Steady-state sustainment of divertor detachmentwith multi-species impurity seeding in LHD
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言語: English
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公開日: 2021-11-04
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キーワード (En):作成者: MUKAI, Kiyofumi, MASUZAKI, Suguru, Hayashi,
Yuki, OISHI, Tetsutaro, SUZUKI, Chihiro, KOBAYASHI,
Masahiro, TOKUZAWA, Tokihiko, TANAKA, Hirohiko,
TANAKA, Kenji, KINOSHITA, Toshiki, SAKAI, Hikona,
PETERSON, Byron J.
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Journal Title
https://doi.org/XXXX/XXXX

Steady-state sustainment of divertor detachmentwith multi-species impurity seeding in LHD
Kiyofumi Mukai1, 2, Suguru Masuzaki1, 2, Yuki Hayashi1, Tetsutaro Oishi1, 2, Chihiro
Suzuki1, 2, Masahiro Kobayashi1, 2, Tokihiko Tokuzawa1, 2, Hirohiko Tanaka3, Kenji
Tanaka1, 4, Toshiki Kinoshita4, Hikona Sakai4, Byron Jay Peterson1, 21National Institute for Fusion Science, National Institutes of Natural Sciences, Toki, JapanThe Graduate University for Advanced Studies, SOKENDAI, Toki, Japan
3
Graduate School of Engineering, Nagoya University, Nagoya, Japan
4
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, Kasuga, Japan2

E-mail: mukai.kiyofumi@nifs.ac.jp
Received xxxxxx
Accepted for publication xxxxxx
Published xxxxxx

Abstract
Multi-species impurity seeding is an advanced operation scenario to mitigate the divertor heatload for the realization of future fusion reactors. In the Large Helical Device (LHD), divertor
detachment is successfully sustained using higher-Z (krypton, Kr) and lower-Z (neon, Ne)superimposed seeding. Emission from Kr impurities is drastically enhanced if it is followed
by Ne seeding. Plasma radiation can be enhanced even at the upstream region in the edge
plasma compared with Ne only seeded plasmas with suppression of impurity accumulation
toward the central plasma. The characteristics of divertor heat load reduction and energy
confinement are comparable between the Kr+Ne seeding and Ne only seeding under the sameradiation fraction. However, while the detachment in Ne only seeding is transient, thedetachment in Kr+Ne seeding is stable. It indicates that multi-species impurity seeding can becompetitive for steady-state operation although further investigation is desired about thebalance between divertor heat load reduction, impurity screening, and confinement
degradation. The Kr emission enhancement is strongly affected by electron density andtemperature at the last closed flux surface resulting in impurity penetration.
Keywords: detachment, divertor, impurity seeding, LHD

and target heat flux by N seeding [2]. In Wendelstein 7-X,
detachment using Ne or N seeding has been investigated [3].
In LHD, we have investigated detachment using Ne, N or Kr
seeding individually [4-6]. However, the characteristics ofdivertor detachment in multi-species impurity seeded
plasmas have not been investigated. Thus, in this study, we
attempted to superimposed seeding using Kr and Ne in LHD.
The cooling rate of Ne has the maximum at the electron
temperature, Te, ~ 30 eV. On the other hand, the rate of Kr
takes a local minimum at Te ~ 30 eV, it is expected that these
impurities could enhance plasma radiation complementarily.

1. Introduction
Divertor detachment is a desirable regime to realize afusion reactor. Divertor heat load should be mitigated stably.
Furthermore, to handle the power exhaust, it is necessary toenhance plasma radiation not only in the divertor region but
also in the upstream region with suppression of dilution.
Therefore, multi species impurity seeding of higher-Z andlower-Z impurities is proposed in JT-60SA [1]. In ASDEX
Upgrade, argon (Ar) and nitrogen (N) seeding was performed
to feedback control main chamber radiation by Ar seedingxxxx-xxxx/xx/xxxxxx

