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Unconventional, promising approaches to inertial fusion energy (IFE) proposed and studied in the framework of IFE keep-in-touch activity at Frascati EURATOM-ENEA Association will be reconsidered for the present more advanced technological context associated to a planned large installations scenario. Then the possibility to generate laser-produced fast heavy-ion sources (e.g., Bi ions, tens GeV) will be explored taking as reference results obtained in the FIGEX experiment scaled toward thermonuclear regimes. A possible dimensioning of such sources will be also given as well as the associated laser requirements.
In several experiments, faster ions were produced from the backside of
solid targets irradiated by powerful laser pulses. The ion acceleration
was considered due to the negative electrostatic sheath formed on the
backside of the target (TNSA), or to the expansion wave starting at the
backside surface, or to the expansion wave and to its embedded
electrostatic rarefaction shock. In this experiment, ions have been
generated by transferring energy to a controlled amount of mass before the
target become transparent by gas dynamic expansion (controlled amount of
mass mode (CAM)). The targets used were thin transparent disks
causally isolated from the holder to trim down, during the
interaction process, unwanted effects due to the surrounding parts. Two
kinds of target corresponding to a different set of parameters were
designed (LARGE and SMALL). Both targets were conceived to survive, in the
actual contrast conditions, to the low power pulse forerunning the giant
laser pulse, bigger margin but lower performances being assigned to LARGE.
For comparison standard square foils under the same focusing conditions,
were also studied (LARGE-LIKE and SMALL-LIKE irradiation).
Irradiation of solid targets by short laser pulses can result in a
production of fast ions. In this paper, two production modes are
discussed: the controlled amount of matter mode (CAM) and the open amount
of matter mode (OAM). The CAM mode is based on laser energy transfer to a
controlled amount of matter before the target becomes transparent to the
laser light due to the gas-dynamical expansion. For the CAM mode, it is
presented a model that allows determining the target parameters, the
focusing conditions, and the pulse duration as a function of the laser
pulse energy, of the aimed energy per nucleon and of the energy transfer
efficiency to the target. The conditions to be this mode experimentally
addressed are indicated. The OAM mode relies on the irradiation of a
target with large ion content by a short laser pulse; in this case, a
small amount of fast ions is emitted from the rear and lateral sides of
the target depending on the laser pulse and focusing parameters. For this
mode, observed in several experiments, a theoretical discussion is
presented. Special attention is devoted to the target normal sheath
acceleration (TNSA) and to expansion wave (EW) mechanisms. The EW process
is discussed in the framework of a two-temperature isothermal model and
some peculiar hydrodynamic processes are discussed.
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.
Experiments have been performed on the interaction
physics of laser light with polystyrene and agar–agar
foams having average densities higher than critical. The
experiments have been performed at the ABC facility of
the Associazione EURATOM-ENEA sulla Fusione, in Frascati.
The main addressed topics have been energy coupling (balance),
diffusion of energy into the target, plasma and dense phase
dynamics, and harmonics generation. The laser light (λ =
1.054 μm) was focused by a F/1 lens to produce on the target
surface about 1.6 × 1014 W/cm2
(≈1015 W/cm2 in the waist, set about
100 μm inside the target). Experiments have shown efficient
energy coupling (>80%) to be attributed to cavity formation
in the low density foam (efficient light absorption) and good
mechanical coupling of the plasma trapped in the cavity to the
dense phase (ablation pressure work). Heat diffusion possibly
plays a transitory role in the initial stages of the interaction
(300–500 ps). Time integrated harmonics measurements
revealed a blue-shifted 2ω and a red-shifted 5/2ω.
We considered an ignition strategy based on energy
injection in the final stages of the target implosion,
but well before stagnation. The injected energy is used
to set on a high adiabat the small portion of the fuel
destined to become the ignition spark. Most of the energy
needed to form the spark is provided by the work of the
imploding target on the spark fuel. This approach is first
introduced by a simple analytical model and then illustrated
by 2D numerical calculations, in which the ignition of
imploding cylinders by heavy ions is simulated. Different
implosion designs and burn modes are presented and discussed.
