J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 400-403

Transient Transport Studies for Particle and Heat Using
Tracer-Encapsulated Solid Pellet lnjection in LHD
TAMURA NAOKi, KHLOPENKOV KONStANtiN V.I, SERGEEV VIAdiMir YU.I'2,KUTEEV Boris V.2, SUDO Shigerul, MUTO Sadatsugul, SATO Kuninorir,
FUNABA Hisamichir, INAGAKI Shigerur, NAGAYAMA Yoshiol, KAWAHATA Kazuorand LHD Experimental Groups I / III

Grad. Univ. Advanced Studies, Toki 509-5292, Japant National Institutefor Fusion Science, Toki 509-5292, Japan2 St. Petersburg State Polytechnical University, St. Petersburg 195251, Russia(Received: ll December 2001 / Accepted: 17 June 2002)

Abstract
In order to study impurity particle transport by a transient response of the tracer ions, TracerEncapsulated Solid PELlet (TESPEL) has been developed. TESPEL consists of polystyrene(-CH(C6H5)CH2-)' as an outer shell and tracer particles as an inner core. In case of the titanium (Ti)tracer, the temporal behavior of line emissions from the highly ionized tracer ions has been clearlyobserved by both X-ray pulse height analyzer (PHA) and a vacuum ultra violet (VUV) spectrometer.
These experimental obtained data are compared with the ones simulated by the impurity transport code,
MIST. Moreover, the propagation of the cold pulse caused by TESPEL injection was observed by a 32channel electron cyclotron emission (ECE) heterodyne radiometer and the heat plasma conductivity has
been evaluated.

Keywords:
impurity transport, tracer-encapsulated solid pellet (TESPEL), diffusion coefficient, peaking factor,
MIST, neutral beam injection, temperature perturbation, cold pulse propagation

1. Introduction

TESPEL consists of polystyrene (-CH(C6H5)CH2-)' as
an outer shell and impurity particles as an inner core. Itis important that TESPEL can make a both poloidally
and toroidally localized impurity particle source as atracer, which is deposited at first within a very small
volume in the plasma. Another merit of TESPEL is theeasy identification of the total amount of the injected
impurities due to the known size of the inner core of theTESPEL. In addition, since the TESPEL injector caninject pellets of different sizes (in a 300-950 pm range),
the TESPEL penetration depth, the perturbation
amplitude of the electron temperature and otherOne of the important issues of fusion research is tounderstand the transport of particles and heat inmagnetically confined plasma. For a better
understanding of these transport mechanism, bothanalysis of the steady state and that of the transient state
are important. Impurity injection (laser blow-off method[], impurity pellet injection [2], etc.) is one of thecommon transient transport study techniques. In order todiagnose impurity transport more accurately, a tracerencapsulated solid pellet (TESPEL) [3,4] has beendeveloped [5] and the TESPEL injector and associatedsystem have been installed recently on LHD [6].
C o rre sp onding autho r' s e -mail : ntamura @ LH D. nifs. acjp
400

@2O02 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchTamura N. ,zt al., Transient Transport Studies for Particle and Heat Using Tracer-Encapsulated Solid Pellet Injection in LHD

parameters can be optimized according to the3. lmpurity Tracer Injection Experiments

experimental objectives.

There have been few studies on impurity transport
in stellarator type devices, while many experiments forimpurity transport have been performed in tokamaks.
Several important results were obtained in Heliotron Eplasmas heated by neutral beam injection (NBI) [13]. It
was observed that the diffusivity has an important role

2. Experimental Apparatus
The Large Helical Device (LHD) [7,8] is theworld's largest heliotron device (poloidal period number
I = 2, and toroidal period number m = l0) with a major
radius R = 3.50-4.05 m, average minor radius a = 0.6m, magnetic field up to 3 T generated by super-on the impurity transport in NBI plasmas, while theinward flow plays only a minor role. However, thisexperiment was performed only with a co-NBI plasma
setup. Thus, it is still an important task to investigate
whether the difference of the NBI direction can cause a

conducting coils. The TESPEL injector has been
installed on LI{D Port #3-O and a TESPEL is injectedfrom the outboard side by means of the pneumatic pipegun technique. The accelerating gas is Helium and its
pressure is 30 atm. The obtained TESPEL velocity issignificant difference of the impurity transport asobserved in tokamaks.

typically in the range of 250-400 m/sec. The 59
TESPELs can be loaded in the rotating disk. TheThe impurity transport in NBI plasmas with various

