Chapter 7
Relativistic Laser and Solid Target
Interactions
7.1 Pre-formed Plasma in Laser-Solid Interaction
7.1.1 Pedestal of Laser Pulse
In the early time of the experimental research of relativistic laser-solid interaction, it
was very difficult to obtain reproducible data from experiments, because it was very
hard technically to obtain the same intensity profile after the pulse compression by
the chirped pulse amplification (CPA). The main technical challenge is so-called
pedestal before the main pulse, ultra-short pulse of relativistic intensity. The
pedestal is orders of magnitude longer pulse with orders of magnitude lower
intensity pre-formed light before the main pulse. In many experiments, the
pre-plasmas produced by the pedestal are formed as seen below. As a result, the
idealistic theory such as the vacuum heating and higher harmonic generations are not
well applicable to a lot of data observed experimentally. A typical pedestal and main
pulse intensity is plotted as a function of time in Fig. 7.1 [1]. The measured contrast
ratio of the initial pulse from a commercial Ti:Sapphire CPA laser is about 2 Â 10
À8
at tens of picoseconds before the main pulse as shown with the black curve. Such a
contrast ratio can be improved to 10
À10 like the red curve. It is noted that
theoretically even more improvement to 10
À13 to 10
À14 is possible.
The effect of the pedestal pulse on the generation of pre-formed plasma before the
arrival of the main pulse is studied by use of hydrodynamic code with reasonable
physics modeling [2]. The parameters of the pedestal and target are taken from a real
experiment, where the aluminum foil is irradiated by a main laser which is 10
19 W/
cm
2 , the contrast ratio of pedestal is 2 Â 10
À9 , and length of pedestal is 1 ns. It is
studied by a hydrodynamic simulation code like the same as explained in Chap. 3.
The intensity of the pedestal beam is 2 Â 10
10 W/cm
2 . It should be pointed out that
the electron-ion collision frequency at low temperature is very high, and the laser
photons are absorbed every 10
–15–16 s in aluminum solid as shown in Fig. 3.16. This
© Springer Nature Switzerland AG 2020
H. Takabe, The Physics of Laser Plasmas and Applications - Volume 1, Springer
Series in Plasma Science and Technology,
https://doi.org/10.1007/978-3-030-49613-5_7
239
Relativistic Laser and Solid Target
Interactions
7.1 Pre-formed Plasma in Laser-Solid Interaction
7.1.1 Pedestal of Laser Pulse
In the early time of the experimental research of relativistic laser-solid interaction, it
was very difficult to obtain reproducible data from experiments, because it was very
hard technically to obtain the same intensity profile after the pulse compression by
the chirped pulse amplification (CPA). The main technical challenge is so-called
pedestal before the main pulse, ultra-short pulse of relativistic intensity. The
pedestal is orders of magnitude longer pulse with orders of magnitude lower
intensity pre-formed light before the main pulse. In many experiments, the
pre-plasmas produced by the pedestal are formed as seen below. As a result, the
idealistic theory such as the vacuum heating and higher harmonic generations are not
well applicable to a lot of data observed experimentally. A typical pedestal and main
pulse intensity is plotted as a function of time in Fig. 7.1 [1]. The measured contrast
ratio of the initial pulse from a commercial Ti:Sapphire CPA laser is about 2 Â 10
À8
at tens of picoseconds before the main pulse as shown with the black curve. Such a
contrast ratio can be improved to 10
À10 like the red curve. It is noted that
theoretically even more improvement to 10
À13 to 10
À14 is possible.
The effect of the pedestal pulse on the generation of pre-formed plasma before the
arrival of the main pulse is studied by use of hydrodynamic code with reasonable
physics modeling [2]. The parameters of the pedestal and target are taken from a real
experiment, where the aluminum foil is irradiated by a main laser which is 10
19 W/
cm
2 , the contrast ratio of pedestal is 2 Â 10
À9 , and length of pedestal is 1 ns. It is
studied by a hydrodynamic simulation code like the same as explained in Chap. 3.
The intensity of the pedestal beam is 2 Â 10
10 W/cm
2 . It should be pointed out that
the electron-ion collision frequency at low temperature is very high, and the laser
photons are absorbed every 10
–15–16 s in aluminum solid as shown in Fig. 3.16. This
© Springer Nature Switzerland AG 2020
H. Takabe, The Physics of Laser Plasmas and Applications - Volume 1, Springer
Series in Plasma Science and Technology,
https://doi.org/10.1007/978-3-030-49613-5_7
239