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Development of a new model for electron-­‐wave interaction in traveling wave tubes

par Elodie PICO - publié le

Thesis advisor : Yves Elskens
E-­‐mail and address : yves.elskens@univ-­‐
+33 -­‐ 491 288 230

Co-­‐advisor : Frédéric André (Thales Electron Devices, Vélizy)
+33-­‐1 30 70 24 80

Description :

Wave-particle interaction is a fundamental process in the physics of warm and natural hot plasmas, of accelerators and of beams ; in particular, it is the basis of electromagnetic radiation amplifiers such as free electron lasers, gyrotrons, travaling wave tubes… Power densities in these devices and their wide frequency spectrum lead to instabilities which are increasingly critical and difficult to simulate nowadays. A microscopic description enables a better understanding of the coupling mechanisms between N particles (xl, pl) and M waves (with phases θj and intensities Ij) using a so-called self-consistent hamiltonian. For N→∞, the dynamics of this system converges to the one described with vlasovian kinetic equations.

Numerical simulation presently relies on two classes of models. Particle-in-cell (PIC) models est on a minimal simplification of the physics equations but imply gigantic computing times, due to the very large number of degrees of freedom. Specialized models, on the contrary, permit only the simulation of particular regimes, albeit with incommensurately shorter computing times. The widely used envelope models are frequency-domain models in which the amplified wave is represented by the
cold wave (propagating in the absence of a beam) modulated by an envelope function varying with the position along the propagation direction. This approach is not suited to the study of nonlinear regimes, such as instability saturation.

The Ph.D. work will focus on the development of simple nonlinear systems and their
numerical implementation toward application to the traveling wave tubes for the industry (Thales Electron Devices, Vélizy) and for fundamental research in our laboratory (Marseilles).

Bibliography :
- F. André, P. Bernardi, N.M. Ryskin, F. Doveil & Y. Elskens, Hamiltonian description of selfconsistent wave-particle dynamics in a periodic structure, Europhysics Letters 03 (2013) 28004.
- Ph.D.s in Marseilles : A. Macor (2007), A. Aïssi (2008, : tel- 00406576), P. Bernardi (2011 : tel-00708349).
- Y. Elskens & D. Escande, Microscopic dynamics of plasmas and chaos (IoP Publishing, Bristol, 2003).

Support :
CNES and Thales