Chapter 6
Relativistic Laser Plasma Interactions
6.1 Charge Separation in Low-Density Plasma
In Sect. 5.3, we have solved two cases for α ¼ 1 and <γ>. Study here the case when
laser is impinging to low-density plasmas and the free electron model (α ¼ 1) is
applicable. In order to make the mathematic simpler, consider the case of circular
polarization. Then, γ ¼ a
2
0 =2 þ 1 γ 0
ð
Þ is constant. Let us estimate under what
condition the free electron model is validated by evaluating the accumulation of
the charge separation from the plasma boundary.
6.1.1 Charge Separation by Photon Force
So far, an infinite plan wave is assumed to describe the electron motion in strong
laser field. This is good assumption if the laser pulse is very long compared to the
laser wavelength, and the focusing diameter is much larger than the laser wavelength. Based on the results above, however, let us consider what kind of physics is
imagined intuitively for a given density of charge neutral plasma. This intuitive
consideration may be important to plasma dynamics in the early time just after the
laser irradiation, and the ions can be assumed not to move because of large mass.
Consider the laser is impinged in the plasma whose electron density is n 0 and the
electrons move forward due to the force in x-direction with the drift velocity
(5.3.25). As shown in Fig. 6.1, it is reasonable to assume that the electrons move
to the right until the time when the effective pressure due to coupling with laser field
will balance with the pull-back force by the charge separation.
Consider one-dimensional system, and assume that the electrons are decelerated
by the electric field E es produced by the charge separation:
© 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_6
203
Relativistic Laser Plasma Interactions
6.1 Charge Separation in Low-Density Plasma
In Sect. 5.3, we have solved two cases for α ¼ 1 and <γ>. Study here the case when
laser is impinging to low-density plasmas and the free electron model (α ¼ 1) is
applicable. In order to make the mathematic simpler, consider the case of circular
polarization. Then, γ ¼ a
2
0 =2 þ 1 γ 0
ð
Þ is constant. Let us estimate under what
condition the free electron model is validated by evaluating the accumulation of
the charge separation from the plasma boundary.
6.1.1 Charge Separation by Photon Force
So far, an infinite plan wave is assumed to describe the electron motion in strong
laser field. This is good assumption if the laser pulse is very long compared to the
laser wavelength, and the focusing diameter is much larger than the laser wavelength. Based on the results above, however, let us consider what kind of physics is
imagined intuitively for a given density of charge neutral plasma. This intuitive
consideration may be important to plasma dynamics in the early time just after the
laser irradiation, and the ions can be assumed not to move because of large mass.
Consider the laser is impinged in the plasma whose electron density is n 0 and the
electrons move forward due to the force in x-direction with the drift velocity
(5.3.25). As shown in Fig. 6.1, it is reasonable to assume that the electrons move
to the right until the time when the effective pressure due to coupling with laser field
will balance with the pull-back force by the charge separation.
Consider one-dimensional system, and assume that the electrons are decelerated
by the electric field E es produced by the charge separation:
© 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_6
203