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Composite double-layer targets for laser-plasma ion acceleration have been drawing much attention in recent years. In this talk I will present two recent (and unrelated) works on double-layer targets.
In the first work, done in collaboration with HZDR, we have coupled a gas jet hundreds of microns thick to a thin foil and successfully accelerated protons. The laser’s shape, duration, energy and frequency are modified during its propagation in the gas, altering the laser-solid interaction leading to proton acceleration. The interaction of the laser with the gas essentially imposes new TNSA ‘initial conditions’. Exploring these conditions while observing the resulting accelerated protons promotes better understanding of the behavior of the TNSA mechanism with respect to its input parameters. The promising results motivate further exploration with this target.
In the second work, done in collaboration with CEA, we study proton acceleration from a gas-foil target using 2D PIC simulations. The target consists of a near-critical-density hydrogen gas layer of a few tens of microns attached to a $2\,\rm \mu m$-thick solid carbon foil with a contaminant thin proton layer at its back side. At optimal gas density, the maximum energy of the contaminant protons is increased by a factor of ~4 compared to a single foil target. This improvement originates from the near-complete laser absorption into relativistic electrons in the gas. Several energetic electron populations are identified, and their respective effects on the proton acceleration are quantified by computing the electrostatic fields that they generate at the protons’ positions. Our analysis also reveals the important role of the neighboring ions in the acceleration of the fastest protons, and the onset of multidimensional effects caused by the time-increasing curvature of the proton layer.