Proceedings of ITC/ISHW2007

Tracer impurity transport inside an IDB plasma of LHD
Naoki TAMURA, Kuninori SATO, Pavel R GONCHAROV, Hisamichi FUNABA,Yuri L IGITKHANOV, Shigeru SUDO, Ichihiro YAMADA, Kazumichi NARIHARA,
Kenji TANAKA, Clive MICHAEL, Kazuo KAWAHATA, Shinji YOSHIMURA, Byron J PETERSON,Ryuichi SAKAMOTO, Masahiro KOBAYASHI, Hiroshi YAMADA, Nobuyoshi OYABU,Akio KOMORI and LHD Experimental Group
National Institute for Fusion Science, Toki 509-5292, Japan
Recently, in LHD, extremely high-density region accompanied with an Internal Diffusion Barrier (IDB) has
been established by optimizing a pellet fueling and a magnetic configuration. In order to understand impurity
transport inside the IDB plasma of LHD, a tracer-encapsulated solid pellet (TESPEL), which can deposit a tracer
impurity locally inside the plasma, injection into the IDB plasma has been performed. In this experiment, titanium(Ti) particles are loaded into the TESPEL as a tracer. The local deposition of the Ti tracer impurity inside the IDB is
successfully achieved with the help of the TESPEL technique, which is confirmed by an optical measurement.
Considering temporal change in both electron density and electron temperature, temporal behavior of line emissions
from the Ti tracer impurity ions measured by a vacuum ultra violet spectrometer indicates the Ti tracer impurity
ions seems to be accumulated inside the IDB.
Keywords: impurity transport, TESPEL, local tracer deposition, ultrahigh density, internal diffusion barrier

1. Introduction
Understanding and control of impurity transport inmagnetically confined plasmas is still an important issue
for achieving a practical fusion reactor, since impurities
can significantly influence the reactor performance throughsuch effects as radiative losses, radiative instabilities, fueldilution and so on. Especially when an “impurityaccumulation” takes place in the core plasma, theseproblems become more acute. Thus impurity transport
study has been performed diligently in many tokamaks andstellarators/heliotrons so far [1-6].
In Large Helical Device (LHD) [7], ultrahigh density
region (the maximum density in the region is up to 1 x 1021m-3) accompanied with an Internal Diffusion Barrier (IDB)
has been established by optimizing a pellet fueling and amagnetic configuration [8, 9]. One of the important points
to be clarified for the IDB plasmas is whether the IDB
causes an impurity accumulation or not. So far, theperformance degradation of the IDB plasmas due to theimpurity accumulation has not been observed despite theexistence of a negative radial electric field. In LHD, atracer-encapsulated solid pellet (TESPEL) [5, 10], whichallow us to obtain a three-dimensionally localized
deposition of the tracer impurity inside the plasma, hasbeen utilized for impurity transport study. When alow-level transport such as an internal transport barrier for
impurity is established inside the plasma, the TESPEL
technique has a great advantage in the study of theauthor’s e-mail: ntamura@LHD.nifs.ac.jp

transport inside and in the vicinity of that region, since ittakes longer for an intrinsic impurity influx from theplasma facing components to arrive at there.
So TESPEL injection has been tried to investigate
impurity transport inside the IDB plasma.

2. Experimental setupThe LHD with a heliotron type magnetic
configuration is the world’s largest helical device and has
superconducting l/m = 2/10 helical coils and 3 pairs ofsuperconducting poloidal coils. In this experiment, theposition of a magnetic axis Rax is 3.85 m and the magnetic
field strength at the axis is 2.571 T. The plasma was started
up by an electron cyclotron heating and heated additionallyby a tangential neutral beam injection (NBI). And then theultrahigh core electron density accompanied with the IDB
is obtained by a multiple solid hydrogen pellet injection[11], in which the size of the pellet and the time interval ofinjection are optimized. In order to introduce a tracer
impurity inside the IDB, a TESPEL is utilized. As a tracer
impurity, titanium (Ti) particles, the total amount of which
is approximately 2 x 1017 particles, are loaded into theTESPEL. The TESPEL injector is installed on theequatorial plane at Port 3-O of LHD and the TESPEL isinjected from the outboard side of the LHD plasma. Adetailed radial profile of electron density ne and electrontemperature Te is measured with a Thomson scatteringsystem. Total radiated power Prad is measured by a wideProceedings of ITC/ISHW2007

