J. Plasma Fusion Res. SERIES, Vol.7 (2006) 27-30

Radial Argon Kα Profile Measured
with Pulse Height Analyzer in LHD
MUTO Sadatsugu, MORITA Shigeru and LHD Experimental Group
National Institute for Fusion Science, Toki 509-5292, Japan（Received: 29 October 2004 / Accepted: 21 December 2005）

Abstract
The first measurement of time-resolved radial profiles of argon Kα lines has been successfully done inLarge Helical Device, using an assembly equipped with conventional semi-conductor detectors and pulseheight analyzers. The fixed argon discharges enabled the radial scanning of the assembly. Results obtained
here indicate that a transport study on impurity also becomes possible with the pulse-height analyzers as wellas the measurement of electron-temperature profile.
Keywords:
argon, titanium, chromium, iron, Kα line, radial profile, Large Helical Device
1．Introduction
Measurement of x-ray spectrum is important toobtain significant information from a core plasma
such as the electron temperature, heavy impurity
concentration, an impurity transport, and the nonthermal electron density. Especially, regarding to theimpurity transport, the measurement of a space- anda time- resolved spectra are essentially requested. Inorder to obtain such spectra in x-ray region, a few
diagnostics have been constructed until now. InJET tokamak a diﬀusion coeﬃcient was evaluatedcombining with an impurity injection [1].
The x-ray signals detected from high-temperatureplasmas are always integrated along the line-of-sight.
An Abel inversion of the detected signal is necessary inorder to obtain the radial emission profile. The Abelinversion strongly depends on the plasma shape andthe line-of-sight of the diagnostics.
In order toeliminate the large error after Abel inversion, thesoft-x-ray diagnostic system installed in Large Helical
Device (LHD; Rax = 3.6 m, a = 0.64 m, B = 2.9 T) was
set at the vertically-elongated plasma cross-section andmeasured perpendicularly to the magnetic axis from justbelow the plasma.
In this paper the measurement on the time-resolved
argon Kα profiles in LHD is presented. Furthermore,inverted emission profile of argon is also reported withKα profiles of intrinsic heavy metallic impurities.

semi-conductor detectors and a movable slit which canscan the sight line of the detectors along the majorradial direction of LHD. The position of the movable
slit is monitored by an encoder. The x-ray signal
detected with the assembly was converted into the lineintegrated radial profile using the encoder signal [2].
The evaluations of electron-temperature profile up to
10 keV and metallic impurities are available, since thedetector covers a broad energy range from 1 keV to
30 keV. A fast pulse-height analyzer (PHA) is also
adopted for digitizing of the amplified signal voltage.
The maximum counting rate of 64 × 4 kcps wasachieved per one detector in which four Si (Li) elements
are mounted. Consequently, the intensity of the line
spectrum has been estimated with a time resolution of
2 msec as the maximum performance. The time- andspace- resolutions are not simultaneously obtained atpresent. In order to obtain many radial points needed
for better radial profile, several fixed discharges werecarried out.
The continuum intensity becomes too high below
an x-ray energy of 4 keV. In order to avoid the signal
saturation of the PHA, 1 mm-thick beryllium isnormally inserted as a filter. The use of the filter,
however, also reduced the Ar Kα intensity, because thex-ray attenuation rate at Ar Kα energy was larger than102 .
In LHD Ar discharges have been carried out for a
specific purpose of high-performance discharges withhigher central ion temperature. In addition, a small

2．Experimental procedure
The first measurement of time-resolved Ar profiles in
LHD was done using an assembly equipped with Si(Li)C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchCorresponding author’s e-mail: mutos@nifs.ac.jp

