We have recently developed an X-ray spectrometer with a high resolution (14 eV) for the reversed
field pinch (RFP) plasma in TPE-RX using a superconductive TES (transition edge sensor) X-ray
microcalorimeter. TPE-RX is one of the three largest RFP machines in the world with major radius
R=1.72m and minor radius α=0.45m, which can produce the deuterium plasma of electron/ion
temperature kT=1.0keV and electron density n=5×10¹⁹ m^-3 with flat-top duration time τ=50ms. The primary purpose of the experiment is resolving characteristic X-ray
lines of impurities (e.g., O, Cr, Fe, Mo) in the 0.5--8 keV range in order to evaluate the
contribution of the line intensities to the continuum spectrum. By subtracting these contribution,
we can obtain the continuum spectrum, then electron temperarure of the RFP plasma more precisely.
It is also possible to diagnose impurity abundance and the ionization status of O, which is
considered to reach marginally equilibrium. The TES microcarorimeter is a detector which measures
the energy of an incident X-ray photon as a temperature rise using the sharp transition edge of
superconductors. To give its maximum performance, the detector is cooled below 100mK by an
adiabatic demagnetization refrigerator (ADR)[1], and the energy resolution of 14eV at 8keV has been attained at the laboratory. The TES detector has been originally developed for observing cosmic X-rays to be installed in a man-made satellite. The paper presents the results of a new collaboration between the space and the fusion plasma fields. In this paper, we present the detector performance, experimental setup, and the first result of the measurement.

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