Magnetic field generation through angular momentum exchange between circularly polarized radiation and charged particles

The interaction between circularly polarized (CP) radiation and charged particles can lead to generation of magnetic field through an inverse Faraday effect. The spin of the circularly polarized electromagnetic wave can be converted into the angular momentum of the charged particles so long as there is dissipation. We demonstrate this by considering two mechanisms of angular momentum absorption relevant for laser-plasma interactions: electron-ion collisions and ionization. The precise dissipative mechanism, however, plays a role in determining the efficiency of the magnetic field generation

Compton harmonic resonances, stochastic instabilities, quasilinear diffusion, and collisionless damping with ultra-high intensity laser waves

The dynamics of electrons in two-dimensional, linearly or circularly polarized, ultra-high intensity (above 10{sup 18}W/cm{sup 2}) laser waves, is investigated. The Compton harmonic resonances are identified as the source of various stochastic instabilities. Both Arnold diffusion and resonance overlap are considered. The quasilinear kinetic equation, describing the evolution of the electron distribution function, is derived, and the associated collisionless damping coefficient is calculated. The implications of these new processes are considered and discussed

The interaction of energetic alpha-particles with intense lower hybrid waves

Lower hybrid waves are a demonstrated, continuous means of driving toroidal current in a tokamak. When these waves propagate in a tokamak fusion reactor, in which there are energetic {alpha}- particles, there are conditions under which the {alpha}-particles do not appreciably damp, and may even amplify, the wave, thereby enhancing the current-drive effect. Waves traveling in one poloidal direction, in addition to being directed in one toroidal direction, are shown to be the most efficient drivers of current in the presence of the energetic {alpha}-particles

Current-drive by lower hybrid waves in the presence of energetic alpha-particles

Many experiments have now proved the effectiveness of lower hybrid waves for driving toroidal current in tokamaks. The use of these waves, however, to provide all the current in a reactor is thought to be uncertain because the waves may not penetrate the center of the more energetic reactor plasma, and, if they did, the wave power may be absorbed by alpha particles rather than by electrons. This paper explores the conditions under which lower-hybrid waves might actually drive all the current. 26 refs

Magnetostatic Ponderomotive Potential in Rotating Plasma

International audienceA new end-plugging method for rotating plasmas is identified and analyzed. It uses the ponderomotive potential associated with an azimuthal magnetostatic wiggler. Studied both analytically and numerically, this process compares favorably to other end-plugging methods in open field line magnetized plasma devices

Guiding center motion for particles in a ponderomotive magnetostatic end plug

International audienceThe Hamiltonian dynamics of a single particle in a rotating plasma column, interacting with an magnetic multipole is perturbatively solved for up to second order, using the method of Lie transformations. First, the exact Hamiltonian is expressed in terms of canonical action-angle variables, and then an approximate integrable Hamiltonian is introduced, using another set of actions and angles. The perturbation introduces an effective ponderomotive potential, which to leading order is positive. At the second order, the pseudopotential consists of a sum of terms of the Miller form, and can have either sign. Additionally at second order, the ponderomotive interaction introduces a modification to the particle effective mass, when considering the motion along the column axis. It is found that particles can be axially confined by the ponderomotive potentials, but acquire radial excursions which scale as the confining potential. The radial excursions of the particle along its trajectory are investigated, and a condition for the minimal rotation speed is derived, in order for particles to remain radially confined. Last, we comment on the changes to the aforementioned solution to the pseudopotintials and particle trajectory in the case of resonant motion, that is, a motion which has the same periodicity as the perturbation

Relativistic Raman instability shifted by half-plasma frequency

A new nonlinear Raman instability in underdense plasma is investigated theoretically. Unlike the usual linear Raman instabilities which grow exponentially in time, this instability takes a finite amount of time to diverse. The explosion time t{sub {infinity}} depends on the initial level of the perturbation. A general set of equations for spatio-temporal evolution of the forward non-linear Raman scattering is derived and its temporal evolution is studied in detail. This new instability results in the generation of forward Raman radiation shifted by half the plasma frequency for laser intensities of order or exceeding 10{sup 18}W/cm{sup 2}, something that has been recently observed

Massive, long-lived electrostatic potentials in a rotating mirror plasma

International audienceHot plasma is highly conductive in the direction parallel to a magnetic field. This often means that the electrical potential will be nearly constant along any given field line. When this is the case, the cross-field voltage drops in open-field-line magnetic confinement devices are limited by the tolerances of the solid materials wherever the field lines impinge on the plasma-facing components. To circumvent this voltage limitation, it is proposed to arrange large voltage drops in the interior of a device, but coexist with much smaller drops on the boundaries. To avoid prohibitively large dissipation requires both preventing substantial drift-flow shear within flux surfaces and preventing large parallel electric fields from driving large parallel currents. It is demonstrated here that both requirements can be met simultaneously, which opens up the possibility for magnetized plasma tolerating steady-state voltage drops far larger than what might be tolerated in material media

Diffraction of Gaussian and Laguerre–Gauss beams from a circular aperture using the moment expansion method

International audienceA method based on the distribution theory is introduced to compute the Fresnel diffraction integral. It is applied to the diffraction of Gaussian and Laguerre–Gauss beams by a circular aperture. Expressions of the diffracting field are recast into a perturbation series describing the near- and far-field regions.</jats:p

Ti:sapphire – A theoretical assessment for its spectroscopy

International audienceThis article tries to theoretically compute the stimulated emission crdss-sections when We know the oscillator strength of a broad material class (dielectric crystals hosting metal -transition impurity atoms). We apply the present approach to Ti:sapphire and check it by computing some emission cross-section curves for both it and o polarizations. We also set a relationship between oscillator strength and radiative lifetime. Such an approach will allow future parametric studies for Ti:sapphire spectroscopic properties. (C) 2016 Elsevier B.V. All rights reserved