Measurement of Fast Transients in Nb3Sn Magnets by Using a Static Harmonic-Coil

An induction-coil magnetometer has been produced at CERN to determine the location of an incipient quench in the HL-LHC Nb$_{\text{3}}$Sn inner-triplet quadrupoles. The instrument, known as a quench-antenna, allows the measurement of the position and propagation of quenches with a reduced number of acquisition channels. This is possible because four layers of nested coils are designed to be sensitive only to the normal and skew sextupole and octupole components. Moreover, the magnetometer allows the study of fast magnetic transients due to flux jumps observed during the ramping of Nb$_{\text{3}}$Sn superconducting accelerator magnets. This paper presents the observation and characterization of flux jumps during powering ramps similar to machine operations. Spatial and temporal distributions of flux jumps are derived from the induced voltages in the quench-antenna as a function of the transport current. Modeling flux jumps as traveling magnetic moments allows the reconstruction of the effect in terms of position and magnitude and, thus, an estimation of the impact on the magnetic field quality.