J. Plasma Fusion Res. SERIES, Vol.7 (2006) 35-39

Modeling of Impurity Transport in High Density Plasma with Highly
Enhanced Radiation Loss on JT-60U
TAKENAGA Hidenobu, ASAKURA Nobuyuki, KUBO Hirotaka, KONOSHIMA Shigeru,
NAKANO Tomohide, PORTER Gary1 , ROGNLIEN Thomas1 and RENSINK Marvin1
Japan Atomic Energy Research Institute∗ , Naka 311-0193, Japan
1 Lawrence Livermore National Laboratory, Livermore, CA 94551-9900, USA（Received: 5 October 2004 / Accepted: 20 May 2005）

Abstract
Impurity transport has been modeled using a 1-D transport code for the main plasma and a 2-D fluid code
(UEDGE) for the divertor and scrape-oﬀ layer plasmas in the JT-60U high β p mode plasma with highly enhanced radiation by injecting seed impurity argon. The density profile of the seed impurity argon estimatedfrom the soft x-ray profile was more peaked by a factor of 2 than the electron density profile. On the other
hand, the density measurement of the fully ionized intrinsic impurity carbon indicated a flat carbon density profile. The radiation profile evaluated from these impurity density profiles was more peaked than themeasurement in the main plasma. The divertor radiation calculated by the UEDGE code with the boundary
condition of the argon density consistent with the 1-D transport analysis was localized around the strike points
compared with the measurement.
Keywords:JT-60U, impurity transport, seed impurity, intrinsic impurity, 1-D transport code, UEDGE code,
radiation profile
1．Introduction
Reduction of heat load localized onto the divertor
plates is a critical issue in magnetic confined fusion devices to mitigate damage of the divertor plates. Highradiation loss in the divertor and scrape-oﬀ layer (SOL)
plasmas enhanced by injecting seed impurity is mosteﬀective for expansion of the heat load onto the wide
area. In JT-60U, the high radiation loss operation has
been developed with the seed impurity neon or argon[1], and the radiation loss power has reached to over
80 % of the net heating power in high density plasmas.
In order to extrapolate these results to ITER and DEMO
reactor, understanding of the impurity transport is important. The impurity transport has been analyzed using a 1-D transport code for the main plasma [2] and a
2-D fluid code [3] or a Monte-Carlo code [4] for thedivertor and SOL plasmas. In these analyses, impurity density and/or transport coeﬃcients were evaluated.
However, detailed comparison of the radiation profile
between modeling and experiment was not performed.
In this paper, the impurity density was estimated using
a 1-D transport code for the main plasma and the 2-D
fluid code UEDGE [5] for the divertor and SOL plasmas in the high β p mode plasma with highly enhanced

radiation by injecting seed impurity argon. The radiation profile was calculated based on the estimated impurity densities for both seed impurity argon and intrinsic
impurity carbon and it was compared with the measurement.
2．Experiments
JT-60U is a large tokamak device with a major radiusof 3.4 m. The typical plasma configuration is shown in
Fig. 1 together with sight lines of the diagnostics related to this study. The W-shaped divertor was adopted,
where the inner and outer inclined divertor plates andthe dome in the private flux region form a W-shaped
surface. The divertor pumping was performed fromboth inner and outer private flux regions. The soft xray emission intensities were measured with 10 sightlines in the main plasma. The radiation loss was measured with 20 sight lines using bolometer arrays. 8 lines(ch 1-8) were arranged for the main plasma side and 12
lines (ch 9-20) were arranged for the divertor region.
The seed impurity of argon was injected into highdensity high β p mode plasmas with an internal transport barrier (ITB) at the plasma current of I p = 1.0
C 2006 by The Japan Society of Plasma

Corresponding author’s e-mail: takenaga.hidenobu@jaea.go.jp∗ Present: Japan Atomic Energy Agency

Science and Nuclear Fusion Research

35Modeling of Impurity Transport in High Density Plasma with Highly Enhanced Radiation Loss on JT-60U

Fig. 1 Typical plasma configuration and diagnostic arrangements related to this study.

