The parameters of radio frequency helium discharge under atmospheric pressure were studied by electrical and optical measurements using high voltage probe, current probe and optical emission spectroscopy. Two discharge modes $alpha$ and $gamma$ were observed within certain limits. During $alpha$ to $gamma$ mode transition, a decrease in voltage (280a€“168 V), current (2.05a€“1.61 A) and phase angle (76$^{ m o}-56^{ m o}$) occurred. The discharge parameters such as resistance, reactance, sheath thickness, electron density, excitation temperature and gas temperature were assessed by electrical measurements using equivalent circuit model and optical emission spectroscopy. In $alpha$ mode, the discharge current increased from 1.17 to 2.05 A, electron density increased from $0.19 imes 10^{12} { m to} 0.47 imes 10^{12} { m cm}^{a?’3}$ while sheath thickness decreased from 0.40 to 0.25 mm. The excitation temperatures in the $alpha$ and $gamma$ modes were 3266 and 4500 K respectively, evaluated by Boltzmanna€?s plot method. The estimated gas temperature increased from 335 K in the ?± mode to 485 K in the ?3 mode, suggesting that the radio frequency atmospheric pressure helium discharge can be used for surface treatment applications.
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