An experimental setup is presented which facilitates the measurement of ion beam emittances of radiofrequency ion thrusters (RIT) based on the slit-wire method. For the present study, the RIT was equipped with a single orifice three-electrode extraction system. First experimental results and comparisons with simulations are shown. Partial agreement between experiment and simulations could be achieved by assuming negligible space charge. This finding is attributed to space charge neutralization due to the single orifice system which produces an unusually high residual gas density between the RIT electrodes. The correspondingly high rate of collisions between the primary ion beam and the residual gas particles produces sufficiently many electrons for space charge compensation.