Fig. 3 (a) Electron density and (b) electron temperature
profiles before (t = 4.7 s : open circles) and after(t = 6.95 s : closed circles) the argon penetration.
Radiation profiles for (c) main plasma region and(d) divertor region before (t = 4.7 s : open circles)
and after (t = 6.95 s : closed circles) the argon penetration.

the Greenwald density (nGW ), which is empirical scaleddensity for high density disruptive limit, around t = 7.5
s. The stored energy (Wdia ) was almost kept at constant value and confinement degradation at high density was suppressed by high-field-side pellet injection,
argon injection and small D2 gas-puﬃng until n = 1
mode appeared at t = 7.1 s. The confinement degradation is observed in many machines when the density
is increased by large D2 gas-puﬃng. The edge localized mode (ELM) observed in the Dα emission intensity (IDα ) became small during t = 6.5 − 7 s. The radiation losses (Prad ) in both main and divertor plasmas
were enhanced after argon injection. At t = 6.95 s,
the total radiation loss power reached up to 90 % of theheating power at n̄e /nGW = 0.92 with high confinement
enhancement factor of 0.96 over the IPB98 (y,2) ELMy
H-mode scaling.
A peaking of the electron density (ne ) profile with
the ITB and a flattening of the electron temperature (T e )profile were observed as shown in Figs. 3(a) and 3(b).
The profile of the soft x-ray intensity was also peakedin the main plasma. The radiation had a hollow profile before the argon penetration in the main plasma as
shown in Fig. 3(c). After the argon penetration, the central radiation was increased. The divertor radiation wasenhanced in the wide region as shown in Fig. 3(d).

Fig. 2 Typical wave-forms in high density and high radiation loss plasma with argon injection. (a) Solidand dotted lines show line averaged electron density (n̄e ) and Greenwald density (nGW ), respectively. (b) Solid and dotted lines show injected
powers from positive and negative ion based neutral beams (P-NB and N-NB). (c) Solid line shows
stored energy (Wdia ). Dotted and dashed lines
show Ar and D2 puffing rates, respectively. (d)D« emission intensity from the divertor region. (e)Solid and dashed lines show total radiation loss
power and radiation loss power from the main
plasma, respectively.

MA and the toroidal magnetic field of BT = 3.6 T as
shown in Fig. 2 [6]. Argon was puﬀed from t = 4.5
s and penetrated into the plasma with a time delay of
0.2 − 0.3 s. The neutral beam (NB) heating power of
PNB = 20 − 30 MW was injected from positive andnegative ion based NBs (P-NB and N-NB). The line averaged electron density (n̄e ) increased and reached to36Modeling of Impurity Transport in High Density Plasma with Highly Enhanced Radiation Loss on JT-60U

3．Methods of analyses
In the main plasma, the profile of the total argon density (nAr ) summed over all ionization states was estimated using a 1-D impurity transport code, where thediﬀusivity (D) and the convection velocity (v) were determined by fitting the calculated soft x-ray profile tothe measurement [2]. The argon radiation coeﬃcient in
the soft x-ray range was taken from the ADAS database[7] considering the JT-60U diagnostic setup (the energy range of 2.7 − 20 keV). The absolute value of n Ar
was determined by the radiation loss in the central region. The radiation loss from argon was evaluated using
the radiative cooling rate calculated in Ref. [8] (ADPAK). The intrinsic impurity carbon density (nC ) was
estimated using the 1-D transport code based on the C6+
density measured with charge exchange recombination
spectroscopy (CXRS). The radiation loss from carbon
was also evaluated using ADPAK. In these analyses, themeasured parameters of the background plasma such as
ne and T e were used.
The impurity transport was modeled using the 2-D
fluid code UEDGE [5] in the divertor and SOL plasmas. Since detailed background plasma parameters
were not measured in the divertor and SOL plasmas,
these parameters were also calculated in the UEDGE
code consistently with impurity and neutral behavior.
In the UEDGE calculation, the boundary conditions at
the core-edge (96 % of magnetic flux) were the bulk
ion density of 1.5 × 1019 m−3 and the power escaping from core to edge of 14 MW divided equally between ions and electrons. The anomalous diﬀusion coeﬃcients were all taken to be 1.0 m2 /s for the thermal
diﬀusivity and 0.25 m2 /s for the bulk particle diﬀusivity. The impurity diﬀusivity was set to be 1.0 m2 /s without the convection velocity. The carbon yield rate was
set to be a Haasz yield for both physical and chemical
sputtering. The Ar15+ density at the core-edge boundary
was scanned in the range of 0.5 − 3 × 1016 m−3 . Recently, drift eﬀects were included in the UEDGE code[9]. However, the drift eﬀects were not included in thismodeling. The impurity ionization, radiation and recombination rates came from ADAS rates for carbon
and impurity transport code STRAHL [10] for argon.

