M2 Intership – Physics – experimental – TP/NC/1

Plasma experiments on earth all need a container: magnetic confinement fusion devices, electrostatic nuclear fusion devices, experimental bench test, satellite thrusters, magnetrons for industrial thin layer depositions, etching devices, decontamination… For unmagnetized plasmas, the Bohm criterion separates the quasi-neutral presheath from the high potential gradient sheath but it has been theoretically demonstrated by simulations that this sheath criterion disappears in magnetic sheath with a low incidence angle1,2 of magnetic field. Several theoretical analysises3,4 explain our experimental results5 exhibiting a thermalization of the Ion Velocity Distribution Function (IVDF) with two physical contradictory phenomena: collisions and ballistic effects. Langmuir probe6 is the most used diagnostic to measure plasma properties by collecting currents through its own surrounding sheath. The interpretation of this diagnostic is an opened issue since a theoretical sheath model is still needed to analyze measurements. For electron emissive surface sheath, if the rate of electron emission is high enough, inverse sheath is predicted7 but is still not experimentally observed. Because of a very low sheath thickness and a high potential gradient, only the non-intrusive laser-induced fluorescence (LIF) diagnostic can be used. This powerful diagnostic of which the supervisor is an expert, allows the space and time Ionic Velocity distribution Function (IVF)8 measurements even if some artefacts have been identified by authors and must be considered9.

This internship proposes to study electrons emissive sheath by a dedicated plasma experiments measuring IVDF in a non-intrusive way by LIF8 diagnostic in a low pressure DC Argon discharge. This study will start with a classical floating sheath study to validate modelisations (kinetic, fluid and PIC simulations developed in another team of the lab) to an electrons emissive sheath. The emissive cathode will be either a heating conducting metallic surface or a heated insulated ceramic.

1. Chodura, R., Phys. Fluids 25, 1628 (1982).
2. Stangeby, P. C., Nucl. Fusion 52, 083012 (2012).
3. Baalrud, S. D. & Hegna, C. C., Plasma Sources Sci. Technol. 20, 025013 (2011).
4. Coulette, D. & Manfredi, G., Phys. Plasmas 22, 043505 (2015).
5. Claire, N., Bachet, G., Stroth, U. & Doveil, F., Phys. Plasmas 13, 062103 (2006).
6. Chen, F. F. Langmuir Probe Diagnostics. IEEE-ICOPAS Meeting (2003).
7. Campanell, M. D., Phys. Rev. E 88, 033103 (2013).
8. Pigeon, V., Claire, N., Arnas, C., Terasaka, K. & Inagaki, S., Phys. Plasmas 27, 043505 (2020).
9. Pigeon, V., Claire, N., Arnas, C. & Doveil, F., Phys. Plasmas 26, 023508 (2019).

M2 Intership – Physics – modelling – TP/YE/1

Wave-particle interaction is a fundamental process in the physics of hot and natural plasmas, accelerators and beams ; it is the basis of wave amplifiers such as free electron lasers, gyrotrons, traveling wave tubes… where a focused electron beam transfers momentum and power to radiofrequency modes of a waveguide. In particular, traveling wave tubes enable the efficient and robust operation of satellite telecommunications and data transmission from space probes, and they enable analysing beam-plasma interactions without the noise and some nonlinearities inherent to plasma.

The power in some of these devices and their broad frequency spectrum lead to instabilities, nowadays increasingly critical and hard to simulate. A microscopic description enables a better understanding of the coupling mechanisms between N particles (xl, pl) and M waves (with phases φj and intensities Ij) using a so-called self-consistent hamiltonian.

For N → ∞ and fixed M, the dynamics of this system converges to the one described by vlasovian kinetic equations. Numerical simulation currently relies on two types of models. Particle-in-Cell (PIC) models rest on a minimal simplification of physics equations but lead to huge computing times, as the number of degrees of freedom is very large. Specialized models, in contrast, allow simulating only particular regimes, but with outstandingly shorter times. The very popular envelope model is a frequency-domain model in which the amplified wave is represented by the cold wave (the wave propagating in the absence of beams), multiplied with an envelope function varying with position along the propagation direction. This frequency-domain approach is not fit for investigating nonlinear regimes, like saturation instabilities and intermodulation effects.

We developed a novel time-domain model with few degrees of freedom thanks to an efficient representation of fields, enabling a realistic simulation of amplification in traveling wave tubes. We confront these simulations with experiment in space traveling wave tubes and in the 4 metre long device which enabled our laboratory to perform the first direct observation of several fundamental processes of this physics, and at CEA CELIA.

The internship may focus on the interaction of the radiofrequency signal with electron pulses, to use the resulting models in particular for the traveling wave tubes of Thales Avionics (Vélizy), in our laboratory (Marseille), and for the acceleration of protons generated by a laser shot on a metallic target (CEA CELIA).

The project may extend to a Ph.D. if possible.

– Y. Elskens & D. Escande, Microscopic dynamics of plasmas and chaos (IoP Publishing, Bristol, 2003).
– D.F.G. Minenna, Kh. Aliane et al., Time simulation of the nonlinear wave-particle interaction in meterslong traveling-wave tubes, Phys. Plasmas 28 (2021) 092110 (15 pp.).
– J.V. Gomes et al., Low-dimensional chaos in the single wave model for the self-consistent wave-particle hamiltonian, Chaos AIP 31 (2021) 083104 (17 pp.).
– Ph.D.s in Marseilles : A. Macor (2007), A. Aïssi (2008), P. Bernardi (2011), S. Théveny (2016), D.F.G. Minenna (2019), Kh. Aliane (in progress). https://phys.org/news/2019-03-traveling-wave-tubes-unsung-heroes-space.html