NBI directions is studied in LHD by means of theTESPEL injection. Figure 1 shows an example of theablation light of TESPEL injected into LHD plasma isobserved by the CCD cameras and the photo-multipliertubes (PMT). These are equipped with thecorresponding interference filters for shell and tracer. By
using the knc,wn TESPEL velocity, the high timeresolved PM]' signal is translated to the TESPEL
position in the plasma. Thus, the location and width oftime evolutions of Ti Ka emission measured by PHA2Oand Ti XIX (,1 = 169.6 A; measured by VUVspectrometer in LHD shot #27864.In this shot, TESPEL- 1.190 sec during co-NBI. The Ti Kaemission rises to its maximum, during approximately
500 ms after the injection and the emission decay timeis estimated as 1.48 sec between the time when the Ti
was injected at tthe tracer deposition can be measured.

Ka emission reaches a maximum value and the time
when NBI was switched off. The profiles of n" and T.

In the case of an impurity transport study, titanium(Ti) was used as a tracer. The typical shell diameter forthis study is about 700 prm and several Ti micro-balls(with 80-100 pm diameter) have been filled inside theTESPEL. The total amount of Ti particle was varied
approximately in the (0.5-3) x l0l7 range.
In experirnents, the temporal and spatial behaviorsof the electr6'n density (n") are measured by a 10channel FIR laser interferometer [9]. The time evolutionof the electron temperature (2") is measured by an ECE
heterodyne radiometer [10] with a high time resolution.

before and after TESPEL injection and their temporal
evolutions are shown in Fig. 2. One can conclude that n"
and 7i perturbations caused by TESPEL injection can be

1.6=(t,1.2o 0.8

The ?" profile is also measured with a good spatial (-2cm) resolution by multi-point Thomson scattering [ 1].
The behavior c'f the Ti line emissions is measured by anX-ray pulse height analyzer (PHA) [12] and a vacuum
ultra violet (\/UV) spectrometer. Two PHA systems
have been used. One system (PHA2O) is installed on theequatorial plane of LHD Port #2-O to observe the LHD
plasma center. Another (PHARD) system located at Port
#2.5-L has 3 channels to observe the radial profile of X-

LHD shot#27864'''-./
exponential (lV,-,

Ti Ka (PHA)

Tixrx (vuv)

cEc) 0.4.N(EE00.5
TESPELiniection rrrrrslosw'
Fig. 1 An example of the temporal behavior of Ti Ka(open circles) measured by PHA2O and Ti XIX(closed diamonds) measured by VUV spectrometer. The dash-dotted line is the exponential fitting
curve for the temporal behavior of Ti Ko. Plasma
condition: co-NBl plasma (P"r, = 1.7 MW) with line
averaged electron density n._0". = 1.8 x 101s m-3,
and central electron temperature i"(0) - 2.1 keV.
NBI was terminated at t = 2.3 sec (vertical dashedray emissions. The energy resolution of the PHA
systems is approximately 300 - 400 eV. Therefore, themeasured Ti Ka emission by the PHA systems contains

Ti Ka line emissions from several charge states (Helike, Li-like, Belike, etc.).

line).
401

Tamura N. et al., Transient Transport Studies for Particle and Heat Using Tracer-Encapsulated Solid Pellet Injection in LHD
LHD#27864^LHD#278U

L3(aLHD#27864, t =1.9 sec

1.4(c)

42
1.20 sec-;2oo 'l.18ss. |.20 sec".

F1

1.22sm1.28 s6c

ct'6

t3ocouz()
trJ

Normalized radius,

0.8.cootl

F't '1.2
0 0.2 0.4 0.6 0.8p (au.)

1.2-olE'er.rroa* l'10.6.N(d0.4Etime (sec)oz

Fig. 2 Profiles of (a) n", (b) l. before and after TESPEL
injection and the temporal evolutions of (c) n"_0",,(d) 7:. TESPEL injection was at t 1.1903 sec.
Same discharge as in Fig. 1.

0.2-01020304050
minor radius (cm)Fig. 3 Calculated profile of line-integrated Ti Ka emis-

neglected when 0.1-0.15 sec has passed since TESPEL

sion and the experimental data measured byinjection. Thus, impurity transport studies wereperformed for times after t = 1.3 sec in this shot.
Simulations were done by using the impurity
transport code, MIST [4]. For those, cylindricalsymmetry was assumed and the radial particle flux f,

PHARD for LHD #27864 at f= 1.9 sec before thetermination of NBl.

