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Erosion of the boron nitride chamber insulator of a high power quasisteady magnetoplasmadynamic thruster has been measured directly using a quartz crystal microbalance (QCM) method. A l-|*-thick layer of boron nitride is sputtered on the QCM, after which it is mounted flush in the boron nitride backplate insulator, and monitored for change in resonant frequency following thruster operation. The measurements confirm the concept of an "onset" arc current, above which severe erosion occurs, but below which long-term thruster operation at interesting performance levels appears promising.

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