Preliminary experimental investigations were conducted to evaluate the effects of a modified cathode shape on ammonia arcjet engine performance. The experiments were conducted with a contoured nozzle arcjet in a fully-instrumented facility which included a thrust stand. The modified cathode tip had a reduced diameter and more acute conical tip with respect to the baseline cathode design. A uniform 15% decrease in arc voltage was demonstrated over a mass flow range of 0.175 to 0.350 g/s. A 4% improvement in thrust efficiency was noted at 22.0 kW. Plans for future cathode testing are discussed with the aim of developing a long-life, high-performance arcjet cathode.