04aE62P
モンテカルロコードを用いた再突入高速イオンの熱流束評価と高速イオン計測との比較
Comparison of Heat Flux Evaluation of Re-entering Fast Ion
Using Monte-Carlo Method with Fast Ion measurement in LHD
關良輔、永岡賢一、渡邊清政、鈴木康浩
Ryosuke SEKI, Kenich NAGAOKA, Kiyomasa WATANABE and Yasuhiro SUZUKI
核融合研
NIFS
In a fusion reactor, it is an important issue to design the blanket. The design of the blanket requires that aheat flux due to alpha particles produced by the fusion is exceedingly-small. Especially, in the fusion reactor likethe Large Helical Device (LHD), the space for installation of the blanket is not enough large because of a small
distance between the plasma boundary and coil. Thus, the detailed analysis of the particle orbits is required. Insuch analyses, a full-orbit tracing of alpha particles using the Monte-Carlo method is usually used because aLarmor radius of the alpha particle is relatively large. However the Monte-Carlo code based on the full orbit
tracing needs a long calculation time. In order to save the calculation time, a heat flux model of fast ions including
the finite Larmor effect is required.
In the LHD, the reactor-relevant high-beta plasmas with the volume averaged beta value, <>~5%, areachieved with about 0.5 T. In the LHD high beta discharge, the Larmor radius of a 180 keV fast ion produced by a
NB is the almost same as that of alpha particle in a reactor. In addition, in the LHD high beta plasma, it is shownthat most of fast ions produced by tangentially injected Neutral Beams are “re-entering fast ions”, which haveclosed orbit across the Last Closed Flux Surface[1]. In the LHD, the power density of their fast ions can bemeasured by a hybrid directional probe[2] and its power density can be compared with the power density
evaluated by a calculation code.
We evaluated the power density of the re-entering fast ions by using a Monte-Carlo code, MORH[3],which can consider “re-entering fast ion”. Then the power density evaluated by the MORH was compared withthe fast ion measurement of the probe. Figure 1 shows the power density of re-entering fast ions. In Fig. 1, avertical axis is the power density of fast ions and horizontal axis denotes the measurement position of the probe.
The power density (blue line) evaluated with guiding center orbits is less than one-tenth of the measurements (redpoints). On the other hand, the power density (green line) evaluated with full-orbits, in which the particle aretraced with including the Larmor movements, is almost same value as the measurements. In the low field (~ 0.5 T),
the Larmor radius of the fast ions produced by NBs is almost same as that of alpha particle produced by a fusion.
The evaluation of the power density of fast ions needs to include the Larmor-movement in such plasma. Inaddition, we have developed a finite Larmor width model,
and the developed model has been introduced to theMORH based on the guiding center orbits for saving thecalculation time. The power density evaluated by using
the guiding center orbit with the developed model is alittle smaller than the measurements. A calculation time ofthe evaluation by the guiding center orbit with thedeveloped model is less than one-tenth of the evaluationby the full-orbit. In this conference, the developed modeland the MORH will be explained in detail. The power
density of the re-entering fast ions will be discussed.
Reference[1] R. Seki, et al., Plasma Fusion Res., 3, 016 (2008).
[2]K. Nagagoka et al., Rev. Sci. Instrum. 79 10E523(2008).
[3] R. Seki, et al., Plasma Fusion Res., 5, 014 (2010).

Comparison of fast ion measurementswith that evaluated by MORH