A Scaling Law for the Snapback in Superconducting Accelerator Magnets

Abstract

Abstract-The decay of the sextupole component in the bending dipoles during injection and the subsequent snapback at the start of beam acceleration are issues of common concern for all superconducting colliders built or in construction. Recent studies performed on LHC and Tevatron dipole magnets revealed many similarities in the snapback characteristics. Some are expected, e.g. the effect of operational history. One particular similarity, however, is striking and is the subject of this paper. It appears that there is a simple linear relation between the amount of sextupole drift during the decay and the magnet current (or field) change during the ramp required to resolve the snapback. It is surprising that the linear correlation between snapback amplitude and snapback field holds very well for all magnets of the same family (e.g. Tevatron or LHC dipoles). In this paper we present the data collected to date and discuss a simple theory that explains the scaling found.Index Terms-Current distribution, decay and snapback, magnetization reversal, superconducting accelerator magnets.

Measurements of the Persistent Current Decay and Snapback Effect in Tevatron Dipole Magnets

“…In addition, an accurate check of the scaling law proposed in [8] was performed. For IP durations greater than 6 s, we found the ∆b 3 sb versus ∆I dependence is linear for the measured magnets.…”

“…[1]- [4] and the references found in [8].) However, recent Tevatron beam tests [6] and stand-alone magnet measurements [9] uncovered a new fast sextupole decay at the beginning of injection.…”

“…In the recently proposed scaling law [8], two main assumptions are made. The first assumption is that the proportionality, described in the second line of (3), is linear without an intercept term.…”

The Development of Superconducting Magnets for Use in Particle Accelerators: From the Tevatron to the LHC

“…A similar situation holds for the snapback dependence on the current. A significant result linking the snapback amplitude and its decay constant in the case of an exponential fit was found during the first years of the LHC dipole production; this scaling law has been proven both for the LHC and Tevatron dipoles [64]. A major effort was carried out to build a LHC field model [66,67] using all the relevant information and permitting the programming of the circuit magnets before day 1 of commissioning.…”

Superconductivity: its role, its success and its setbacks in the Large Hadron Collider of CERN

“…The amplitude of the jump varies with the cable and is impossible to predict for each type of cable -the mean value and its variance had to be carefully evaluated from a limited set of magnet measurements in order to set up a pre-programmed b3 correction by dedicated sextupole families. For the LHC a scaling law capable of describing the snapback behaviour has been successfully worked out [16], as shown in Figure 7. sextupole components, b3, to change by 3 units (from 1.2 to 4.2) at constant main field, see central graph.…”