Temporal Evolution Of The Current-voltage Characteristic of a Probe in the Wake of an Object Immersed in a Pulsed Flowing Plasma

Measurements of the current-voltage characteristics of a Langmuir probe in the near wake of a disk immersed in a pulsed flowing plasma have been made. A 1 cm diameter biasable sphere was placed 10 cm downstream of a 10 cm diameter disk immersed in a Mach 6 pulsed flowing plasma. The sphere was situated as to be in the ion-free near wake region of the flowing plasma stream when the sphere was unbiased. The current-voltage characteristic of the sphere was observed as a function of time as the sphere bias was scanned from -5000 V to +1000 V. The collected current is found to be monotonically increasing with increasing positive bias voltage but exhibits a threshold voltage for current collection as the bias voltage becomes more negative. The plasma potential throughout the wake region was measured with the aid of an emissive probe. Potential measurements were made for a sphere bias voltages below, at, and above the current collection threshold for a number of times during the wake formation period. Predictions from the particle trajectory code SIMION are compared with data, showing excellent agreement in the prediction of the current collection threshold.