Gas heating in low-pressure microwave argon discharges

Both an energy balance equation and a collisional-radiative model were developed in order to discover which process is responsible for gas heating in a low-pressure argon discharge. In this way, for a wide range of plasma conditions, the space-charge field contribution to gas heating was found to be negligible compared to that resulting from elastic collisional processes, although the value of the former is higher than the latter when calculating the absorbed power per electron. This is due to (1) the heating associated with the space-charge field only being effective in the plasma sheath, which is very close to the vessel inner wall. (2) The vessel temperature value at the external wall is taken as a boundary condition, as a result of which the space-charge field influence on gas heating is indirectly imposed on the model. The results of the collisional-radiative model take into account the influence of gas heating on the electron temperature and on the argon low-lying excited levels. Two different zones have been found. The first corresponds to low electron densities, in which the gas temperature remains constant, whereas in the second (high electron densities) the heating of the gas takes on great importance. These results compare well with experimental data.