1© xxxx IOP Publishing Ltd

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The rest of the paper consists as follows. The
experimental setup is shown in section 2. In section 3,
steady-state sustainment of divertor detachment with Kr+Ne
seeding is shown and Kr emission enhancement due to Ne
seeding is discussed. The paper is summarized in section 5.

seeded 0.4 s after the Kr seeding for 25 ms with the flow rateof 1.8 Pa·m3/s and with the amount of 0.045 Pa·m3. After Kr
seeding but before Ne seeding, -0.4 s < t - tNe < 0.0 s,
increase of total plasma radiation power, Prad, is insubstantial
as shown in FIG. 2 (d). As shown in FIG. 2 (e), Kr emission
is slightly enhanced. However, the enhancement of Prad is
0.08 MW and Prad at t - tNe = 0.0 s is 0. 55 MW. SIsat, allshown in FIG. 2 (f), which is the summation of ion saturation
current in all divertor probes, gradually decreased by 10%.
<qdiv> shown in FIG. 2 (g), which is the averaged divertor
heat flux in all divertor probes, gradually decreases by 20%.
τE, exp/τE, ISS04 shown in FIG. 2 (h) gradually decreases and is
0.95 at t - tNe = 0.0 s. Here, τE, exp is experimental energy
confinement time calculated by Wp/(PNBI, abs-dWp/dt) whereWp and PNBI, abs are plasma stored energy and absorbed NBI
heating power, respectively. τE, ISS04 is energy confinement
time estimated by ISS04 scaling [12].
The results after the Kr seeding but before Ne seeding

2. Experimental setup on LHD
The experimental setup on LHD is shown in FIG. 1. In
this study, the magnetic configuration is set with the major
radius of the magnetic axis, Rax = 3.60 m. The direction of
the toroidal magnetic field is counterclockwise and its
strength Bt = 2.75 T. The natural error field is compensatedusing a perturbation coil system. Plasmas are generated using
electron cyclotron heating (ECH) and are sustained using
three tangential neutral beam injections (NBIs). Deuterium
gas is puffed from the lower port between the toroidal
sections #3 and #4 (3.5-L). Impurity gases (Ne and Kr) are
puffed from the 5.5-L port. Total plasma radiation, Prad, andradiation brightness profiles are measured by resistive
bolometer arrays [7]. Ne and Kr emission profiles aremeasured with EUV spectroscopy [8] and SOXMOS [9-11].
Divertor heat load is evaluated by Langmuir probe arrays [6].

3. Divertor detachment with multi-species impurity
seeding
3.1 Steady-state sustainment of divertor detachmentwith Kr+Ne seeding
The waveform in Kr and Ne superimposed seeded plasma
is shown in FIG. 2. As shown in FIG. 2 (a), the plasma is
generated using ECH and is sustained using NBI. Lineaveraged electron density, ne, bar, shown in FIG. 2 (b), before
impurity seeding is 1 × 1019 m-3. Kr is seeded 0.4 s before
Ne seeding for 5 ms with the flow rate of 3.0 Pa·m3/s withthe amount of 0.015 Pa·m3, as shown in FIG. 2 (c). Since theplasma response to Kr is slow compared with Ne, Ne is

FIG. 2. Steady-state sustainment of divertor detachment for ~ 1
s using Kr+Ne superimposed seeding. Pre-seeded Kr emission is
drastically enhanced after Ne seeding.

FIG. 1. Experimental setup on LHD.

