The neutral temperature within a cylindrical, inductively coupled plasma source has been studied for rare gas and molecular plasmas using the technique of optical emission thermometry. By adding small quantities of N2 to the gas feeds as an actinometer, the neutral temperature of the discharge can be estimated by simulation and fitting of the rotationally unresolved second positive band (C3Πu–B3Πg). In this work, the neutral temperature was estimated using this technique for flowing discharges of argon, helium, neon, nitrogen, and oxygen as a function of pressure and power. It was found that the neutral temperature for all of the discharges studied increased roughly proportional to the logarithm of the pressure. An increase in neutral temperature was also observed with increases in power; however, the dependence did not follow a simple functional form. The rare gases exhibited temperatures significantly above room temperature under high power (1200 W) and high pressure (∼1 Torr) conditions with argon approaching 2000 K. Molecular discharges such as N2 and O2 exhibited significantly higher temperatures (approaching 2500 K) than the rare gases even though they are expected to have lower plasma densities at the same pressure and power. It is believed that Franck–Condon heating of the gases during electron impact dissociation, vibrational excitation/thermalization, and exothermic wall reactions may all play important roles in producing such elevated temperatures. Simple, zero-dimensional plasma modeling indicates that neutral temperature elevation will result in significant increases in discharge electron temperature and electron-impact reaction rate coefficients under the same operating conditions.

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