High temperature superconducting tapes for undulator technology

The latest generation light sources use undulator magnets to produce photon beams with a high brilliance. An undulator consists of a sequence of bending magnets. Electrons move in an undulator following a sinusoidal pattern. Each time that the electron beam is bent, it emits synchrotron radiation, which, at specific wavelengths, interferes constructively with the one emitted at all other places where it is bent along the undulator. State of the art for undulators is the permanent magnet technology. Superconducting undulators (SCUs) can produce, for the same geometry, a higher peak field on axis than permanent magnet undulators, further improving the tunability and brilliance of existing light sources. It is possible to tune the emitted photon energy by changing the operating current. Recently, it has been demonstrated for the first time that a full scale length superconducting undulator, conduction cooled and wound with NbTi wire, could reach, with an electron beam, a higher peak field than what is reachable with permanent magnets cooled at cryogenic temperatures [1]. Replacing the NbTi wire with another conductor with a higher engineering current density J_e makes it possible to increase the on axis peak field even further. A possibility is using NbSn₃ wire. However, due to the necessity of thermal treatment after winding, it is very challenging to use NbSn₃ for maintaining the high mechanical tolerances needed to generate a high quality magnetic field, which is necessary to maximize the constructive interference of the photon beam. An appealing alternative to NbTi wire is high temperature superconducting (HTS) tape, which together with a higher J_e has the additional advantage of having a larger temperature margin. This means that it is possible to deal with a larger beam heat intake, making the operation of the magnet more robust. Moreover, operation of the magnet at higher temperatures than 4 K translates into a more efficient way to remove heat loads.

HTS tapes, especially rare earth–barium copper oxide (REBCO) coated conductors, can be used for planar SCUs in geometries similar to the ones used for NbTi wire [2–4]. The recent paper [4] explores applying HTS tapes to such a geometry. The authors propose a novel scalable scheme for a planar superconducting undulator without resistive joints and with partial insulation. Minimizing the number of resistive joints, each having a typical resistivity of 40–50 nΩcm² in cold conditions [5], is recommendable to reduce the heat load input. The same paper [4] tackles the relevant issue of magnet insulation. The scheme developed includes partial insulation on the part of the magnet far from the beam to reduce the field settling time, relevant for tuning the undulator with an electron beam, keeping J_e as high as for a non insulated magnet.

Another proposal for a HTS tape stacked undulator has been made for free electron laser applications [6]. To define the current path allowing a sinusoidal magnetic field on axis, the HTS tape is structured, which can be performed using lithography [6] or a high power laser [7]. For such an application, uniformity of the current along the conductor is a must to reach the high field quality necessary for an undulator.

Important issues to be addressed for the application of HTS tape to accelerator magnets are quench detection and protection, as well as radiation hardness. Quench detection and protection are different from what is required and are well established for low temperature superconductors (LTS), and proper solutions need to be developed. Since the normal zone propagation velocity is much slower in HTS tape (1–10 mm s⁻¹) than in LTS (1–2 m s⁻¹), to avoid damage of the HTS tape, quench detection in a HTS magnet needs a faster response time (<5 ms), a smaller quench detection voltage threshold (∼100 μV), and faster quench protection [8]. Concerning radiation hardness, few experiments have been performed. While NbTi magnets have been widely applied in accelerators demonstrating to be radiation hard, a similar experience is missing for HTS tape magnets. It is of particular importance to demonstrate that the conductor quality does not degrade with radiation exposure.

The companies producing HTS tapes are making important improvements to increase J_e by doping REBCO tapes with Zr [9] and reducing the thickness of the substrate from 50 μm to 30 μm [10]. However, uniformity, repeatability, mechanical properties, available length, and production rate of high quality HTS tapes need to be improved for applications in superconducting undulators to produce full scale length magnets that can be reliably operated in storage rings and free electron lasers.