This paper reports some of the studies on nonconventional ICF
approaches performed at the ICF Physics and Technology Laboratory
of the AEEF in Frascati, Italy. Having as reference potential
difficulties associated to the conventional central spark ignition
(fuel mixing) and to the usual approach to fast ignition by
laser (transfer and coupling of the energy pulse, fast electrons
energy tuning), we have made explorative work on possible
alternatives. The performances of targets ignited near stagnation
by pulses of heavy ion beams (HIB) or by macroparticle impact
were previously studied. The needed driver energy, the power,
and the beam quality requirements, as well as the level of
synchronization the implosion and the igniting pulse have been
found. More recently, to relax some requirements on the HIB
beam parameters set by the previous approach, the injected entropy
approach (IE) has been introduced. In this method, the conditions
for spark formation are set in the final stages of the implosion,
when the spark fuel size is a few times the final size at
stagnation (volume a few tens of the final). Energy is injected
at this time to set the spark fuel on a high adiabat. In this
paper, for illustration and comparison purposes, some relevant
results we previously obtained for near-stagnation ignition
are first introduced and critically reviewed. The new IE method,
after a short analytical introduction, is presented and illustrated
by the results of extensive 2-D numerical simulations. The
considered cases refer to imploding cylinders of finite length.
As required by this approach, one or two opposing beams axially
injected additional energy, whereas the acceleration stage of
the cylindrical low-entropy implosion was assumed driven by
a different driver. Heavy ion beams, soft X-rays (SXR), and
laser generated light ion beams were considered as vectors for
the entropy injection. Issues related to the feasibility of
these generators are discussed. The study was made for various
initial conditions leading to different ignition modes and burn
propagation. The most recent results on the injected entropy
method to the ignition of high gain targets are included.
