28D31P
中性粒子がダイバータプラズマの非接触フロントの安定性に与える効果
Effects of Neutral Particles on the Stability of Detachment Fronts in DivertorPlasmasaa

東郷 訓、b中村 誠、b清水 勝宏、c滝塚 知典、b星野 一生、a小川 雄一
Satoshi Togo, bMakoto Nakamura, bKatsuhiro Shimizu, cTomonori Takizuka, bKazuo Hoshino, aYuichiOgawaa

東大大学院新領域創成科学研究科、b日本原子力研究開発機構、c阪大大学院工学研究科
aGraduate School of Frontier Sciences, University of Tokyo, bJapan Atomic Energy Agency, cGraduate
School of Engineering, Osaka University
1. Introduction
Reduction of the divertor heat load is one of thecrucial issues in designing next generation tokamaks
such as ITER and DEMO. In order to resolve this issue,
operation with detached divertor plasmas is considered
to be a promising way [1].
In order to model SOL-divertor plasmas,
two-dimensional (2D) codes, such as SONIC [2] andSOLPS [3], and point divertor models have been used.
It is considered, however, that 2D codes arecomputationally massive to focus on studying eachphysical phenomenon in plasmas. On the other hand,point divertor models are very easy, but have not
reproduced detached divertor plasmas so far. Thus
one-dimensional (1D) codes [4, 5] have been used togain physical insights of detached divertor plasmas.
In our previous works [6, 7], we adopted a simple
model for the neutral particles assuming that theneutral flux decays exponentially due only toionization reaction. In the presentation, we will show
the effects of neutral particles on the detachment fronts
by introducing a time-dependent neutral model. Here
we show some preliminary results in the attachedregime.
2. ModelThe 1D divertor model we use analyses aSOL-divertor plasma along the magnetic field. Refer [6,7] for detailed explanations of geometry, plasma fluid
equations and the boundary conditions.
The neutral model we have introduced instead of thesimple one is described as follows;∂n n ∂ ∂n  n+ α (n n v FC ) + β  − λCX vth n  + n∂t ∂s ∂s  τ n

(1)= ni ne σv rec − n n ne σv ion ,Γn,us = 0 , Γn,dp = η trap Γi ,dp .

(2)
Equation (1) is the particle conservation equation. The
coordinate s is in the poloidal direction. We canchoose the transport model from the convection modeland the diffusion model considering the charge
exchange reaction [8] by switching the value of(α , β ) as (1,0) or (0,1) . The third term in the LHS

is the particle loss term whose time constant τ n is afree parameter. Equations (2) represent the boundary
conditions. The parameter η trap is the recycling rateof the ion flux.
3. Results
The steady state neutral density profiles for different
transport mechanisms when τ n is set to be infinity
are shown in Fig. 1 (a). The mean free path of neutral
particles becomes longer in the diffusion case due tocharge exchange reaction. The effect of the loss term isshown in Fig. 1 (b). When τ n is short enough to becomparable with the time constants of the diffusion
term and source terms, the neutral density becomes
lower.
Diffusion
Convection

(a)

(b)

Fig. 1 Steady state neutral density profiles for different
transport mechanism when τ n is set to be infinity(a) and those for different values of τ n in thediffusion case (b).

References[1] ITER Physics Expert Group on Divertor, Nucl. Fusion
39 2391 (1999).
[2] H. Kawashima et al., Plasma. Fusion. Res. 1, 031(2006).
[3] R. Schneider et al., Contrib. Plasma Phys. 46, 3 (2006).
[4] R. Goswami et al., Phys. Plasmas 8 857 (2001).
[5] W. Fundamenski et al., J. Nucl. Mater. 290–293 593(2001).
[6] M. Nakamura et al., Plasma Fusion Res. 6, 2403098(2011).
[7] S. Togo et al., Plasma Fusion Res. 7, 2403087 (2012).
[8] R. Schneider et al., Contrib. Plasma Phys. 46 3 (2006).