Plasma and Fusion Research: Regular Articles

Volume 14, 3403057 (2019)

Comparative Analysis of Impurity Transport in the Peripheral
Plasma in the Large Helical Device for Carbon and Tungsten
Divertor Configurations with EMC3-EIRENE∗)
Mamoru SHOJI1) and Gakushi KAWAMURA1,2)
1)2)

National Institute for Fusion Science, National Institutes of Natural Sciences, Toki 509-5292, Japan
Department of Fusion Science, Graduate University for Advanced Studies (SOKENDAI), Toki 509-5292, Japan(Received 27 December 2018 / Accepted 17 February 2019)

Impurity transport in the peripheral plasma in the Large Helical Device for carbon and tungsten divertor
configurations is analyzed using a three-dimensional edge plasma simulation code (EMC3-EIRENE). Long pulse
plasma discharges in LHD have often been interrupted by the emission of iron dust from the surface of thevacuum vessel. The iron ions in the peripheral plasma originating from the iron dust can enhance the sputtering of
impurities on the divertor plates. The influence on the peripheral plasma by the sputtered impurities is investigatedin the iron dust emission case for both the carbon and the tungsten divertor configurations. The simulation reveals
that the dependence of the radiation power by the sputtered impurities on the plasma density and the iron emission
rate is quite diﬀerent between the two divertor configurations. It is found that, for the tungsten divertor, a highplasma density operation is advantageous for controlling the radiation power and the impurity ion content. In
the high plasma density operation, the radiation power by tungsten ions is suppressed to less than that by carbon
ions for the carbon divertor even in the case of high iron dust emission rates due to the reduced sputtering rate bylowered divertor electron temperatures.
c 2019 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: impurity transport, tungsten, carbon, divertor, EMC3-EIRENE, LHD
DOI: 10.1585/pfr.14.3403057

1. Introduction
Recent long pulse discharges in the Large Helical Device (LHD) have often been interrupted by a carbon dust
emission from the divertor region caused by the exfoliation of carbon dominant mixed-material deposition layers [1, 2]. The conventional carbon divertor plates will be
replaced to tungsten plates in near future for suppressing
the deposition layers. It is expected that the newly installed
tungsten divertor plates will contribute to the extension of
the duration time of the long pulse discharges as a resultof the low sputtering rate of tungsten. However, the sputtered tungsten can interrupt the plasma discharges by thehigher cooling rate due to highly charged tungsten ions in
the plasma. For adopting the tungsten divertor, there isa concern about the sustainment of long pulse discharges.
That is, electric arcing on the surface of the vacuum vessel
has been frequently observed during the plasma discharges
with abrupt increase in the iron ion intensity in the plasma,
where large amounts of iron dust were released by the electric arcing [3]. The dust enters the peripheral plasma, andiron ions are produced because iron is the main componentof the vacuum vessel (SUS316L). The iron ions can trigger
author’s e-mail: shoji@nifs.ac.jp∗) This article is based on the presentation at the 27th International Toki
Conference (ITC27) & the 13th Asia Pacific Plasma Theory Conference(APPTC2018).

the physical sputtering of tungsten on the divertor plates byacceleration of the iron ions by the sheath potential formedin front of the divertor plates.
In order to understand the characteristics of the tungsten divertor compared to that of the conventional carbon
divertor, a fully three-dimensional edge plasma simulation
code (EMC3-EIRENE) is applied [4, 5]. The influenceon the peripheral plasma by the sputtered impurities (carbon and tungsten) is surveyed in various plasma densities,
plasma heating powers, and iron emission rates for both
the divertor configurations. The simulations contribute to
not only finding the optimized experimental condition forsuppressing the influence of the sputtered impurities, butalso to understanding the characteristics of both the carbon
and the tungsten divertor configurations in more detail.

