FRIB chopper monitor system calibration process

At FRIB, a chopper in the low energy beamline is used for beam power control as well as machine protection through beam mitigation. An FPGA based monitoring system was developed to govern the chopper’s operation and ensure that it is functioning properly. This chopper monitoring system conducts checks of the incoming chopper control signal, the high voltage being applied to the chopper plates, and the current flow to and from the plates as the voltage is applied and removed. A curve fitting, or calibration, is required to convert from a digital value to a user readable physical quantity, and vice-versa for the input of acceptable threshold settings for machine protection purposes. As the chopper is functionally a capacitor, the current flow, as high voltage is applied or removed, will vary with level of HV applied and with any change in the capacitive load. This reality makes calibration of the chopper current readings difficult, and an analytical approach was adopted to overcome these difficulties and ensure satisfactory functioning of the monitoring system. This analytical process and a subsequent calibration of the chopper monitor will be discussed in detail in this paper.