Soft x-ray measurement of the toroidal pinch experiment RX reversed field pinch plasma using transition edge sensor calorimeter

A superconductive transition edge sensor (TES) calorimeter is for the first time applied for the diagnostics of the reversed field pinch plasma produced in the toroidal pinch experiment RX (TPE-RX), and the instrumental system is fully described. The first result from the soft x-ray spectroscopy in 0.2-3 keV with an energy resolution similar to 50 eV are also presented. The TES calorimeter is made of a thin bilayer film of titanium and gold with a transition temperature of 151 mK and its best energy resolution at our laboratory is 6.4 eV, while it was significantly degraded by about a factor of eight during the plasma operation. The TES microcalorimeter was installed in a portable adiabatic demagnetization refrigerator (ADR), which is originally designed for a rocket experiment. The detector box is carefully designed to shield the strong magnetic field produced by the ADR and TPE-RX. The ADR was directly connected to TPE-RX with a vacuum duct in the sideway configuration, and cooled down to 125 mK stabilized with an accuracy of 10 mu K rms using an improved proportional, integral, and derivative (PID) control method. Thin aluminized Toray Lumirror or Parylene-N films were used for the IR to UV blocking filters of the incident x-ray window to allow soft x-rays coming into the detector with good efficiency. TPE-RX was operated with the plasma current of I-p=220 kA, and the wave forms of the TES output for every plasma shot lasting similar to 80 ms were obtained with a digital oscilloscope. The wave forms were analyzed with the optimal filtering method, and x-ray signals were extracted. A total of 3472 counts of x-ray signals were detected for 210 plasma shots during the flat-top phase of t=35-70 ms. Combined with the data measured with a lithium drifted silicon detector in the 1.3-8 keV range, spectral features are investigated using a spectral fitting package XSPEC. The obtained spectrum is well explained by thermal plasma emission, although an impurity iron-L line emissions at variously ionized states are dominant around 0.7-1.2 keV. At least three different temperature components ranging T=350-900 eV are required to account for the spectral shape, while the average temperature is consistent with the ruby laser Thomson scattering measurement. (c) 2006 American Institute of Physics.