24C01
Operations of divertor heat load mitigation on LHD
LHDにおける受熱板熱負荷軽減運転
Noriyasu Ohno1, Tsuyoshi Akiyama2, Suguru Masuzaki2, Masahiro Kobayashi2and the LHD experiment group2
大野哲靖1, 秋山毅志2，増崎 貴2，小林政弘2，LHD実験グループ2
1

2

Nagoya University, Chikusa-ku, Nagoya, 464-8603 Japan
名古屋大学 〒464-8603 名古屋市千種区不老町

National Institute for Fusion Science, 322-6 Oroshi-cho, Toki-shi, 509-5292, Japan
核融合科学研究所 〒509-5292 土岐市下石町322-6

Experiments on divertor heat load mitigation are conducted on LHD. Ne seeding can increase the radiation
intensity by a factor of two and decrease the ne and Te on divertor plates by half without deterioration ofcore confinement. The resonant magnetic perturbation field of the m/n=1/1 mode can also induce thedetachment and stabilize the position of the radiation region at the island. These schemes can be combined
and increase the radiation further.

2. Impurity Gas Puffing
In present medium/large fusion devices, a plasma
facing material is carbon, and it works as adominant radiator to reduce the edge Te. However,carbon will not be utilized in fusion reactors toFig. 1. Typical time evolutions of (a) plasma stored
energy (Wp) and NBI power (PNBI), (b) line averaged
density (ne,bar), radiation power (Prad) and gas-puffing, (c)electron density (ne,div) and temperature (Te,div) in thedivertor (torus inboard-side).

1008 (b)016Ne puffing

H2 puffing

4800.6 (c)

040

0.4

20

0.2
03

3.5

4

4.5
time (s)50
5.5Prad (MW)0.5ne,bar (1019m-3)
ne,div (1019m-3)

20Te,div (eV)

1 (a)PNBI (MW)

reduce tritium retention in the vacuum vessel and to
avoid severe erosion of plasma facing components.
Instead, metallic materials such as tungsten will be
used for plasma facing materials. Therefore, it isconsidered that impurities such as Ne seeding arenecessary to enhance the radiation loss in the edge
region.
Figure 1 shows time evolutions of plasma
parameters of a Ne seeding discharge with a line
average density (ne,bar) of 5×1019 m-3. The Ne gas ispuffed from t = 4 s for 120 ms. The Ne gas flux is ~1.6 Pa·m3/s, and it is about 10 % of H2 gas flux at t
= 4 s. Total radiation power (Prad) rises from 2 MW(~0.15×PNBI) to 4 MW (~0.3×PNBI) during the Ne
Wp (MJ)

1. Introduction
In magnetically confined fusion reactors, divertor
plates will be exposed to the extremely high energy
flux more than 10 MW/m2 from a plasma, which isbeyond the present engineering limit of the divertor
structure. Although great efforts have been paid todevelopment of divertor materials and theirstructures so far, it is crucial to reduce the divertor
flux. It is one of the key issues to realize fusion
reactors because its economic merit depends on themaintenance and replacement of the divertor plates.
The power dispersal via energy loss such asradiation by impurity species is, at the moment, apossible scheme to reduce the divertor power load
lower than the engineering limit. The operation ofthis scheme is called “divertor detachment”.
Impurity seeding experiments for optimization ofedge parameters and impurity species have been
conducted in tokamaks.
On the other hand, several experiments tomitigate the heat and particle loads have started on
the Large Helical Device (LHD). LHD has adifferent peripheral magnetic structure, such as theexistence of the stochastic layer, and edge ne and Te
profiles from those in tokamaks. Investigation ofoperations for heat and particle loads mitigation on
LHD is not only useful for helical reactors but also
for toroidal plasmas.

Normalized n e profile Normalized T e profileby profile at t=3.95 s by profile at t=3.95 s

1.5

10

#10383301

0.5
t=4.05 s (during Ne puff)
t=4.35 s (after Ne puff)

10
H puff 3.5L (V)

(b)
10.5U

Iis (A)00.5

103000 I

0.50
3.6

30
205

10
1.5(a)ne (10 19 m-3)

2000
LCFS

3.8

4

4.2

4.4

4.6

R (m)

Fig. 2. Normalized radial profiles of ne and Te profiles by
those at t=3.95s (before Ne seeding) in the samedischarge as Fig. 1.

puffing. It is clearly shown that both divertor ne(ne,div) and Te (Te,div) decreased with the Ne seeding.
This means that particle and heat loads to thedivertor are reduced. The Ne seeding does notstrongly affect global parameters such as the plasma
stored energy, Wp, and ne,bar.
Figure 2 shows radial profiles of ne and Te
normalized by those before the Ne seeding (t=3.95s). While ne and Te change in the stochastic region
and near the LCFS in the core plasma, the central neand Te around R~3.6 m do not change. They are
preferable features of the Ne seeding.
3. Inducing Detachment and Stabilization ofRadiation Region with RMP
Thermally unstable plasmas after the detachmentfrom the divertor plates are observed in variousdevices. These instabilities lead to difficulty ofcontrol of the radiation level and location, theX-point MARFE with deterioration of core plasma
performance.
In LHD, it is found that the resonant magnetic
perturbation (RMP) field of the n/m=1/1 mode has
a stabilizing effect on the strongly radiating plasma,
where the detachment operation is successfully
sustained without deterioration of the core plasma
performance up to the end of the discharge2).
Without the RMP, otherwise, the plasma leads toradiative collapse. From the measurements ofbolometer and AXUVD, the strong radiation layer
is considered to be stabilized around X-point of theisland.
Figure 3 shows time traces of ne,bar, the divertor
particle flux and the ramp-up perturbation coilcurrent for the m/n=1/1 RMP field. After theinitiation of the discharge at t=3 s, ne,bar is gradually

LID

(A)

1000
0(c)

4

6

8time (s)10Fig. 3. Time traces of (a) line averaged density and gas
puff wave form, (b) the divertor particle flux, (c) RMP
coil current.

increased by gas puffing and it reaches the flat top~8×1019 m-3, while the perturbation field continues
to increase during the discharge as shown in the coilcurrent. By keeping the density constant after t=5 s,
only the perturbation field increases and the plasma
goes to the detached state at t=6.3 s as shown in thedivertor particle flux reduction. The detachment issustained up to the end of the gas puffing at t=9 sec.
The detachment transition occurs at the coil currentof slightly above 2000A. The timing of the coilcurrent ramp up is shifted back and forth in time
while keeping almost the same density flat top toconfirm the effect of the RMP field. It is found thatthe detachment onset is accordingly shifted backand forth in time. These results demonstrate that theRMP field is a new control knob of the divertor
detachment.
The compatibility of the island induced
detachment with the impurity gas seeding has also
been confirmed. The additional Ne seeding to theisland detached plasma increases the radiation
intensity by a factor of 1.4, while keeping the stabledetachment.
4. Summary
On LHD, the impurity gas seeding successfully
mitigates heat and particle loads to the divertor
plates. The RMP field can induce the detachment. Itis possible to combine these two schemes andfurther radiation can be obtained.
References[1] N. Asakura et al., Nucl Fusion 49 (2009) 115010.
[2] M. Kobayashi et al., Physics of Plasmas 17 (2010)
056111.