A fast and accurate wire scanner instrument for the CERN accelerators to cope with severe environmental constraints and an increased demand for availability

The Beam Instrumentation group of the European Organization for Nuclear Research (CERN) has been developing an instrument called the Beam Wire Scanner (BWS) for the past few years. This system is used to measure the size of proton beams in the Large Hadron Collider (LHC) and its injector chain. An electro-mechanical system moves a very thin wire of 30 μm through the particle beam and measures the induced radiation losses generated by this interaction. The actuator, based on a Permanent Magnet Synchronous Motor (PMSM), a solid rotor resolver and an in-house designed high precision optical encoder are located in underground installations and have to cope with large irradiation levels. Another difference with respect to its predecessors is the placement of all moving parts in the vacuum. The control electronics is situated far away from the beam tunnels to minimize the destructive impact of ionizing particles. Challenges arise from the long distance between these two parts, up to 250 meters, and the high scanning speed of the wire of up to 20 ms^1, with a target position accuracy as low as 5 μm rms. This paper describes the challenges of the BWS design, details the current status and introduces the philosophy of its conception to the IEEE control application community.