depends on the scale-length of pre-formed plasma as suggested in Fig. 7.5 for the
case of laser intensity 2 Â 10
19 W/cm
2 and very short pulse 20–25 fs. A systematic
comparison among three different lasers has been done to compare the intensity
dependence of laser absorption, etc. [6]. Attention is focused on the effect of
intensity contrast and consequent energy fraction of the pedestal by ASE to the
main laser energy. Experimental data for the best contrast case with Astra Gemini
laser are obtained under the following condition [7].
The laser energy is 12 J and the pulse duration of 50 fs. Its wavelength is 800 nm.
The laser is irradiated on a flat thin aluminum foil of 0.1 μm with p-polarization at
angle of 35 degree, and intensity is varied from 10
17 W/cm
2 to 10
21 W/cm
2 . The
focusing diameter is 2.5 μm. The intensity contrast ratio of pedestal to the main pulse
is 10
À10 , while the energy ratio is measured to be 2 Â 10
À5 , which value was almost
the best in 2009. In high contrast and very short pulse, some fraction of laser energy
10
11
12
t / T
L / λ = 0
a
b
c
x / λ
L / λ = 0.02
L / λ = 0.2
10
8
6
4
2
0
-0.1
10 0
10 -1
10 -2
0
2
4
6
8
0
2
4
6
Harmonic number
Norm.energy
ω /
ω
0
ω p
ω p
8
0
2
4
6
8 10
-0.1
X cr
X cr
n
e /n
cr
-0.8
-0.4
log(|j
⊥ (ω)/j
⊥max |)
50
40
30
20
10
-2
-1
0
0
0
0
Fig. 7.6 Electron density (top), spectra of the transverse current (middle), and harmonic spectra
(bottom) for p-polarized excitation with a 0 ¼ 0.3. The circles (bottom left) represent the spectrum
calculated for a harmonically oscillating mirror with X 0 /λ ¼ 0.06. The dashed lines indicate the
critical surface (middle) and ω p (bottom). Red dotted lines represent ω p (x). [Figure 3 in Ref. 5]
7.2 Laser Absorption at Solid Targets
245