Proceedings of ITC/ISHW2007

Radial Profile of Kα line energy shift from Metallic Impurities
measured with X-Ray Pulse Height Analyzer in LHD
Sadatsugu MUTO, Shigeru MORITA, and LHD Experimental Group
National Institute for Fusion Science, Toki 509-5292, Gifu, Japan
Radial profiles of metallic impurity have been successfully obtained in Large Helical Device, using an assembly
equipped with conventional semiconductor detectors and soft x-ray pulse-height analyzers. Long pulse discharges
have enabled the radial scanning of the assembly to measure the radial profiles of Kα lines of metallic impurities.
The assembly of soft x-ray pulse-height analyzer makes it possible to obtain the concentrations of the impurities inconnection with a cord calculation.
Keywords: Pulse Height Analyzer, Iron, chromium, titanium, Large Helical Device.

1. Introduction
Measurement of x-ray energy spectrum is important toinvestigate fundamental properties of magneticallyconfined high temperature plasma, since the spectrum
reflects information on electron temperature, impurity,and non thermal electron. [1] Recently, it is intended torise up the spectral, special and temporal resolution in the
x-ray diagnostics to obtain significant information such as
the shift of magnetic axis, electron temperature profile,and especially impurity transport. [2] However, in an
x-ray region it is fundamentally difficult to getsimultaneously the energy and another resolution
mentioned above. Recent technology has not yield any
applications which can fully resolve the problem.
An assembly of pulse height analyzer (PHA) has been
constructed in Large Helical Device (LHD). The
assembly is equipped with a utility called a radial
scanning system which modulates and identifies the sightline of PHA along the major radius direction of LHD. The
scanning range of the system fully covers the plasma inthe major radial direction. With the system the radial
profile of x-ray spectrum is obtained. Accordingly, Abelinversion of x-ray spectrum is available, although signals
obtained with an x-ray detector are line integrated along
the sight line of the detector,
It is the most important advantage of the PHA that theenergy resolution is enough to distinguish each Kα line ofimpurity and the absolute number of photons can beobtained. Especially, Kα lines of impurities heavier thanargon have been observed in LHD. [2]The radial profiles of Kα lines emitted from metallic
impurities such as titanium, chromium, and iron havebeen successfully observed with the assembly. Theenergy shift of Kα line is also observed. In addition to theexperiment the intensity profile of Kα line emitted from
author’s e-mail: journal@jspf.or.jp

respective charge state of metallic impurity is calculated
as a function of electron temperature and density profiles.
As a result absolute density has been estimated incomparison with the experimental results and thecalculation.
In this article experimental results in LHD arepresented on the radial profiles of Kα line observed withthe assembly. Furthermore, the analysis on the density of
the respective charge state is also reported.

2. Assembly for x-ray measurement
The profiles of Kα lines have been carried out in LHD
using the assembly. The performance of the assembly has
been reported with observed Kα lines of metallic
impurities and continuum in a reference [3]. It must bementioned here that the energy resolution of the PHA is
160 eV at 3.2 keV and the maximum counting rate is 10kcps in the present experiment. The sight line of thedetector is modulated along the major radial direction of
LHD in a few hundred milliseconds. Next scanning starts
in a time interval of a few seconds. Then, scanning isavailable approximately four times in ten seconds. It isalso the advantage of the assembly to permits adjustable
acceptance of the sight line and scanning time. As a result,the spatial resolution is approximately 20 millimeters on
the central cord.
The observed x-ray spectrum must be inverted to a
local emissivity profile. The x-ray intensity obtained withthe assembly is line integrated. The calculation process of
the inversion is qualitatively reported with thearrangement of the assembly. [2]3. Experimental results
Examples of x-ray spectra obtained in the present
experiment are shown in Fig.1. Taking into account thetransmission rate of a beryllium filter with a thickness ofProceedings of ITC/ISHW2007
1 mm, the spectrum has to be modified. Then, the real
intensity is stronger in the energy range below 4.0 keV.
The spectra consist of emissions from two different
origins. One is the continuum emission from electrons as
bremssstrahlung. The other is the Kα lines emitted frommetallic impurities. The radiation loss due to metallic
impurities is comparable with that of the continuum in theenergy range above 4.0 keV as is shown in the figure.
Accordingly, the intensity of the Kα lines must be
estimated by subtracting the continuum. The intensities ofthe spectra are strongly dependent on the electrontemperature. Particularly, it is available to estimate theelectron temperature from the continuum. In addition to
the continuum it is also confirmed that Kα line emitted
from iron at a radius of ρ = 0.5 shifts toward lower energy
side than that of ρ = 0.2. This fact is one of significant
results in order to obtain the density profile of respective
charge state of metallic impurity, since lower charge stateof ion emits lower energy photon.

