  We comment on Marvel Fusion's proposal to use ultra-short, intensive laser
pulses impinging on nanostructured targets for the production of hot ions, with
the ultimate aim of energy production through the p-11B fusion reaction. We
show how a minimum requirement on the line density of the heated region arises
also for such schemes aiming at ignition or just beam-energy amplification,
which already for the D-T reaction are prohibitive if no pre-compression of the
target is foreseen. We conclude that advanced ultra-short laser pulses can be
of use for fusion applications only if the fusion reaction time can be brought
down to sufficiently short values. This can happen only via strong
pre-compression of the target, which appears difficult to reconcile with a
nanostructured target and in particular with the fuel assembly geometries shown
by the proponents.
