In a high frequency oscillator system a series oscillator circuit includes a load coil inductively coupled with a plasma generator. A capacitance is in series with the coil between the drain and source terminals of a transistor. A capacitive or inductive feedback responsive to the oscillation is connected to the gate terminal of the transistor. In another embodiment two sections of a series circuit are connected by the load coil, each section including a separate transistor. A first capacitance is connected between the coil and the first transistor drain, and a second capacitance is connected between the coil and the second transistor drain. A first capacitive feedback is connected between the second section and the first transistor gate, and a second capacitive feedback is connected between the first section and the second transistor gate. For alternative inductive feedback, one feedback is connected between the first section and the first transistor gate, and the other is connected between the second section and the second transistor gate.