We adopted saturation spectroscopy for observing a Doppler-free spectrum of the Balmer-α line of atomic hydrogen in a linear magnetized plasma source. The spectrum was composed of a broadband offset and many peaks which were assigned as fine-structure components of the Balmer-α line with Zeeman splitting. We examined the amplitudes of the offset and line components as functions of the pump laser power and the discharge gas pressure. The saturation parameter or the amplitude of the saturation spectrum was discussed by referring to the theory of saturation spectroscopy. In addition, the physical origin of the broadband offset component was speculated on the basis of the pressure dependence of the saturation spectrum.

We observed optical emission of molecular hydrogen in a recombining hydrogen plasma with an electron temperature of 0.1 eV and an electron density of 3 × 10 12 cm −3 . The optical emission intensities of molecular hydrogen in the recombining plasma were roughly 10%-45% of those in an ionizing plasma with an electron temperature of 4 eV. The ratio was greater for a transition line originated from an excited state with a larger vibrational quantum number. Because of the low electron temperature of 0.1 eV, the production processes of excited states are not considered electron impact excitation in the recombining plasma. Possible recombination processes are discussed which produce excited states of molecular hydrogen in the recombining plasma.

Optical emission spectra in the wavelength range of molecular hydrogen were observed in both ionizing and recombining plasma modes of hydrogen discharges. The optical emission spectrum from the ionizing plasma was dominated by the Fulcher-α system (a 3 Σ + g − d 3 Π − u ) of molecular hydrogen. On the other hand, the optical emission spectrum from the recombining plasma was composed of many lines, and was completely different from the spectrum of the ionizing plasma. The many peaks observed from the recombining plasma were assigned to the Recombining plasmas are important in nuclear fusion research since they are closely related to plasma detachment, which is considered to be a promising approach for avoiding excess heat flux to the divertor plate in a nuclear fusion machine [1]. It is well known that in a recombining plasma, highly excited states are populated significantly via three-body recombination, and the optical emission spectrum is considerably different from that of an ionizing plasma [2]. In the case of a recombining hydrogen plasma, the optical emission spectrum of atomic hydrogen has been investigated well, however, the optical emission spectrum in the wavelength region of molecular hydrogen has not been investigated thoroughly. Therefore, in this study, we examine the optical emission spectrum in the wavelength region of molecular hydrogen in a recombining hydrogen plasma.The plasma source [3] was a linear machine with a uniform magnetic field of 350 G along the cylindrical axis. An rf power source at 13.56 MHz was connected to a helical antenna wound around a glass tube with an inner diameter of 1.6 cm. The glass tube was attached to a stainless steel vacuum chamber. A plasma column with the diameter same as the glass tube was confined radially by the external magnetic field. We used pure hydrogen for discharge in this experiment. The plasma was produced in a pulsed mode with a repetition frequency of 10 Hz and a discharge duration of 4 ms to avoid over-heating of the author's e-mail: sasaki@nuee.nagoya-u.ac.jp machine. The diagnostic method was conventional optical emission spectroscopy, which was performed in the downstream region at a distance of 22 cm from the helical antenna using a spectrograph with a focal length of 50 cm. The spectrum was recorded using a charge-coupled device camera with a gated image intensifier (ICCD camera). The gate of the ICCD camera was opened from 3.95 to 3.99 ms after the initiation of the pulsed discharge.Two types of discharge were observed when the gas pressure was higher than 30 mTorr. Low rf power resulted in a low-density mode discharge with an electron density of the order of 10 11 cm −3 . A high-density mode discharge with an electron density of the order of 10 12 cm −3 was obtained at rf powers higher than ∼1.5 kW [3]. On the other hand, when the gas pressure was lower than 30 mTorr, a low-density mode discharge was obtained even at a high rf power of 3 kW. In the high-density mode discharges at gas pressures lower than 50 mTorr and all the low-density mo...