23P151-P
Hot Electron Measurement using Electron Spectrometer on FIREXin Gekko XII
激光12号高速点火実験における電子スペクトロメーターによる高速電子計測
Tetsuo Ozaki, Mayuko Mayuko1, Hiroyuki Shiraga1, Hiroshi Azechi1, Hitoshi Sakagami andFIREX Group1
尾﨑 哲、古賀麻由子1、白神宏之1、疇地宏1、坂上仁志、FIREX実験グループ1
National Institute for Fusion Science, 322-6, Oroshi, Toki, Gifu, 509-5292, Japan
核融合科学研究所 〒509-5292岐阜県土岐市下石町322-6
1
Institute of Laser Engineering Osaka Univ., 2-6, Yamada-oka, Suita, Osaka, Japan
大阪大学エネルギー学研究センター, 〒565-0871大阪府吹田市山田丘2-6
The high-energetic electron measurement is one of the most important issues to research the ignition
mechanism. Hot electrons are generated by a strong electric field based on the interaction between the highintensity laser and the produced pre-plasma. The pre-formed plasma is found to be created by the
imploding laser in addition to the pre-pulse of LFEX. It is important to suppress the pre-formed plasma inorder to achieve the high coupling efficiency between the imploded core and the hot electron.

1. Introduction
It is important to deposit hot electrons to theimploded core in order to realize fast ignition.
According to simulations of the integrated target
with a gold (Au) cone of 7 µm-thickness and 500µm diameter deuterium polystyrene (thickness of 6µm) with the combination of LFEX (maximumenergy 10 kJ, 1.06 µm, ω) and Gekko XII (20 kJ,0.53 µm, 2ω) lasers, the suitable energy of the hot
electron to obtain high coupling efficiency is lessthan 1 MeV. The energy of the hot electron depends
on the scale length of the pre-formed plasma, whichis created mainly by the pre-pulse. The hot electron
penetrates the imploded core when the energy is toohigh. Therefore the suppression of the pre-pulse isessential to achieve fast ignition. Here we measure
the hot electron energy spectra by using the electron
spectrometer [1]. The effective temperatures (Teff),
which are induced from the slopes of the spectra arecompared at various targets and conditions.
2. Relation between pre-formed plasma and Teff
From the results of a PetaWatt laser [2] and thesimulation, the absorption and heating efficienciesof the laser can be expected to improve at the shortpulse and the high intensity. We compare the Teff
between 1.5 ps and 4 ps pulses. At the short pulse,
Teff is larger but it is larger than the expected value
from the scaling. The relation between the scale
length and Teff can be modified from the Puhkov [3]scaling as,
TeffR(α/3.546)（I/I18）1/2G（2.465 L1/4 -2.223）, (1)

100

10

Plain,Cone& Integrated
AlAu
Cone-wireCone+CDblock
CHConeOnly
WCone+CuPlate
wo Implosion
Implosion 9beams
Implosion 12beams
Pukhov(Te,15μm)
Pukhov(Te,30μm)
Pukhov(Te,60μm)
Pukhov(Te,150μm)
Pukhov(Te,300μm)
Pukhov(Te,600μm)

1 17181920101010102
LFEX(W/cm )(1.5ps,50µµ mφ
φ ,70%x50%)
Fig. 1. Teff vs LFEX power in various targets.
Teff depends on the target shape due to the pre-plasma
formation.