2. Setup of Impurity Transport Simulation in the Peripheral Plasma
The EMC3-EIRENE calculates a steady-state solutionof the three-dimensional profile of peripheral plasma parameters including the eﬀect of the impurity ion transport.
The simulation is performed in a three-dimensional gridmodel as shown in Fig. 1. This model includes an ergodic
layer around the Last Closed Flux Surface (LCFS), divertor legs, divertor components, and the vacuum vessel for

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Fig. 1 A perspective view of a three-dimensional grid model
used in the EMC3-EIRENE for a typical magnetic configuration (Rax = 3.60 m) in the case of the iron dust emission. The poloidal cross-section of the grid cells for the
LHD peripheral plasma at the toroidal angle where theiron source exists (φ = 10.625◦ ) is described as a groupof small fine black cells.

one-half of the helical coil pitch angle (0◦ ≤ φ ≤ 18◦ intoroidal direction φ), in which the radial position of themagnetic axis Rax is set to be 3.60 m (one of the mosttypical magnetic configurations in LHD). At both toroidal
edges of the model, a periodical boundary condition isadopted. The material on the surface of the vacuum vessel is regarded as iron. All the divertor components consistof tungsten/carbon for tungsten/carbon divertor configurations, respectively.
The impurity ion density profile in the peripheral
plasma is obtained by solving the particle and the momentum balance equations of the impurity ions along
magnetic field lines under the assumption that the impurity ion temperature is equal to the plasma ion temperature. The radiation power by the impurities andthe ionization/recombination coeﬃcients are derived fromthe database on the Atomic Data and Analysis Structure(ADAS) [6]. The position of the iron source locates ona surface on a helical coil can in the inboard side of thetorus. This position corresponds to the location where electric arcing was actually initiated at the termination of along pulse discharge [3]. A number of test particles representing iron atoms (two million at the maximum) with akinetic energy corresponding to the boiling temperature ofiron (0.26 eV) are launched from this position to deuterium
plasmas.
The perpendicular particle and thermal diﬀusion coeﬃcients (D⊥ and χ⊥ ) are assumed to be 0.5 m2 /s and1.5 m2 /s, respectively. The profile of the plasma parameters and the impurities is obtained as a converged solution. It is found that the EMC3-EIRENE cannot provide

the converged solution when the averaged divertor electron
temperature is less than several eV. Three input parameters must be defined for carrying out the simulation. The
first and second ones are the plasma heating power PLCFS
and the plasma density nLCFS
at the core-SOL boundarye(defined just inside the LCFS). The third parameter is theemission rate of the iron atoms I Fe0 released from the position of the iron source. The bulk neutral particles (deuterium atoms and molecules) are recycled in the grid model(no bulk particle sink exists). The impurity ions arriving
at the core-SOL boundary are reflected to the peripheral
plasma while maintaining the charge states of the ions. The
impurity atoms/ions reaching to the divertor components
or the vacuum vessel are treated along the following procedures.
The original EMC3-EIRENE does not include physical sputtering (chemical and self-sputtering have alreadybeen implemented in the code). Thus, a variable chemical sputtering rate has been newly introduced as a pseudosputtering rate which includes both the chemical and thephysical sputtering rates [7,8]. The value of this sputtering
rate is changed during the iterative calculation processes
for obtaining the converged solution. The sputtering rate
is varied with the averaged electron and ion temperaturein front of the divertor plates taking account of the charge
state of the incident impurity ions [9]. An energy of 10 eV
is adopted as the kinetic energy of the sputtered tungsten
atoms from the tungsten divertor plates by both the physical (Fe→W) and the self-sputtering (W→W) [10]. An energy of 0.05 eV is set for the kinetic energy of the sputtered carbon atoms from the carbon divertor plates, whichequals the energy of the typical divertor plate temperaturein plasma discharges. The kinetic energy of the carbon
atoms sputtered by the self-sputtering process (C→C) is set
to be 20 eV [7]. The reflection of the incident iron atoms
and the tungsten/carbon atoms are not included. The influence of this assumption on the simulation results is estimated to be less than a few percent at most because of
the much lower reflection coeﬃcients than the sputtering
yields of the impurities.

3. Impurity Transport Analysis
3.1 Simulation results for the carbon divertor
Figure 2 shows the poloidal cross-section of the calculated profile of the carbon and iron ion densities, the radiation power by total carbon ions (including carbon ionswith all charge states), and the electron temperature in the
peripheral plasma for the carbon divertor. The emission
rate of the iron atoms I Fe0 is set to be 6.24 × 1019 atoms/s.
The plasma heating power and the density at the core-SOL
boundary are set to be 12 MW and 1 × 1019 m−3 (a low
plasma density condition), respectively. This figure showsthat while the singly charged carbon ions (C1+ ) is localized
in both the divertor legs and the outer edge of the ergodic

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Fig. 2 The poloidal cross-section of the profile of the singly charged carbon ions (a), highly charged carbon ions (b), the total carbon ion
density (c), the total iron ion density (d), the radiation power by the carbon ions (e), and the electron temperature in the peripheral= 1 × 1019 m−3 , and I Fe0
plasma (f) for the carbon divertor configuration in a low plasma density condition (PLCFS = 12 MW, nLCFSe= 6.24 × 1019 atoms/s).

