24-347 Advanced Shaping of Wave-particle

il y a 2 semaines

Marseille, France CNES - Centre National d'Etudes Spatiales Temps plein

Doctorat, 36 mois
- Temps plein
- Aucune expérience exigée
- Maitrise, IEP, IUP, Bac+4
- Telecommunications

**Mission**:
In a nutshell [1], a TWT is a radiofrequency device in which an electron beam is injected in a radiofrequency (RF) waveguide (typically a helix or a folded waveguide), with a beam velocity slightly larger than the RF wave phase velocity. The beam-wave coupling transfers momentum and energy from the electrons to the waves, thereby amplifying them with a gain up to 50 dB. Appropriate collection of the beam after the interaction, and adapted waveguide design enable an electric efficiency up to 60%. Similar physical processes are also fundamental in wave-particle interactions in plasmas, in gyrotrons, in free electron lasers and many other devices.

In this Ph.D. we propose to continue the efficiency improvements permitted by tapering, namely the possibility to change at will the wave propagation velocity using profiles for the helix geometry (typically its pitch). First works were begun with the concept of “deep taper” and should soon be tested experimentally. There also appeared recently the idea of “phase jump”, which consists in causing the wave phase to jump rather than profiling it progressively [2], following an approach developed for free electron lasers [3]. In theory, important benefits are obtained for TWTs with a poor interaction efficiency, which is typically the case for TWTs operating in Q-band. One research line will be to explore these new ideas and extend them.

The second research line will aim at a numerical model more performant than the current industrial ones for simulating this interaction in the presence of tapers. Too large departures are noted between previsions and experimental results. After closing several tracks in previous works, our hypothesis to explain these discrepancies is that reflections intrinsically related with pitch changes in the helix generate standing waves which cannot be neglected any longer. Our novel interaction model DIMOHA [1, 4, 5] enables already a first consideration of these reflections in simple cases. A good agreement with experiment was obtained in the previous Ph.D. [1] for a simple reflection at the TWT output. We must now enhance this model to represent best the propagation in the tapered helix.

[1] Kh. Aliane et al., IEEE Trans. El. Dev. 68 (2021) 6476 ; hal : 03528645.

[2] F. Lan, IEEE Trans. El. Dev. 69 (2022) 4586.

[3] A. Mak et al., Phys. Rev. Accel. Beams 20 (2017) 060703.

[4] D.F.G. Minenna, Kh. Aliane et al., Phys. Plasmas 28 (2021) 092110 ; hal : 03344311.

[5] D.F.G. Minenna et al., IEEE Trans. El. Dev. 66 (2019) 4042 ; hal : 02280734.

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**Profil**:
Master in physics or engineering - plasmas, electrodynamics, wave-particle interactions

**LABO**:
PIIM

**Mot du recruteur**:

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