[en] Disruptions in beam-heated plasmas on PBX were studied by classifying precursor oscillations detected by a soft x-ray imaging system and Mirnov coils. A data base consisting of 80 discharges was constructed. In low I/sub p/ operation, attainable β/sub T/ values are limited by disruptions preceded by m = 1 activity at the center and coupled an m/n = even/1 mode. In high I/sub p/ and high β/sub T/ operation with high indentation, the sawtooth activity disappears. At these disruptions the plasma loses vertical position or distorts on a relatively fast time scale (<100 microseconds)