Experimental researches on the operational performance and the arc structure of a quasisteady magnetoplasmadynamic (MPD) arcjet was made to develop a high-performanceaccelerator in space. The results showed that the current pattern, which depended strongly on gas species and discharge current levels, influenced the thruster performance. It was found that the plasma produced in the discharge chamber was compressed radially and that 50% on the input power was consumed in the cathode region. From the plasma potential distribution, a positive voltage slope and mass starvation were found to be present near the anode surface beyond the predicted critical current level.