24P043-P
Dynamic Divertor Concept by Plasmoid Ejection in TS-4 Spherical Tokamak
Experiment
TS-4球状トカマク実験のプラズモイド放出を用いた動的ダイバータの提案	 	 
Shizuo Inoue,	  Yasushi Ono, Shingo Ito,	  Michiaki Inomoto
井上静雄, 小野	  靖，伊藤慎悟, 井	  通暁Graduate School of Engineering, The University of Tokyo
2-11-16, Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
東京大学工学系研究科 〒113-0032	  東京都文京区弥生2-11-16
This paper proposed a new type of dynamic divertor by use of intermittent plasmoid ejection from a main
plasma to a divertor coil. Our 2-D MHD simulation demonstrated for the first time how the plasmas andcoil conditions could control plasmoid dynamics for this divertor action. This dynamic divertor has four
essential steps. First, current drive and heating causes the main detached plasma to expand around the divertor region, causing a plasmoid formation. Second, the expanding core plasma finally pinches off thesmall plasmoid. Next, the plasmoid isolated from the main plasma is cooled down by Argon gas puffing
and finally is connected with the divertor plate. The series of divertor actions are expected to reduce theheat road to divertor plate significantly, suggesting a new type divertor useful for heavy heat road fromtype I Edge-Localized Mode (ELM). We will present its simulation result and the corresponding experiments of plasmoid ejection in TS-2 and TS-4 merging experiments.

1. Introduction
Two essential issues for future divertors of fusion reactors is to reduce heat load to divertor
plate and also to exhaust efficiently the heliumash which pile up in core plasma. A serious
problem is concentration of high-energy heat flux
damages divertor plate, generating impurities fromdivertor wall. Especially, a large heat flux such as
Type I Edge-Localized Mode (ELM) damages thedivertor plate heavily, resulting in replacing divertor plate frequently. Currently, various schemes
such as detached divertor or pebble divertor has
been tested and improved for this purpose [1]. We
developed a new dynamic divertor to solve thisproblem. Its new concept is to utilize periodic
plasmoid ejection from the main plasma to the divertor coil, improving significantly insulation of thedivertor plate from the main plasma [2]. This paper
addresses the basic motion of dynamic divertor withsimulation and experimental data.
2. Concept of Dynamic Divertor
This paper describes the basic concept and motion of dynamic divertor in comparison with classicdivertor. While the classic divertor’s plate is connected with main plasma by divertor coil’s magnetic field line. On the other hand, the plasmoid
connects indirectly the dynamic divertor’s platewith the main plasma. The dynamic divertor action
has 3 essential steps.

Fig.1. (a)Classic divertor. (b) Dynamic divertor.

First, current drive and heating causes the main detached plasma to expand around the divertor region,
forming a plasmoid which is a magnetic island
ejected from main plasma together with Helium
ashes. Second, the expanding core plasma finallypinches off the small plasmoid and obviously Helium ashes are also emitted with plasmoid. The
plasmoid isolated from the main plasma is cooled
down by Argon gas puffing and finally is connectedwith the divertor plate. The influence of impurities
is suppressed because of distance from the plate tothe main plasma much longer than the classic divertor and argon gas cooling of plasmoid under thecondition isolated from the main plasma.
3. Simulation Model
The main plasma has to produce plasmaid periodically for the dynamic divertor motion. At first,
we solved magnetohydrodynamic equations tostudy global plasma motion, of main plasma andSOL/Divertor plasma. Our two dimensional MHD
simulation [3] proved for the first time basic motions of the dynamic divertor and controllability ofplasmoid ejection cycle [4]. The equation to besolved is the resistive MHD equations based on theexplicit finite difference method with second-orderaccuracy both in space and time. The spatial derivative of a variable was based on the Taylor expansion up to the second order of the grid separation!x and the temporal evolution is solved by thesecond-order Lax-Wendroff method. The initialcondition of the MHD simulation is provided by theGrad-Shafranov equation and we apply the perfectconducting wall as boundary condition. We also
add ohmic coil heating effects at boundary as generating electric field.
4. First results of experiment and simulation
Figure 2 shows simulation data of one cycle operation for the dynamic divertor. From 0.63 to
1.51[TA], the main plasma is amplified by theOhmic heating coil, causing a plasmoid formation.
The plasmoid is pushed by PF1 coil and is detachedfrom the main plasma in 1.56[TA]. Finally, theplasmoid is pulled by PF2 coil and connected todivertor plate on the simulation boundary. ThisMHD simulation does not include the gas puff’seffect or ionization effects. In the series of motion,
the divertor plate is connected indirectly to main
plasma through the ejected plasmoid, removing direct heat flow into the divertor. The distance frommain plasma to the divertor plate is longer than theclassic’s, improving the shield effect for our divertor plate and also space for argon gas puff.
5. Summary
We show our initial experimental and numericalresults of the proposed new dynamic divertor usefulfor heavy heat load from Type I ELM. In poster
session, we will present the concept of dynamic
diverter and how to obtain such divertor motion bycoil current using both of numerical simulations andTS-4 experiment.

Fig.2 Time evolution of poloidal flux contour during
intermittent plasmoid ejection. The PF1 coil current isused to produce the plasmoid and the PF2 coil current toconnect the plasmoid with the divertor plate.
References[1] T. Suzuki, M. Akiba, and M. Saito：“Comprehendingthe Divertor Structure”, J. Plasma Fusion Res. , Vol.
82, No. 10, p. 699 (2006) (in Japanese).
[2] 小野靖：22 年度科学研究費補助金：挑戦的萌芽
研究 22656208「ダブレット配位を用いた動的ダイバータの提案」[3] Y. Ono, and M. Katsurai：“Three-dimensional numerical simulation of the multi-helicity magnetohydrodynamic relaxation process in low-q spheromaks”, Nucl. Fusion, Vol. 31, p. 233 (1991).
[4] S. Inoue, Y. Ono, S. Ito : “MHD Simulation of Dynamic Divertor by Plasmoid Ejection”, to be published.