J. Plasma
J.
Plasma Fusion
Fusion Res.
Res. SERIES,SERIES,Vol.
Vol.6 6(2004)(2004)634–637
000–000

Applicability of the TESPEL Charge-Exchange Recombination
Spectroscopy Diagnostics for Impurity Transport Study on LHD
KALININA Diana, SUDO Shigeru1, TAMURA Naoki1, SERGEEV Vladimir Yu.2and LHD Experimental group
Graduate University for Advanced Studies, Kanagawa, 240-0193, Japan
1
National Institute for Fusion Science, Toki, 509-5292, Japan
2
State Technical University, Politechnicheskaya 29, St. Petersburg, Russia(Received: 9 December 2003 / Accepted: 1 April 2004)

Abstract
Using the theoretical optimization of the Tracer Encapsulated Solid Pellet (TESPEL) Charge eXchange
Recombination (CXR) signals for various impurities in the visible spectral range under the conditions of the Large
Helical Device (LHD) plasma, the TESPEL CXR Spectroscopic (CXRS) experiments with tracer materials, such as
fluorine and magnesium and impurity amounts of about 1017–1018 particles have been done. For comparison with theTESPEL pellet case, the CXR emissions from other materials after the pure impurity injection, such as neon andcarbon with bigger impurity amount (2 × 1019) were investigated. Adaptability of the TESPEL CXRS diagnostics inthe ultra soft X-ray (USXR) spectral range have been discussed.
Keywords:TESPEL, impurity pellet, impurity transport, CXRS

1. Introduction

magnitude smaller CXR signal value for Li tracer material.
This explains why no detectable signal has been observed in
LHD TESPEL experiments with LiH tracer before. Therefore,
based on the theoretical calculation of the operational rangeof the TESPEL CXR diagnostics in the visible spectral range
under the conditions of the LHD plasma (ENBI = 150 keV)[2], the TESPEL CXRS experiments with tracer materials,
such as fluorine ( λF(n=10→9) = 479.3 nm) and magnesium(λMg(n=12→11) = 478.9 nm) have been carried out. From simple
estimations the increase of the CXR signals in these cases up
to the signal level that was observed on the CHS machinewith LiH tracer is expected.

The properties of local impurity transport with fairly
high-accuracy could be obtained by means of the tracerencapsulated solid pellet (TESPEL) injection, which has beenutilized previously on the Compact Helical System (CHS) andnow on the Large Helical Device (LHD) [1]. TESPEL
consists of polystyrene (-CH (C6H 5) CH2-) as outer shell(typically 0.5~0.9 mm diameter) and tracer particles as an
inner core (~0.2 mm size). The flexible choice of the tracer
particle is one of the important characteristics of the TESPEL
method. The behavior of impurity tracer ions deposited locallyin the core plasma by means of TESPEL injection can bemeasured by the observation of the line emission due to thecharge exchange reaction of the injected impurities. Thismethod allows us to estimate the local impurity transportcoefficients, which are already obtained by Li III on CHS. InTESPEL CXRS experiments with the LiH tracer on LHD,
however, Li III ( λLi = 449.9 nm) emission in the visible
spectral range could not be measured [2,3]. At a fixedmagnetic geometry of the experiment, and Neutral Beam
Injection (NBI) energy and power, the CXR signal isCX
proportional to 〈σ (n,n–1)〉, the capture cross-section of thecharge exchange reaction, and the density of the injectedimpurity nuclei NZ. In this case, the theoretical estimations
with small values of both capture-radiation cross-section andNBI density flux in the case of LHD result in two orders of

2. Setup of TESPEL CXRS diagnostic onLHD
The basic idea of this method is illustrated in Fig. 1.
When injected TESPEL enters into the plasma, the outer layer
is ablated first, delivering the tracer core to the inner plasma
region. After ionization, the fully stripped tracer ions movealong the magnetic field lines forming a toroidal annular Xions domain as shown in Fig. 1, which later diffuses in theradial direction. The long time scale diffusion in the radial
direction of the fully ionized impurity ions from this localized
domain can be measured by the observation of charge
exchange recombination (CXR) emission intensity due to the©2004 by The Japan Society of Plasma
Science and Nuclear Fusion Research

Corresponding author’s e-mail: diana@LHD.nifs.ac.jp1634

Kalinia D. et al., Applicability of the TESPEL Charge-Exchange Recombination Spectroscopy Diagnostics for Impurity Transport Study on LHD

Fig. 1 Schematic view of the TESPEL pellet injection system.

