Characterization of an Atmospheric-Pressure Helium Plasma Generated by 2.45-GHz Microwave Power

An atmospheric-pressure helium plasma generated in a 2.45-GHz microwave-induced cylindrical plasma torch has been investigated. The atmospheric-pressure helium plasma can be ignited at a microwave power of less than 400 W and sustained at as low as 100 W, at a gas flow of 50 L/min. The electron temperature T
_e
, electron density n
_e
, and gas temperature T
_g
measured by optical emission spectroscopy are 0.28-0.5 eV, 10
¹⁵
-10
¹⁶
cm
^-3
, and 1000-2000 K, respectively, at a power of 200-400 W and a gas flow rate of 20-50 L/min. In addition, it is noted that T
_e
≈ T
_v
(vibrational temperature) >; T
_g
≈ T
_r
(rotational temperature) for these experiment parameters. These results show that the helium plasma generated in this atmosphericpressure plasma torch is in partial local thermodynamic equilibrium (pLTE).