23D12
Control of impurity influx with external perturbation field in high-density
plasmas on LHD
LHD 高密度プラズマにおける外部摂動磁場による不純物流入の抑制
Yukio Nakamura, Tomohiro Morisaki, Kimiharu Ohyama, Naoki Tamura, Byron Jay Peterson,
Masahiro Kobayashi, Ryuichi Sakamoto, Gen Motojima
中村幸男、森崎友宏、大山公治、田村直樹、B.J. Peterson、小林政弘、坂本隆一、本島 厳
National Institute for Fusion Research322-6, Oroshi-cho, Toki 509-5292, Japan
核融合科学研究所 〒509-5292 土岐市下石町322-6
Impurity behavior in high-density plasmas with super dense core (~2x1020m-3) has been investigated.
Radiation profile measurements showed a remarkable temporal increase of core radiation due to intrinsic
metallic impurities. The operational regime is in impurity accumulation window on n - Pnbi diagram for
impurity behavior. Impurity influx control with an external induced magnetic island at the plasma edge wasdemonstrated and impurity accumulation in the core was suppressed.

1. Introduction
Super Dense Core (SDC) plasmas with Internal
Diffusion Barrier (IDB) for particle transport can beproduced by particle fueling with pellet injection inLHD. The development of repetitive pellet injectorenables us to maintain the SDC plasmas for longtime. As a result, a quasi-steady state discharge withIDB (ne-bar = 2 x 1020 m-3) was achieved over 3 s. In
this discharge, radiation profile measurements by
AXUVD silicon photodiode arrays showed atemporal peaking of radiation profile. Therefore,
impurity behavior in quasi-steady state discharges
with internal diffusion barrier is studied and thencontrol of impurity influx with an external
perturbation field is investigated.
2. Impurity behavior in SDC plasmas
2.1 Super Dense Core (SDC) plasma
A super dense core operational regime was foundin LHD [1-2]. SDC plasmas are spontaneously
produced by injection of multiple pellets in themagnetic configuration with the major radius ofmore than R = 3.75 m. A core region with densities
as high as 1.0 x 1021 m-3 and temperatures of 0.85
keV is confined by an internal diffusion barrier(IDB) with very high-density gradient, which islocated at normalized minor radius ρ ~ 0.5. The
SDC plasmas can be also maintained by controlling
the plasma density with repetitive pellet injection.
2.2 Impurity behavior in long pulse discharge
Figure 1 shows a typical long pulse SDC
discharge with IDB. The average plasma density
rapidly builds up to a high-density level of 3 x 1020
m-3 with high repetitive pellet injection. Then theaverage plasma density is controlled so as to keep
more than 2 x 1020 m-3 by a feedback control loopwith CO2 laser interferometer signal. In spite ofkeeping the average plasma density, the density
profile changes with time. The density profile
becomes peaked remarkably until 5.2 s, and thenthe central density decreases and the density in theperipheral region increases with time. The radiation
profiles are measured by a system of two
20-channel fan-beam cameras with AXUVD silicon
photodiodes. Local radiation is reconstructed with atomographic imaging technique. As shown in thelast column, a remarkable peaking of radiation
profile is observed despite of keeping the density
and temperature profiles. This suggests that
impurities are accumulated in the core plasma. The
suppression of impurity accumulation in the second
half period of the discharge is related to impurity
shielding due to the increase of edge density.

Fig. 1. Long pulse SDC discharge with IDB

2.3 n - Pnbi diagram for impurity behavior
Impurity behavior in LHD can be distinguishedin the operational regime on n - Pnbi diagram asshown in Fig. 2. The impurity transport largely
depends on radial electric field and impurity
shielding effect in ergodic layer [3], which are
unique features of helical system. In this diagram,we can find the impurity accumulation window [4]in ion root regime (Er < 0) and impurity screening
region in high-density regime. The impurity
behavior in SDC plasmas is consistent with that in
the operational regime on n - Pnbi diagram.
15w/o imp. accu.
(positive E)

w imp. accu.
(negative E )r

10w/o imp. accu.
(screening)P
0=0.8 x n (!=1)

NBI

01

2
3
4
5
19 -3
n (!=1) (10 m )Fig. 3. Discharges with and without LID field
8/5

1.5e

6

5.0

7eFig. 2. n - Pnbi diagram for impurity behaviort = 4.7 s104258
104268!=1!=10.54. SummaryLong pulse SDC discharges showed impurity
accumulation just after plasma density ramp up
phase. The successful control of impurity

3.0
2.0

!=1!=1

1.0
0.0

3.0

3.5

4.0
R (m)

0.0

4.5

3.0

3.5

4.0
R (m)

4.5

5.0
1042581042684.0
Radiation (a.u.)

3. Control of impurity influx
In the previous LHD experiments [5], theinfluence of magnetic islands on impurity behavior
was observed by using external perturbation coils,
which produce an n/m = 1/1 magnetic island near
the plasma edge. This method is applied to SDC
discharges and the impurity accumulation control isdemonstrated. Figure 3 shows typical discharges
with and without the perturbation field. In this case,the perturbation field is applied before the discharge
and decreased with time. The plasma density inboth discharges is kept constant level (~2 x 1020m-3). In the discharge without the perturbation filed(#104258), a remarkable peaking of radiation
profile and the increase of metallic impurity line
intensity (Fe XIX) are observed after the density
ramp up phase as shown in Figs. 3-4. On the otherhand, we can see the suppression of impurity
accumulation by reducing the impurity influx in thedischarge with the perturbation filed (#104268).
The physical mechanism of impurity accumulation
suppression will be investigated in future.

t = 4.7 s

1042581042684.0

1.0
Te (keV)

5ne (10 20m-3)

P

NBI(MW)raccumulation was demonstrated with a perturbation
field (a magnetic island near the plasma edge).

t = 4.7 s

3.0
2.0
1.0
0.0

05
1015
Channel Number

20Fig. 4. Profiles of temperature, density and radiation

References[1] N. Ohyabu, T. Morisaki, S. Masuzaki, et al., Phys.
Rev. Lett. 97 (2006) 055002.
[2] R. Sakamoto, M. Kobayashi, J. Miyazawa, et al.,Nuclear Fusion 49 (2009) 085002.
[3] Y. Feng, M. Kobayashi, F. Sardei et al., NuclearFusion 49 (2009) 095002.
[4] Y. Nakamura, Y. Takeiri, B. J. Peterson, et al.,Plasma Phys. Control. Fusion 44 (2002) 2121.
[5] Y. Nakamura, Y. Takeiri, R. Kumazawa, et al.,Nuclear Fusion 43 (2003) 219.