This research paper describes the design, setup, and application of a moving multi-Langmuir probe array that measured and mapped ion flux and plasma source efficiency in a flowing plasma with large (∼1.5 T) external magnetic fields in the VASIMR (Variable Specific Impulse Magnetoplasma Rocket) VX-100 experiment. The probe array was swung through an argon plasma revealing a detailed 2-dimensional cross section. Neutral input flow and probe collector bias voltage were scanned from pulse to pulse in an effort to find optimal settings. Secondary electron emission was found to be an important factor that artificially enhanced the measured flux at large negative bias voltages. The most accurate bias voltages were found to be around -20 volts where the flux enhancing effects were minimized. Variations of this probe will be used in more advanced plasma rocket study.