J. Plasma Fusion Res. SERIES, Vol.7 (2006) 81-84

Soft X-Ray Measurement of the TPE-RX Reversed Field Pinch
Plasma Using High Spectral Resolution TES Microcalorimeter
SHINOZAKI Keisuke, HOSHINO Akio, ISHISAKI Yoshitaka, MORITA Umeyo,
OHASHI Takaya, MIHARA Tatehiro1 , MITSUDA Kazuhisa2 , TANAKA Keiichi3 ,YAGI Yasuyuki4 , KOGUCHI Haruhisa4 , HIRANO Yoichi4 and SAKAKITA Hajime4
Department of Physics, Tokyo Metropolitan University (TMU), Tokyo 192-0397, Japan
1 Cosmic Radiation, the Institute of Physical and Chemical Research, Wako 351-0198, Japan
2 Institute of Space and Astronautical Science (ISAS/JAXA), Sagamihara 229-8510, Japan
3 SII Nano-Technology Inc. (SIINT), Matsudo 270-2222, Japan
4 National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8568, Japan（Received: 5 October 2004 / Accepted: 10 January 2006）

Abstract
We report the first result of soft X-ray spectroscopy for the Reversed Field Pinch (RFP) plasma in TPERX using a high spectral resolution (FWHM ∼ 50 eV) superconductive transition edge sensor (TES) microcalorimeter. Total 3472 count of X-ray signals were detected in 0.2–3.0 keV for 210 plasma shots during
the flat-top phase (35–70 ms) after the pile-up rejection. Combined with the 1.3–8 keV range data with aSiLi detector, we examined the wide band X-ray spectrum of the RFP plasma. The obtained spectrum isdominated by thermal plasma emission in the soft X-ray band ( 5 keV), although at least four diﬀerent
temperature component is required to account for the whole band spectral shape. The average temperature isconsistent with the value by the ruby laser Thomson scattering method. The spectrum also indicates that 85%of the flux in 0.7–1.2 keV is dominated by the Fe-L complex with variously ionized states, and that the ironabundance to deuterium is comparable to the value derived for carbon and oxygen with a VUV spectrometer.
Keywords:
reversed field pinch, TPE-RX, X-ray, TES, microcalorimeter
1．Introduction
In the soft X-ray energy range of 0.2–8 keV, there
exist K and L emission lines and absorption edges fromvarious elements, e.g., C, O, Cr, and Fe, which contain much information on the physical state of the Xray emitting plasma (e.g., temperature, density), element abundance, and motion of ions. A typical energy
diﬀerence between the fine structures of these lines is
5–20 eV, hence a spectrometer with an energy resolution of a few eV is strongly desired to resolve them.
For practical applications, it is also required to sustain
a high count rate ( 1 kc/s). Recently, a transition edge
sensor (hereafter TES) was proposed as an extremely
sensitive thermometer for X-ray microcalorimeters [1].
TES microcalorimeters also have a merit that the response time is significantly improved due to a strong
electro-thermal feedback [2]. We are now developing aTES microcalorimeter array for future Japanese X-ray
astronomy missions [4,8]. To date, an energy resolution of ∼ 6 eV in the energy range of  10 keV has beenachieved with a single pixel device in our laboratory [3].

The TES microcalorimeter is a detector which measuresenergy of an incident X-ray photon as a temperaturerise using the sharp transition edge of superconductors.
To give its maximum performance, the detector must
be cooled below ∼ 100 mK by an adiabatic demagnetization refrigerator (ADR). We have also developed aportable ADR system for ground experiments, which isbased on the system originally designed for the rocket
experiment [6]. Temperature stability of  10 µK andholding time of ∼ 24 h at 125 mK are attained so far.
This paper presents the first result of a new collaboration between the astrophysics and the fusion plasma
fields. The deuterium plasma radiation and impurity
line emissions from a large RFP device, the TPE-RX
experiment [12,11], are investigated using the TES microcalorimeter installed in the ADR, which is directly
connected to TPE-RX with a vacuum duct. TPE-RX is
one of the three largest RFP machines in the world withmajor radius R = 1.72 m and minor radius a = 0.45 m.
C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchCorresponding author’s e-mail: shino@phys.metro-u.ac.jp

81Soft X-Ray Measurement of the TPE-RX Reversed Field Pinch Plasma Using High Spectral Resolution TES Microcalorimeter

