A simple double probe for energetic plasma beam experiments (1016/cm3, 10 cm/&mgr;sec) is reported. By capacitive discharge, the probe can be used to obtain the entireI–Vcurve in a single shot, so that the electron temperature and the ion density can be easily estimated. For supersonic conditions, the ion saturation current is limited by the incident ion flux instead of the factor (kTe/M)1/2, as in the case of a stationary plasma.