24P035-P
Investigation of the detached divertor model in the SONIC simulation
SONICシミュレーションにおける非接触ダイバータモデルの検討	 
Kazuo Hoshino, Katsuhiro Shimizu 1, Makoto Nakamura、Tomonori Takizuka 1、Nobuyuki Asakura、Tomohide Nakano 1
星野一生、清水勝宏 1、中村誠、滝塚知典 1、朝倉伸幸、仲野友英 1
Japan Atomic Energy Agency
2-166 Oaza-Obuchi-Aza-Omotedate, Rokkasho-mura, Kamikita-gun, Aomori 039-3212, Japan
1 Japan Atomic Energy Agency
801-1, Mukouyama, Naka 311-0193, Japan
日本原子力研究開発機構 〒039-3212 青森県上北郡六ヶ所村大字尾駮字表舘2番166
1 日本原子力研究開発機構 〒311-0193 茨城県那珂市向山 801-1In order to improve the detachment modeling, effects of various processes are investigated by using a suiteof integrated divertor codes SONIC. The increase in the radial diffusion loss at the private edge decreases
the ion density and the ion flux by 15%. The supersonic flow effect on flux reduction is small because thesupersonic flow occurs just in front of the target under the present simulation condition. Due to effects ofthe wall pumping, the poloidal profile of the ion flux is significantly changed. However the change in theion flux at the target is small. The weak rollover of the ion flux appears due to the combination of effects of
the radial diffusion loss and the wall pumping.

1. Introduction
The large heat load to the divertor plate is oneof the crucial issues for magnetic fusion devices.
The detached divertor operation is the mostpromising candidate to reduce the heat load and it
has been achieved in many devices [1].
A number of divertor codes have beendeveloped to interpret and predict theSOL/divertor
plasmacharacteristics.
Toinvestigate the detached divertor plasma, thevarious physics processes, such as the atomic andmolecular processes, drift, current and so on,
have been implemented into the divertor codes.
However, it seems to be still challenging toreproduce the detached divertor plasma observedin the experiments, especially significant
reduction of the particle flux onto the divertor
target [2].
In this study, several models and processes areevaluated for improvement of the detachment
characteristics simulated by a suite of integrated
divertor codes SONIC [3, 4].
2. SONIC result with Standard Setup
The SONIC code is applied to the study of
JT-60U divertor plasmas [5]. The ion and electron
input power across the core interface boundary(r/a~0.9) is set to Q i=Q e=1.6MW, respectively.
The ion flux across the core boundary, Γin, and D 2
gas puff rate, Γpuff, are adjusted for a parameter
survey. The anomalous transport coefficients forthe particle ( D! = 0.3 m 2 / s ) and heat
( ! !i = ! !e = 1.0 m 2 / s ) are assumed to be spatiallyconstant. The recycling coefficient R at thedivertor and the wall is set to 1, i.e. saturated wall
is assumed. An effective pumping speed is set toS pump = 30 m 3 / s . The chemical sputtering yield isassumed to be Ych=3 % on all plasma facingwalls.

Fig. 1 Detachment characteristics in the SONIC
simulation.

The electron temperature Te and the ion flux Γdat the outer strike point are plotted as a functionof the mid-plane density nmid in Fig. 1. With
increasing nmid, Te significantly decreases and Γdinitially increases. Although Te becomes less thana few eV, Γd does not decrease. The rollover ofthe ion flux, which is a feature of the detachment,
cannot be reproduced in the standard setup for theSONIC simulation.
3. The radial diffusion loss from the private edge
In the SONIC code, the radial decay length λ ofdensity is given as the boundary condition at theprivate side edge. The decrease in λ corresponds toincrease in the radial diffusion at the private edge.
To investigate effects of the radial diffusion loss
from the private edge, a radial decay length ofdensity λ is changed. Figure 2 shows the radial
profile of the ion density along the inner divertor
target. With decreasing λ, particle loss by the radial
diffusion increases. As a result, the ion density ni inthe private region decreases. Although decrease inni leads to decrease in the ion flux Γd near the strike
point, the change in Γd is 15% at most.
4. The supersonic flow in the divertor region
At the sheath entrance, M=1 (M: Mach number)is generally fixed in the plasma fluid model.
However, The solution of the supersonic flow(M>1) at the sheath entrance was expected
analytically or numerically as was discussed in Ref.
[6-9]. To investigate effects of the supersonic flowon the detachment, the boundary condition of themomentum at the target is improved to allow thesupersonic flow. The SONIC simulation shows thesolution of the supersonic flow in the detachment
state. Although ni at the target significantly
decreases, change in Γd is small. If the supersonicflow and the resultant decrease in the ion density
occur along the field line from the X-point to thetarget, significant effect on the divertor plasma dueto the change in the plasma-neutral interaction, suchas ionization, charge-exchange etc, is expected.
However, in the present simulation condition, thesupersonic flow occurs just in front of the divertor
target. As a result, the change in Γd is small because
the ion flux is conserved from the upstream.
5. The wall pumping
The effect of the wall pumping is investigated bychanging the wall recycling coefficient R. As therecycling coefficient in the exhaust chamberdecreases, the electron temperature at the strike
point increases. Because the ionization front moves

Fig. 2 Radial profile of the ion density ni along theinner divertor target. Radial decay lengths at theprivate region boundary (dsep~0.01 m) are set to be
λ=0.03 m, λ=0.01 m and λ =0.001 m.

to the target from the X-point, the poloidal profileof Γd is significantly changed. However, the change
in the ion flux at the outer target is relatively small.
6. Synergetic effects and future work
By taking into account both effects of the large
radial diffusion by the small radial decay length andthe wall pumping effect, the weak rollover of theion flux appears. However, reduction in Γd after therollover is still small compared with experimental
data.
For further understanding of the formation of thedetached divertor plasma, evaluation of adependence of the detachment characteristics on theanomalous transport coefficients, the hot-electroneffects, is in progress and will be discussed in thepresentation.
Acknowledgments
This study is partially supported by aGrant-in-Aid for Scientific Research on Priority
Areas of MEXT (19055005) and Grant-in-Aid forYoung Scientists (B) of JSPS (23760817).
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