Abstract: A substrate processing apparatus includes a vacuum chamber with a processing zone for processing a substrate using plasma and at least one magnetic field source configured to generate one or more active magnetic fields through the processing zone. The apparatus also includes a magnetic field sensor configured to detect a signal representing the one or more active magnetic fields, and a controller coupled to the magnetic field sensor, and the at least one magnetic field source. The controller is configured to detect a target value corresponding to at least one characteristic of the one or more active magnetic fields, set an initial current through the at least one magnetic field source, the initial current corresponding to the target value; and adjust a subsequent current through the at least one magnetic field source based on the detected signal representing the one or more active magnetic fields.