04aE25P
トロイダルプラズマにおける外部磁場揺動による強制磁気再結合
Forced reconnection by external magnetic perturbation in toroidal plasmas
石井康友、安積正史1)
Yasutomo Ishii, Masafumi Azumi
日本原子力研究開発機構、高度情報科学技術研究機構1)
JAEA, RIST1)
Magnetic perturbations applied to magneticallyconfined plasma cause the forced magnetic
reconnection and lead to magnetic island formation.
Such a change of the magnetic structure affects theplasma performance. The forced magnetic
reconnection process has been studied in many
theoretical, numerical and also experimental works.
However, especially in the theoretical studiesand/or numerical simulation ones, many works
treated the slab geometry or the cylindricalgeometry focusing on fundamental process of themagnetic reconnection. On the other hand,magnetic confined plasma aiming for nuclear fusion
reactor is confined in the toroidal geometry. In thetoroidal geometry, if an external magnetic
perturbation with a single helicity is applied, many
modes with different helicities are excited throughthe side band mode couplings and affect themagnetic configuration. In this study, we will reportresults obtained by numerical simulation study
about effects of the external magnetic perturbation
applied to tokamak plasma with toroidal symmetry,especially about feature of externally driven
magnetic islands.
Figure 1 shows the time evolution of the magnetic
island width excited by applying the external
magnetic perturbation with m/n=5/1 at the plasma
surface to toroidal plasma, which has the aspectratio of 4 and has the safety factor at the magnetic
axis q_0=2.01 and at the plasma surface q_a=6.11.
The initial equilibrium is stable for the tearing
mode and the resistivity is10^-6, which is constantin the radial direction in this simulation. In thisconfiguration, there is a possibility that m/n=5/1,4/1
and 3/1 harmonics interacts with each other. The
external perturbation evolves as the followings;
psi(5/1)=10^-4 t/250 : 0 < t < 250=10^-4 (500-t) /250 : 250 < t < 500=0 : 500 < t.
As shown in fig.1, for the magnetic island withm/n=5/1, while outer magnetic island width evolves

according to the time evolution of the externalperturbation, inner magnetic island width continues
to grow even in the decreasing phase of the externalperturbation. After some time, the inner magnetic
island width begins to decrease. This feature iscaused by the difference between the process ofcompressing magnetic field in the increasing phaseof the external magnetic perturbation and that ofrelaxation process of the magnetic field in thedecreasing phase of the external magnetic field.
After disappearance of the external magnetic
perturbation, m/n=5/1 magnetic island continues
with longer time scale.
In this configuration, magnetic islands withm/n=4/1 and m/n=3/1 are also excited by thetoroidal mode coupling with the magnetic
perturbation with m/n=5/1. However, these sideband modes evolve in more slow time scale asshown in fig.1. In the low resistivity parameterregion, the time evolution of magnetic island after
disappearance of the external magnetic perturbation
becomes slow. Therefore, this shows that, even after
the disappearance of the external magnetic
perturbation, there is a possibility that externallydriven magnetic island affects the plasma
performance for a long time.

Fig.1 Time evolution of magnetic islands excited by
m/n=5/1 external magnetic perturbation.