shown for three times when the wave-breaking happens. It is seen that many fluid
elements are oscillated at the same point in the top of the left figures, and the next
time, the small fraction of the elements are trapped by the wave phase in the next
neighbor to be continuously accelerated selectively. The trapped electrons are
ejected with higher velocity from the region of resonant point. They are called
high-energy electrons or hot electrons.
From Fig. 3.24, in order to obtain substantial absorption in finite angle θ, the
density scale length L should be of the order of the laser wavelength (kLsinθ ~ 1). It
looks like not the case of laser plasma where hydrodynamic expansion makes the
density scale length longer as the time goes. It will be clear, however, in the later
session that the laser photon pressure will modify the density profile near the critical
density and makes it of the order of unity.
As shown in Fig. 2.42, a small fraction of electrons deviates from its initial phase
and escapes from the plasma oscillation region with high energy. This is one
example of high-energy electron production process, and the absorbed laser energy
is converted not to the bulk electron heating but is given to selected electrons in
plasma. All collisionless heating processes are due to such energy deposition
physics, and laser energy is once transferred to the selected electrons and relaxes
to the thermal energy of the bulk electrons through classical collisional process.
The energy of the high-energy electron is roughly estimated in this case like:
E he ¼ mV
2
¼ eLE d
ð3:9:16Þ
This is the work done by laser electric field over the distance L. This is intuitively
reasonable evaluation. Roughly evaluate the energy of high-energy electrons accelerated in such scheme. Using the value of laser field E ¼ 6 Â 10
8 V/m at
I L ¼ 10
15 W/cm
2 , the energy is about 6 keV for L ¼ 10 μm.
3.10 Vacuum Heating
When an ultra-short laser pulse is irradiated on solid density target, it is already
discussed about the classical absorption in Sect. 3.2. If the laser intensity is high
enough and the collisional absorption can be neglected, what kind of collisionless
absorption process may happen? It is proposed [21] that the following physical
process happens in a very short time, during which the ions have no time to move,
and the electrons in the solid target are affected by a strong laser field penetrated into
the solid surface due to the plasma skin effect, which is the same as the tunneling
effect of the laser field into the higher-density region. This is the case of
p-polarization as shown in Fig. 3.33, where the oscillating electric field penetrates
into solid plasmas over the skin depth. This electric field drives the following
electrons motions in the solid and vacuum regions.
3.10 Vacuum Heating
123
elements are oscillated at the same point in the top of the left figures, and the next
time, the small fraction of the elements are trapped by the wave phase in the next
neighbor to be continuously accelerated selectively. The trapped electrons are
ejected with higher velocity from the region of resonant point. They are called
high-energy electrons or hot electrons.
From Fig. 3.24, in order to obtain substantial absorption in finite angle θ, the
density scale length L should be of the order of the laser wavelength (kLsinθ ~ 1). It
looks like not the case of laser plasma where hydrodynamic expansion makes the
density scale length longer as the time goes. It will be clear, however, in the later
session that the laser photon pressure will modify the density profile near the critical
density and makes it of the order of unity.
As shown in Fig. 2.42, a small fraction of electrons deviates from its initial phase
and escapes from the plasma oscillation region with high energy. This is one
example of high-energy electron production process, and the absorbed laser energy
is converted not to the bulk electron heating but is given to selected electrons in
plasma. All collisionless heating processes are due to such energy deposition
physics, and laser energy is once transferred to the selected electrons and relaxes
to the thermal energy of the bulk electrons through classical collisional process.
The energy of the high-energy electron is roughly estimated in this case like:
E he ¼ mV
2
¼ eLE d
ð3:9:16Þ
This is the work done by laser electric field over the distance L. This is intuitively
reasonable evaluation. Roughly evaluate the energy of high-energy electrons accelerated in such scheme. Using the value of laser field E ¼ 6 Â 10
8 V/m at
I L ¼ 10
15 W/cm
2 , the energy is about 6 keV for L ¼ 10 μm.
3.10 Vacuum Heating
When an ultra-short laser pulse is irradiated on solid density target, it is already
discussed about the classical absorption in Sect. 3.2. If the laser intensity is high
enough and the collisional absorption can be neglected, what kind of collisionless
absorption process may happen? It is proposed [21] that the following physical
process happens in a very short time, during which the ions have no time to move,
and the electrons in the solid target are affected by a strong laser field penetrated into
the solid surface due to the plasma skin effect, which is the same as the tunneling
effect of the laser field into the higher-density region. This is the case of
p-polarization as shown in Fig. 3.33, where the oscillating electric field penetrates
into solid plasmas over the skin depth. This electric field drives the following
electrons motions in the solid and vacuum regions.
3.10 Vacuum Heating
123