Collective Flux Jumps Observed During Operation of the EDIPO Magnets

The tilted head race track coils of the EDIPO test facility are wound with cable-in-conduit conductors made by high current density
$\hbox{Nb}_{3}\hbox{Sn}$
strands,
${\rm J}_{\rm c}=2400\ \hbox{A/mm}^{2} $
at 12 T 4.2 K. Since the commissioning of the coils, frequent voltage spikes were observed at the quench detection taps during the low current range of operation, completely disappearing above 2 T. The large amplitude of some spikes triggered the quench detection system: the parameters for protection (voltage thresholds and validation time) had to be modified for the voltage pairs monitoring the windings. The occurrence of partial flux jumps in the high current density strands is known from former characterization. The fast data acquisition of the quench detection system, with 800 Hz sampling frequency, allows investigation of the dynamics and propagation of the voltage spikes in the various layers of the winding. From the collected data of several runs of operation, statistics of amplitude, initial location, propagation and reproducibility were drawn, looking also for correlation with the operation history, e.g., first cool-down, ramp up versus ramp down, ramp rate, etc. The energy released by the flux jumps is conservatively estimated by the time integration of the voltage spikes and discussed in terms of energy margin and heat removal rate in the layers. In conclusion, the limitation of the use of high current density
$\hbox{Nb}_{3}\hbox{Sn} $
strands in high current conductors is discussed.