An apparatus for controlling the 
voltage applied to a shield interposed between an 
induction coil powered by a power supply via a matching network, and the 
plasma it generates, comprises a shield, a first feedback circuit, and a second feedback circuit. The power supply powers the shield. The first feedback circuit is connected to the 
induction coil for controlling the power supply. The second feedback circuit is connected to the shield for controlling the 
voltage of the shield. Both first and second 
feedback circuits operate at different frequency ranges. The first feedback circuit further comprises a first controller and a first sensor. The first sensor sends a first 
signal representing the power supplied to the inductive coil to the first controller. The first controller adjusts the power supply such that the power supplied to the 
inductor coil is controlled by a first 
set point. The second feedback circuit further comprises a second sensor, a second controller, and a variable 
impedance network. The shield is powered via a variable 
impedance network. The second sensor sends a second 
signal representative of the 
voltage of the shield to the second controller. The second controller adjusts the variable 
impedance network such that the voltage of the shield is controlled by a second 
set point.