24C07
Reduction of the divertor heat load with impurity seeding in LHD
大型ヘリカル装置における不純物ガスパフを用いたダイバータ熱負荷軽減
Suguru Masuzaki1, Tsuyoshi Akiyama1, Masahiro Kobayashi1, Izumi Murakami1, Noriyasu Ohno2
and the LHD experiment group1
増崎 貴1，秋山毅志1，小林政弘1，村上 泉1，大野哲靖2，LHD実験グループ1
National Institute for Fusion Science, Oroshi-cho 322-6, Toki, 509-5292, Japan
核融合科学研究所 〒509-5292 土岐市下石町322-6
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

2. Neon seeding experiment
Figure 1 shows waveforms of plasma parameters
during discharges with and without Ne seeding. Negas was puffed for 120 ms from t = 4s with the fluxof ~ 1.6 Pa·m3/s. This flux was about 10 % of thehydrogen fueling flux. With Ne seeding, the total

1.1
10.9

n e, div (10 19m -3)

P rad (MW)n e,bar (10 19m -3)

0.8
644
2
00.6
0.3
020T e, div (eV)

1. Introduction
Reduction of heat and particle loads to divertor
is a crucial issue to realize fusion reactor.
Divertor detachment is a favorable operation forthe purpose. To achieve divertor detachment,
reduction of electron temperature (Te) in
scrape-off-layer (SOL) is necessary. One of theeffective methods to reduce Te is radiation
enhancement in the SOL plasma. In present
medium/large fusion devices, plasma facing
material has been carbon, and carbon works as
dominant radiator for reduction of Te. However,carbon will not be utilized in fusion reactor to
reduce tritium retention in vacuum vessel and to
avoid large erosion of plasma facing components,and metallic material such as tungsten will beplasma facing material. Therefore, it isconsidered that impurity such as neon seeding isnecessary to enhance radiation loss in SOL. Intokamaks, impurity seeding experiment has been
conducted, and reduction of Te in SOL has been
observed [1]. Against this background, impurity
seeding experiment was conducted in LHD which
has unique edge magnetic field line structure suchas existence of stochastic layer in SOL. In LHD,
neon (Ne), argon (Ar) and nitrogen (N2) wereseeded, respectively. In this presentation, theresults of the impurity seeding experiment will beshown.

Stored energy (MJ)

Impurity seeding for the reduction of the divertor heat load was conducted in LHD. Neon, argon andnitrogen were seeded, respectively, and enhanced radiation loss and reduction of the divertor heat load
were observed for all cases without drastic change of global plasma parameters such as stored energy andline averaged density. The ratio of the radiation power to the plasma heating power was limited by theradiative collapse of plasma, and that was almost same for the three impurities.

100
3.4

3.6

3.8

4
4.2
time (s)4.4

4.6Fig. 1 Typical waveforms of plasma parameters
during discharges with (red) and without (blue) Ne
seeding. Ne gas was puffed for 120 ms from t = 4s.
Plasma heating power with NBI was about 13 MW.

radiation power (Prad) was almost doubled, theplasma stored energy and the central line-averaged
density (ne,bar) slightly decreased and increased,Isat (A)

0.2

0.1

w/o New/ Ne
0-120-100 -80 -60 -40 -20
position (mm)020

Fig. 2 Ion saturation current profile on a divertor
plate at t = 4.2s in the discharges shown in Fig. 1.

respectively, and both the density (ne,div) andtemperature (Te,div) on a divertor plate decreased.
Figure 2 shows ion saturation current (Isat) profiles
on a divertor plate at t = 4.2s in the discharges withand without Ne seeding shown in Fig. 1. The drastic
reduction of Isat was observed at large Isat positions
such as position of -96mm and 78 mm. The
magnetic field lines structure was considered torelate with this change of Isat profile. The effect ofthe seeding such as reduction of temperature waslimited at the peripheral and the scrape-off layer.
The maximum Prad was obtained after thetermination of the seeding, and Prad decreased witha long time constant of ~ 4.7s. It suggested thatrecycling of Ne was large. To increase Prad, seeding
amount of Ne was increased. However, plasma
collapsed radiatively in the region Prad > ~ 0.3 times
plasma heating NBI power (PNBI) in the discharge
condition shown in Fig. 1.
Ne seeding in lower density discharge (ne,bar ~2×1019m-3) than that in the discharge shown in Fig.
1 was also conducted, and reduction of divertor
loads was also observed. In this case, Prad/PNBI
reached 0.5, though the effect of the seeding
reached deeper inside the last closed flux surface
than that in higher density discharge case, and ne
and Te in peripheral region largely increased anddecreased, respectively.
Ne seeding was also applied to Super Dense Core(SDC) discharges with pellet fueling [2], andreduction of the divertor loads was confirmed.
In LHD, other methods to reduce the divertor
loads such as divertor detachment with magnetic
island formation [3] and SERPENS mode [4] havebeen conducted. But they cannot be applied toinward shifted magnetic axis configurations up tonow, and they need high density plasma. It should
be noted that Ne seeding can be applied to inward
shifted configurations and relatively low density
plasma (ne,bar ~ 2×1019m-3).
3. Other impurity seedingN2 and Ar were also examined as radiators tounderstand the effect of the difference of their
atomic and molecule processes in plasma on theradiation enhancement.
In both seeding cases, radiation enhancement wasobserved as in the case of Ne seeding. The plasma
response to Ar seeding was roughly same as that inNe seeding discharge.
For the case of N2 seeding, Prad increased only
during the N2 gas-puffing. After the termination ofthe puffing, Prad decreased immediately. Itsuggested that N2 recycling coefficient was muchsmaller than that of Ne and Ar.
For all these three impurities, the Prad/PNBI limit
for the radiative collapse was almost same.
4. Summary
Impurity seeding for the enhancement ofradiation and reduction of the divertor heat load
was conducted in LHD using Ne, Ar and N2gas-puffing, respectively. For all cases, enhanced
radiation state was observed. In the state, the effectof the seeding on the plasma performance wassmall. However, once the radiation power exceeded
the limit, plasma collapsed radiatively. The limit
was ~ 0.3 times plasma heating power except low
density discharge case. Recycling coefficient of N2
was observed to be much smaller than that of Neand Ar.
Modeling including atomic and molecule
processes and transport is necessary to understandthe experimental results and to develop plasma
operation scenarios with impurity seeding in future
fusion reactor.
References[1] For example, Asakura, N. et al. Nucl. Fusion 49(2009) 115010.
[2] Ohyabu, N. et al. Phys. Rev. Letters 97 (2006)
055002.
[3] Kobayashi, M. et al. Phys. Plasma 17 (2010)
056111.
[4] Miyazawa, J. et al. Nucl. Fusion 46 (2006) 532.