1. magnetic confinement - ENEA - Fusione

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5. INERTIAL CONFINEMENT
5.3 Theory
a
Target
Laser spot
Opaque mask
400 µm
Table 5.I - Single foil simulations
b
5.2 - a) Relative positioning
and sizes of target
and laser spot. The spot
image was taken without
target and combined with
the typical cross section
of a foam target. The
relative positioning is that
adopted in the experiments.
b) Masking of the
laser spot. Although completely
opaque, the mask
is represented as partially
transmitting to show the
relative hole -spot
positions and sizes.
Case number Focalization Foil transparency Transit time
(cm) (tb ns) (ts, ns)
1 -0.015 0.79 0.62
2 -0.030 1.1 0.76
3 -0.045 1.37 0,85
main physical parameters (typical velocities, burn through time, etc…). Then a
structure composed by 3 parallel layers was considered. The material was assumed
to be CH and the foil thickness d=0.5 µm. In the multi-foil simulations the spacing
between them was s=75 µm (that is an average density of 6.7 mg).
The material was irradiated by 1.054 µm radiation, focused along the negative
direction of the z-axis (the optical axis) according to a F/1 geometry. The focal spot
was set at different positions along the z-axis for different cases. The pulse of laser
power, triangular as time waveform, started at t=0, achieved the maximum at t=0.7
ns and was set to zero at t=2 ns. The total energy used was 40 J. The solid material
was set on the positive portion of the z axis, starting at z=0.
In the single foil simulations the d=0.5 µm foil was set between z=0 and z=0.5 µm,
and irradiated by focusing with F/1 optics behind the target at z=-0.015 cm, or at
z=–0.03 cm or at z=-0.045. Some of the findings are reported in table 5.I.
The quantity t b in table 5.I represents the time when the foil becomes transparent to
the laser light (due to ablation and transverse expansion), whereas t s represents the
time when the accelerated matter is displaced by a distance, towards negative z,
s=75 µm, the “pores” size. For the cases listed in table 5.I, in spite of the twodimensional
effects, t b >t s . This means that, in a multi-layer structure, matter will be
accumulated from the irradiated layer upon the following one, so that the light will
become faced with an even thicker foil (and so on). In other words a sort of
snowplough process occurs, as mentioned in previous papers in which the
structured nature of the material was not considered. In the following we report
some results for case 3. In figure 5.3 the fraction of absorbed light is represented
(abscissa is time in ns). The absorption drops sharply near the transparency time t b .
In the following some quantities are represented just before t b . In figure 5.4 the