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Author et alindicated that Kr only seeding is not effective for the divertor
detachment. However, after the Ne seeding at t - tNe = 0.0 s,
SIsat and <qdiv> drastically decreases with the increase of Prad.
The detachment is sustained stably until the termination of
the NBI heating. The sustainment of the detachment can
extend if there is no limitation on the pulse length of the NBI
heating. Here, as shown in FIG. 2 (b), ne, bar increases from0.96 × 1019 m-3 (averaged -0.1 s £ t - tNe £ 0.0 s) to 1.76 ×
1019 m-3 (averaged from 0.4 s £ t - tNe £ 1.0 s) after the Ne
seeding. When the seeded Ne are fully ionized, ne, bar canincrease to 0.36 × 1019 m-3. The Te at the magnetic axis islower than the ionization energy of KrXXVII, 2.9 keV.
Therefore, when each seeded Kr generates 25 electrons, ne, bar
can increase to 0.30 × 1019 m-3. Since the total ne, bar increaseof 0.80 × 1019 m-3 is larger than the increase 0.66 × 1019 m3
due to the impurity seeding, wall recycling should bechanged. NBI port-through power is constant, however, theheating efficiency improved by the ne, bar increase. Therefore,
PNBI, abs shown in FIG. 2 (a) increases after the Ne seeding.
At 0.4 s £ t - tNe £ 1.0 s, Prad increases to 1.61 MW andradiation fraction, frad = Prad/PNBI, abs, was 0.40. <qdiv>
averaged 0.4 s £ t - tNe £ 1.0 s decreases by 0.92 comparedwith <qdiv> averaged -0.5 s £ t - tNe £ -0.4 s before the Kr
seeding. τE, exp/τE, ISS04 averaged 0.4 s £ t - tNe £ 1.0 s was 0.80.
Time evolutions of KrXIX and NeVIII emissions shown in
FIG. 2 (e) had quite interesting behaviors. After the Ne
seeding, not only NeVIII emission but also KrXIX emission
is drastically enhanced after t - tNe = 0.15 s. The behavior of
the impurities is discussed in the next section.
The waveform of Ne only seeded plasma is shown in FIG.
3. PNBI, abs and ne, bar before the Ne seeding shown in FIG. 3(a) and (b) were comparable with the Kr+Ne superimposed
seeded plasma shown in FIG. 2. Ne is seeded from t - tNe =0.0 s for 40 ms as shown in FIG. 3 (c) with the flow rate of
1.8 Pa·m3/s and the amount of 0.072 Pa·m3. The maximum
frad shown in FIG. 3 (d) is also comparable with those of FIG.
2. In the Ne only seeded plasma, after the Ne seeding SIsat, alland <qdiv> reduction with Prad and NeVIII enhancement isobserved as shown in FIG. 3 (d) - (g). The most importantdifference between Kr+Ne and Ne only seeded plasma is thatthe detachment in Kr+Ne seeded plasma is sustained stably
while the detachment in Ne only seeded plasma is transientand is reattached in 0.2 s. In the Ne only seeded plasma, at t tNe = 0.17 s, frad had the maximum value of 0.40 which is
10% higher than that in Ne only seeding with multi-pulses[5]. <qdiv> averaged 0.1 s £ t - tNe £ 0.25 s is 0.77 lower
compared with that averaged from 0.0 s £ t - tNe £ 0.1 s
before the Ne seeding as shown in FIG. 3. (g). τE, exp/τE, ISS04
averaged 0.10 s £ t - tNe £ 0.25 s shown in FIG. 3 (h) is 0.90.
Magnetic field connection length, Lc, and typical Isat
profiles on a divertor plate in Kr+Ne and Ne only plasmas
are shown in FIG. 4. The private-side peak of the Isat profile

FIG. 3. Divertor detachment using Ne seeding with the
maximum frad ~ 0.4 which is comparable with FIG. 2. The
detachment disappears 0.2 s after the Ne seeding.

would be attributed to the magnetic field change due to theedge pressure gradient inside the core region [13]. FIG. 5
shows the toroidal profile of the SIsat ratio before impurity
seeding and after impurity seeding. Here, SIsat is thesummation of ion saturation current in 40 divertor probes ina toroidal section. In the Kr+Ne plasma, the peak value of
SIsat slightly decreases. However, the reduction was not
significant and Isat increases at some positions on the divertor
plate. At this time, the SIsat ratio had toroidal asymmetry as
shown in FIG. 5. It implies that Kr ions are localized along
magnetic field lines connected to the position of impurity
seeding and are not expanded to all toroidal sections. The
SIsat ratio decreases in toroidal sections #2, 7, 8, 9, and 10
while it does not decrease in toroidal sections #4 and 6. Kr
and Ne seeding are applied from toroidal section #5.5 asmentioned in section 2. After the Ne seeding, Isat on thedivertor plate decreases in both Kr+Ne and Ne only seeded3

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FIG. 5. toroidal profile of SIsat ratio before impurity seeding andafter impurity seeding. While toroidal asymmetry was observed
after Kr seeding, SIsat decreased in all toroidal sections after Ne
seeding in both plasmas.