Fig. 3 The poloidal cross-section of the profile of the singly charged carbon ions (a), highly charged carbon ions (b), the total carbon ion
density (c), the total iron ion density (d), the radiation power by the carbon ions (e), and the electron temperature in the peripheral= 6 × 1019 m−3 , andplasma (f) for the carbon divertor configuration in a high plasma density condition (PLCFS = 12 MW, nLCFSe
I Fe0 = 6.24 × 1019 atoms/s).

layer (Fig. 2 (a)), the highly charged carbon ions (C6+ ) aredistributed around the core-SOL boundary (Fig. 2 (b)). The
density of the total iron ions is also distributed around thecore-SOL boundary (Fig. 2 (d)).
Figure 3 presents the poloidal cross-section of the calculated parameters for a high plasma density conditionare 12 MW and 6 × 1019 m−3 , rewhere PLCFS and nLCFS
e
spectively. Being diﬀerent from the simulation for the low
plasma density condition, the total iron ion density is locally high near the iron source (Fig. 3 (d)). The radiation
power by the total carbon ions is observable in both theouter edge of the ergodic layer and the divertor legs as

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Fig. 4 The poloidal cross-section of the profile of the singly charged tungsten ions (a), moderately charged tungsten ions (b), the total
tungsten ion density (c), the total iron ion density (d), the radiation power by the tungsten ions (e), and the electron temperature=1×in the peripheral plasma for the tungsten divertor configuration in a low plasma density condition (PLCFS = 12 MW, nLCFS
e1019 m−3 , and I Fe0 = 6.24 × 1019 atoms/s).

shown in Fig. 3 (e). It has been recognized that the significant diﬀerence of both the iron/carbon ion density and theradiation power by the carbon ions for the low and highplasma densities is explained by the balance between thethermal force and the friction force in the two plasma density conditions [11].

3.2Simulation results for the tungsten divertor

Figure 4 illustrates the poloidal cross-section of thecalculated profiles in the low plasma density condition
for the tungsten divertor. While the singly charged tungsten ions (W1+ ) exist in the outer edge of the ergodic
layer and in the divertor legs (Fig. 4 (a)), the moderately
charged tungsten ions (W22+ ) are distributed around thecore-SOL boundary (Fig. 4 (b)), which is the same as thosefor the carbon divertor in the low plasma density condition(Figs. 2 (a) and (b)).
Figure 5 gives the poloidal cross-section of the calculated profiles in the high plasma density for the tungsten
divertor. The density of the singly charged tungsten ions
(W1+ ) is quite low and it is localized near the tungsten
divertor plates (Fig. 5 (a)). Compared to the simulations
for the low plasma density, the total tungsten ion density(Fig. 5 (c)) and the radiation power (Fig. 5 (e)) are significantly suppressed. The total iron density is concentrated
near the iron source (Fig. 5 (d)). This considerable diﬀerence for the low and high plasma densities is also explainedby the balance between the two forces in the two diﬀerent
plasma density conditions [11].

4. The Dependence of the Impurity
Transport on the Plasma Density
4.1 Simulation results for the carbon divertor
The impurity transport simulation was performed inand plasma heating powers
various plasma densities nLCFS
e
PLCFS for the carbon divertor. The ranges of the both twoparameters are between 1 × 1019 m−3 and 7 × 1019 m−3 ,and between 1 ∼ 2 MW and 12 MW, respectively. These
parameter ranges thoroughly cover the most of typical operational conditions in LHD plasma discharge experiments[12]. Figure 6 gives the simulation results which show theradiation power by the carbon ions (PCRad ), the total carbon
ion content in the peripheral plasma, the averaged divertor electron temperature, the production rate of the singly
charged carbon ions, the radiation power by the iron ions,and the total iron ion content, respectively. The radiation
power and the impurity ion content are calculated by summing those with all charge states in all grid cells.
While the divertor electron temperature decreases
with the plasma density (Fig. 6 (c)), the radiation power by
the carbon ions (Fig. 6 (a)) and the carbon ion production
rate (Fig. 6 (c)) gradually increase with the plasma density,
which is explained by the almost constant sputtering rate ofcarbon which is irrelevant to the divertor plasma temperature. In other words, the sputtered carbon flux linearly increases with the deuterium ion flux onto the divertor plates.
The radiation power by the iron ions (Fig. 6 (e)) and theiron ion content (Fig. 6 (f)) do not significantly change withthe plasma density and the plasma heating power because

