Observing the temperature curve of Fig.3.3, one can easily notice that
heat shield temperature steadily drops as tungsten increases in diameter
except for g1mm_d1mm (meaning graphite thickness of 1mm and tungsten
diameter of 1mm) curve. This is because the 1mm cross sectional area of
tungsten is too small for the current flow to be effective on the heat
shield. This causes thermal stress along the tungsten rod (1865K at the
tip but just 1000K contacting the graphite). Hence, the current flowing
through the heat shield is limited by 1mm tungsten diameter. However,
both ‘g1mm_d1mm’ and ‘g1mm_d2mm’ curves give stagnation values of
1030K and 1240K respectively.