Fig. 4 (a) Carbon density profiles. Circles show the measured C6+ density. Solid and dotted lines show
the calculated C6+ density and total carbon density, respectively. (b) Carbon radiation profile. (c)Solid line shows total Argon density profile. Electron density profile is also shown by dashed line
for comparison. Both profiles are normalized at
r / a = 0.65. (d) Argon radiation profile.

m/s were assumed. The density profile calculated in
the 1-D transport code is also shown in Fig. 4(a). The
calculated C6+ density profile was consistent with themeasured profile. Figure 4(b) shows the radiation profile calculated from the nC profile shown in Fig. 4(a).
The radiation from carbon impurity was localized in theedge region. The calculated n Ar is shown in Fig. 4(c),where D = 1.0 m2 /s and v = D · ∇ne /ne in the regionof r/a ≥ 0.65, v = 2 × D · ∇ne /ne in the region ofr/a < 0.65 were assumed to fit the calculated soft x-ray
profile to the measured one. The large inward convection velocity of about −8 m/s was assumed at r/a ∼ 0.6
due to strong density ITB. In the central region, Ar18+ ,Ar17+ and Ar16+ were main contributors and their density ratios were estimated to be 40 %, 40 % and 20 %,respectively. The nAr profile was more peaked by a factor of 2 than the ne profile. The radiation from argon
impurity was peaked in the central region as shown in
Fig. 4(d).
The total radiation loss power calculated with the
UEDGE code increased from 6.6 to 8.4 MW by increasing the ratio of nAr /ne at the core-edge boundary (nAr /ne )core−edge from 0.14 to 0.93 %. The radiation from argon increased from 0.3 to 3.2 MW and theradiation from carbon decreased from 5.7 to 4.7 MWwith increasing (nAr /ne )core−edge . The radiation fromdeuterium was almost constant (∼ 0.6 MW) and hada small contribution to the total radiation. During this(nAr /ne )core−edge scan, ne and T e at the outer strike point

4．Results of analyses
4.1 Calculations of 1-D transport code and UEDGE
code

Impurity transport was analyzed at t = 6.95 s in
the discharge shown in Fig. 2. The C6+ density measured with CXRS indicated a flat profile in the regionof r/a < 0.8 as shown in Fig. 4(a). In order to reproduce the flat density profile, D = 1.0 m2 /s and v = 037Modeling of Impurity Transport in High Density Plasma with Highly Enhanced Radiation Loss on JT-60U

decreased from 2.6×1020 to 1.9×1020 m−3 and from 2.7
to 1.3 eV, respectively, with increasing (n Ar /ne )core−edge .
The detachment process was gradually progressed at theouter divertor. On the other hand, the inner divertor was
detached and ne and T e at the inner strike point werein the ranges of 1.8 − 2.8 × 1019 m−3 and 0.68 − 0.72
eV, respectively. The increase in radiation was related
to the decrease in ne and T e at the outer divertor. At(nAr /ne )core−edge = 1 %, ne at the outer strike point waslargely decreased to 7.2 × 1019 m−3 indicating the detachment. At the same time, T e at the outer strike point
was decreased to 1 eV. The total radiation largely increased to 9.7 MW, where the radiations from both carbon and argon were increased to 5.1 MW and 4 MW,
respectively. The ratio of the radiation from argon to
the total radiation reached to 40 %4.2 Comparison of radiation profile between measurement and calculation