27Radial Argon Kα Profile Measured with Pulse Height Analyzer in LHD

amount of Ar gas is routinely puﬀed at the beginningof discharges to measure ion temperature from ArXVII
Doppler broadening. Then, the argon becomes a regularelement for in LHD discharges. The Ar discharges
were produced and heated by three tangentially injected
neutral beams (NBI). The line intensity of Ar Kα
x-ray lines was remarkably enhanced compared withKα lines from intrinsic heavy impurities in H2 and Hedischarges. Therefore, the measurement of Ar Kα
profile was clearly improved with decreasing a staticerror.
In order to analyze the radial profile of Ar Kα lines,
the integration equation derived from the arrangementof the assembly is taken into account; ∞22dx f
ρ +x ,(1)F(ρ) = 2Fig. 2 Typical spectrum from Ar discharges in LHD.

shown in Fig. 1. The Kα emissions from titanium,
chromium, and iron appear at 4.7 keV, 5.6 keV, and6.6 keV, respectively. The continuum x-rays originally
increase when the x-ray energies decrease. However,
the observed intensity begins to decrease below 4.5 keV.
This is simply explained by the reduction of thetransmission rate in the used beryllium filter. Thesignals below 2.0 keV are discriminated because of thesignal noise from a preamplifier of the detector.
In the case of Ar discharges an x-ray spectrum wasobtained as is shown in Fig. 2. The Ar Kα line appears
at 3.2 keV. Taking into account the transmission rateof the Be filter, the real Ar Kα emissions are 2 orders
stronger than those of metallic Kα lines as mentioned
above. In Fig. 2 the intensity of continuum is negligible
in compared with Ar line, while the continuum
intensity is comparable to the metallic impurity case(also see Fig. 1). Accordingly, the radiation loss in Ar
discharges is dominated by the Ar line emissions in an
x-ray region higher than 3 keV. This situation is quite
diﬀerent from the case of H2 and He discharges.
Figure 3 shows typical profiles of metallic Kα lines
obtained with the assembly in a single shot. The
electron temperature profile measured from thecontinuum is also indicated. In the case of H2 and He
discharges, long pulse discharges become available [3].
In these discharges, the electron temperature, density,and the intensity of impurity lines were constant at least
over 8 sec. In order to obtain the profile the movable slit
was continuously scanned at a constant velocity.
Figure 4 shows a typical profile of Ar Kα lines
which was plotted at 3.2 keV as shown in Fig. 2. The
electron temperature at plasma center was 5.0 keV. Theline intensity of Ar Kα was not constant and stronglytime-dependent, because a strong Ar gas was puﬀed at
the beginning of the discharges. This situation is quite
diﬀerent from the long pulse H2 and He discharge cases.
Then, the measured line profile could not be obtained in0

where f (ρ) is a radial distribution of impurity lines.
The impurity Kα profile observed with the assembly isexpressed by F(ρ) in Eq. (1). Of course, it is assumed
that the x-ray intensity from impurities is constant along
a magnetic surface. If F(ρ) is a gaussian, f (ρ) is easilyderived as expressed by Eqs. (2) and (3),1f (ρ) = √ F(ρ),πσ(2)∵ F(ρ) ≡ e−(ρ/σ) ,(3)2where the normalized plasma radius and the widthof the Gaussian profile are denoted by ρ and σ,
respectively.
3．Results and discussions
In H2 and He discharges of LHD an x-ray spectrum
was obtained with the assembly at center code as isFig. 1 Typical spectrum obtained from H2 and He
discharges in LHD. Spectrum was measured withthe PHA assembly. Lines denoted with Kα (Ti), Kα(Cr) and Kα (Fe) indicate the Kα lines of titanium,chromium, and iron, respectively.
28

Radial Argon Kα Profile Measured with Pulse Height Analyzer in LHD

Fig. 5 Typical radial emissivity of Kα line emitted fromargon (chain line) and heavy metallic impurities
such as titanium (dotted line), chromium (brokenline), and iron (solid line). Kα profiles were fittedby Gaussian.

