A high dispersion and high time-resolution spectrometer equipped with two slits in output ports is developed to observe variation of Doppler broadening in the high-temperature bubbles [K. Terasaka, et. al., Phys. Plasmas 25, 052113 (2018)]. Reduction of Doppler broadening in a line spectrum of helium ion is experimentally observed when the bubbles emerge in the viewing chord of spectrometer. The results suggest possibilities of ion temperature reduction during the high-temperature bubbles.
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