where α, I, I18 and L are the coefficient factor of 1.5
MeV, the laser intensity in W/cm2, the normalized
factor of 1018 and the scale length in µm,respectively. Teff increases over the scaling value by
using the short pulse of LFEX. This suggests theeffect of the strong pre-pulse in the short pulseduration (=high intensity).
To reduce the pre-pulse, a saturable absorber isinstalled in the LFEX laser oscillator. Teff canslightly decrease due to one tenth of the pre-pulse
reduction. However the higher Teff is not only due
to the pre-formed plasma by the pre-pulse because
Teff in the integrated target is higher than that in theplain target (see Fig. 1). The saturable absorbershould be improved because the pre-plasma
suppression becomes worse after many dischargesof the LFEX.
In the integrated target, there is a possibility thatthe imploding lasers create the pre-formed plasma
by irradiating the Au cone, in addition to thepre-plasma formed by the pre-pulse of the LFEX.
Although the omega component of the GXII laser
before the wavelength conversion has a different
focal point, it may partially irradiate the inner partof the Au cone. The imploded core may collidewith the cone tip and form the pre-plasma at theinner part of the cone.
We compare the Teff in plain, cone and integrated
targets. The Teff in the cone is higher than the Teff in
the plain target. The scale length of the pre-plasma
in the cone formed by the pre-pulse of the LFEX islonger than that in plain target because thepre-plasma jets from the cone surface make highdense pre-plasma on the axis of the cone. In theintegrated target, the electron acceleration
mechanism is complicated. The effects ofpre-formed plasmas by the implosion and theimploding laser should be considered in addition tothe pre-plasma formed by the pre-pulse. As shownin Fig. 1, the Teff in the implosion is obviously
higher than the Teff in the cone.
Part of omega light irradiates the inner cone. In
the standard integrated experiments, 9 beams of 12
beams of GXII are used in order to avoid direct
irradiation to the cone. Omega lights from 3 beams
around the cone partially irradiate the inner cone.
The distance between the position irradiated by theomega light and LFEX laser is 250 µm. The
clearance is only 200 µm because the focal point ofthe LFEX laser fluctuates around ±50 µm. The
pulse width and the intensity of the imploding laserare 1 ns and 1012 W/cm2, respectively. According tothe simulation, the growth of the pre-formed plasma
becomes 200 µm. Therefore the plasma pre-formedby the imploding laser affects the generation of thehot electron.
The imploded core may destroy the cone tip andmay affect the hot electron spectrum. We also
perform the integration experiment using full beams(12 beams) of the GXII laser in addition to 9 beam
implosion. In the 12 beams case, the focal positionis shifted in order to avoid the direct irradiation of
the neighbouring 3 beams to the cone. The
elongated cone is used in order to avoid theirradiation of omega component of theneighbouring 3 beams to the inner cone. According
to 2-dimentional X-ray streak results, the imploded
core at 12 beams irradiation does not move unlike
at 9 beams. In the 9 beams case, the imploded core
moves toward the cone tip. The pre-formed plasma
may be created by collision of the imploded corewith the tip at 9 beams. The Teff at 9 beams should
be higher than the Teff at 12 beams. However both

values of Teff are not so different as shown in Fig.1.
This means the collision of the imploded core withthe cone tip is not essential in the current situation.
The clearance between the LFEX and theirradiating position by the omega light is 200 µm.
The clearance between the 2-omega light and theouter cone is only 3 µm. More X-rays at 12 beams
are observed than at 9 beams by using a simple
X-ray monitor [4]. In the 12 beams case, 2-omega
light may directly irradiate the outer cone and create
pre-plasma in the cone inner. This assumption canbe verified by the results shown in Fig. 2. In Fig. 2,the Teff is shown as a function of the difference
between the implosion and the LFEX injection
timing. In both the 9 beams and 12 beams cases,
maximum Teff can be obtained when the implosion
corresponds to the LFEX injection. Teff decreases
because the super high dense plasma explodes
toward the LFEX laser path after implosion. If thepre-formed plasma is created by the imploding laser,
this result can be explained without contradiction.
Te vs Delay

20 ●LFEX<500J▼LFEX>500J(9b)▲LFEX>500J(12b)151050-300-200-1000Delay (ps)100200Fig. 2. Teff vs LFEX injection timing.
Maximum Teff can be obtained when theimplosion corresponds to the LFEX injection.

3. Summary
The high coupling efficiency between theimploded core and the hot electrons can be obtainedat the short pulse duration of the LFEX. To achieve
high coupling efficiency, it is important to suppress
the pre-formed plasma. The pre-formed plasma isfound to be created by the imploding laser inaddition to the pre-pulse of LFEX by thecomparison between the experiment and thescaling.
References[1] T. Ozaki, et al, JOP:Conf. Ser.2, 244, 022056(2010).
[2] R. Kodama, et al, Nature 418, 933-934(2002).
[3] A. Pukhov, et al, POP, 6, 2847(1999).
[4] T. Ozaki, et al, to be submitted.