Effect of plasma hydrodynamics on laser-produced bremsstrahlung MeV photon dose

Abstract

We detail a laser plasma experiment aimed at enhancing laser to MeV electron energy coupling and then the x-ray dose produced when a short pulse laser propagates through a long preformed plasma. This study can be of interest not only for radiography of high areal mass objects requiring large doses but also for radiation safety of large scale laser facilities such as LMJ or NIF able to produce long preformed plasmas through which a short pulse laser can propagate. A low-intensity (∼1014 W/cm2) ns beam explodes a thin foil deposited on a high-Z solid target to generate an underdense plasma. An intense (>1018 W/cm2) and short (<1 ps) laser pulse then (with an adjustable delay δt) interacts with this plasma and produces multi-MeV electrons. These high-energy electrons are converted into a bremsstrahlung emission of MeV x-ray photons in the high-Z target. In a second target design, a vacuum gap between the foil and the conversion target is also tested to let the plasma expand on both sides of the foil, increasing the interaction length even more. Results show how the vaporization of the foil produces an underdense plasma over several hundreds of micrometers which significantly enhances x-ray doses, with harder x-ray spectra obtained at an optimum delay, δt, until the short pulse laser is affected by refraction. Increasing the interaction length with gap targets is at the origin of a much more complex plasma hydrodynamics involving on-axis plasma stagnation which delays the optimum time for the maximum x-ray dose production.