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Thermal Temperature Measurement of a Plasma by Alexandrite Effect Spectropyrometer

The thermal temperature of a 100kW dc plasma torch is measured (Fig. 1). The collimator of the alexandrite effect spectropyrometer is directed toward the nozzle of the plasma torch to measure the thermal temperature of the plasma jet.

Fig.1. The layout of the non-transferred 100 kW dc plasma torch and temperature measurement setup.

Argon was used for the plasma torch, and the power is kept at 20 kW and current at 100 A. The location of collimator, along the axis-direction of the plasma, is 1.5 m away from the torch head. Fig. 2 shows a measurement window of the spectropyrometer of the argon plasma. The temperature (11,329 K) and the emission spectrum are simultaneously displayed. The average measured thermal temperature is 11178 + 382 K for 12 measurements. The spectral power distribution consists of continuum underlying distribution and spectral lines.

Fig.2. A temperature measurement of the plasma torch by the alexandrite effect spectropyrometer.

The spectropyrometer has a spectral correction function, which can be used to correct the spectral power distribution by deleting the spectral lines in Fig. 3. The corrected spectral power distribution of the plasma is shown in Fig. 6, and the thermal temperature is 9949K.

Fig. 3. The corrected spectral power distribution of the plasma jet.

The very strong spectral lines of the plasma jet can cause an error for directly measuring thermal temperature by the spectropyrometer. However, the measurement error caused by the spectral lines can be corrected using the spectral correction function of the spectropyrometer. The thermal temperature calculated from the corrected continuum underlying spectral power distribution of the plasma jet is 9949K with an estimated standard deviation of about + 340K (see Fig. 3).

Fig. 4 shows the further corrected spectral power distribution of the plasma jet according to that of the relative spectral power distribution of a blackbody. The calculated thermal temperature changes to 10106 K + 345K, which is slightly higher than the calculated thermal temperature of 9949K + 340K in Fig. 6. The thermal temperature calculated from the further corrected spectral power distribution in Fig. 7 is the true thermal temperature of the measured plasma jet, and it is more accurate than that calculated from the corrected spectral power distribution in Fig. 3. However, the difference between the two corrected thermal temperatures is about 1.5%,   which is not significant for the thermal temperature measurement of the plasma jet.

Fig. 4. Corrected spectral power distribution according to that of thermal radiator.

            In fact, the stronger the spectral lines, the less accurate the directly measured thermal temperature by the spectropyrometer. The ratio between the directly measured thermal temperature and the corrected true thermal temperature can be used to indirectly estimate the thermal equilibrium state of the thermal plasma jet. The ratio of the true thermal temperature to the directly measured temperatures is a measure of how well the plasma approximates a blackbody, and the ratio is defined as the blackbody level (BL) of a thermal plasma:

                                                                                                           (1)

where TT is the calculated true thermal temperature (in Kelvin) and MT is the directly measured thermal temperature (in Kelvin). When the BL of a thermal plasma is 1, such as that of the Sun, the thermal plasma is a blackbody in the state of complete thermodynamic equilibrium. When the BL of a thermal radiator is less than 1, it does not reach the complete thermal equilibrium. The smaller the BL is, the less the thermal plasma reaches the complete thermal equilibrium. The BL of the thermal plasma jet is:

            In general, the directly measured thermal temperature is always higher than the true thermal temperature calculated by the corrected spectral power distribution of a thermal plasma, thus the BL is always less than 1.

Link: Peir-Jyh Wang, Chin-Ching Tzeng, and Yan Liu, 2010. Thermal Temperature Measurements of Plasma Torch by Alexandrite Effect Spectropyrometer, www.hindawi.com/journals/aot/2010/656421.html.