23P123-B
Influence of High Energy Electrons on ECRH in LHD
LHD における ECRH による高エネルギー電子の発生と加熱への影響
Kubo Shin1,2) , Takahashi Hiromi1) , Shimozuma Takashi1) , Yoshimura Yasuo1) , Nishiura Masaki1) , Igami Hiroe1) ,Ogasawara Shinya2) , and Makino Ryohei2)
1,2)
1)
久保 伸 , 高橋 裕己 , 下妻 隆 1) , 吉村 泰夫 1) , 西浦 正樹 1) , 伊神 弘恵 1) , 小笠原 慎弥 2) , 牧野 良平 2)
1)

National Institute for Fusion Science,322-6 Oroshi-cho, Toki 509-5292, Japan
Dept. of Energy Science and Technology, Nagoya Univ., Nagoya 464-8463, Japan1)
核融合科学研究所 〒 509-5292 岐阜県土岐市下石町 322-62)
名古屋大学大学院工学研究科エネルギー理工学専攻 〒 464-8463 名古屋市千種区不老町2)

The central bulk electron temperature of more than 20 keV is achieved in LHD as a resultof increasing the injection power and the lowering the electron density near 2 × 1018 m−3 . Such
collision-less regime is important from the aspect of the neoclassical transport and also the potential structure formation. The presences of appreciable amount of high energy electrons areindicated from hard X-ray PHA, and the discrepancy between the stored energy and kinetic energy estimated from Thomson scattering. ECE spectrum is also sensitive to the presence of highenergy electrons are discussed by solving the radiation transfer equation. The ECRH power absorptions to the bulk and the high energy electrons are dramatically aﬀected by the accelerationand the confinement of high energy electrons. The heating mechanisms and the acceleration process of high energy electrons are discussed by comparing the experimental results and ray tracing
calculation under assumed various density and mean energy of high energy electrons.
1.

Introduction

Recent upgrade of the ECRH system in LHD [1] enabled
to study the plasma confinement properties at the far low
collisional or collision-less regime in the helical system
where specific confinement features are predicted fromthe neo-classical transport theory. These specific features
includes confinement degradation in the so called ”1/ν”
regime where the ripple loss become dominant but radial
electric field can drastically alter the situation [2]. In order
to discuss the confinement properties in such collision-lessregime, the accurate estimation of the behavior of the highenergy electrons as well as that of the bulk electrons temperature is required, since the high energy electrons canabsorb injected ECRH power at the relativistically down
shifted frequency and can deposit their energy to the bulk
electrons out of the region where they absorb the energy
from injected EC wave. Thus, the power deposition profile
which is the key parameter in the transport analysis canbe much aﬀected by the presence of high energy electrons.
One of the most reliable and well established methods toexperimentally deduce the power deposition profile is using the transient analysis of the electron cyclotron emission(ECE) [3].
It is known that the presence of high energy electrons
can alter the ECRH absorption, mainly due to the finite
Larmor eﬀect. In an inhomogeneous magnetic field configuration, the relativistic down shift of the cyclotron resonance causes the shift in the absorption region and also
the appearance of the higher harmonic resonances for highenergy electrons. The ECRH heating scenario in such low
collisional plasma have to be explored taking the relativistic eﬀect into account.
2. Achievement of high electron temperature and production of high energy electrons
Using upgraded ECRH (of more than 3MW injectionpower), the plasma parameter region of the low density andhigh temperature is explored. With the careful adjustmentof the magnetic field and the focal point, the central bulk
electron temperature exceeded 20 keV at the center. Thisbulk electron temperature is derived by averaging over several shots of YAG-Thomson scattering data. It is confirmedthat these estimations of the bulk electron temperature are
not aﬀected by the presence of high energy electrons discussed hereafter.
High temperature low density plasma produced by
ECRH normally contains appreciable fraction of high energy electrons. In Fig.1 are over plotted the time evolutions of the plasma parameters for high ne (1.2×1019 m−3 )low (0.2×1019 m−3 ) and very low (0.1×1019 m−3 ) density
shots ( as indicated in the column b) )with identical ECRH
injection pattern as shown in a). It should be noted that thediamagnetic stored energy (Fig.1c) for the low and very
low density cases are almost comparable up to 3.8 s eventhough the diﬀerence in the density is about factor 3. Furthermore, the stored energy for the very low density casegradually approaches to that for high density case toward
the end of the shot. These stored energy behaviors are well
explained by the production and acceleration of high en-