23P057-P
Profile Control of Bootstrap Current in Tokamak Fusion Reactor
トカマク核融合炉におけるブートストラップ電流の分布制御
Junji Mano, Kozo Yamazaki, Tetsutarou Oishi, Hideki Arimoto, Tatsuo Shoji
真野純次、山﨑耕造、有本英樹、大石鉄太郎、庄司多津男
Nagoya University
Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
名古屋大学 〒464-8603 名古屋市千種区不老町
For the commercialization of steady-state tokamak reactor, it is necessary to keep the burning
plasma with a high bootstrap current ratio. In order to control burning plasma having high autonomy withlow external current, we consider the current density profile affecting particle and heat transports. In thisstudy, we suspected that peak of bootstrap current density profile moves to the center of plasma, because ofsinking and diffusion of current. It is concluded that external current-drive is required at both the centerand periphery of the plasma in order to maintain steady-state profile of temperature and density in thehigh bootstrap current ratio.

1. Introduction
In Tokamak Fusion Reactor, burning plasma
should be maintained by only non-inductive
current-drive for the achievement of steady-state
reactor. Total non-inductive current is a summationof a bootstrap current proportional to the pressure
gradient of plasma and an externally driven current,such as neutral beam current-drive. But in order toestablish a commercial reactor postulating
steady-state operation, it is necessary to reduce theamount of external current drive and to cover themajority of the plasma current with bootstrap
current. Burning plasma has high autonomy, so weneed to consider the current density profile withlittle disturbance including changes in particle andheat transports.
In this study we conducted an analysis of time
evolution of the current density profile for burning
plasmas by using 2.0-dimensional equilibrium,
1.5-dimensional-transport code (TOTAL code [1]).

0.8neteti

60

0.6

40

0.4

20

0.2

000.5
r/a

108

Fig.1. Profiles of electron temperature Te, ion
temperature Ti and electron density ne.

1.6jt
jbs
jext

6

1.2

q
4

0.8

2

0.4

000.5

1

j[MA/m 3]

temperature[keV]80

2. Analysis Method
First we performed iteration computationconsidering the difference between trial value andtarget value of the total plasma current, such thatthe current profile becomes consistent with thegiven profile of the safety factor. After this plasma
equilibrium iteration analysis, we evaluate thebootstrap current ratio and the total current density
profile during steady state which is obtained fromprofiles of both the external driving current and thebootstrap current given in the end.
When we assume high safety factor area in thecenter of plasma, which is weak poloidal magnetic
field,

q1

electron density [×10 20/m 3]

100

We assume that the reactor size is that of ITER.
In order to get much bootstrap current, the internaltransport barrier (ITB) is required, and here we usedcurrent-diffusive ballooning mode model [2,3] as aheat transport model.

0r/a

Fig.2. Profiles of safety factor q, total current density jt,
bootstrap current density jbs and external current densityjext.

[MW]alph

3. Results
The radial profiles of steady-state temperatureand electron density are shown in Fig. 1. The
profiles of safety factor, total plasma current
density, bootstrap current density and externalcurrent density are shown in Fig. 2 in the case ofdriving a current at both the center and the half
normalized minor radius. Time evolution of alphaheating power in both cases is shown in Fig. 3.
In the case of driving a current only at the plasma
center, the ratio of the bootstrap current Ibs and thetotal plasma current Ip, Ibs/Ip is 0.981. However,electron density cannot be maintained and thenuclear fusion reaction rate becomes lower. In thiscase, the peak of bootstrap current and theminimum of safety factor approaches into the centerof plasma. It is reported form existing simulation
study that because of sinking and diffusion ofcurrent, external current-drive is required in order tomaintain steady-state current profile [5]. The
simulation code used here does not include sinking
and diffusion dynamics of current, though same
phenomenon comes about.
In the case of driving a current at both the centerand the half normalized minor radius, the ratio Ibs/Ip
is 0.901 and the profiles of temperature and electron
density can be maintained throughout more than
200 second. In this case, the peak of bootstrap
current and the minimum of safety factor keep thesame position.
From this we suspected that since plasmapressure is modified by the change in q profile due
to bootstrap current change, peak of bootstrap
current density profile moves to the plasma center.
And we concluded that to maintain the steady-state
temperature and electron density profile of ITER inhigh bootstrap current ratio in the present model,
external current-drive is required at both the centerand the periphery of the plasma.

both center and peripherycenter160P

there is a problem that confinement of fast particle
become worse [4]. We performed two simulation
cases; driving a current only in the center of plasma
and both in the center and at the normalized minor
radius r/a=0.5 of plasma.

120

80

4000

100

200time[s]300400Fig.3. Time evolution of alpha heating power in bothcases.
References[1] K.Yamazaki and T. Amano, Nucl. Fusion 32(1992) 633.
[2] K.Itoh et al., Plasma Phys. Control. Fusion 36(1994) 279 .
[3] A.Fukuyama et al.,Plasma Phys. Control.Fusion 37(1995) 611 .
[4] T.Suzuki, J. Plasma Fusion Res. 83 (2007) 434 (inJapanese)[5] A. Fukuyama, J. Plasma Fusion Res. 79 (2003)
1117 (in Japanese)