denum limiters (98.5 mmφ) are attached for the purposeof protection against the strong heat load. Total number of limiters is 244, and 34 are installed in the port
section.
The measurement was conducted on 16–18 Aug
2004 at AIST and 210 plasma shots were obtained witha suﬃcient quality for the analysis. Each shot was basically generated in every 5 min interval, which is determined by the TPE-RX specifications. The deuteriumgas pressure was kept constant at 0.4 mTorr during thedischarge, and the plasma current I p was ∼ 220 kA during the flat-top phase (Fig. 2). Typical plasma parameters on this condition is that electron density n el =5 × 1018 m−3 , electron temperature T el = 600 eV, ion
temperature T i = 300 eV, and the plasma beta βp = 0.07
[11]. The ADR was cooled down to 125 mK and theTES was operated at 58 mΩ with the bias voltage of
2.4 µV. In fact, the operating temperature and resistance are slightly higher than the optimal point, because
we found a significant increase of low-frequency noise
when the TES current was increased. In order to calibrate the detector gain continuously, a 55 Fe radioactive isotope was installed inside the ADR at an adequate position not obstructing the line of sight but the5.9 keV (Mn-Kα) X-rays from the isotope coming into
the detector. The count rate was about 0.09 c/s, whichis enough to monitor the gain, while the probability thatX-rays from the isotope came into the detector during
the plasma shot (< 100 ms) is negligible. From the analysis of the calibration isotope data, it is demonstrated
that the energy resolution (FWHM) was 19.2±0.8 eV at
5.9 keV and 14.6 ± 0.3 eV at 0 keV. However, it is foundthat the energy resolution was degraded to ∼ 50 eV during the plasma shots due to the noise increase probably
aﬀected by the changes of the magnetic field generated
by the plasma current.

The primary purpose of the experiment is resolving
characteristic X-ray lines of impurities (e.g., O, Cr, Fe,Mo) in 0.2–8 keV to evaluate the contribution of line
intensities to the continuum spectrum.
2．Experimental setup
The TES is made of a thin bilayer film of titanium(40 nm thick) and gold (110 nm) with 0.5 mm × 0.5 mm
size, which is suspended by a silicon-nitride bridge(1 µm thick 700 µm wide) as a weak thermal link tothe silicon substrate. An X-ray absorber made of gold
(0.3 mm × 0.3 mm wide, 300 nm thick) is depositedon the TES, and a sapphire collimator with 0.2 mmφ
300 µm thick is attached in front of the absorber. Detailed design, fabrication and its best performance aredescribed in [3,10]. The TES microcalorimeter was installed in the ADR which was horizontally connected
to No.15 port section of TPE-RX with a vacuum ductas shown in Fig. 1. The distance between the inner vessel and the detector surface was 2.40 m. We set sevenorifices (32 mmφ, 3 mm thick), to avoid stray X-ray detection by the reflection inside the duct. An adjustable
slit is placed at the port section to regulate the X-rayflux, which was fixed to 5 mm width 0.5 mm height
during the measurement. There is a set of five aluminized myler films (Al : 37 nm, C10 H8 O4 : 540 nm) inseries inside the ADR for thermal and optical shielding
to block infrared to ultraviolet light. The detector eﬃciency, including the entrance window transmission andthe TES microcalorimeter quantum eﬃciency, is indicated in Fig. 3 (d). The TPE-RX vacuum vessel is madeof stainless steel SUS316L (Fe: 66 %, Cr: 17 %, Ni:14 %). Inside the vessel, many mushroom-type molyb-

3．Data analysis and results
Figure 2 shows the typical waveform of the TES
microcalorimeter output obtained by a digital oscilloscope (2 MHz sample rate, 13 bit resolution) during aplasma shot. The waveform acquisition is triggered by
the plasma current generation at t = 0 of the x-axis. The
X-ray pulses from the detector have a shape of exponential decay ∝ exp (−t/τe f f ), in which τe f f  200 µsthroughout our measurement. When the gate valve wasopened, soft X-ray signals were detected around 20–
80 ms, while there were not when closed. We accumulate the X-ray pulses in the time duration of 35–
70 ms corresponding the flat-top of I p to make an energy spectrum. The energy of each pulse is determinedby the optimal filtering method commonly used for theFig. 1 The experimental setup between the ADR andTPE-RX.

200TES Output (mV)

300(b)

250

100

20050

150

0100-50

50-1000-150-200-50-50-25

0255075

100

125

Plasma Current (kA)

150

-100time (ms)Fig. 2 A typical waveform during a plasma shot (black)and the plasma current (gray).
82

Soft X-Ray Measurement of the TPE-RX Reversed Field Pinch Plasma Using High Spectral Resolution TES Microcalorimeter

(b)

(a)
O-Kedge
CKedge

Fe-L

Mo-L

Fe-K
Cr-K

(d)

(c)

TES

SiLi

Fig. 3 (a) The TES (thick crosses) and SiLi (thin crosses) spectra ﬁtted with the best ﬁt model. (b) Same as (a), but without
impurities lines. (c) The best ﬁt model and each component. (d) Detector efﬁciencies of the TES microcalorimeterand SiLi including the window transmission and the detector quantum efﬁciency.

