A discharge tube made of a dielectric material extends through a hole of a rectangular waveguide and through a coaxial microwave cavity so as to be coaxial with the central axis of the cavity. Also provided is a method of inhibiting the Ras pathway by administering a lipophilic derivative of ascorbic acid. A distortion trigger compares a measured average distortion signal and the distortion threshold and reduces the controllable gain of the amplifier if the average distortion signal exceeds the distortion threshold. Predicted values from a mathematical model are compared to the sensor feedback, and the estimated basepoint is selectively adjusted in response to a difference between the predicted values and the sensor feedback in order to reduce the difference.