Fig. 3 Typical radial profiles of Kα line emitted fromheavy metallic impurities such as titanium (soliddiamonds), chromium (solid squares), iron (solidcircles) and electron temperature (open circles).
Kα profiles were fitted by Gaussian. The data
corresponding to each point were obtained with anaccumulation time of 240 msec.

Fig. 6 The time evolution of Ar Kα lines. Ar gas puffing isdone at t = 500 msec and t = 1000 msec.

Fig. 4 Typical radial profiles of Ar Kα lines. Solid line
indicates a Gaussian fitting. The data are obtainedwith an accumulation time of 50 msec.

A diﬀusion coeﬃcient and a convective velocity ofparticles are basically a function of radial position ofplasmas. The radial flux is derived from a density
profile through a transport equation;a single shot. Several fixed discharges were necessary
to measure the Ar profile. For the purpose the positionof the slit was scanned shot by shot. Time- and spaceresolved Ar Kα profile was, thus, successfully obtained.
Figure 5 shows the radial emissivity of the Kα lines
emitted from Ar, Ti, Cr, and Fe. The radial emissivityof the Kα lines is calculated through the Abel inversion
satisfying Eq. (1). The measured Kα profiles of Ti, Cr,Fe, and Ar are fitted by a single gaussian as is shownin Figs. 3 and 4. As expressed by Eqs. (2) and (3),the radial emissivity can be simply obtained using theGaussian fitting.
The temporal behavior of Ar Kα lines is measured
with a time resolution of 25 msec as shown in Fig. 6.
The Ar Kα begins to increase after Ar gas puﬃng at
t = 500 msec and decays at t = 700 mse. Using thisbehavior the study on impurity transport in core plasma
becomes possible.

∂ r, t),n(r, t) = ∇ · Γ(∂t r, t) = −D(r)∇n(r, t) + V( r)n(r, t),Γ((4)(5)

 V,
 and the scalars n(r), D are
where the vectors r, Γ,radial position, particle flux, convective velocity,density profile, and diﬀusion coeﬃcient, respectively.
From Eq. (4) the diﬀusion coeﬃcient is obtained asD(0)  ρ −2  ρ 2(6)e σ −1 +1 ,D(ρ) =2
σwhere D(0) is the diﬀusion coeﬃcient at plasma center.
In Eq. (6), the density profile is assumed to beρn = n 0 e− σ2· e− τ ,t(7)where t, τ, σ and n0 are time, density decay time,29

Radial Argon Kα Profile Measured with Pulse Height Analyzer in LHD

the Eq. (10). From Eq. (9) the convective velocity at
plasma edge is also estimatedto be 1 m/s (inward).

gaussian width and central density, respectively. Inaddition, the convective velocity is also assumed byV(ρ) = −Va ρ,(8)aVa τ = − ,2(9)4．Conclusions
In LHD a time evolution of Ar Kα profile has beensuccessfully obtained by several fixed Ar discharges
using an assembly equipped with conventional semiconductor detectors. In the present experiment themost important profit of the assembly is conspicuouslyindicated. Especially, the radial emission profile of K αemitted from argon, titanium, chromium, and iron are
derived.

where a is averaged plasma radius. Thus, the diﬀusion
coeﬃcient and the inward velocity become a simple
function of the decay time. Here, the value of thediﬀusion coeﬃcient at plasma center is given by
D(0)τ = 2

 aσ 22.

(10)

As mentioned above, the time evolution of density
distribution is necessary in order to obtain thediﬀusion coeﬃcient and the convective velocity. A
code analysis implies that the width of Ar
distribution is approximately 0.47 (= σ) in minor
radius. Consequently, the diﬀusion coeﬃcient at theplasma center is roughly inferred to be 0.2 m2 /s through

[1]
[2]
[3]

30

References
R. Gianalla et al., Nucl. Fusion 34, 1185 (1994).
S. Muto et al., Fusion Eng. Des. 34-35, 205(1997).
S. Muto et al., Rev. Sci. Instrum. 72, 1206
(2001).