Neutral gas temperature of adamantane in a radio-frequency inductively coupled plasma electrothermal micro-thruster using optical emission spectroscopy

The gas temperature of adamantane plasma in a 13.56 MHz radio-frequency (RF) inductively coupled plasma was measured using optical emission spectroscopy. Rovibrational band fitting of the Second Positive System of nitrogen gas (N₂) from the addition of N₂ and Swan system of homo-nuclear carbons (C₂) from adamantane dissociation were used to obtain the gas temperature through the rotational temperature under the assumption of rotational-translational equilibrium. The measured temperature ranged from 400 to 700 K, increasing with RF power range from 10 to 100 W and adamantane flow rate from 0.25 to 1 mg s⁻¹, with discharge pressures up to a few Torr. The energy released during adamantane dissociation contributed to a slight temperature increase compared to conventional pure N₂ plasma. Adding a large amount of helium (He) acted as a quencher, reducing the gas temperature. In an adamantane-He mixture, the gas temperature remained stable at mid-range power and at lower adamantane flow rates. Justification of gas temperature and a simple power balance model were described. Applications of adamantane spectroscopy include propulsion, plasma processing, and astrophysics, with the inclusion of future studies.