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Fig. 5 The poloidal cross-section of the profile of the singly charged tungsten ions (a), moderately charged tungsten ions (b), the total
tungsten ion density (c), the total iron ion density (d), the radiation power by the tungsten ions (e), and the electron temperature=6×
in the peripheral plasma for the tungsten divertor configuration in a high plasma density condition (PLCFS = 12 MW, nLCFS
e1019 m−3 , and I Fe0 = 6.24 × 1019 atoms/s).

Fig. 6 The simulation results showing the dependence of the radiation power by the carbon ions (a), the total carbon ion content (b), thedivertor electron temperature (c), the production rate of the singly charged carbon ions (d), the radiation power by the iron ions (e),and the heating power PLCFS for the carbon
and the total iron ion content (f) in the peripheral plasma on the plasma density nLCFS
edivertor.

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Fig. 7 The dependence of the radiation power by the tungsten ions (a), the total tungsten ion content (b), the divertor electron temperature(c), the production rate of the singly charged tungsten ions (d), the radiation power by the iron ions (e), and the total iron ion
and the heating power PLCFS for the tungsten divertor.
content (f) in the peripheral plasma on the plasma density nLCFSeof the constant emission rate of the iron atoms in the simulation.

4.2

Simulation results for the tungsten divertor

For the tungsten divertor, the radiation power by thetungsten ions (Fig. 7 (a)), the total tungsten ion content(Fig. 7 (b)) in the peripheral plasma, and the singly charged
tungsten ion production rate (Fig. 7 (d)) drastically decrease with the plasma density. These simulation results
are completely diﬀerent from those for the carbon divertor(Figs. 6 (a), (b), and (d)). The simulation has revealed that≥ 4 × 1019 m−3 )this trend for high plasma densities (nLCFS
e
is explained by the synergy eﬀect of both the reduction of
the physical/self-sputtering rate due to the lowered divertor electron temperature and the dominant friction force on
the tungsten ions in the peripheral plasma [9]. The simulation results indicate that, for the tungsten divertor, a higher
plasma density operation is eﬀective for suppressing thetungsten ions induced by the iron dust emission. For example, in the low plasma density and high plasma heat= 1 × 1019 m−3 and PLCFS =ing power condition (nLCFS
e
12 MW), the simulation predicts that the radiation powerby tungsten ions (PWRad ) in the tungsten divertor configuration is larger than that by carbon ions (PCRad ) in the carbon

divertor configuration by about 20. On the other hand, in= 7 × 1019 m−3
the high plasma density condition (nLCFS
e
LCFS
and P
= 12 MW), the radiation power by tungsten ions)issignificantly
reduced to lower than that by carbon(PW
Rad
ions (PCRad ) by about three-orders of magnitude as shownin Fig. 6 (a) and Fig. 7 (a).

5. The Dependence of the Impurity
Transport on the Iron Emission
Rate
5.1 Simulation results for the carbon divertor
The impurity transport in the LHD peripheral plasma
is investigated in various iron emission rates for the carbon divertor in the case of a plasma heating power PLCFS
of 12 MW. Figure 8 presents the dependence of the radiation power by the carbon ions, the total carbon ion content, the divertor electron temperature, the production rateof the singly charged carbon ions, the radiation powerby the iron ions, and the total iron ion content in various plasma density conditions. The radiation power by
the carbon ions (Fig. 8 (a)) and the total carbon ion content (Fig. 8 (b)) do not significantly change with the ironemission rate. The production rate of the singly charged

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Fig. 8 The dependence of the radiation power by the carbon ions (a), the total carbon ion content (b), the divertor electron temperature(c), the production rate of the singly charged carbon ions (d), the radiation power by the iron ions (e), and the total iron ion content(f) in the peripheral plasma on the iron emission rate I Fe0 and the plasma heating power PLCFS for the carbon divertor in the caseof PLCFS = 12 MW.

