plasma condition. It is not simple to show how to design the laser and solid target to
obtain absorption efficiency as high as possible.
Regarding the effect of pre-formed plasmas, it is concluded that the pre-formed
plasmas help increasing the absorption rate. Comparing the absorption rate from
Fig. 7.8 with pre-formed plasma by pedestal beam to that shown in Fig. 7.21, it is
possible to conclude that at a given intensity, the absorption rate increases with the
decrease of the pre-formed plasma density, where the laser interacts with the plasma.
This is roughly speculated from (7.6.1) to (7.6.2) as follows. From both relation, we
can obtain an approximate relation:
R %
n h
2I L
1 þ
Mn i u i
m e n h c
m e c
2
ð7:6:3Þ
Roughly speaking, the decrease of the density n h and n i reduces the reflection
fraction for a given laser intensity I L . The main reason for the scattering of the
experimental and computational data of laser absorption in Fig. 7.21 is speculated to
be due to the density of laser-plasma interaction region.
It is, therefore, suggested that multilayer target with a low-density foam layer in
front of the main solid target may increase the energy conversion from relativistic
laser to the plasmas. Recently, structured targets are used to control not only laser
absorption but also other characteristics such as hot electron beam collimation,
enhanced X-ray, gamma-ray emission, and so on. Let us see briefly the physics of
the interaction of relativistic ultra-short laser with such targets.
7.7 Enhanced Coupling with Foam Layered Targets
We now know that clean ultra-short relativistic lasers irradiated on solid targets are
convenient to the generation of higher harmonics via solid surface oscillation.
However, it is not convenient to obtain higher laser energy absorption. In general,
higher laser absorption is reported, however, for the case with pre-formed plasma by
the pedestal (see Fig. 7.8). With an advancement of the laser technology, the fraction
of the pedestal to the main laser, the contrast ratio, decreases dramatically so that the
pre-formed plasma effect due to the pedestal is not significant in the recent
experiments. As a result, it becomes clear that the laser absorption without
pre-formed plasma is not efficient in irradiating on the conventional solid targets.
Now, a lot of research is focused on a variety of target design so that higher
absorption efficiency is expected. As known well, laser energy is absorbed via a
variety of collisionless nonlinear plasma process. Therefore, the increase of laser
absorption means how highly laser energy is converted to the generation of hot
electrons, the number of particles, and the temperature of the hot electrons. One
simple way is not to reduce the energy of pedestal and provides pre-formed plasma
by pedestal or another laser beam on purpose. Even in this case, controlling the
7.7 Enhanced Coupling with Foam Layered Targets
263
obtain absorption efficiency as high as possible.
Regarding the effect of pre-formed plasmas, it is concluded that the pre-formed
plasmas help increasing the absorption rate. Comparing the absorption rate from
Fig. 7.8 with pre-formed plasma by pedestal beam to that shown in Fig. 7.21, it is
possible to conclude that at a given intensity, the absorption rate increases with the
decrease of the pre-formed plasma density, where the laser interacts with the plasma.
This is roughly speculated from (7.6.1) to (7.6.2) as follows. From both relation, we
can obtain an approximate relation:
R %
n h
2I L
1 þ
Mn i u i
m e n h c
m e c
2
ð7:6:3Þ
Roughly speaking, the decrease of the density n h and n i reduces the reflection
fraction for a given laser intensity I L . The main reason for the scattering of the
experimental and computational data of laser absorption in Fig. 7.21 is speculated to
be due to the density of laser-plasma interaction region.
It is, therefore, suggested that multilayer target with a low-density foam layer in
front of the main solid target may increase the energy conversion from relativistic
laser to the plasmas. Recently, structured targets are used to control not only laser
absorption but also other characteristics such as hot electron beam collimation,
enhanced X-ray, gamma-ray emission, and so on. Let us see briefly the physics of
the interaction of relativistic ultra-short laser with such targets.
7.7 Enhanced Coupling with Foam Layered Targets
We now know that clean ultra-short relativistic lasers irradiated on solid targets are
convenient to the generation of higher harmonics via solid surface oscillation.
However, it is not convenient to obtain higher laser energy absorption. In general,
higher laser absorption is reported, however, for the case with pre-formed plasma by
the pedestal (see Fig. 7.8). With an advancement of the laser technology, the fraction
of the pedestal to the main laser, the contrast ratio, decreases dramatically so that the
pre-formed plasma effect due to the pedestal is not significant in the recent
experiments. As a result, it becomes clear that the laser absorption without
pre-formed plasma is not efficient in irradiating on the conventional solid targets.
Now, a lot of research is focused on a variety of target design so that higher
absorption efficiency is expected. As known well, laser energy is absorbed via a
variety of collisionless nonlinear plasma process. Therefore, the increase of laser
absorption means how highly laser energy is converted to the generation of hot
electrons, the number of particles, and the temperature of the hot electrons. One
simple way is not to reduce the energy of pedestal and provides pre-formed plasma
by pedestal or another laser beam on purpose. Even in this case, controlling the
7.7 Enhanced Coupling with Foam Layered Targets
263