  A coated hollow core microsphere is introduced as a novel target in
ultra-intense laser-matter interaction experiments. In particular, it
facilitates staged laser-driven proton acceleration by combining conventional
target normal sheath acceleration (TNSA), power recycling of hot laterally
spreading electrons and staging in a very simple and cheap target geometry.
During TNSA of protons from one area of the sphere surface, laterally spreading
hot electrons form a charge wave. Due to the spherical geometry, this wave
refocuses on the opposite side of the sphere, where an opening has been laser
micromachined. This leads to a strong transient charge separation field being
set up there, which can post-accelerate those TNSA protons passing through the
hole at the right time. Experimentally, the feasibility of using such targets
is demonstrated. A redistribution is encountered in the experimental proton
energy spectra, as predicted by particle-in-cell simulations and attributed to
transient fields set up by oscillating currents on the sphere surface.
