On the use of Langmuir probes for the diagnosis of atmospheric thermal plasmas

The thermal and fluid dynamic disturbances caused by the insertion of a Langmuir probe into a flowing thermal plasma are examined. Attention is paid to a 5-mm-long, free burning 200-A argon plasma at atmospheric pressure. It has been found that the disturbances caused by the probe are so severe that probe measurements cannot refer directly to an undisturbed plasma. The difficulties in interpreting the probe voltage-current (V-I) characteristics are discussed. The use of a Langmuir probe as a general diagnostic tool for the plasma conditions investigated is not yet a practical proposition.