Fig.2 Typical radial profiles of Kα line emitted frommetallic impurities of titanium, chromium, and iron,respectively. The intensities are line integrated along thesight line of the detector. Filled circles, filled rectangular,and filled diamond represent the line intensity of iron,chromium, and titanium, respectively. Solid lines are
fitting functions for the observed profiles. Each function
is a single Gaussian. Open circles with error bars
represent electron temperature estimated from continuum.
The accumulation time of x-ray for each point is 240
milliseconds.

4. Results of analysis and discussions

Fig.1 Typical x-ray energy spectra inverted from theradial profile of the spectrum which has been observedwith the assembly in a single discharge. Solid and brokenlines represent the spectra locally emitted from the radiusof ρ = 0.2 and ρ = 0.5, respectively. The measurement has
been carried out in hydrogen and helium discharges
heated by NBI. Kα emissions from titanium, chromium
and iron appear at 4.7 keV, 5.6 keV and 6.6 keV,respectively. The dotted line represents the transmission
rate of the beryllium filter. The sign of ‘ρ’ represents thenormalized radius of the LHD plasma.
Figure 2 shows typical profiles observed with theassembly. The radial profile of electron temperature
estimated from continuum is also indicated. At present
the flat top duration of the discharge is approximately 8sec. Especially, the electron temperature, density, and theintensity of x-ray have been maintained to be constantduring the accumulation time of the PHA. Consequently,
the intensities of Kα lines reflect only the amount of theimpurities.

Figure 3 shows the intensity of the iron Kα lines
emitted from two different positions in LHD plasma. In
the figure results from a cord calculation is also indicatedas vertical lines. The energy of line is depending oncharge state as is indicated in the figure. Accordingly, theKα line spectrum reflects the amount of respective chargestate. In addition it is shown in the figure that theemission is mainly contributed from helium, lithium,beryllium, and boron like ions. In the present experiment
the electron temperature is approximately 2 keV at theplasma center. Then, it is suggested from the calculationand the experimental results that there is much lower
contribution from hydrogen like ion than helium like ion.
In the calculation the diffusion coefficient is assumed tobe 0.2 m2/s. This assumption is consistent with the result
from an argon transport experiment in LHD. [2]
It is obtained with the assembly that the intensity of theKα line shifts toward lower energy side as the normalizedminor radius increases as is shown in Fig.3. This fact
qualitatively reflects that Kα lines from lower charge
states increase at higher radius, since the electron
temperature decreases. The shift is able to be qualitatively
explained using Fig.4. In the figure the most intense line
is emitted from lithium like ion in the region within ρ =0.6. However, the intensity of lower charge state thanlithium like ion rapidly increase in the region higher than
ρ = 0.5. On the contrary, helium like ion decreases.

Proceedings of ITC/ISHW2007
Consequently, it is qualitatively consistent with theexperimental result shown in Fig.3 that the cord
calculation predicts the spectrum to shift toward lower
energy side.

Fig.3 Energy shift of iron Kα line. The spectra are
obtained from the inversion process of the experimental
result and the subtraction of continuum. The spectra arealso modified in consideration with the reduction of theberyllium filter. The solid line and the gray line represent
the spectra emitted from the radius of ρ = 0.2 and theradius of ρ = 0.5, respectively. The solid vertical line
means the result from the cord calculation.

