  The declared aim of Marvel Fusion is the realization of a reactor based on
the aneutronic fusion of proton and boron-11 nuclei in the near future, making
use of latest-day laser technology and nano-structured materials. The aim of
the preprint quoted above is to demonstrate the feasibility of fusion energy
gain from the irradiation of an initially uncompressed target consisting of an
array of nano-wires, by a femto-second laser. This proposal is in apparent
contrast to text-book wisdom, which postulates - even for the fast-ignitor
concept, and a DT fuel mix - a target density about 1000 times that of solid
state. The novel, optimistic predictions of Ruhl and Korn are based, however,
not on rigorous estimates, but only on parametric dependencies, extrapolated
far beyond their conventional limits of validity. The authors invoke the
effects of self-arising magnetic fields to result in additional magnetic
confinement, but we show their model to contain intrinsic contradictions. The
conclusion from out note is that ultra-fast, high power lasers can be of use to
inertial fusion energy production only, if the intrinsic time-scale for fusion
reactions can be brought down to a level more commensurate with the Laser pulse
length. Involving magnetic fields does not change this basic limitation, as the
Alfv\'en time will be similar to the kinetic disintegration time. Given that
the fusion energy gain as function of particle energy has a maximum (for DT at
about 170 keV) overheating will not be helpful so that the required enhancement
of the fusion reaction rate can only be achieved by strong pre-compression of
the target.
