7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Study on impurity transport model based on
JT-60U experimental data

Sota Takemoto1, Takaaki Fujita1, Atsushi Okamoto1, Tomohide Nakano2, Nobuhiko Hayashi2
1
Nagoya University, 2 National Institutes for Quantum Science and Technology (QST)e-mail (speaker): takemoto.sota.d8@s.mail.nagoya-u.ac.jp
Tungsten (W), which is to be used as a divertor target
material in ITER and DEMO reactors, is a high-Z element,
and there is concern about its high radiation loss whenintroduced into plasma as an impurity. Elucidating themechanism of impurity accumulation is one of thechallenges to realize thermonuclear fusion reactors.
In the large tokamak device JT-60U, W accumulation has
been observed to increase with increasing toroidal rotation(𝑉𝑡 ) in the opposite direction to the plasma current in Hmode plasmas [1]. To understand this phenomenon,
impurity transport simulations have been studied.
To understand this phenomenon, impurity transport
simulation studies have been conducted with theintegrated transport code TOTAL. In previous studies,pinch models (PHZ pinch and Er pinch) [2] proposed by
Hoshino et al. have been incorporated into TOTAL to
examine whether they can reproduce the toroidal rotation
dependence of tungsten accumulation [3].
The purpose of this study is to examine the validity ofthe tungsten transport model by comparing the radial
distribution and time evolution of tungsten accumulation,
which have not been compared before, in order to further
improve the impurity transport calculations.
In order to compare the radial distribution of W
accumulation, it is necessary to relate the tungsten densitycalculated by TOTAL to the experimental data from JT60U. The soft X-ray intensity reflects the W density.
Therefore, the soft X-ray intensity distribution is selected
as the experimental data to be compared in this study. The
soft X-ray array in JT-60U has 16 poloidal lines of sightand observes radiation in the range of 2.7 keV to 20 keV.
In the TOTAL code, soft X-ray intensities are calculated
taking into account bremsstrahlung radiation (𝑃𝑏𝑟𝑒𝑚 ) fromfree electrons and line radiation from tungsten ions (𝑃𝑙𝑖𝑛𝑒 ).
This is written as 𝑃𝑆𝑋 = 𝑓𝑏 𝑃𝑏𝑟𝑒𝑚 + 𝑓𝑙 𝑃𝑙𝑖𝑛𝑒 where 𝑓𝑏
and 𝑓𝑙 are correction factor that takes into account theobserved energy range (2.7 keV to 20 keV). At present, 𝑓𝑙
is assumed to be 1/10 because it is difficult to get theenergy spectrum of the line radiation of tungsten ions.
Another consideration is that the soft X-ray intensity
distribution is given by the line integral in experiment. We
assume a cylindrical line of sight and integrate the local
soft X-ray intensities along the line of sight.
In this study, two cases are selected for comparison: (a)a case with small 𝑉𝑡 and small W accumulation, and (b)a case with large 𝑉𝑡 and large W accumulation. In
TOTAL's transport calculations, the rate equation withradial transport is solved for W q+ (0 ≤ q ≤ 74).

𝜕𝑛𝑘1 𝜕(𝑉 ′ 𝛤𝑘 )=− ′𝜕𝑡𝑉 𝜕𝜌
+[𝛾𝑘−1 𝑛𝑘−1 + 𝛼𝑘+1 𝑛𝑘+1 − (𝛾𝑘 + 𝛼𝑘 )𝑛𝑘 ]𝑛𝑒 ,𝛤𝑘 = −(𝐷𝑘 NC + 𝐷𝑘 AN )𝜕𝑛𝑘+ (𝑉𝑘 NC + 𝑉𝑘 AN )𝑛𝑘 ,𝜕𝜌where 𝛾𝑘 is the ionization coefficient and 𝛼𝑘 is therecombination coefficient. The subscripts NC and AN
represent neoclassical and anomalous transport, and thePHZ pinch and Er pinch models are considered in 𝑉𝑘 AN.
The W atoms are injected through the plasma surface. The
background plasma parameters ( 𝑛𝑖 , 𝑇𝑒 𝑒𝑡𝑐. ) are fixedbased on the experimentally obtained profiles.
Simulation results are shown in Figure 1. Figure 1(a)reproduces the weak peak of soft X-ray intensity observedin the case of low tungsten accumulation. Here, scales ofthe vertical axes are adjusted to match the heights of peaks.
Figure 1 (b) shows the calculation results for the case withhigh tungsten accumulation on the same scale as Figure 1(a), which reproduces a strong peak reflecting W
accumulation but overestimates it.
Comparative method of time evolution is underconsideration. In order to compare the time evolution, it isnecessary to consider the time evolution of the pinch
velocity and diffusion coefficient due to changes in thebackground plasma. Detailed discussion, including details
under consideration regarding time development, will begiven in the presentation.
This work was supported by QST Research
Collaboration for Fusion DEMO and QST Research
Collaboration for Tokamak Core Plasma.
References[1] T. Nakano, J. Nucl. Mater. 415, S327-S333 (2011)[2] K. Hoshino, et al., Nucl. Fusion 51, 083027 (2011)[3] Y. Shimizu, et al., Plasma and Fusion Res. 10,3403062(2015)(a)(b)

Figure 1 Comparison of soft X-ray intensity distribution(a) in small 𝑉𝑡 and small W accumulation and (b) in large
𝑉𝑡 and large W accumulation.