Abstract
The in-vacuum undulator with a permanent magnet at room temperature is a mature technology and is widely used; with a short period length in a medium-energy facility, it can enhance photon brilliance in the hard x-ray region. A cryogenic permanent magnet has been investigated as an in-vacuum undulator; this undulator will become the best prospective device to satisfy the requirements of a photon source with great brilliance in the hard x-ray region. For the further hard x-ray region, a superconducting wiggler can provide great flux with a continuous spectrum, whereas a superconducting undulator will provide great brilliance with a discrete spectrum. High-temperature superconducting wires are highly promising for use in the development of superconducting undulators; unique algorithms for their development with an extremely short period in a small-magnet gap have been devised. Some out-of-vacuum planar undulators with special functions must also be fabricated to enable diverse applications in various light-source facilities. This article describes current and future developments for insertion devices in storage-ring and free-electron-laser facilities and discusses their feasibility for use therein.
Highlights
A third- or fourth-generation synchrotron facility focuses mainly on either enhancing output at the greatest photon energies or extending the range of tunable photon energy at medium electron-energy facilities
Few transitional out-of-vacuum planar insertion devices are used in recent advanced light sources, such as the planar elliptically polarized undulator (EPU) of the Apple-II structure, which is still popular in medium-energy storage rings
Many in-vacuum undulators have been utilized in insertion devices—especially at advanced medium-electronenergy facilities
Summary
A third- or fourth-generation synchrotron facility focuses mainly on either enhancing output at the greatest photon energies or extending the range of tunable photon energy at medium electron-energy facilities. In most medium-energy facilities, a short-period undulator with a small magnet gap is the only way to attain the hard x-ray range. Despite the frequent use of an out-of-vacuum insertion device with a hybrid or pure structure in the past two decades, the period length and magnet pole gap in these ID are large, making impossible the achievement of great photon brilliance at hard x-ray energies in the medium-energy facilities. The in-vacuum undulator (IU) [1,2,3] with a permanentmagnet or hybrid structure at room temperature is a mature technology and has been widely used in the past decade This IU can support a short-period-length undulator (minipole undulator) in a medium-energy facility (about 3 GeV), subsequently enhancing the photon brilliance in the hard x-ray region.
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