Normal Zone Propagation in Superconducting Focusing Solenoids and Related Quench Protection Issues

Abstract

Abstract-Superconducting solenoids are increasingly used as focusing lenses in transport channels of proton linear RF accelerators. If these accelerators employ superconducting RF cavities, each focusing lens is usually comprised of three coils connected in series: a main coil, which provides the needed focusing strength, and two bucking coils, that help to reduce magnetic field outside the lens.When a normal zone develops in any of the coils in a focusing lens, it propagates with a direction and a rate which depends on the coil and the specific part of the coil in which the quench first occurred. As a result of this propagation process (quenching), the temperature and/or voltage of parts of the lens can exceed safe limits, thus compromising lens reliability. On the other hand, the negative impact of quench events can be significantly mitigated if an external resistor is used to absorb a part of the energy stored in the magnetic field. This paper presents the main results of a solenoid quench protection study based on computational modeling of normal zone propagation in solenoid lenses being built for a superconducting linear RF accelerator under development at Fermilab.

Superconducting Solenoid Lens for a High Energy Part of a Proton Linac Front End

“…The model solves the 2D problem of heat and quench propagation in the radial and longitudinal directions, which were shown for this case to exceed the propagation rate along the strand [13]. The original code has evolved to include a convenient user interface for making changes to the many solenoid parameters, geometry, material properties, and quench origins.…”

“…The original code has evolved to include a convenient user interface for making changes to the many solenoid parameters, geometry, material properties, and quench origins. The improved code allows for input of magnetic field maps and coil self-and mutual-inductances [14].…”

“…This latter feature is necessary to reflect the behavior observed in the actual quench tests, because the model does not consider eddy current heating during the rapid current discharge, which might induce a secondary quench. Simulation of quench development was then made for several configurations of external protection resistors: a) a single dump resistor in parallel with the solenoid [14], b) dump resistors across either both BCs, or both MCs [16], c) dump resistors in parallel with each of the coils [17]. A systematic study was made to determine the optimum resistor values in each case, to minimize the peak temperature and voltage to ground.…”

Design and Performance of Focusing Lenses for Installation Into Superconducting Cryomodules of a Proton Linac

“…The increased focusing strength needed for the lenses in the SSR2 section is achieved by increasing the solenoid coil length (aperture, coil peak field, and margin being equal). Quench protection of these magnets has been extensively modeled [7]- [8], and the stored energy in the SSR2 solenoid is such that the superconductor is at risk to high temperatures and voltages if no special care is taken. A prototype design for a lens in the SSR2 section is similar to that of a lens in the SSR1 section, but the longer main coil is split into two coils separated by an insulating wall [9].…”