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A Penning ion source with self-heated cathodes has been applied in a miniature cyclotron. Optimizing the formation of negative hydrogen ions in the ion source needs the knowledge of plasma parameters. Optical emission spectroscopy is introduced as a noninvasive and in situ diagnostic tool for line-of-sight averaged plasma parameters in this paper. The relative intensities of neutral atomic hydrogen emission spectral lines are used to evaluate the electron temperature of hydrogen plasma. The experimental results show that the relative intensities of Balmer lines (Hα, Hβ, and Hγ) display significant dependence on gas flow rate, not sensible to arc current and magnetic field. The calculated electron temperature is in the range of 4000-7000 K in the experiments. The correlations of electron temperature with arc current, gas flow rate, and magnetic field are discussed, respectively.