The AB-BI (Beam Instrumentation) group designs, builds, maintains and improves the diagnostics instruments that allow the observation of the particle beams and collision parameters of all CERN accelerators and transport lines with the precision required to diagnose, tune, operate and improve the installations. This is a very widespread activity which demands competence in the following fields: precision mechanical engineering, ultra-high vacuum technology, electrical engineering including high voltages, electronics engineering in particular in the radio-frequency domain, engineering on electromagnetic sensors, engineering on interceptive sensors, with which the charged particle beams interact directly (solids, liquids, various gazes). Digitization of signals, software engineering (real time and off-line analysis), and profound understanding of accelerator operation and accelerator theory.

The machine performance relying strongly on instrumentation, the BI group is engaged in research and development to always better observe the beam parameters. The range of activities spans from accelerator physics (machine optics, particle tracking, polarization, beam physics) over detector technology (gas chambers, electromagnetic pickups, silicon detectors, optical detectors) to electrical, mechanical and software engineering. Design goals are high functionality within the specifications of the users, high reliability and maintainability of the systems. The BI group hosts fellows and doctoral students. For the LHC project it runs collaborations with TRIUMF (Canada), INDORE (India), LBNL (Berkeley, US) and BNL (Brookhaven, US).

The importance of beam instrumentation is nicely documented in the introduction to a course on bunched beam diagnostics written by R.H. Siemann: " There is remarkable diversity in the field of accelerator physics". Look at the proceedings of any conference. Papers range from Hamiltonian dynamics to the latest improvements of performance in accelerator X to the engineering of specialized equipment. Beam Diagnostics is somewhere in the middle of this. Diagnostic instruments make the connection between theories and observations, and the capabilities of these instruments influence strongly our ability to improve performance."