FIG. 4. (a) Lc profile on a divertor plate located on the right-side
in the toroidal section #7 and typical Isat profiles in the (b)Kr+Ne seeded plasma and (c) Ne only seeded plasma. After the
Ne seeding, Isat drastically decreases while the reduction ismodest after the Kr seeding.

plasmas. The SIsat ratio also decreases in all toroidal sections
in both plasmas. It should be emphasized again that the
detachment in Kr+Ne seeded plasmas is sustained while thedetachment in Ne seeded plasma is transient.
Plasma radiation profiles measured with the resistive
bolometer arrays in Kr+Ne seeded plasma and Ne only
seeded plasma are shown in FIG. 6. Here, the radiation is thepower irradiated onto the bolometer arrays and reff, min isdetermined as the minimum reff on the bolometer sightlines.
In the Kr+Ne seeded plasma, radiation is enhanced at |reff,min/a99| ~ 0.8 compared with the radiation profile in Ne only
seeded plasma. It indicates that plasma radiation is enhanced
at the upstream region using higher-Z impurity under thesame frad. Optimal impurity seeding conditions should be
investigated to obtain further radiation enhancement also at
the downstream region simultaneously. It is notable that theradiation profile in the Kr+Ne seeded plasma is hollow
although the path length of the sightline in the core plasma islonger than that in the edge plasma. It indicates that theaccumulation of the seeded impurities toward the central
plasma could be suppressed during the detachment. FIG. 7
shows the time evolution of Te at various radial positions.

FIG. 6. Plasma radiation profiles in Kr+Ne seeded plasma and
Ne only seeded plasma. Plasma radiation was enhanced at the
upstream region using higher-Z impurity under the same frad.

FIG. 7. time evolution of Te profile. Reduced Te due to Ne
seeding is recovered at |reff/a99| ~ 0.8 after t – tNe = 0.2 s.

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While Te decreases after the Ne seeding in all radial positions,
Te is recovered at |reff/a99| ~ 0.8 after t - tNe = 0.2 s. It isconsistent with the suppression of the impurity accumulation
towards the central plasma.
FIG. 8. shows the radial profiles of ne, Te, and electronpressure, pe, in Kr+Ne seeded plasma and in Ne only seeded
plasma. Profiles are comparable at reff/a99 > 0.9 although corene is higher in the Kr+Ne seeded plasma. The Ne seeding
amount in Ne only seeded plasma is 1.6 times that in Kr+Ne
seeded plasma. Since the amount of deuterium puff is thesame in both discharges, the higher core ne in Kr+Ne seeded
plasma is caused by the Kr ionization. FIG. 9 shows therelation between frad and τE, exp/τE, ISS04. frad in the Ne only
seeded plasmas is plotted as the averaged value in thetransient detachment and frad in the Kr+Ne seeded plasma isplotted as the averaged value in the steady state, 0.4 s £ t - tNe£ 1.0 s. In the Ne only seeded plasma, τE, exp/τE, ISS04 degradeswith the increase of frad. In the Kr+Ne seeded plasma, while
the detachment can be sustained, τE, exp/τE, ISS04 was 10%
lower than that in the Ne only seeded plasmas under thesame frad. Optimization of impurity screening may improvethe confinement degradation in the Kr+Ne seeded plasma
from the Ne only seeded plasmas. Moreover, ne fluctuations
can be considered as a candidate of the τE, exp degradationfrom τE, ISS04 during the detachment phase. FIG. 10 shows thetime evolutions of the ne fluctuation profile measured with
two-dimensional phase contrast imaging (PCI) [14]. Both in
the Kr+Ne seeded plasma and Ne only seeded plasma, nefluctuation is enhanced after the Ne seeding during thedetachment especially at 0.7 < |reff/a99| < 0.8. In order toimprove the energy confinement during the detachment, thefluctuation should be suppressed. FIG. 11 shows the relation
between the seeded impurity amount and the frad. Higher fradis obtained using Kr+Ne seeding compared with the Ne only
seeded plasmas under the same seeded impurity amount. In
order to sustain the detachment in Ne only seeded plasmas aslong as that in Kr+Ne seeded plasma, several times Ne
injection, i.e., further amount of Ne is required. It indicatesthat multi-species impurity seeding is competitive for steadystate operation without dilution.