LHD shot#27864

1.6'-'-(a) Ti Ka (PHA2O)

for the i-th charge state ion was modeled by a simple--D=90Ocm'?/ec.Cv=2

1.2

diffusive-convective model: li = -DiQ)dn/0r - ViQ)ni,
where n1 is the density of the impuritl, DiQ) is the radial
diffusion coefficient andV;(r) is the convective velocity,respectively. In our studies, the diffusion coefficient isassumed to be radially constant and the convective
velocity term is represented as V, = - Cv(2rD,la2),

D =,15O cm'/sc. Cv = 0.. ... Io.e?
g= 1966 6m?src. Cv = 3

0.4.i.
oEoo4
N

where Cv is the dimensionless peaking factor.

E362
t1Figure 3 shows the calculated profile of Ti Kaemission integrated along the PHARD line of sight andthe experimental one for LHD #27864 at t = 1.9 sec
before the termination of NBI. One can see from thisfigure that the radial profile is weakly sensitive to Cv(b) Tixrx (vuv)!.

1)

0!ii'i,c.

:ti

3 .c.
1.5

2

2.5

time (sec)Fig. 4 Comparison of normalized temporal evolutions ofthe (a) Ti Ka (open circles) and (b) Ti XIX (openvalues. Thus, only a 0 - 3 range of Cv could beevaluated from Fig. 3. Since this steady-state profilediamonds) with those calculated by MIST withseveral sets of D and Cv (dash-dotted line: D =analysis gives only the ratio of the convective fluxes todiffusive ones, the temporally evolving data are used forfurther analysis. Comparison of the calculated temporal

450 cm2/sec. Cv = 0, dashed one: D = 900 cm2/sec,Cv = 2, dotted one: D = 1800 cm2/sec, Cv = 3). Cal-culations were stopped at t = 2.3 sec when theNBI was terminated. The intensity of Ti Ko is normalized by the value at the time when the Ti Koemission reaches a maximum and that of Ti XIX

evolutions for several sets of D and Cv with themeasured ones is shown in Fig. 4. A fairly good
agreement is obtained with the value of D = 900 cm2lsec
and Cv = 2 (i.e. V(a) = Jl cm./sec, where V(a) is theconvective velocity at the plasma edge) as a result oftrial and error. From this analysis, the Ti ion transport isexplained with the value of D = (450-1800) cm2lsec andCv = 0-3. Similar analysis has been performed forbalanced NBI and counter-NBl plasmas with line-

is normalized by the value at t= 2.15 sec.

averaged electron density ne_bar in the range of (l-2)x10re m-3. The pair of D = 900 cm2lsec and Cv = 0 forbalanced NBI case and D = 700 cm2lsec and Cv = 0 for402

Tamura N. et aI., Ttansient Transport Studies for Particle and Heat Using Tracer-Encapsulated Solid Pellet Injection in LHD

counter-NBl case are obtained as the best fit.
In Heliotron E, no strong dependence of theimpurity transport on the impurity species (Si, Fe, etc.)
was observed t151. Thus, the obtained transportcoefficients by means of TESPEL with Ti tracer in LHD

plasmas with co-NBI. These results in LHD were almost
consistent with the findings of Heliotron E. However, inour experiment, many parameters were fixed. Thus theparametric dependencies of impurity transport onelectron density, NBI power and others remain as
objectives of future studies. TESPEL injection has
produced the electron temperature perturbations withsmall perturbations of other plasma parameters. Theare compared with those by laser blow-off techniquewith silicon impurity in Heliotron E t131. The D valueof (70G-900) cm2/sec, obtained in LHD is close to onesin Heliotron E. Small Cv 3 2 values (i.e. small inward

resulting cold pulse propagations were clearly measured
by an ECE heterodyne radiometer. Since the TESPEL
injector can inject various sizes ofTESPEL, the electron
heat diffusivity, X", can be studied in a widespread
plasma region.

flow) for co-NBI, counter-NBl and balanced NBI
plasmas obtained in LHD are also consistent with thefinding for co-NBI plasmas in Heliotron E [2]. For
further comparison, dependence of impurity transportcoefficients on plasma parameters (electron density, NBI
power and so on) should be obtained in LHD.

Acknowledgments
This work was supported partly by grant No.

4. Gold Pulse ExperimentL00537 ofJapan Society for the Promotion of Science.

TESPEL has produced the electron temperature
perturbation u'ith small perturbations of other plasma
parameters. Thus, the values of electron heat diffusivity,7", in the interior of the plasma were evaluated fromanalysis of the cold pulse propagation caused byTESPEL injection [16]. For one, the value of Xewas

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