23P112-P
MHD Simulation of Multi-pulsed Current Drive byCoaxial Helicity Injection in Spherical Torus
同軸ヘリシティ入射による球状トーラスのマルチパルス電流駆動の MHD シミュレーションTakashi Kanki1), Masayoshi Nagata2) and Yasuhiro Kagei3)
神吉隆司1), 永田正義2)，影井康弘3)
1)

Japan Coast Guard Academy 5-1 Wakaba, Kure, Hiroshima 737-8512, Japan1)
海上保安大学校 〒737-8512 広島県呉市若葉町5-1
2)Graduate School of Engineering, University of Hyogo 2167 Shosha, Himeji, Hyogo 671-2280, Japan2)
兵庫県立大学大学院工学研究科 〒671-2280 兵庫県姫路市書写2167
3)
Research Organization for Information Science & Technology 2-4 Shirane, Shirakata, Tokai,Ibaraki 319-1106, Japan3)
高度情報科学技術研究機構 〒319-1106 茨城県那珂郡東海村白方白根2-4
A 3-D MHD simulation of a multi-pulsed current drive by coaxial helicity injection on a spherical torus(ST) plasma is carried out for the high safety factor q range (q>2) of a corresponding experiment conductedon the HIST device. The simulation results show that during the driven phase, the poloidal flux and thetoroidal current are amplified by the axisymmetric merging of the pre-existing ST plasma with ejected one
which involves magnetic reconnection. Of particular interest is observed during the decay phase that the ST
approaches the axisymmetric MHD equilibrium state without flow due to the dissipation of magnetic
fluctuations to rebuild the closed flux surfaces.

1. Introduction
Coaxial helicity injection (CHI) which has beenused as non-inductive current drive and plasma
startup by a magnetized coaxial plasma gun (MCPG)
is one of the more attractive methods in spheromak
and spherical torus (ST) because of its possibility ofhigh efficiency. However, a critical issue of the CHI
is the evidence that during the sustainment, thecentral open flux column (COFC) develops a helicaldistortion due to the nonlinear growth of the n=1
kink instability and the subsequent magnetic
reconnection destroys the closed flux surfaces. Inorder to improve this demerit of the CHI, themulti-pulsed operation of CHI (M-CHI) that causesa stepwise buildup of the magnetic field of theconfiguration by pulsing a MCPG has been proposedto achieve quasi-steady state with increase of thecurrent flowing in closed flux surfaces region. The
M-CHI has been for the first time demonstrated inthe SSPX device [1] and performed in the HIST
device [2] to obtain both improved energy
confinement and sustainment. The purpose ofpresent study is to investigate the effects of theM-CHI on dynamics of the high-q ST by using
resistive nonlinear 3D-MHD simulation [3].
2. Simulation Model
We use a 3D full-toroidal cylindrical (r, θ, z)geometry. The simulation domain consists of a gun
region and a confinement region. The insertion of a

toroidal field current Itf along the geometry axis
inside the central conductor produces a vacuum
toroidal field, creating a tokamak. The governing
equations are the set of nonlinear resistive MHD
equations. To solve the equations, we use thesecond-order finite differences method for thespatialderivatives
andthefourth-order
Runge-Kutta method for the time integration. A bias
magnetic flux penetrates electrodes at the inner andouter boundaries of the gun region, and an electricfield is applied to the gap between electrodes in theshape of periodic pulses. We use a perfectconducting boundary at the wall of the confinement
region. The initial conditions are given by anaxisymmetric MHD equilibrium of high-q ST withfinite pressure gradients, which can be obtained bynumerically solving a Grad-Shafranov equation
under these boundary conditions. In the simulation,
the mass density is spatially and temporally
constant and no-slip wall condition is assumed at allboundaries. We also impose that the heat flux canpass through all the boundaries.
3. Simulation Results
As shown in Fig.1, during the driven phase fromt=600 to 800τA, the poloidal flux and the toroidal
current are amplified by the axisymmetric mergingof the pre-existing ST plasma with ejected one
which involves magnetic reconnection, because then=0 mode is dominant to other modes. As shown inFig.1. Time evolution of vector plots of poloidal field Bp on the poloidal cross section at the θ =0 plane.

Fig.2. Time evolution of vector plots of poloidal flow velocity vp and contours of toroidal flow velocity vt on thepoloidal cross section at the θ =0 plane.

Fig.2, in this merging process, due to the Lorentz
force, the strong poloidal flow which moves fromthe gun to confinement region occurs, and thetoroidal flow is driven in the same direction as thetoroidal current in the COFC region. During thedecay phase from t=800 to 900τA, the ST
approaches the axisymmetric MHD equilibrium
state without flow due to the dissipation ofmagnetic fluctuations to rebuild the closed flux

surfaces. At t=804τA, the poloidal flow whichmoves from the confinement to gun region isinduced due to the pressure gradients.
References[1] B. Hudson et al.: Phys. Plasmas 15 (2008) 056112.
[2] M. Nagata et al.: Proc. 23rd IAEA Fusion Energy
Conference, Daejon, Korea, 2010, EXC/P2-04.
[3] T. Kanki et al.: Plasma Fusion Res. 5 (2010) S2055.