3.2. Kr emission enhancement due to Ne seeding
In the Kr+Ne superimposed discharges, KrXIX emission
is drastically enhanced as shown in FIG. 2 (e). On the other
hand, in the Kr only seeded plasmas, radiative cooling is noteffective. The reduction of edge Te is modest and frad is up to
0.21 [5]. These results indicate that Kr ions are transported
toward the upstream region in Kr+Ne superimposed
discharges. FIG. 12 shows the radial profiles of seeded
impurity ion emission intensity (KrXIX and NeVIII). The
difference of the peak position between FIG. 6 and FIG. 12
can be considered that Kr and Ne ions with higher charge
state can exist and that the spread of the bolometer sightline

FIG. 8. Radial profiles of (a) ne, (b) Te, and (c) pe in Kr+Ne seeded
plasma and in Ne only seeded plasma measured with Thomson
scattering. Profiles are comparable at reff/a99 > 0.9 although
core ne is higher in the Kr+Ne seeded plasma.

is larger than that of the EUV spectroscopy. While NeVIII is
exhausted with the reattachment in the Ne only seeded
plasma, KrXIX and NeVIII are retained at reff/a99 ~ 1 in the
Kr+Ne seeded plasma. Negative radial electric field, Er,5

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FIG. 11. Relation between the seeded impurity amount and frad.
Higher frad is obtained using Kr+Ne seeding compared with the
Ne only seeded plasmas.

FIG. 9. Relation between frad and τE, exp/τE, ISS04. In the Kr+Ne
seeded plasma, τE, exp/τE, ISS04 comparable to that in the Ne only
seeded plasmas under the same frad is sustained.

region, impurities are screened by the increase of the ion
temperature gradient and Mach number. Therefore, whenplasmas are in a certain window on the n-T diagram,
impurity accumulation occurs.
Time evolutions of ne, LCFS - Te, LCFS characteristics in the
Kr+Ne superimposed discharges are shown in FIG. 13. ne,
LCFS, Te, LCFS are plotted every 0.1 s. The color of the symbols
corresponds to the intensity of the KrXIX emission. Threecases of the discharges are plotted on FIG. 13. In the firstcase KrXIX emission is not enhanced after the Ne seeding(open squares). In the second case KrXIX emission isenhanced and sustained (closed circles). In the last case
KrXIX is enhanced but radiatively collapsed (open triangles).
Note that the amount of the seeded impurities is the sameamong all cases. (ne, LCFS, Te, LCFS) is around (0.9 ´ 1019 m-3,
240 eV) after Kr seeding but before Ne seeding in all cases.
After the Ne seeding, due to the ionization of the seeded Ne,(ne, LCFS, Te, LCFS) changes towards higher-ne, LCFS and lower-Te,
LCFS. In the stably detached discharge shown in FIG. 2, 0.2 s
after the Ne seeding (t - tNe = 0.2 s), (ne, LCFS, Te, LCFS)
changed to around (1.6 ´ 1019 m-3, 150 eV). Then, the KrXIX
emission is enhanced. One the other hand, in the case that
KrXIX is not enhanced, Te, LCFS reduction is smaller than thatin the other two cases. In the radiatively collapsed case, thene, LCFS increase is higher than that in the other two cases.
These results lead to two conditions; (i) when Te, LCFS is
reduced below ~ 150 eV by the Ne seeding, KrXIX isenhanced and (ii) when the ne, LCFS increases above 2 ´ 1019
m-3 after the Ne seeding, the plasma radiatively collapses.
The behavior of KrXIX emission enhancement is various
although the amount of the seeded Kr and Ne are the sameamong the discharges. It indicates that impurity transport isdifferent among the three cases. ne, LCFS and Te, LCFS after Kr
seeding but before Ne seeding in all cases are similar.
However, the ne, LCFS and Te, LCFS are near the border between