microcalorimeter pulse analysis [9]. The average andstandard deviation of the detected count per shot are
23 ± 5 c/shot, corresponding to ∼ 660 c/s. The pileup events are rejected using the time interval between
the two sequent events to be greater than 200 µs. We
also rejected pulses whose shape deviates significantly
from the pulse template. As a result, 3472 count per
210 shots (7.35 s exposure) remains in the energy rangeof 0.2–3 keV, after rejecting 948 pulses. The obtainedenergy spectrum is indicated in Fig. 3 (a). In the spectral analysis, we also utilize a spectrum which was obtained with a SiLi detector at the same port with similarparameters of TPE-RX in October 2003. The SiLi detector has 4 mmφ aperture with 4.48 mm thick and theenergy resolution is ∼ 200 eV at 5.9 keV. A beryllium
window with 87.7 µm thick was placed in front of thedetector to constrain signals in hard band (see Fig. 3d).
There are 4513 count per 16 shots (0.56 s) in 1.3–8 keV.
These spectra are examined using a spectral fitting
package XSPEC v11.3 (http://xspec.gsfc.nasa.gov/),
which is commonly used in the X-ray astronomy
field. A thermal equilibrium plasma emission model,
MEKAL [7], is used to account for the incident X-ray
spectrum. Figure 3 (c) represents the best fit model,
which is overlaid on Fig. 3 (a) after convolving the detector response. At least 4 thermal plasma + 1 powerlaw + 3 gaussian components are required to obtain areasonable fit with χ2 /d = 203.0/151 (d is a degree offreedom). Emissions from impurities in plasma are considered in the MEKAL model for C, O, Fe, and Ni, andthe abundance ratio of Fe/Ni is fixed for the SUS316L

value. The TES microcalorimeter and the SiLi detector
spectra are fitted simultaneously with the same model
parameters except for the overall normalization. Number of fitting parameters are 23 in total. The best fit
parameters are summarized in Table 1.
4．Discussion
The obtained X-ray spectra require at least four temperature component, ∼ 78, 350, 814, and 900 eV, plus
power-law with photon index Γ ∼ 2.4. The non-thermal
power-law component is only visible at hard band (5 keV), and the soft X-ray band is dominated by thethermal plasma emission. There are little diﬀerence between the spectrum in the 35–50 ms duration and that in
50–70 ms, although the latter is slightly harder than theformer. It is suggested that there would be temperature
gradient from the plasma center to the edge in the lineof sight. If we ignore the 78 eV component, the averagetemperature of the plasma is 550 eV, weighted with theMEKAL model normalization ∝ nel ni dV. This temperature is consistent with the value (∼ 600 eV) by theruby laser Thomson scattering method.
Although three lines are detected in the SiLi spectrum, their line center energies are consistent withneutral Mo-L (2293 eV), Cr-K (5415 eV), and Fe-K(6404 eV), respectively. Their origins are thought to befluorescence X-rays from the molybdenum limiters andthe SUS316L vessel hitted by high energy electrons. On
the other hand, 85 % of the flux in 0.7–1.2 keV is dominated by Fe-L complex, and probably Cr-L, at variously
ionized states (Fig. 3 (b)). This is a clear evidence that83

Soft X-Ray Measurement of the TPE-RX Reversed Field Pinch Plasma Using High Spectral Resolution TES Microcalorimeter

Table 1 List of the best ﬁt parameters

MEKAL model: FX = 8.79 erg cm−2 s−1 (0.2–10 keV)
kT 1 = 78+10norm1 = 75000+18000
FX = 8.364−10 eV,−14000 ,+33
norm2 = 477+105,
FkT 2 = 350−48 eV,
X = 0.22−126+124+35
kT 3 = 814−78 eV, norm3 = 85−84 ,FX = 0.06+151+47
FX = 0.15
kT 4 = 900−820 eV, norm4 = 209−55 ,+23−5−5[C/D] = 16−7 × 10 , [O/D] < 1.2 × 10−5[Fe/D] = 1.12+0.35−0.20 × 10

power-law model:+3.07Γ† = 2.35+0.14−0.10 , norm = 2.73−2.18

FX = 0.016

gaussian model:
E1 = 2309+15−16 eV,
E2 = 5444+71−63 eV,E3 = 6386+45−42 eV,FX = 0.005
S 1‡ = 0.98+0.20−0.21
S 2 = 0.049+0.020−0.009S 3 = 0.053+0.015−0.014σ1 = 105+21−18 eV,
σ2 = 165+163−82 eV,σ3 = 124+76−64 eV,†

Spectral photon index.

‡photons cm−2 s−1 .
(Errors are 90% confidence level)

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the iron ions exist in the plasma as an impurity, and thatthey are collisionally interacting with thermal electrons.
The iron abundance to deuterium ions is evaluated to be[Fe/D] = 1.1 × 10−5 , which is comparable to [C/D] or[O/D] measured in the VUV band [5]. It is anticipatedthat high Z impurities enter the plasma due to the sputtering caused by the plasma-wall interaction.
Actually, Cr-L line data are not included in theMEKAL model, which is partly responsible for the fit
residuals around the Fe-L. It also need attention that
the MEKAL model assumes the collisional equilibrium,
whereas it is marginal for TPE-RX. As shown in Table
1, [C/D] and [O/D] are constrained from the X-ray spectrum, too. However, their abundances are determined in
the energy range where our device has very little detective eﬃciency, hence the systematic error is probably significant. It is essential to replace the X-ray window with C- or O-free material, such as thin beryllium(∼ 1 µm).
We thank Prof. Kuni Masai for useful discussions
and comments.

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