Fig. 1 Typical waveforms of the IDB plasma withTESPEL injection. A vertical dashed line denotes theTESPEL injection time, t ~ 1.09 s.
angle metal foil bolometer, which is installed at the same
port (Port 3-O) as the TESPEL injector and plasma
radiation light is done by absolute extreme ultra violet
silicon photodiode (AXUVD) arrays, which are installed atPort 3.5U and 4-O on a semi-tangential cross-section inLHD [12]. The temporal behavior of the emissions in avacuum ultra violet (VUV) domain from the Ti tracer ionsis measured by a Schwob-Fraenkel soft x-ray
multi-channel spectrometer (SOXMOS, typical exposure
time ~ 50 ms).

Fig. 2 Radial profile of (a) electron density ne and(b) electron temperature Te measured by theThomson scattering system. TESPEL is injected at t~ 1.09 s.

3. Tracer Impurity Injection into IDB Plasma
Figure 1 shows typical waveforms of the IDB plasma
with the TESPEL injection. In this discharge, as can beclearly seen in the Hα signal, the multiple solid hydrogenpellet injection is performed from t = 0.71 s to t = 0.86 s
and then the ultrahigh density region with the IDB isestablished. And then The TESPEL is injected at t ~ 1.09 s.
At this time there is a very sharp peak in the Prad. This isbecause the wide angle bolometer, which is installed at thesame port as the TESPEL injector, can also observe theemissions from the TESPEL ablation cloud. After thissharp peak, the Prad is higher than that during the period
from the end of the multiple pellet injection through until
the time of the TESPEL injection (the period of the IDB
plasma without the Ti tracer impurity) and remains at thehigher level for about 300 ms. And then the Prad is
gradually decreased, which is started before NBIs (#2 and#3) are turned off. At about t = 1.3 s, a small peak can beseen in the signals of the Prad and the C IV. This has no
relationship to the TESPEL injection. This might bebecause carbon flakes fell into the plasma.

Fig. 3 (a) Electron density and electron temperature
measured by the Thomson scattering system at t =1.0 s before the TESPEL injection as a function ofnormalized averaged minor radius and (b) electron
density gradient and TESPEL ablation emissions asa function of the normalized averaged minor radius.
Ti-tracer would be deposited between ρ = 0.4 and ρ= 0.5.

Proceedings of ITC/ISHW2007
Figure 2 shows radial profiles of ne and Te during theperiod from t = 1.0 s to t = 1.6 s. The ultrahigh density
region fades gradually during this period and vanishedcompletely at t = 1.6 s. The TESPEL injection, which isdone at t ~ 1.09 s, seems to have little impact on the IDB.
After the TESPEL injection (from t = 1.2 s), the Te profile
becomes the hollow profile, which remains until t = 1.5 s.
This could be ascribed to the accumulation of the Ti tracer
impurity inside the IDB.
Figure 3 (a) shows ne and Te at t = 1.0 s before theTESPEL injection as a function of the normalized averaged
minor radius. To project the R-coordinate on theρ-coordinate, the three-dimensional free boundary MHD
equilibrium code VMEC is used. As seen in Fig. 3(a), theultrahigh density region can be seen in a wide region of theplasma (The value of ne with over 1.0 x 1020 m-3 is seen
inside ρ ~ 0.75.) As can be seen in Fig. 3(b), the TESPEL
penetrates deeply inside the IDB plasma and consequently
the Ti tracer is deposited between ρ = 0.4 and ρ = 0.5. Thus
the deposition of Ti tracer impurity inside the IDB is
successfully achieved.

