Comparison of Pyrex plate and spectroscopic temperature measurements for gas temperature monitoring in a nitrogen afterglow

The temperature time evolution of a Pyrex plate is determined by measurements of its reflectance when placed in the early afterglow of a microwave discharge in flowing nitrogen. For comparison, simultaneous monitoring by optical emission spectroscopy is performed both in the discharge and the so-called short-lived afterglow. After t 2 min, the spectroscopic and plate temperatures are shown to be very close and can then be assimilated to the gas temperature. To account for the temporal evolution of the plate temperature, a global thermal diagnostic is made. The system, consisting of the gas, the plate and its holder, is analysed through a thermal balance taking conductive, radiative and convective effects and plasma de-excitation into account. The calculated plate temperature is deduced numerically from the energy equation and compared to the measured one. A fairly good agreement is achieved, showing a reasonable appreciation of the heat exchanges governing the plate temperature evolution.