04aE34P
JT-60Uにおける電流ホール平衡の過渡応答の解析	 
Analysis of transient response of current hole equilibria in JT-60U	 
藤田隆明1, 林伸彦2, 本多充2, 濱松清隆2
FUJITA Takaaki1, HAYASHI Nobuhiko2, HONDA Mitsuru2, HAMAMATSU Kiyotaka2
1

1

名大工、2原子力機構
Graduate School of Engineering, Nagoya University 2Japan Atomic Energy Agency

A nearly zero current density region in thecentral part of a tokamak plasma, "current hole", isobserved [1]. Increase in the off-axis noninductive
current jNI causes reduction in the central toroidal
electric field Eφ(0) and the central current densityj(0) through the Faraday’s law, resulting in thecurrent hole. The fact that no significant reversed
current has been observed so far suggests that j(0)does not become negative even when Eφ(0) isnegative, which is called "current clamp". When jNI
is present in the central region, this condition could
be generalized such that j(0) is not negative fornegative σEφ(0)+jNI, though evaluation of jNI and
the electrical conductivity σ is not confirmed yet in
the current hole. In JT-60U, co and counter ECCDwere applied in the current hole, but neither positive
nor negative current was observed during theECCD period (0.8 s). This is the first experimental
observation of current clamp. Furthermore, it
showed that the j(0) does not become positive insome cases even in presence of positive σEφ(0)+jNI,
which was not predicted by any theory.
In the analysis of the above experiment, Eφ(ρ)
was evaluated as the time derivative of the poloidal
flux Ψ, E φ = − ∂Ψ ∂t , by analysis of equilibrium
evolution using MSE. As a different approach, inthis study, evolution of j(ρ) and the Eφ(ρ) isanalyzed by a 1.5D transport code TOPICS.
€Diffusion of j(ρ) is calculated in a usually way evenin the current hole region, but the safety factor to beused for the equilibrium and the transportcoefficients was restricted to be less than somethreshold value (typically 30) [3]. Initial results areshown in Fig. 1. Note that though Te(0) and thecurrent hole radius are similar to those in theexperiment, adjustment of the ne, Te, Ti and q
profiles, which are so far calculated using atransport model (CDBM), is not completed yet. InFig. 1 (a), jEC is assumed to be the same as theEC-driven current evaluated by a Fokker-Planckcode (EC-Hamamatsu) for an equilibrium with
nested flux surfaces. An off-axis peak in j(r) ispredicted to be formed within 0.8 s, though its
height is significantly smaller than jEC because ofremaining negative loop voltage. This does notagree with the experiment. In Fig. 1 (b), a flat jEC
within the current hole was assumed, corresponding
to no confinement in the current hole. In this case, achange of j(ρ) is predicted only around the edge ofthe current hole. This change also seems to disagreewith the experimental observation, but more precise
adjustment of ne, Te, Ti and q profiles is needed
before reaching a conclusion on validation of thismodel. Results of improved simulation will bereported in the conference.

Fig. 1. Evolution of calculated current density profile
after EC injection at t = 5.2 s. (a) With a jEC profile
predicted by a Fokker-Planck code. (b) With a jEC profileflat in the current hole region. The EC-driven current is0.22 MA and the plasma current is 1 MA.
References[1] T. Fujita, Nucl. Fusion 50 (2010) 113001.
[2] T. Fujita et al., Phys. Rev. Lett. 95 (2005) 075001.
[3] N. Hayashi et al., Nucl. Fusion 45 (2005) 933.