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 measuremen ts of its reflectance when placed in the early afterglow of a microwave dis charge in flowing nitrogen. For comparison, simultaneous monitoring by opti cal emission spectroscopy is performed both in the discharge and the so-cal led short-lived afterglow. After t congruent to 2 min, the spectroscopic an d 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 o f the gas, the plate and its holder, is analysed through a thermal balance taking conductive, radiative and convective effects and plasma de-excitatio n into account. The calculated plate temperature is deduced numerically fro m the energy equation and compared to the measured one. A fairly good agree ment is achieved, showing a reasonable appreciation of the heat exchanges g overning the plate temperature evolution.