TEMPORAL STUDY OF THE CURRENT COLLECTED BY A LANGMUIR PROBE BIASED TOHIGH VOLTAGES IN THE WAKE BEHIND A CONDUCTING BODY

The temporal evolution of the voltage-current characteristic of a sphe rical Langmuir probe in the near-wake region of a conducting body is i nvestigated in a pulsed plasma device. Measurements of the current col lected by a sphere placed on axis behind a conducting disk are present ed as a function of the bias voltage and time. The bias voltage on the collecting sphere is large enough, ranging from +500 to -3000 V, to a ttract ions or electrons across the wake region. In addition, plasma p otential profiles as a function of sphere bias voltage and time throug hout the nearwake and midwake region are presented. It is found that f or a negative sphere bias a threshold voltage exists for current colle ction. This threshold voltage decreases during the formation of the wa ke behind the conducting disk. Potential measurements indicate that th e sheath region around the negatively biased sphere contracts as the w ake region establishes itself. This contraction of the sheath region i ncreases the electric field attracting ions to the sphere and decrease s the threshold voltage for current collection. For positive sphere bi as voltage the current collected by the sphere does not exhibit a thre shold voltage but is monotonically increasing.