After reading the full text article I was surprised to find the protons from the aluminium foil/picosecond pulse interaction actually come from hydrocarbon impurities that were on the back surface of the foils.

If they don't guard that hydrocarbon film on the back of the aluminium from the Boron-ionizing nanosecond pulse then they don't get protons. So that is why the second thinner aluminium film is there; to shield the impurity layer on the thick aluminium film.