Focusing an intense and relativistic electron beam for flash radiography

Flash radiography is an imaging technique used at CEA/DAM aiming at studying dense objects in fast hydrodynamic evolution. The X-rays beams required to probe such materials must have the following characteristics: photons with energies of the order of magnitude of a few M eV, a delivered dose of several hundred of rad and a pulse duration less than a hundred of nanoseconds. As part of the Simulation program, CEA/DAM has the EPURE installation which is dedicated to produce this radiation. The principle is based on the interaction between an intense beam of relativistic electrons with a high atomic number converter target such as tantalum. The X-rays are generated by Bremsstrahlung. The incoming beam must have a high current (2 kA), relativistic electrons (20 MeV ), and last approximately < 100 ns to produce the X-rays required by the technique. The dimensions of the electron spot on the converter target are the same order of magnitude as those of the X-ray source. Reducing the size of the X-ray source improves the resolution of X-ray images. The goal of this Ph.D thesis is to investigate ways of improving the focusing of electron beams for flash radiography. Two aspects are examined. The first concerns the emission of ions from the converter target during its irradiation by the electron beam. These ions, expanding in the beam volume, can disturb the focal spot. The second aspect concern the possibility to focus an electron beam thanks to plasma. This latter has theoretically a fewer chromatic aberration than a solenoid, which is commonly used to focus an electron beam in our context. Moreover, the study of the interaction between the electron beam with plasma will bring experimental data in a context where there is a lack of it. (author)