Design of near-critical gaseous targets

Up to now the main method to accelerate protons and ions using high-power lasers in the so-called TNSA (Target Normal Sheath Acceleration) mechanism where a laser impinges a solid target foil, and particles are accelerated in the target normal direction due a space-charge electric field formed as fast electrons are accelerated. However, at a high repetition rate operation, the use of solid foils has some inherent limitations such as the destruction of the target or the generation of debris. The interest of targets that could overcome these limitations leads to the development of new target concepts: liquid, gaseous, cryogenic, etc. This course will be focused on the production of gaseous targets, and more specifically, the generation of near-critical targets by supersonic gas jets. Some of the advantages of such targets are continuous refreshment of the target or high repetition rate operation. The basics of the expected acceleration mechanisms will be shown and an introduction of compressible flow theory will be given in order to design such targets.