FIG. 10. Time evolutions of ne fluctuation profile measured with
two-dimensional phase contrast imaging (PCI). In the bothcases, ne fluctuation is enhanced after the Ne seeding during
the detachment especially 0.7 < |reff/a99| < 0.8.

forms after t - tNe = 0.4 s as shown in FIG. 2 (i). It canpromote the retention of the Kr ions. Although negative Er
forms also in the Ne only seeded plasma as shown in FIG. 3(i) after t - tNe = 0.3 s, the plasma is almost reattached at thistime.
The impurity accumulation behavior in LHD can becharacterised by the relation between ne and Te at the LCFS
(ne, LCFS, Te, LCFS), the so-called n-T diagram [15, 16]. In the
study, the accumulation and screening of intrinsic impurities(carbon and iron) was investigated in the steady-state NBIheated plasmas. The main gas was hydrogen while the maingas in this paper is deuterium. In the lower-ne, LCFS andhigher-Te, LCFS region on the n-T diagram, impurities arescreened by positive radial electric field, Er. In the higher-ne,
LCFS and lower-Te, LCFS region, impurities are also screened by
friction force in the ergodic layer. In the high-power
discharges, i.e., in the higher-ne, LCFS and higher-Te, LCFS

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FIG. 13. Relation between ne, LCFS, Te, LCFS, and KrXIX emission in
Kr+Ne superimposed seeded discharges. Although the amountof the seeded impurities is the same, plasma response isdifferent depending on the ne, LCFS and Te, LCFS of the backgroundplasmas.

evaluated for carbon and iron. These results imply that asmall difference in the ne, LCFS - Te, LCFS characteristics before
the Ne seeding strongly affects the response of the Kr
enhancement. The relation between ne, LCFS 0.1 s before the
Ne seeding, ne, LCFS (t - tNe = -0.1 s), and Dne, LCFS, DTe, LCFS,
DKrXIX, and DNeVIII are shown in FIG. 14. Smaller ne, LCFS
makes the larger impurity screening and larger ne, LCFS makes
the larger impurity penetration. The differences are
determined by the subtraction of those 0.1 s before Ne
seeding from those 0.2 s after Ne seeding. Dne, LCFS increases
with the increase of ne, LCFS (t - tNe = -0.1 s). Since theamounts of the seeded Kr and Ne are the same, enhancementof Ne ionization due to an increase of Ne penetration at theergodic layer can be considered as one of the candidates forthe increase of Dne, LCFS. When ne, LCFS (t - tNe = -0.1 s) isbelow 0.8 ´ 1019 m-3, KrXIX emission is not enhanced while
the NeVIII emission intensity increases. When ne, LCFS (t - tNe= -0.1 s) is above 0.8 ´ 1019 m-3, further DTe, LCFS decrease isobserved and KrXIX emission intensity is enhanced while
the NeVIII emission intensity saturates. It indicates thationization of Kr atoms and transport of Kr ions changes at ne,
19-3
LCFS (t - tNe = -0.1 s) ~ 0.8 ´ 10 m .
In order to investigate the momentum balance, e.g. friction
force and thermal force, in the ergodic layer, data of ion
temperature and effective ion charge is required. Er
measurement in core plasma is also required to investigate
the mechanism to suppress the impurity accumulation toward
the central plasma. Simulation study is also needed for the
discussion. Control of the edge profile using divertor
pumping and feedback control using fast Thomson scattering
and/or ECH in the edge plasma are required to obtain further