line at the λF(n=10→9) = 479.3 nm wavelength- and magnesium powder for observing magnesium (Z= 12) CXR line at λMg(n=12→11) = 478.9 nm.
In our experiments, the outer diameter of TESPEL has ranged
from 700 to 900 µm. Since it takes much time to evaluate theamount of these impurities due to its amorphous shape, we
assumed here that the 50 % (~ 2 × 10–12 m–3) by volume of
the inner core of the TESPEL is occupied by the tracer
material. Then, the total amounts of the impurities injectedby TESPEL approximately are 1017 for the F tracer, and 9 ×
1016 for the Mg tracer, respectively. The expected brightness ofthe visible CXR light should be about 1.5 × 1011 (ph/cm2sr s)
and 10 11 (ph/cm 2sr s) for Mg and F, respectively. For
comparison it is two orders larger than that in the Li impurity
case (109 (ph/cm2sr s)) [4]. In the case of the F tracer, a small
signal was observed at one of the eight channels. In the caseof the Mg tracer, the CXR emission signal was not observed.
As was mentioned before, the CXR signals for the fixed
geometry are proportional both to the capture cross-section
and to the density of injected impurity nuclei. According to
the theoretical calculations the tracer material was chosen as
the impurity with the optimal value of the cross-section for
LHD plasma conditions, so the possible reason for nodetectable signal could be in the small amount of the injected
impurity into the LHD plasma. For comparison with theresults, the CXR emissions of the other impurities, such asneon (λNe(n=11→10) = 524.9 nm), boron (λB(n=7→6) = 494.6 nm)
and carbon (λC(n=8→7) = 529.3 nm), were investigated. This
could be done after the impurity injection, through gas puffing
on pure pellet injection. Unfortunately it is difficult toCX reaction of injected impurity nuclei IZ (of nuclear chargeZ) with the neutral beam injection (NBI) hydrogen atoms(assumed to be in their ground state). In our experiments, theCXR signal is measured within a certain visible spectral
range. The IZ–1 ions can also exist in the peripheral plasmawith low electron temperature due to recombination of IZ ions.
For this reason, the IZ light is observed simultaneously at thelocation on the NBI path and, for reference, at the neighboring
port without the NBI (see in Fig. 1). The light emission fromthe two arrays of the photo detectors having different radial
positions in the NBI port together with the reference detector
array in the port without NBI, is transmitted through theoptical fibers with 200 µm in diameter to a spectrometer withan Intensified CCD camera. The typical exposure time of thissystem is 15 ms, therefore the instrument can provide multiple
spectra during a plasma discharge with time interval of about
40 ms. The subtraction of the reference signals from the CXR
detector signals can be interpreted as pure CXR signals.

3. Experimental results in the visible
spectral range
The operational limits of the use of TESPEL CXRS
diagnostics in the visible spectral range on LHD have been
determined which provide a maximum CXR signal assumingthat the complete ionization time has to be much smaller than
the impurity transport time [2]. Under our experimental
conditions (electron temperature Te = 1–3 keV and density ne= 0.5–2 × 1013 cm–3 with NBI power PNBI = 3 MW), there are
two types of impurity tracers:- a Teflon (C2F4)n block for observing fluorine (Z = 9)2635

Kalinia D. et al., Applicability of the TESPEL Charge-Exchange Recombination Spectroscopy Diagnostics for Impurity Transport Study on LHD

Fig. 2

Ne IV emission at the location of NBI (CXRS) and the reference location for two different channels (ρ = 0.9(1), ρ = 0.8(2)) beforeNe pellet injection t = 0.4 sec (a), just after t = 0.3 6sec (b) and 350 ms after injection (d).

estimate the impurity amount after the neon gas puff orboronization of the LHD vessel, so we cannot make acomparative analysis of the CXR signals with TESPEL. In
the case of the pure impurity pellet, the total amount of theimpurity in that is approximately 3 × 1019 and 1 × 1020 forneon and carbon, respectively, which is several orders ofmagnitude more than in the case F or Mg TESPEL. However,
the radiation cross-section for neon (11 × 10–18 cm2) under
LHD conditions is almost the same as for fluorine (9 × 10–18cm2). Therefore, due to the larger amount of neon impurity,from theoretical estimation the CXR signal should be morethan two orders of magnitude larger than was expected forthe fluorine case. One example of the signals from detectors
with NBI and without NBI just before and after Ne pellet
injection for the two different channels is presented in Fig. 2.
in the case of the magnetic axis Rax = 3.75 m and magnetic
field B = 2.6 T. These two channels are corresponding to thedifferent radial positions indicated by ρ-values. As shown in
Fig. 2a no signals are detectable before pellet injection forboth the CXRS and Reference channels cases. At the moment
after pellet injection the signals are detectable in the several
channels and the maximum appears later for the outer radii(effective radius (ρ = 0.9), since a pure pellet begins to ablate
from the plasma edge (Fig. 2b). In this case, the relative
intensity of the CXR signal measured at the plasma periphery
was more than one order of magnitude larger than the F tracer
CXR signal level measured, as it was expected from theFig. 3Comparison of the CXR signal intensity measured inthe visible spectral range from the TESPEL (fluorine (λF= 479.3 nm), magnesium (λMg = 478.9 nm)) and fromthe pure impurity pellet (neon (λNe = 524.7 nm) andcarbon (λC = 529 nm)). The amount of the impurities
was different.