Figure 5 shows comparison of the radiation profile
between measurement and calculation (a) in the main
plasma region and (b) in the divertor region. Here, thecalculated results of 1-D transport code with argon andcarbon were used for the main plasma and the UEDGE
calculation result with (n Ar /ne )core−edge = 0.75 % wasused for the SOL plasmas in Fig. 5(a). In Fig. 5(b),the UEDGE calculation results with (n Ar /ne )core−edge =0.14, 0.75 and 1 % were used together with the 1-D
transport results with carbon and argon. Absolute n Ar
was determined by adjusting the calculated radiation
summed over the 1-D transport and UEDGE results tothe measurement at the center bolometer chords (ch 8).
In the line integral of the calculated radiation, size ofthe sight line was considered. The value of n Ar was estimated to be about 1.6 % of ne in the central region and
0.7 % of ne in the edge region. The calculated radiation loss was more peaked than the measurement in themain plasma region as shown in Fig. 5(a). When possible misalignment of the sight lines was considered (halfchannel), the variation of the radiation loss was relatively large in the edge region (ch 4). However, the calculated radiation profile was still peaked. It is necessary
to evaluate combination of D and v for satisfying both
soft x-ray and radiation profiles in future work. Furthermore, the edge channels (ch 1-3) were much smaller
than the measurement. The radiation in the SOL plasma
could be larger than the present modeling. In the hightriangularity configuration, the large radiation at edge
channels viewing the SOL plasma was often observed[11]. Since eﬀect of secondary sepratrix is stronger inthis configuration near upper inside corner, this eﬀect
on the radiation should be also investigated in future

Fig. 5 Comparison between calculated radiation profileand measured one in (a) the main plasma region and (b) the divertor region. Circles show
the measurements. (a) Solid line shows the calculated radiation profile summed over the radiations from 1-D transport code results with bothcarbon and argon and from UEDGE result with(nAr / ne )core-edge = 0.75 %. Dotted line shows the calculated radiation profile summed over the radiations from 1-D transport code result with carbon
and UEDGE result with (nAr / ne )core-edge = 0.75 %.
(b) Dashed, solid and dotted lines show the radiation profile calculated using UEDGE results with(nAr / ne )core-edge = 1 %, 0.75 % and 0.14 %, respectively.

work.
The divertor radiation had peaks at both strike points(ch 11 for inner and ch 19 for outer divertor) as wellas the measurement. The peak at the inner strike point
was almost constant even when (n Ar /ne )core−edge was increased. The peak at the outer strike point was largely
enhanced with (nAr /ne )core−edge = 1 % and was larger
than the peak at the inner strike point. The calculated radiation with (nAr /ne )core−edge = 0.75 %, which was consistent with (nAr /ne )core−edge calculated using 1-D transport code, was consistent with the measurement at the
inner strike point within the ambiguity (± 15 %) of themeasurement. At the outer strike point, the eﬀect of
misalignment of the sight lines was large due to strong
localization of the radiation in front of the outer diver-38Modeling of Impurity Transport in High Density Plasma with Highly Enhanced Radiation Loss on JT-60U

Acknowledgements
The authors wish to thank Dr. N. Oyama for establishment of the computer system for UEDGE code and
Dr. Y. Kamada for the support on this study.

tor plate. Therefore, when possible misalignment (halfchannel) was considered, the radiation loss at the outerstrike point increased to the same value as the measurement. On the other hand, the radiation in the divertor
region (ch 9&10 for inner and ch 17&18 for outer divertor) was smaller than the measurement, indicating thatthe calculated radiation was localized around the strike
points compared with the measurement.

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[1]

5．Summary
Impurity transport was modeled using a 1-D transport code for the main plasma and the 2-D fluid code
UEDGE for the divertor and SOL plasmas in the highβ p ELMy H-mode plasma with argon injection on JT60U. The calculated radiation profile was more peaked
than the measurement in the main plasma, and the calculated radiation was much smaller than the measurement in the edge region. The expanded radiation zone
into the wide SOL plasma should be investigated in future work. The radiation calculated with the UEDGE
code was localized around the strike points comparedwith the measurement. The systematic parameter scansuch as diﬀusivity and recycling coeﬃcient is necessary
to reproduce the measured radiation profile as well as
the considering the drift eﬀects.

39