The general aspects of the undulator technology are described in close relation with both the accelerator technology and the spectral performance as light sources. In addition, we show the latest technology for future synchrotron radiation facilities such as cryogenic permanent magnet undulators or HTSC (high temperature superconductor) undulators.

A 5 GeVclass EnergyRecovery Linac (ERL) is a promising device for future light sources. In order to realize a light source of this kind, it is essential to demonstrate the ERL scheme. We have decided to build a small test facility, the Compact ERL (C-ERL) for this purpose. The C-ERL is outlined and the status of its development is reported.

The International Linear Collider (ILC) will require about 15,000 1.3 GHz superconducting radio frequency (SRF) accelerating cavities with high accelerating gradient (>35 MV/m) in its main linac. The high yield (80%) of successful high gradient cavities is necessary. Both of the yield and the accelerating gradient of SRF cavities does not reach the required level at present. We think that the gradient of the SRF cavities is limited by irregularities on the interior surface of the cavities, for example, fine dusts (1 μm), balls (100 μm) and pits (100 μm): electrons emitted from the fine dusts by the tunnel effect are accelerated in the electric field, and consume the stored energy of the cavities (Field Emission). The balls and pits cause a breakdown by a magnetic field enhancement or a thermal current concentration (Thermal Breakdown). To prevent these problems the interior surface are treated by polishing and rinsing. The relation between the surface states and the gradient limitations, however, is still not clarified. To study the relation, we are developing an optical inspection system of the interior surface.