BPM Technologies for Quadrupolar Moment Measurements

Quadrupolar moment measurements based on electromagnetic pick-ups (PU), like BPMs, have attracted particular interest as non-intercepting diagnostics to determine the transverse beam size. Here, the second-order moment, which contains information about the beam size, is extracted from the BPM electrode signals. Despite the simplicity of the concept, quadrupololar measurements have always been challenging in practice. This is related to the fact that the quadrupolar moment constitutes only a very small part of the total PU signal, which is dominated by the contributions of beam intensity and position. In this study we discuss the limitations of absolute quadrupolar measurements if applying traditional BPM technologies, and we propose a new approach to efficiently overcome them via movable PUs. Moreover, we highlight the potential use of BPMs as an emittance measurement system during the energy ramp at synchrotrons by performing differential quadrupolar measurements, which show a remarkably higher accuracy than absolute measurements. Dedicated studies using different types of BPMs in the Large Hadron Collider (LHC) at CERN demonstrated promising results.Quadrupolar pick-ups (PU) have attracted particlular interest as candidates for non-intercepting beam size and emittance measurements. However, their application has been proven to be limited. Two fundamental factors make beam size measurements with quadrupolar PUs exceptionally challenging: first, the low quadrupolar sensitivity of PUs and second, the parasitic position signal incorporated into the measured quadrupolar quantity. In this paper, the basic concepts of the quadrupolar measurements are reviewed with a special focus on the challenging nature of the measurements. Additionally, the potential use of existing beam position monitor (BPM) technology is studied. Recent tests performed with BPMs in the Large Hadron Collider (LHC) are discussed. Preliminary measurements demonstrate promising results