A continuously variable microwave circuit capable of being tuned to operate under a plurality of distinct operating conditions, comprising: a waveguide comprising an adjustable tuning element having a core configured to protrude into the waveguide; an actuator in operative communication with the adjustable tuning element, wherein the actuator is operable to selectively vary a length of the core that is protruding into the waveguide so as to minimize reflected microwave power in the plasma asher; and a controller in operative communication with the actuator, wherein the controller is configured to selectively activate the actuator upon a change in the plurality of operating conditions.