Design of ${\rm Nb}_{3}{\rm Sn}$-Based Short Period Model Superconducting Helical Undulator

Abstract

Superconducting undulators are being constructed to satisfy an increasing need within the synchrotron radiation community for higher and higher energy photon beams. The paper focuses attention on the design of a helical undulator for the ILC photon (hence positron) source. Finite element method (FEM) modeling was performed to extend the design of our existing 14 mm period undulator design to shorter periods with a view towards the final design, construction and testing of a 10 mm undulator. To launch this task we modeled the properties of 10 mm period undulators formed around bore tubes of the 6 mm and 8 mm OD with winding-pack cross-sections of 3 mm 6 mm and 3 mm 11 mm. These windings do allow a pole width of 2 mm and can take advantage of the corresponding about 0.2 T field increment due to it. In order to reach the target bore field of 1.1 T it is necessary to either reduce the bore size and tighten the associated tolerances, or to use a higher performing strand. In the present work we have chosen to optimize the undulator design basing it on the properties of an advanced tube-type strand keeping the bore tube diameter at 6-8 mm in order to see the needed performance to meet this undulator specification.

A model superconducting helical undulator fabricated using a small filament, tube-type, multifilamentary Nb3Sn wire

“…These results indicate that the λ = 10.7 mm, B o = 1 T criteria cannot be met using the Summers- [13] or Godeke-[14] extrapolated internal-Sn data specified in [12]. Clearly, improved strand and/or winding techniques will be required if a λ = 10 mm undulator is to be realized [15]. To simultaneously achieve λ = 10.7 mm and B o = 1 T is difficult, as shortening λ decreases the radial on-axis field per 1 Ampere turn of the undulator winding (the winding is becoming 'more solenoidal').…”

“…However, this is less effective due to the smaller contribution of the more distant turns to the on-axis magnetic field. To correct for these issues one needs to (i) reduce the beam tube aperture size [15] and/or (ii) use a higher performance (in terms of its critical current density and stability) superconducting strand. For this reason, so far research has been focused mainly on 14 mm period superconducting undulators [3][4][5][6][7].…”

“…There we see the possibility for a greater current margin not only at 4.2 K but also at higher temperatures. Our best conductor now reaches 2450 A mm −2 at 12 T 4.2 K [15]. We studied the properties of a model helical undulator 250 mm long (λ = 14 mm) the winding of which was made using a tube-type multifilamentary Nb 3 Sn conductor and its helical poles were made of 1016 low carbon steel.…”