Laser-accelerated proton beams: Isochoric heating and conversion efficiency

One possible route to fusion energy is the fast ignition variant of inertial confinement fusion. In this scheme, pre-compressed fuel is heated to thermonuclear ignition temperature by a charged particle beam. This charged particle beam is generated by the interaction with matter of a petawatt power laser at relativistic intensity. Relativistic electrons accelerated by such a laser were originally proposed as the ignition source in fast ignition.;To measure the conversion efficiency of laser light into accelerated protons of energy greater than a few MeV, radiochromic film as a proton beam diagnostic has been developed. This is applied in an experiment on a 150 J, 0.5 ps laser. A conversion efficiency scaling of 1targetthicknes s is found. This is explained with a simple analytical model that agrees with numerical plasma simulations. The conversion efficiency unexpectedly increases with laser pulselength (with constant laser energy). A tentative explanation is that this increase is due to rapid target expansion, so that relativistic electrons spend a smaller fraction of their time in the target bulk where energy is lost to channels other than proton acceleration. The peak observed conversion efficiency is 4.5%, which might be increased by thinning the target and/or depositing a hydrogen-rich layer on the rear surface.;In this thesis a different route to ignition is explored experimentally: laser-accelerated proton beams. These proton beams are driven by the laser-accelerated relativistic electrons and have ∼MeV mean energy. They have been shown to originate from the rear surface of thin foils at an angle approximately normal to the target surface with a very low transverse temperature, making it possible to focus them by curving the target rear surface. A proton focusing experiment is presented that was carried out on a 300 Joule, 700 fs laser, where the position of best focus was determined and solid density matter was isochorically heated to about 80 eV. A code was written to model this heating that self-consistently calculates heating of a partially ionized plasma by a velocity-dispersed proton beam.