Fig.4 The radial profiles of iron Kα line derived fromthe cord calculation. Each line represents the emission
from respective charge state. The lines emitted fromlower charge state than carbon is too low to indicate.
The present experimental result on the energy shift
gives rise to important information concerning the density
profile of respective charge state of the metallic impurity.
It is also the important advantage of the assembly tomeasure the photon number in addition to the energy
resolution. Especially, the assembly makes it possible thatthe local intensity of the spectrum is obtained from theline integrated spectrum by the inversion process.
Accordingly the absolute density profile of respective
charge state is able to be quantitatively investigated to fit

the spectrum by the cord calculation. Figure 5 shows theobserved line spectra fitted by calculated functions in thecase of ρ = 0.2. The calculation is consistent with theobserved Kα lines emitted from iron, chromium, andtitanium.

Fig.5 Typical Kα line spectrum obtained from theinversion process and the subtraction of the continuum at
the radius of ρ = 0.2. The reduction of the intensity due tothe filter is also taken into account. Filled circles andsolid line represent the observed spectrum and fitting
functions obtained by the cord calculation inconsideration with the energy resolution of the PHA. The
vertical lines also mean the results of the cord calculation
without the energy resolution.

Fig.6 The radial profiles of Kα lines emitted fromtitanium, chromium, and iron. Filled circles, filled
rectangular, and filled diamond represent the line
intensity of iron, chromium, and titanium, respectively.
Solid line, broken line, and dotted line are fitting
functions for the observed profiles of iron, chromium,and titanium, respectively.
In the case of chromium and titanium the intensity
profile of the Kα line is different from that of iron. The
intensity of Kα line emitted from higher charge stateincreases and that from lower charge state decreases asthe atomic number decreases. Particularly, the intensity ofProceedings of ITC/ISHW2007
Kα line emitted from beryllium like ion is much lower
than that from lithium and helium like ion in the case oftitanium. The spectrum of titanium consists of only theemission from lithium and helium like ion as is indicatedby vertical lines in Fig.5. Although the energy differenceof the emission from respective charge state is muchsmaller than the energy resolution of the PHA, the line
width of the observed spectrum becomes gradually wider
as the atomic number increases.
Radial profiles of Kα lines are also fitted. Figure 6shows the observed radial profile of Kα lines andcalculated functions. The intensity is obtained from theline spectrum by energy integration. In the case ofchromium and titanium, the fitting is consistent withinerror bars. However, the observed intensity of irondecreases more rapidly than the result on the calculationin the region higher than ρ = 0.3. The assumption oftemperature profile in the cord calculation seems to beslightly different from the actual profile. The intensity issensitively depending on the profile of the electron
temperature.

Figure 7 shows the density profile of titanium which isestimated by the fittings of spectrum and intensity profile.
In the present experiment the amounts of bear andhydrogen like ion are much lower than helium like ion,
although the atomic number of titanium is lowest in theobserved impurities. It is commonly shown in Fig.7-9
that the helium like ion is dominant around the plasma
center.
The concentrations of titanium, chromium, and iron areestimated to be 9.7×10-4 %, 2.1×10-3 %, and 5.9×10-3 %at the plasma center, while the electron density is
3.6×1013 cm-3. Then, the densities of the metallic
impurities are much lower than that of the electron. It isdue to the difference of the x-ray photo-emission
efficiency between the bremssstrahlung and the Kα linesthat the intensities are comparable in the range higher
than 4.0 keV. The efficiency of the Kα line isapproximately four orders higher than that of the electronin x-ray region.

spectively.

Fig.9 The radial density profile of respective charge
state of iron.
Fig.7 The radial density profile of respective charge
state of titanium. Only the charge states mainly
contributing to the spectrum are shown.

5. Summary
Development of an assembly equipped with a radial
scanning system attained a remarkable progress on
measuring radial profile of x-ray spectrum in connection
with conventional utilization of PHA. The line spectra
have been successfully observed with good resolution
enough to analyze the energy shift of the K α line. From
the analysis the radial density profile of respective charge
state is estimated in the case of titanium, chromium, andiron, respectively.

References[1] S.Muto et al., Rev.Sci.Instrum. 74, 1993(2003).
[2] S.Muto et al.,J.Plasma Fus.Res.SERIES, to be published.
[3] S.Muto et al., Rev.Sci.Instrum. 72, 1206 (2001).

Fig.8 The radial density profile of respective charge
state of chromium.