Proceedings of International Conference on Physics in ... - KEK

Reflected light intensity (μJ/sr/nm)
0.5
0.4
0.3
0.2
0.1
0
12 14 16 18 20 22
Wavelength (nm)
Figure 2: Observed spectrum <strong>of</strong> the reflected laser light in
the flying mirror experiment [12].
was measured with a grazing-incidence flat-field spectrograph
in the forward (0 ◦ ) direction. The observed spectra
distributed in the range <strong>of</strong> 7–15 nm. The upshift factors
were 50–114.
Although the experiment showed a somewhat coherent
effect, the reflected photon number was smaller than the
theoretical expectation. In the 2nd experiment[12], a headon
collisions setup <strong>of</strong> the driver and source pulses was employed
thanks to the improvement <strong>of</strong> the laser system and
a more powerful laser was used. The driver and source
laser had peak powers <strong>of</strong> 15 TW(400 mJ/27 fs) and 1.2
TW(42 mJ/ 34 fs), respectively. Reflected source pulses
were observed with an imaging spectrograph that covered
the wavelength range <strong>of</strong> 12-25 nm at the angle range <strong>of</strong>
9 ◦ − 17 ◦ . The result is shown in Fig. 2. The reflected photon
number increased to half <strong>of</strong> the theoretical cusp model
(see Table 2).
To increase the reflectivity further, we can use a variant
<strong>of</strong> Eq. (5) assuming ωd ̸= ωs and θ = 0,
Rδ ≈
( ) 2
ωd 1
ωs 2γ3 ph
. (9)
If ωd > ωs, we obtain a gain <strong>of</strong> (ωd/ωs) 2 . For example,
we can use a frequency doubling crystal for the driver laser.
Table 2: Comparison <strong>of</strong> reflectivities and photon numbers
between the theoretical model and the experiment.
Reflectivity Photon number
Theory 4×10 −5 1.5×10 10
Experiment 3×10 −6 1.1×10 9
Exp. (corrected) 2×10 −5 7.9×10 9
There are no measurement so far on the pulse duration
<strong>of</strong> a reflected pulse from flying mirrors. However, the large
photon number obtained in the experiment implies that the
coherent effect <strong>of</strong> the electrons exists. Therefore, we may
Intensity (arb. u.)
-3
-3
0
300 400 500
Figure 3: Observed spectrum in a high-resolution particlein-cell
simulation.
be allowed to assume that the pulse duration is compressed
down to the theoretical model prediction.
Most serious and to be confirmed is the spot size <strong>of</strong> the
reflected pulse. There are many points that degrades the
focus spot such as a surface error from an ideal parabola,
surface roughness, alignment error to the flying mirror,
etc. These points have not yet been addressed both experimentally
and theoretically. We have just started to estimate
the roughness <strong>of</strong> the flying mirror. As a preliminary
result we have obtained that the surface roughness
in two-dimensional particle-in-cell simulations is smaller
than the simulation resolution 0.02 µm. Further analysis
is needed to determine the focusability <strong>of</strong> the reflected
pulse, especially at low densities (high γph) in Table 1. We
also obtained the upshift factor <strong>of</strong> ωr/ωs ∼500 in a highresolution
particle-in-cell simulation as shown in Fig. 3.
Here the driver laser has an energy <strong>of</strong> 2.3 J and a pulse
duration <strong>of</strong> 20 fs, and the focused irradiance <strong>of</strong> 3.7×10 19
W/cm 2 (a0 = 4.1). The source laser has a wavelength <strong>of</strong>
2.4 µm in this simulation. The plasma density is 1.5×10 19
cm −3 and mesh sizes are ∆x =0.8 nm and ∆y=20 nm. Although
the resolved wavelength is larger than the requirement
in Table 1, water-window or shorter wavelength Xrays
are generated in the simulation.
CONCLUSION
A relativistic flying mirror is examined as a tool to access
ultra-high electromagnetic field irradiance. We carved
out laser and plasma parameters assuming plasma and laser
parameters that are not far from present-day laser technologies.
We reviewed the recent experimental progress <strong>of</strong> the
flying mirror. The concept is in progress showing that the
reflectivity is close to the theoretical estimate using a cusp
model. Still further study is necessary to intensify the focused
laser.