvén spectrum on equilibrium parameters.     

Double-gap Alfvén eigenmodes: revisiting eigenmode interaction with the alfvén continuum

A new type of global shear Alfvén eigenmode is found in tokamak plasmas where the mode localization is in the region intersecting the Alfvén continuum. The eigenmode is formed by the coupling of two solutions from two adjacent gaps (akin to potential wells) in the shear Alfvén continuum. For tokamak plasmas with reversed magnetic shear, it is shown that the toroidicity-induced solution tunnels through the continuum to match the ellipticity-induced Alfvén eigenmode so that the resulting solution is continuous at the point of resonance with the continuum. The existence of these double-gap Alfvén eigenmodes allows for potentially new ways of coupling edge fields to the plasma core in conditions where the core region is conventionally considered inaccessible. Implications include new approaches to heating and current drive in fusion plasmas as well as its possible use as a core diagnostic in burning plasmas.     

Comportement thermique d'un générateur à sorption solide

The thermal behaviour of a solid sorption generator of active carbon/alcohol machine, is studied during heating and cooling phases with a preheated air flow. A bicylindrical walls generator that contains 0.9 kg of a granular adsorbent in the presence of residual gas is tested during a cycle of an average duration of 3 hours; the grains of active carbon are rod-shaped of 0.003 m diameter and 0.008 m average length. The thermal contact conductance of adsorbent to the wall has an important influence on the rate of heat transfer between the generator and the external source of heat. In the absence of alcohol, heat transfer occuring without mass transfer in active carbon is essentially due to the conduction. A numerical bidimensional model allows one to justify experimentally the observed evolution and proposes thermal contact conductance between active carbon pellets and the generator wall. A parametric study of the thermal contact conductance gives 6.5 W·m⁻²·K⁻¹ as the best value. A simulation of heating and cooling phases with average conductance values between 5 and 30 W·m⁻²·K⁻¹ gives model estimated heating and cooling phases duration.     

Nonlinear Coupling of Reversed Shear Alfvén Eigenmode and Toroidal Alfvén Eigenmode during Current Ramp

Two novel nonlinear mode coupling processes for reversed shear Alfvén eigenmode(RSAE) nonlinear saturation are proposed and investigated. In the first process, the RSAE nonlinearly couples to a co-propagating toroidal Alfvén eigenmode(TAE) with the same toroidal and poloidal mode numbers, and generates a geodesic acoustic mode. In the second process, the RSAE couples to a counter-propagating TAE and generates an ion acoustic wave quasi-mode. The condition for the two processes to occur is favored during current ramp. Both the processes contribute to effectively saturate the Alfvénic instabilities, as well as nonlinearly transfer of energy from energetic fusion alpha particles to fuel ions in burning plasmas.     

Observation of odd toroidal Alfvén eigenmodes

Experimental evidence is presented for the existence of the theoretically predicted odd toroidicity induced Alfvén eigenmode (TAE) from the simultaneous appearance of odd and even TAEs in a normal shear discharge of the joint European torus. The modes are observed in low central magnetic shear plasmas created by injecting lower hybrid current drive. A fast ion population was created by applying ion cyclotron heating at the high-field side to excite the TAEs. The odd TAEs were identified from their frequency, mode number, and timing relative to the even TAEs.     

Numerical simulation of the multiple reversed shear Alfvén eigenmodes associated with the triangularity Alfvén gap

It was found that there are multiplicity of low shear toroidicity-induced Alfv′en eigenmodes in a zero beta limit if the inverse aspect ratio is larger than the magnetic shear at the mode location(Candy 1996 Phys. Lett. A 215 299). Because the reversed shear Alfv′en eigenmode(RSAE) and even the RSAE associated with the non-circular triangularity-induced Alfv′en eigenmode(NAE) gap(NAE–RSAE) usually reside near the shear-reversal point, the condition that the inverse aspect ratio is larger than the magnetic shear is naturally satisfied. For this reason, we numerically investigate the existence of multiplicity of core-localized NAE–RSAEs and mode characteristics in the present work. We firstly verify the existence of the multiplicity for zero beta plasma by using a D-shaped equilibrium. It is pointed out that, for a given toroidal mode number, the Alfv′en cascade spectrum accommodates down-sweeping and up-sweeping modes above and below the NAE range of frequencies. An analytical model for the existence of multiple RSAE modes is in good agreement with the simulation results. One notices that the triangularity has a greater effect on the odd-type modes than that on the even-type modes: the odd-type modes come into existence because of the plasma triangularity.     

Parameterization of the nuclear Hulthén potentials

Within the formalism of supersymmetry-inspired factorization method, a two-term nuclear Hulthén potential has been developed and parameterized to reproduce the nucleon–nucleon scattering phase shifts for P and D partial wave states.     

Conjugaisons finies et milieux continus en relativité générale

We present here the main elements of a mechanics of relativistic continua relying upon a concept of 《finite conjugacy》 between two relativistic motions described by two unit vector-fields u and u' defined on two different relativistic manifolds $\text{M}$-and $\text{M}$'.This purely relativistic, global, and intrinsic theory leads, together with a new approach of the deformation tensors in relativity, to a differential system of equations for the conjugacies which is neither under-determined nor over-determined. A rough study of the propagation of the conjugacy-waves shows then that it is advisable to consider the notion of a finite conjugacy as a satisfying relativistic extension of the classical and tridimensional notion of a finite deformation in mechanics, and to identify the spatial conjugacy-waves obtained with the ordinary acoustic waves.Drastic particularizations of the space-times $\text{M}$ and $\text{M}$', of the motions u and u', of the admissible types of conjugacies and of the elastic behaviour of the continua under study allow to recover, as very important but particular cases, the tridimensional non-relativistic theory of elasticity for finite deformations and non-linear behaviour, as well as the main theories of relativistic elasticity already proposed by Mmes Choquet-Bruhat and Lamoureux-Brousse, Rayner, Carter and Carter-Quintana, Grot-Eringen…. The obtained system of equations generalizes also to the finite case some aspects of the infinitesimal theory of Weber and Papapetrou.     

Laboratory realization of an Alfvén wave maser

It is demonstrated that a frequency selective Alfvén-wave resonator can be realized by applying a nonuniform magnetic field to a plasma region bounded between a cathode and a semitransparent mesh anode. When a current threshold is exceeded, selective amplification results in a highly coherent, large amplitude wave that propagates into an adjacent plasma column.     

Chaotic regime of Alfvén Eigenmode wave-particle interaction

The chaotic regime in Alfvén eigenmode wave-particle interaction is identified for the first time in the tokamak plasma of the Joint European Torus. The Alfvén modes are driven by energetic hydrogen minority ions produced by ion cyclotron resonance heating. The experimental signatures of the chaotic regime include spectral broadening, phase flips, and nonperiodic amplitude variations. These phenomena are found to be consistent with a general nonlinear theory of kinetic instabilities near stability threshold developed by Berk, Breizman, and Pekker.