23P080-P
Physical Study on Robustness of ITER Machine against Disruptions
ITER装置のディスラプションに対する健全性の物理的検討
Seiji Miyamoto1, Masayoshi Sugihara2, Yukiharu Nakamura3, Kichiro Shinya4, Ryuji Yoshino2,Tatsuo Sugie1 and Yoshinori Kusama1
宮本斉児1, 杉原正芳2，中村幸治3，新谷吉郎4，芳野隆治2，杉江達夫1，草間義紀1
1

Japan Atomic Energy Agency, 801-1 Mukoyama, Naka, 311-0193, Japan
ITER Organization, Route de Vinon sur Verdon, 13115 St. Paul lez Durance, France3
Nippon Advanced Technology, 3129-45 Muramatsu, Tokai-mura, Naka-gun, Ibaraki, 319-1112, Japan4Toshiba Nuclear Engineering Service, 8 Shin-sugita, Isogo-ku, Yokohama, 235-8523, Japan1
日本原子力研究開発機構 〒311-0193 那珂市向山801-1
2ITER機構 Route de Vinon sur Verdon, 13115 St. Paul lez Durance, France3
日本アドバンストテクノロジー 〒319-1112 茨城県那珂郡東海村村松3129-45
4
東芝原子力エンジニアリングサービス 〒235-8523 横浜市磯子区新杉田町82

Investigation of robustness against vertical displacement events (VDEs) and disruptions is an important
issue in designing ITER machine. In recent years, analysis of halo current and vertical force on the vacuum
vessel generated by VDE and disruption has been carried out using the DINA code. In this paper,
derivation of the maximum halo current and vertical force is described in view of physical constraints on
the current quench duration. The electrostatic sheath formed in front of the wall that bounds the plasma
gives rise to such a constraint on halo current generation. Updating the halo current model in DINA toinclude sheath effects, analysis was done consistently with power balance during current quench. Some
important issues related to VDE, intervention of vertical stabilization system in VDE, over-current of
in-vessel VS coils and position control of runaway electron beam, are also discussed.

1. ITER disruption study
The VDEs and subsequent plasma disruptions
release severe electromagnetic load on thevacuum vessel of tokamak devices. To assure
robustness of the vessel and in-vessel components
against the load, a lot of simulations have been
carried out for ITER using the DINA code [1,2].

Fig. 1. Structure model of ITER. Sequence ofplasma cross-sectional change during VDE is alsoshown.

The structure model of ITER in DINA consists ofpoloidal field (PF) coils, vacuum vessel (VV),blanket modules and in-vessel vertical stabilization(VS) coils (Fig. 1). Halo current generation issolved consistently with Grad-Shafranov equation.
The largest halo current and hence the largest
vertical force occurs in a downward VDE caseaccompanied by slow current quench (CQ). In thepast simulations, electron temperature during CQ,
Te=55eV, was somewhat artificially chosen to
produce the CQ time extrapolated from existing
experiments. In this study, physical constraints onTe (thus CQ time) were employed to derive possible
maximum of halo current and vertical force.
2. Extension of halo current model
The current quench is a dissipation process ofplasma magnetic energy (plasma current) throughjoule loss. The joule loss to the plasma is eventually
emanated through impurity radiation and sheathtransmission heat. Hence it follows that Te, which
determines the radiation and sheath transmission
cannot be arbitrarily high. The highest Te occurs
when the impurity radiation is negligibly small. The
plasma resistivity determined by this highest Te
gives rise to the minimum (slowest) rate of CQ.
The electrostatic sheath modifies the CQ
behavior through sheath potential generation in
front of the wall. First, the halo current that flows inthe wall is regulated under ion saturation current by
the sheath potential. Accordingly, plasma resistancebecomes higher and the CQ time will be short.
Second, the joule input to the halo plasma is also
regulated because of the halo current regulation.
The maximum Te determined by power balance
between the joule input and the sheath transmission
is about 30 eV in the case of ITER.
Results of simulations with and without sheathmodel are compared in Fig. 2. The CQ time
becomes short because the halo resistance increases
effectively due to the sheath potential generation.
On the other hand, the same level of halo current isgenerated even if the sheath model is included. The
quickened CQ induces destabilizing eddy current in
the vessel, so that the plasma moves downward
further to generate halo current required to stabilize
the plasma itself (bottom plot in Fig.2.)

Fig. 2. Effect of sheath on CQ behavior.

3. Influence of VS control
Vertical stability (VS) control during VDE has apotential influence on the vertical force on the VV.
If a radial magnetic field is mistakenly applied tothe direction of enhancing the vertical displacement
further, a larger force will appear in the vessel.
Actually this is the case of maximum force inpresent experiments.
To assess this force, a model of the VS control
circuit composed of external PF coils wasimplemented in DINA. In the case of ITER,
because the time constant of the PF coils will bemuch longer than VDE event duration, it is likelythat possible increase will be rather small. The
result of simulation confirmed that the force wasincreased by up to 10% but still within the design
margin even if the worst case was considered.
4. Induced VS coil current by VDE
In-vessel VS coils will be introduced in the ITER
VV to improve control margin of the VS system(Fig. 1). VDE and subsequent CQ induce huge eddy
current in the in-vessel components including theVS coils. To avoid excessive over current in thesecoils, the upper and the lower VS coils areconnected in anti-series such that the inductions of
the two coils offset each other. A model of in-vessel
VS circuit was newly installed in the DINA codeand the coil current was documented for supporting
the design.
5. Runaway electron control
Generation of runaway electron (RE) is anotherconcern in the robustness of the VV. Large plasmaof ITER size is prone to generate RE. Without
appropriate measures, REs would hit the wall
eventually resulting in a significant damage.
Primary candidate of RE suppression is therepetitive injection of gas. However it is still
desirable to spare alternative schemes for backup.
Vertical position control of RE beam will be a key
point if it could keep the beam from touching thewall until the beam energy is sufficiently dumped
due to collisions with neutral gas.
Control boundary of RE beam was exploredusing the in-vessel VS circuit model of DINA. It
was revealed that vertical position control of RE
beam would require a high runaway current(~12MA) together with some extension of thepresent design limit of VS coil current.
6. Summary
Halo current model was extended to include
effects of sheath. Simulation result shows that theCQ time should be shorter by 1/2 than the present
specification of ITER VDE scenario to satisfy thepower balance during CQ. However, the same levelof halo current is generated, indicating that the halocurrent must be consistent with the requirement forstabilizing the vertical movement of the plasma.
The vertical force on the VV will be reduced
somewhat owing to the shorter CQ time.
Robustness of the VV was confirmed even if the(wrong) intervention of VS system was considered.
Possibility of the RE beam control still remains
unresolved. It should be noted that the modeluncertainty is large in RE generation and detailed
model development is necessary for further study.
References[1] H. Fujieda et al: JAEA-Reserch 2007-052 (2007)[2] M. Sugihara et al: Nucl. Fusion 47 (2007) 337.
Disclaimer: The views and opinions expressed herein do
not necessarily reflect those of the ITER organization.