A fast sextupole probe for snapback measurement in the LHC dipoles

In superconducting particle accelerators a fast change of the magnetic field occurs during the first few seconds after the start of an energy ramp. Standard magnetic measurements using a coil rotating at 1 Hz do not have the time resolution required to completely resolve this phase, usually called snapback. For this reason we have developed a new and fast system dedicated to sextupole measurements. The basic component consists of three Hall plates mounted on a ring. In an ideal case this arrangement compensates the main dipole field and produces a signal proportional to the sextupole only. Mechanical tolerances and differences in the sensitivity of the Hall plates are compensated by instrumentation amplifiers and an in situ fine adjustment of the probe orientation. Using this hybrid compensation technique we have measured sextupole variations in an LHC dipole prototype during snapback at a rate of 5 Hz. In this paper we present details on the device and the results of our measurements.