(a)

(b)

4. Temporal behavior of tracer impurity deposited
inside the IDB
Figure 4(a) shows temporal evolutions ofsight-line-integrated signals of AXUVD arrays, which are
not equipped with any optical filters. Just after the TESPEL
injection, the signals of the all channel in both arrays areincreased drastically. Shortly thereafter the signals of theall channels except the channel 13 are decreased gradually(the signal of the channel 13 in both arrays is increased
more and sustained.). Considering the geometry ofline-of-sight of AXUVD arrays as shown in fig. 4(b), theprofile of plasma radiation light become extremely peaky
after the TESPEL injection. Moreover, taking the gradualdecrease in ne and no change in Te during the period from t= 1.2 s to t = 1.5 s into account, the signals of AXUVD
arrays suggests that the Ti tracer impurity is accumulated
inside the IDB. Figure 5 shows temporal behavior of VUVemissions (Ti XV (O-like), 11.50 nm and Ti XI (Mg-like),
8.772 nm) from the Ti tracer ions measured with theSOXMOS. Just after the TESPEL injection, Both VUVemissions are increased rapidly and then are decreased
gradually. When two NBIs (#2 and #3) are turned off at t =1.5 s, the Ti XV intensity is drastically decreased, but Ti XI
is not. Taking the gradual change in ne and no change in Te
during the period from t = 1.2 s and t = 1.5 s into account,the experimental results obtained by the VUV spectrometer
again suggest that the Ti tracer impurity deposited insidethe ultrahigh core density region is accumulated.
In order to estimate quantitatively impurity
transport inside the ultrahigh core density region, impurity

Fig. 4 (a) Temporal evolutions of AXUVD arrays’
signals. TESPEL injection time is t ~ 1.09 s. (b)Line-of-sight of corresponding AXUVD arrays.
Displayed magnetic surface is just an example.

Fig. 5 Temporal evolutions of (a) Ti XV (O-like,11.50 nm) and (b) Ti XI (Mg-like, 8.772 nm)
measured with a VUV spectrometer. NBI #2 and #3
turned off at t = 1.5 s and NBI #1 turned off at t =2.3 s.

Proceedings of ITC/ISHW2007
transport analysis, taking temporal change in ne and Te intoaccount, will be performed with 1-D transport code, such
as MIST and Strahl. Further TESPEL injection
experiments will be performed to obtain the tracer
deposition inside and outside the IDB, allowing us to know
qualitatively the effect of IDB on impurity transport.

5. Concluding remarks
Recently, in LHD, the ultrahigh core electron density
accompanied with the internal diffusion barrier has beenobtained. In order to investigate impurity transport inside
the IDB, Ti tracer impurity injection into the ultrahigh coredensity region was tried by means of TESPEL. The local
deposition of the Ti tracer impurity in the ultrahigh coredensity region is successfully achieved. Temporal behaviorof the line emissions from the Ti tracer impurity ion has
been clearly observed with the VUV spectrometer. Taking
temporal change in both ne and Te into account, the Ti
tracer impurity ions seems to be accumulated inside theIDB.

Acknowledgement
The authors acknowledge all of the technical staff ofNIFS for their excellent support. They also would like tothank Prof. O. Motojima (Director of NIFS) for his
continuous encouragement. This work is partly supported
by a Grant-in-Aid for Scientific Research (B) (No.
19340179) from Japan Society for the Promotion ofScience, a Grant-in-Aid for Young Scientists (B) (No.
19740349) from MEXT Japan and a budgetary
Grant-in-AidNo.
NIFS07ULHH510
andNIFS07ULHH524 of the National Institute for Fusion
Science.

References
[1]
[2]
[3]

R. Guirlet et al., Nucl. Fusion 48, B63 (2006).
D. Stutman et al., Phys. Plasma 10, 4387 (2003).
K. Ida et al., Plasma Phys. Control. Fusion 45, 1931
(2003).
[4] Y. Nakamura et al., Nucl. Fusion 43, 219 (2003).
[5] N. Tamura et al., Plasma Phys. Control. Fusion 45, 27(2003).
[6] H. Nozato et al., Phys. Plasmas 13, 092502 (2006).
[7] O. Motojima et al., Phys. Plasmas 6, 1843 (1999).
[8] H. Yamada et al., 34th EPS Conf. on Plasma Physics(Warsaw, Poland) (2007) I4.004.
[9] R. Sakamoto et al, this conference.
[10] S. Sudo et al., Plasma Phys. Control. Fusion 44, 129(2002).
[11] H. Yamada et al., Fusion Eng. Des. 49-50, 915 (2000).
[12] B. J. Peterson et al., Plasma Phys. Control. Fusion 45,1167 (2003)