A system for controlling RF power supplies applying power to a load, such as a plasma chamber, includes a master power supply and a slave power supply. The master power supply provides a control signal, such as a frequency and phase signal, to the slave power supply. The slave power supply receives the frequency and phase signal and also receives signals characteristic of the spectral emissions detected from the load. The slave RF power supply varies the phase and power of its RF output signal applied to the load. Varying the power controls the width of an ion distribution function, and varying the phase controls a peak of the ion distribution. Depending upon the coupling between the RF generators and the load, different spectral emissions are detected, including first harmonics, second harmonics, and, in the case of a dual frequency drive system, intermodulation distortion.