Time-dependent plasma temperature measurements of the high-pressure sodium arc

The plasma temperature of the high-pressure sodium arc has been measured as a function of time and power input on 60-Hz sine-wave operation. A maximum temperature of 4370 °K is found at power input above about 100 W/cm3 of arc tube volume. At zero current the temperature falls to 3300 ° K. Square-wave operation indicates that temperature decay to this value occurs in 0.3 msec. Since local thermodynamic equilibrium (LTE) exists in the hot central core of the plasma, the electron density can be calculated using the Saha equation to decay from 1016 to 1015 cm−3 during this period. This decay in electron density is accompanied by an increase of arc reignition voltage of more than a factor of 4.