higher energy in the foam plasma. The physics is as follows. Focus laser on the foam
surface makes well-defined plasma channel confining most of the laser energy
because of the plasma self-focusing in Sect. 6.5. Then, the repelled electrons from
the channel by the ponderomotive force induce radial ambipolar field. Then, some
electrons oscillate in the potential of the ambipolar field. This is called betatron
oscillation. Since the electrons have also the velocity in the channel direction, some
of them can resonate with the laser electric field to keep accelerated by the laser
electric field while increasing the amplitude of the betatron oscillation [23]. The
authors in [21] concluded that the channel formation and betatron acceleration
enhance the hot electron generation in the foam plasma region.
7.8 Efficient Absorption in Structured Targets
How to increase absorption efficiency of relativistic ultra-short lasers is a critical
issue for any kind of application via plasma formation. Since the laser-plasma
interaction in this regime is due to collisionless interactions and most of absorbed
energies are converted to the energy of relativistic high-energy electrons. A simple
strategy to enhance the collisionless interactions is to increase the surface and
volume of the interaction region. However, pre-formed plasmas are not controllable
as seen already. Since the laser cannot penetrate to the solid density, the use of
low-density material like type of foam is beneficial for higher absorption, higher
number of hot electrons, and their temperature. However, the interaction of
relativistic laser and near critical density plasma is subject to many nonlinear
instabilities, such as parametric instabilities, self-focusing, density modification,
etc. For better coupling of laser and targets, several structured targets are proposed
to keep the higher energy conversion to plasma by controlling plasma density profile
interacting with lasers.
7.8.1 Micro-pillar Array Targets
Micro-pillar or nano-pillar arrays are proposed for laser-matter interaction region for
efficient absorption and generation of higher energy hot electrons [24]. In Fig. 7.32,
a schematic is shown for target structure used for micron size pillar target
experiment. The target has a pillar of 1.5 μm in diameter spaced with 2.5 μm to
the next pillar as shown in a stretched view. The laser is PHELIX, GSI, 1 μm
wavelength, 500 fs pulse duration. Intensity is 1 Â 10
17 W/cm
2 (spot size;
300 μm) and 2 Â 10
18 W/cm
2 (spot size; 100 μm); thus, the number of pillars in
the spot is 7000 and 800, respectively. ASE pedestal contrast is 10
À10 to keep ultrahigh contrast in the experiment. It is demonstrated in this experiment that the total
amount of hot electrons are substantially enhanced by use of the micro-pillar targets
compared to the flat solid targets.
7.8 Efficient Absorption in Structured Targets
273
surface makes well-defined plasma channel confining most of the laser energy
because of the plasma self-focusing in Sect. 6.5. Then, the repelled electrons from
the channel by the ponderomotive force induce radial ambipolar field. Then, some
electrons oscillate in the potential of the ambipolar field. This is called betatron
oscillation. Since the electrons have also the velocity in the channel direction, some
of them can resonate with the laser electric field to keep accelerated by the laser
electric field while increasing the amplitude of the betatron oscillation [23]. The
authors in [21] concluded that the channel formation and betatron acceleration
enhance the hot electron generation in the foam plasma region.
7.8 Efficient Absorption in Structured Targets
How to increase absorption efficiency of relativistic ultra-short lasers is a critical
issue for any kind of application via plasma formation. Since the laser-plasma
interaction in this regime is due to collisionless interactions and most of absorbed
energies are converted to the energy of relativistic high-energy electrons. A simple
strategy to enhance the collisionless interactions is to increase the surface and
volume of the interaction region. However, pre-formed plasmas are not controllable
as seen already. Since the laser cannot penetrate to the solid density, the use of
low-density material like type of foam is beneficial for higher absorption, higher
number of hot electrons, and their temperature. However, the interaction of
relativistic laser and near critical density plasma is subject to many nonlinear
instabilities, such as parametric instabilities, self-focusing, density modification,
etc. For better coupling of laser and targets, several structured targets are proposed
to keep the higher energy conversion to plasma by controlling plasma density profile
interacting with lasers.
7.8.1 Micro-pillar Array Targets
Micro-pillar or nano-pillar arrays are proposed for laser-matter interaction region for
efficient absorption and generation of higher energy hot electrons [24]. In Fig. 7.32,
a schematic is shown for target structure used for micron size pillar target
experiment. The target has a pillar of 1.5 μm in diameter spaced with 2.5 μm to
the next pillar as shown in a stretched view. The laser is PHELIX, GSI, 1 μm
wavelength, 500 fs pulse duration. Intensity is 1 Â 10
17 W/cm
2 (spot size;
300 μm) and 2 Â 10
18 W/cm
2 (spot size; 100 μm); thus, the number of pillars in
the spot is 7000 and 800, respectively. ASE pedestal contrast is 10
À10 to keep ultrahigh contrast in the experiment. It is demonstrated in this experiment that the total
amount of hot electrons are substantially enhanced by use of the micro-pillar targets
compared to the flat solid targets.
7.8 Efficient Absorption in Structured Targets
273