intensity is at 1.5 ps; then the second beam arrives to accelerate the electron mainly
in the expanding plasma region (x > 55 μm). During this time the electron gains large
amount of energy as seen in the time evolution of its Lorentz factor in Fig. 7.12b.
This re-circulating acceleration is maintained by very high sheath potential, and the
time evolution of the rear side of the potential is shown in Fig. 7.12c. It is noted that
the laser-focusing diameter was about 200 μm and the electron bounce distance is
about 30 μm; therefore, 1D PIC simulation is applicable to this case.
Summarize the present understanding about the laser absorption for the ultrashort relativistic intensity pulse irradiated on solid targets. In the case of ultra-short
pulse with the pulse duration less than 100 ~ 200 fs, the absorption rate is very
sensitive to the pre-pulse contrast ratio as shown in Fig. 7.8. With enough
pre-formed plasmas, almost 100% of absorption is measured. Once the pulse
duration becomes longer, the absorption rate is high as seen in Fig. 7.10, even if
the pedestal is suppressed. For long pulse the ion motion is effective to form the
plasma during the main pulse, and the absorption is enhanced if the re-circulation of
high-energy electrons is expected for the large focal spot size, where the electrons
are confined by sheath potentials at the front and rear side of targets.
7.4 Hole Boring by Ponderomotive Force
We have discussed the experimental and computational study on the laser absorption
near the solid surface with and without the pre-formed plasma, ion motions,
re-circulation, and so on. Here consider how the laser ponderomotive force interacts
with the plasma relatively lower density than the normal solid density. Strong holepunching phenomena, now-called hole boring, has been demonstrated with 2D PIC
simulation [11]. Plasma slab with electron density of 4n c (critical density) is
irradiated at 1.2 Â 10
19 W/cm
2 (a 0 ¼ 3). The laser is p-polarized in x-y of the 2D
plane and irradiated normally on the slab plasma. No resonance absorption is
observed, while the laser ponderomotive force of JxB shown in (6.3.7) affects the
plasmas non-adiabatically, resulting the laser absorption via hot electron generation.
In the simulation, the focusing width is 14 c/ω (~2.5 μm) in the simulation size in
y-direction of 40 c/ω. Due to the non-oscillatory component of the ponderomotive
force in (6.3.7), the plasma surface in the laser focal spot is found to move to the laser
propagation direction. This is the boring by the light beam pressure. Evaluate the
pressure of the irradiated laser. The photon momentum p ph and the laser energy flux
I L are defined as
p ph ¼ ħk ¼ ħω=c,
I L ¼ ħωN ph c
ð7:4:1Þ
Then, the photon pressure P L is defined and given as
252
7 Relativistic Laser and Solid Target Interactions
in the expanding plasma region (x > 55 μm). During this time the electron gains large
amount of energy as seen in the time evolution of its Lorentz factor in Fig. 7.12b.
This re-circulating acceleration is maintained by very high sheath potential, and the
time evolution of the rear side of the potential is shown in Fig. 7.12c. It is noted that
the laser-focusing diameter was about 200 μm and the electron bounce distance is
about 30 μm; therefore, 1D PIC simulation is applicable to this case.
Summarize the present understanding about the laser absorption for the ultrashort relativistic intensity pulse irradiated on solid targets. In the case of ultra-short
pulse with the pulse duration less than 100 ~ 200 fs, the absorption rate is very
sensitive to the pre-pulse contrast ratio as shown in Fig. 7.8. With enough
pre-formed plasmas, almost 100% of absorption is measured. Once the pulse
duration becomes longer, the absorption rate is high as seen in Fig. 7.10, even if
the pedestal is suppressed. For long pulse the ion motion is effective to form the
plasma during the main pulse, and the absorption is enhanced if the re-circulation of
high-energy electrons is expected for the large focal spot size, where the electrons
are confined by sheath potentials at the front and rear side of targets.
7.4 Hole Boring by Ponderomotive Force
We have discussed the experimental and computational study on the laser absorption
near the solid surface with and without the pre-formed plasma, ion motions,
re-circulation, and so on. Here consider how the laser ponderomotive force interacts
with the plasma relatively lower density than the normal solid density. Strong holepunching phenomena, now-called hole boring, has been demonstrated with 2D PIC
simulation [11]. Plasma slab with electron density of 4n c (critical density) is
irradiated at 1.2 Â 10
19 W/cm
2 (a 0 ¼ 3). The laser is p-polarized in x-y of the 2D
plane and irradiated normally on the slab plasma. No resonance absorption is
observed, while the laser ponderomotive force of JxB shown in (6.3.7) affects the
plasmas non-adiabatically, resulting the laser absorption via hot electron generation.
In the simulation, the focusing width is 14 c/ω (~2.5 μm) in the simulation size in
y-direction of 40 c/ω. Due to the non-oscillatory component of the ponderomotive
force in (6.3.7), the plasma surface in the laser focal spot is found to move to the laser
propagation direction. This is the boring by the light beam pressure. Evaluate the
pressure of the irradiated laser. The photon momentum p ph and the laser energy flux
I L are defined as
p ph ¼ ħk ¼ ħω=c,
I L ¼ ħωN ph c
ð7:4:1Þ
Then, the photon pressure P L is defined and given as
252
7 Relativistic Laser and Solid Target Interactions