FIG. 12. Radial profiles of seeded impurity ion emission (KrXIX,
NeVIII) intensity in Kr+Ne seeded plasma (a, b) and in Ne only
seeded plasma (c). While NeVIII is exhausted with thereattachment in the Ne only seeded plasma, KrXIX and NeVIII
are retained at reff/a99 ~ 1 in the Kr+Ne seeded plasma.

screening and accumulation areas in the lower-ne, LCFS andhigher-Te, LCFS region [15, 16] although these areas are7

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Author et alheat load reduction, impurity screening, and confinement
degradation. The Kr emission enhancement is strongly
affected by ne, LCFS and Te, LCFS resulting in impurity
penetration. Feedback control of these parameters using fastThomson scattering and/or ECH in the edge plasma may be
helpful to obtain further radiation enhancement and further
sustainment of the detachment.

Acknowledgements
The authors thank the LHD technical staff for their
excellent support in the LHD experiments. This work was
supported by JSPS KAKENHI Grant Number 17K14900,
19K14689 and by the LHD project budget (ULHH038,
ULGG801).

References[1] GAŁĄZKA, K. et al. 2018 Contrib. Plasma Phys. 58 751-757[2] KALLENBACH, A. et al, 2012 Nucl. Fusion 52 112003[3] EFFENBERG, F et al, 2019 Nucl. Fusion 59 106020[4] MASUZAKI, S. et al. 2013 J. Nucl. Mater. 438 S133
[5] MUKAI, K. et al. 2015 Nucl. Fusion 55 083016[6] TANAKA, H. et al. 2017 Nucl. Mater. Energy 12 241-246[7] MUKAI, K. et al. 2020 Plasma Fusion Res. 15 1402051[8] ZHANG, H. et al. 2015 Jpn. J. Appl. Phys. 54 086101[9] SCHWOB, J. L. et al.1987 Rev. Sci. Instrum. 58 1601
[10]SUZUKI, C. et al. 2015 J. Nucl. Mater. 463 561-564[11]
SUZUKI, C. et al. 2019 Nucl. Mater. Energy 19 195-199[12]
YAMADA, H. et al. 2005 Nucl. Fusion 45 1684
[13]
TANAKA, H. et al. 2018 Plasma Phys. Control. Fusion
60 125001[14]
TANAKA, K. et al. 2008 Rev. Sci. Instrum. 79 10E7021[15]
NAKAMURA, Y. et al. 2014 Plasma Phys. Control.
Fusion 56 075014[16]
NAKAMURA, Y. et al. 2017 Nucl. Fusion 57 056003

FIG. 14. Background ne, LCFS dependence of (a) Dne, LCFS, (b) DTe,
LCFS, (c) DKrXIX, and DNeVIII. When the background ne, LCFS is
above 0.8 ´ 1019 m-3, further Te, LCFS reduction and KrXIX
enhancement occurres.

radiation enhancement and further sustainment of thedetachment.

4. Summary
Multi-species impurity seeding is an advanced operation
scenario to mitigate the divertor heat load for the realizationof future fusion reactors. In the Large Helical Device (LHD),
divertor detachment is successfully sustained using higher-Z(Kr) and lower-Z (Ne) superimposed seeding. Emission fromKr impurities is drastically enhanced if it is followed by Ne
seeding. Plasma radiation can be enhanced even at the
upstream region in the edge plasma compared with Ne only
seeded plasmas with suppression of impurity accumulation
toward the central plasma. The characteristics of divertor
heat load reduction and energy confinement are comparable
between the Kr+Ne seeding and Ne only seeding under thesame frad. However, while the detachment in Ne only seedingis transient, the detachment in Kr+Ne seeding is stable. It
indicates that multi-species impurity seeding can becompetitive for steady-state operation although further
investigation is desired about the balance between divertor

8