The Wall Current Transformer - a New Sensor for Precise Bunch-by-Bunch Intensity Measurements in the LHC

The Wall Current Transformer (WCT) is a new bunch-by-bunch intensity monitor developed by the CERN Beam Instrumentation Group to overcome the performance issues of commercial Fast Beam Current Transformers (FBCT) observed during Run 1 of the LHC. In the WCT the large magnetic cores commonly used in FBCTs are replaced with small RF transformers distributed around the beam pipe. Rather than directly measuring the beam current, the WCT measures the image current induced by the beam on the walls of the vacuum chamber. The image current is forced to flow through a number of screws which form the single-turn primary windings of the RF transformers. The signals of the secondary windings are combined and the resulting pulse is filtered, amplified and sent to the acquisition system. This paper presents the principle of operation of the WCT and its performance based on laboratory and beam measurements.