theoretical calculations. Based on these experimental resultsfrom the CXR emissions of neon and carbon pellets thesimple estimation of optimizations of the TESPEL CXRSdiagnostics are shown in Fig. 3.
3636

Kalinia D. et al., Applicability of the TESPEL Charge-Exchange Recombination Spectroscopy Diagnostics for Impurity Transport Study on LHDrange under the usual LHD plasma conditions we took halfof the maximum possible tracer amount. The increase by afactor of more than one order of magnitude of the estimated
CXRS signal in the USXR spectrum has a clear advantage incomparison to the measurements in the visible spectral range.
In the nearest future a new diagnostics system including ahigh throughput planar multilayer monochromator for theultrasoft X-ray (USXR) TESPEL CXRS experiments withfluorine and magnesium tracer materials is planned to beinstalled for the next step.

Judging from these experimental results, the CXR signals
in the visible range can be increased by several times byincreasing the tracer amount up to the TESPEL limit as shownin Fig. 3, if we assume the same level of the coefficient ofvisible light emission for each tracer material. Theoretically,
the inner core volume in the current TESPEL setup can beincreased up to 6 × 10–11 m–3 that is several times more thanthat we usually used. In this case the resulting tracer amountwill be increased up to 2 × 1018. According to the Fig. 3, theCXRS signal in this case should be increased by several times
in comparison with the observed signal from fluorine tracer
with a smaller impurity amount (1017). The amount ofimpurity in the recent experiments was increased only up to3 × 1017 for the fluorine and up to 8 × 1017 for the magnesium
case, due to some technical problems to produce a pellet witha limited tracer amount. According to Fig. 3 the CXR signal
for fluorine TESPEL should be increased by two times. As
for the magnesium TESPEL case with the higher amount of
the impurity we cannot extrapolate to a real value, since inthe experiments with the smaller magnesium tracer amountthe CXRS signals were not detectable. Unfortunately thesubtracted CXR signals in the recent experiments are still
marginal in some channels, thus we cannot utilize them fortransport study on LHD with good quality.

5. Conclusions
The anticipated increase of the TESPEL CXR signals in
the recent experiments, even with limited tracer amounts ofmagnesium and fluorine was not observed. So we cannot
utilize the visible spectral range for transport study on LHD
with good quality.
The applicability of the TESPEL CXRS diagnostics inthe USXR spectral range in comparison with that in thevisible spectral range has been estimated. The increase by afactor of more than one order magnitude in the USXR spectral
range has a clear advantage to the TESPEL CXRS diagnostics
in the visible spectral range.

Acknowledgements
4. Discussion

The authors would like to thank members of LHD
experimental groups and NIFS personnel for their kindcooperation.

As an alternative method to obtain larger CXRS signals,
it would be preferable to use measurements in the low
wavelength region, such as the Ultra Soft X-Ray (USXR) andVUV (Vacuum UltraViolet) spectral ranges [5]. As is wellknown, for the ∆n = 1 transitions at shorter wavelengths in
the case of small n the capture radiation cross-section will beincreased by several times. From similar theoreticalestimations that have been done for the visible spectral range,
for the VUV and USXR spectral range the brightness of theCXR signals for the TESPEL case is expected to be about
4.2 × 1012 (ph/cm2sr s) for F IX line at 8 nm and up to 1013
for Mg XII line at 4.5 nm. For the estimations of theapplicability of TESPEL CXRS diagnostics in the USXR

References[1] K. Khlopenkov and S. Sudo, Plasma Phys. Control.
Fusion 43, 1547 (2001).
[2] V.Yu. Sergeev et al., Plasma Phys. Control. Fusion 44,277 (2002).
[3] R.K. Janev, L.P. Presnyakov and V.P. Shevelko, Physics
of Highly Charged Ions (Berlin: Springer, 1983).
[4] S. Sudo et al., Plasma Phys. Control. Fusion 44, 129(2002).
[5] Isler, Plasma Phys. Control. Fusion 36, 171 (1994).

4637