A Simple Modification of DC Current Septa to Reduce Current Density by Half

TL;DR

This paper proposes a simple modification to DC current septa in accelerators, significantly reducing current density and enabling the use of copper coils instead of superconducting materials, thereby improving efficiency and design flexibility.

Contribution

It introduces a novel septum modification that halves current density, allowing copper coils in high-energy accelerators like CEBAF, which was previously impractical due to high current densities.

Findings

Current density reduced by approximately 50%.

Copper coil implementation becomes feasible with the modification.

Field containment outside the septum remains effective.

Abstract

Circular accelerators typically have one injection and one extraction septum magnet. CEBAF is a recircuating electron linac which has a total of 27 DC current septa and one Lambertson. Current densities range from 28-48 A/mm 2 . Current sheet widths are 5-24 mm, turns count 5-24 and lengths 1000-3000 mm. A design exercise to increase the beam energy to 22 GeV is underway. Since doubling the current density in copper is not practical in the CEBAF layout a conductively cooled superconducting septum concept was examined. Putting the current sheet and its cryostat between the poles as in a standard current sheet septum would have required 90 mm pole gap. The poles were brought to 40 mm separation and the steel notched for the 90 mm cryostat. The field in the bore increased while the field outside the current sheet remained close to zero as in a conventional septum. Required current density dropped enough that a copper coil became possible. Two examples will be shown, the one discussed above and the modification of the 3000 mm septum with 0.92 T bore field.