17 Décembre – Thesis defense - Thomas Lahens

14 h Amphi C - Building A29 (Talence campus)

Propagation of an intense relativistic electron beam for flash radiography in a cold plasma.

At the department of military application at cea, flash radiography is being used to image high velocity matter (few km/s), with high density. For this purpose, X-rays need to have an energy around 10MeV, to be short (60ns) and produce a dose of several hundreds of rad. To generate this kind of X-rays, an intense relativistic electron beam (typically 20MeV, a few kA during 60ns) is focused on a conversion target. During the beam/target interaction, a certain amount of the beam energy is converted into heat. Because the energy deposition is very abrupt, we observe temperature and pressure of the order of 1eV and 1Mbar. Under such extreme conditions, matter is vaporised and a plasma plume expand around the conversion target. In the context of prospective studies on a multi-pulse flash radiographic chain, our goal is to study the influence a this plasma plume on the quality of the successive X-ray pulses. To do so we want to design an experiment where we propagate a flash radiographic electron beam in a plasma. First, jointly with a bibliographic study, we made calculations using the envelop equation, modified to take into account the influence of the plasma. The key parameter in beam/plasma interaction is the ratio of their respective electronic densities. Consequently to this preliminary study, we concluded that for the design of our experiment, we needed plasmas with electronic densities between 10^10 cm^(-3) and 10^12 cm^(-3).
For this purpose, we took interest in glow discharges and design a test bench to characterize them with different type of diagnostics. We performed measurements with a langmuir probe, a radiofrequency interferometer and a new diagnostic based on capacitive coupling with the plasma and we came to the conclusion that the maximum electronic density in glow discharges was of the order of 10^10 cm^(-3). Although this does not cover the whole range of electronic density, we designed a device capable of generating a glow discharge and that we could clip on the beam pipe of an accelerator. We called this device the plasma cell. In parallel, in order to be able to sweep the whole range of electronic densities, we develop an inductive heating system for the glow discharges. Interferometric measurements shows that this system allows us to reach electronic densities of the order of 10^13 cm^(-3) even though some work is required to improve its reliability before we can use it on the plasma cell.
We tested  the first version of the plasma cell on the FEVAIR facility at CEA-CESTA (4MeV, 2kA, 60ns). During this experimental campaign, most of the characteristics of the plasma cell were successfully tested, especially one of the most critical one : the plasma/vacuum interface. We achieved propagation of the FEVAIR beam through the plasma cell and measured the beam net current at different axial positions, as well as the beam profil at its exit. First, we observe that gas pressure was acting on the beam from a few 10^(-2) mbar, which is the minimu pressure at which we are able to generate a glow discharge. Besides, this effect is predominant on the effect of the glow discharge. In addition to that, we saw that peripheral electrons were hiting the cell, causing an electrical charge and influencing the beam propagation.
This obervations have inspired some improvements on the plasma cell : its evolution will be shorter and equiped with an inductive heating system based on the one we develop during this thesis. On top of that, this experimental campaign emphasize the importance of a detailled description of the beam and its interaction with the gas in the cell, in this kind of regime.

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