When compared with the standard central self-generated
spark, or the fast ignition approaches, the ignition by
entropy injection is potentially promising of some distinct
This paper is devoted to the investigation of powerful
laser pulse interaction with regularly and statistically
volume-structured media with near critical average density
and properties of laser-produced plasma of such a media.
The results of the latest experiments on laser pulse interaction
with plane foam targets performed on Nd-laser facilities
“ABC” in the ENEA-EURATOM Association (Frascati,
Italy) and “MISHEN” in the Troitsk Institute
of Innovation Thermonuclear Investigations (TRINITI, Troitsk
Russia), and J-laser “ISKRA-4” in the Russian
Federal Nuclear Center, All-Russian Scientific Research
Institute of Experimental Physics (RFNC-VNIIEF, Sarov,
Russia) are presented and analyzed. High efficiency of
the internal volume absorption of laser radiation in the
foams of supercritical density was observed, and the dynamics
of absorbing region formation and velocity of energy transfer
process versus the parameters of porous matter
are found. Some inertial confinement fusion (ICF) applications
based on nonequilibrium properties of laser-produced plasma
of a foam and regularly structured media such as the powerful
neutron source with yield of 109–1011
DT-neutrons per 1 J of laser energy, laser-produced X-ray
generation in high temperature supercritical plasma, and
the compact ICF target absorbers providing effective smoothing
and ablation are proposed.
The evolution of thin foils accelerated over distances much greater than their in-flight thickness has been studied for a classical, low power density regime (5 × 1012 and 2 × 1013 W/cm2 at λ = 1.054 μm, 0.15 THz bandwidth). Two illumination modes have been used, one of these being based on a near-field, multi-lens array ISI smoothing, the other on focusing by F/1 aspherical lenses. The results seem to be encouraging with regard to the possibility of getting smooth, spatially uniform acceleration over distances of the order of 100 × IFFT (In-Flight-Foil Thickness). The experiments with nonsmoothed beams have shown a sort of refractive interchange stabilization for the initial on-target imprinting of the illumination non-uniformities.
A spherical scheme is introduced for indirect drive, having as reference the application to inertial fusion energy. The concept uses the transmission of a radiation wave through a high-Z, low-density spherical radiator surrounding the capsule to be imploded. The low conversion efficiency to in-cavity radiation is somewhat compensated by using modest radiator-to-capsule gaps, the smoothing effects on the longest dominant nonuniformity wavelengths due to this separation being of the order of 10. Another factor of the order of 100–500 is related to the consequent use of a large number of beams. From the application of the loop condition relative to power plants, it is found that this approacrTbecomes practicable if drivers with efficiencies greater than 10% are assumed and if the fraction of circulating power is allowed to be 0.5, this implying a unitary cost of energy about a factor 1.5 greater than that associated to the more conventional fraction of 0.25.
The results of the interaction of laser beams integrated by an optical system composed of two arrays of 256 elements each are presented. Optical dark-field shadowgraphy as well as interferograms show significant smoothing effects when compared with experiments performed in the same conditions but without beam integrators.
A study of the space-time structure of the light reflected when a plastic target (CH)n is irradiated at a maximum power of 1012 watt/cm2 by a Nd glass laser pulse of nanosecond duration is described. The reflected light has been used for time integrated spot imaging, whereas the time dependence of the reflected power at different angles has been studied by optical guides of different length coupled with a single fast photodiode. It has been found that: 1) the spot images are formed by a high contrast stationary speckle pattern, indicating the presence of a random phase mixing in the back-scattered light; 2) the interaction is characterized by a high reflection in the initial stages (t ≤ 0·3 ns) followed by almost complete absorption.
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