S. Meassick et C. Chan, TEMPORAL STUDY OF THE CURRENT COLLECTED BY A LANGMUIR PROBE BIASED TOHIGH VOLTAGES IN THE WAKE BEHIND A CONDUCTING BODY, J GEO R-S P, 99(A10), 1994, pp. 19597-19607
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.