carbon ions (Fig. 8 (d)) shows a similar trend to that of theradiation power and the carbon ion content. The radiation power by the iron ions (Fig. 8 (e)) and the total ironion content (Fig. 8 (f)) increase with the iron emission rate.
The divertor electron temperature slightly decreases with
the iron emission rate (Fig. 8 (c)).
Figure 9 depicts the poloidal cross-section of the profile of the radiation power by the total iron ions, by the total carbon ions, and the electron temperature profile in the
carbon divertor for low and high iron emission rates (I Fe0= 6.24 × 1018 and 1.03 × 1021 atoms/s, respectively). The
is set to beplasma density at the core-SOL boundary nLCFSe
1 × 1019 m−3 . It should be noted that the profile of the radiation power by the carbon ions in the low iron emission
rate is quite diﬀerent from that in the high iron emission
rate (Figs. 9 (b) and (e)) in spite of the equivalent radiation
power by carbon ions (PCRad ) in both cases (Fig. 8 (a)). For
the low iron emission, the radiation power in the outer region of the ergodic layer is higher than that near the coreSOL boundary as shown in Fig. 9 (b). For the high ironemission, the radiation power near the core-SOL boundary is higher than that in the outer region (Fig. 9 (e)). Thisdiﬀerence is explained by the plasma cooling rate of the
carbon ions which strongly depends on the electron tem-

perature. It is found that the cooling rate of carbon ionsreaches the maximum value at an electron temperature ofaround less than 100 eV [13]. The region with the highest
radiation power of carbon ions (PCRad ) in both the low andhigh iron emission rates (Figs. 9 (b) and (e), respectively)
corresponds to the position where the electron temperature
is in this temperature range (< 100 eV) shown as light blue
areas in Figs. 9 (c) and (f), respectively. In the case of the
low iron emission rate, the region where the electron temperature is several tenths eV locates near the outer edge ofthe ergodic layer and the divertor legs. On the other hand,for the high iron emission rate, this region is distributed
around the core-SOL boundary.

5.2 Simulation results for the tungsten divertor
Figure 10 shows the dependence of the above six parameters on the iron emission rate for the tungsten divertor. Both the radiation power by tungsten ions and the total
tungsten ion content (Figs. 10 (a) and (b)) rise in the casewhere the iron emission rate is less than about 1 × 1020
atoms/s. When the iron emission rate is higher than thisvalue, these two parameters (PW
Rad and QW+ ) tend to saturate and decrease. This trend is qualitatively explained by

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Fig. 9 The poloidal cross-section of the profile of the radiation power by the iron ions (a), by the carbon ions (b), and the electron
temperature (c) for a low iron emission rate in the carbon divertor configuration. The profile of the radiation power by the iron
ions (d), by the carbon ions (e), and the electron temperature (f) for a high iron emission rate is also shown.

Fig. 10 The dependence of the radiation power by the tungsten ions (a), the total tungsten ion content (b), the divertor electron temperature(c), the production rate of the singly charged tungsten ions (d), the radiation power by the iron ions (e), and the total iron ion
content (f) in the peripheral plasma on the iron emission rate I Fe0 and the plasma heating power PLCFS for the tungsten divertor in
the case of PLCFS = 12 MW.

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Fig. 11 The poloidal cross-section of the profile of the radiation power by the iron ions (a), by the tungsten ions (b), and the electron
temperature (c) for a low iron emission rate in the tungsten divertor configuration. The profile of the radiation power by the iron
ions (d), by the tungsten ions (e), and the electron temperature (f) for a high iron emission rate is also depicted.

the dependence of the production rate of the singly charged
tungsten ions (S W1+ ) on the iron emission rate as shown in
Fig. 10 (d). The divertor electron temperature drops with
the increase in the iron emission rate I Fe0 , which is promi≤ 3 × 1019 m−3 ). In
nent in lower plasma densities (nLCFS
ethis simulation, no converged solutions are obtained when
the iron emission rate is more than 6 - 7 × 1020 atoms/s
because of the excessively low divertor electron temperature (Fig. 10 (c)). The temperature drop is caused by theradiation cooling by the iron ions, leading to the saturation of the tungsten ion production rate due to the lowered physical/self-sputtering processes (Fig. 10 (d)). This
simulation proves that a catastrophic increase in the radiation power by tungsten ions due to the physical/selfsputtering of tungsten does not occur even for excessively
higher iron emission rates. This is because the sputtering
rate of tungsten is reduced in the lowered divertor electron
temperature (Fig. 10 (c)) for the high iron emission rates.
For example, the radiation power by tungsten ions (PW
Rad )
LCFS
19 −3
= 1 × 10 m ) reaches
in the low plasma density (nethe maximum value at an iron emission rate of 2.5 × 1020
atoms/s, in which the radiation power (PWRad ) is larger thanthe power (PCRad ) for the carbon divertor by about one order of magnitude in the lower plasma density (Fig. 8 (a)and Fig. 10 (a)). On the other hand, the maximum radiation
power by tungsten ions in a medium plasma density (nLCFSe= 4 × 1019 m−3 ) decreases to less than that for the carbon
divertor by a factor of about 2. This simulation indicates
that, for the tungsten divertor, higher plasma density oper≥ 4 × 1019 m−3 ) are favorable for controlling
ations (nLCFSe

the maximum radiation power by tungsten ions (PWRad ) inthe case of excessively high iron emission rates.
Figure 11 presents the poloidal cross-section of the
profile of the radiation power by the total iron ions, by the
total tungsten ions, and the electron temperature profile in
the tungsten divertor for low and high iron emission rates(I Fe0 = 6.24 × 1018 and 5.62 × 1020 atoms/s, respectively).
is set
The plasma density at the core-SOL boundary nLCFSe
to be 1 × 1019 m−3 . The simulation shows that the radiation power by the iron ions and the tungsten ions near
the core-SOL boundary increases with the iron emission
rate. The profile of the radiation power by the tungsten
ions does not qualitatively change with the iron emission
rate (Figs. 11 (b) and (e)), which is diﬀerent from that by
the carbon ions for the carbon divertor configuration. The
electron temperature in the peripheral plasma significantly
drops with the iron emission rate as indicated in Figs. 11 (c)and (f). That is, while the electron temperature near the
core-SOL boundary in the low iron emission rate is about
1.0 keV, the temperature decreases to about 0.3 keV in thehigh iron emission rate. The main cause of this temperature drop is the enhanced radiation power by both the ironand the tungsten ions in the ergodic layer around the coreSOL boundary for the high iron emission rate as shown in
Figs. 11 (d) and (e).

6. Summary
In order to understand the characteristics of the tungsten divertor configuration in LHD plasma discharges, thecomparative analysis of the impurity transport in the pe-3403057-9

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Volume 14, 3403057 (2019)ripheral plasma was performed for both the carbon and thetungsten divertor configurations using the EMC3-EIRENE
in the iron dust emission case. The simulation reveals theobservable diﬀerence of the dependence of the impurity
transport on the plasma density in the two divertor configurations. For the carbon divertor, the radiation power, thecarbon ion content, and the carbon ion production rate increases with the plasma density. On the other hand, forthe tungsten divertor, the radiation power, the tungsten
ion content, and the tungsten ion production rate significantly decrease with the plasma density, which indicatesthat higher plasma density operations are advantageous forsuppressing the influence by impurity ions for the tungsten
divertor. The investigation of the dependence of the impurity transport on the iron emission rates shows the quite different trend in both the two divertor configurations. For the
carbon divertor, the radiation power by carbon ions does
not significantly change with the iron emission rate. For
the tungsten divertor, while the radiation power by tungsten ions rises in the lower iron emission rates, the radiation power tends to saturate and decrease in the higher ironemission rates. The simulation proves that a catastrophic
increase in the radiation power by sputtered tungsten inexcessively higher iron dust emission cases does not occur
due to the lowered divertor electron temperature, which isa favorable characteristic in the tungsten divertor configuration to sustain long pulse discharges in LHD. The simulation also reveals that, for the tungsten divertor, higher≥ 4 × 1019 m−3 ) are efplasma density operations (nLCFS
efective for controlling the maximum radiation power bytungsten ions even in the case of excessively higher ironemission rates, in which the radiation power (PWRad ) is supC
pressed to less than that (PRad ) for the conventional carbon
divertor by a factor of about 2.

Acknowledgments
This work is performed under the auspices of the NIFS
Collaboration Research program (NIFS18KNXN359).
One of the authors (M.S.) would like to thank Y. Feng for
permission to use the EMC3-EIRENE. He is also grateful for the computational resources of the LHD numerical analysis server and the plasma simulator in NIFS.
This work is supported by JSPS KAKENHI Grant Number 16H04619.
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