Beam-induced heat load in in-vacuum undulators with a small magnetic gap

摘要

For hard X-rays users in a synchrotron radiation source, the development of an in-vacuum mini-gap undulator with a short period is a recent trend. The performance of the undulator can become degraded by large beam-induced heat load that arises from both the synchrotron radiation (SR) and the image current. SR is responsible for avalanche meltdown of the magnet foil, whereas the image current mainly decreases the threshold of power to cause an avalanche meltdown. To maximize the performance of the in-vacuum undulator, we hence consider the most appropriate conditions for the beam in a storage ring, such as a long bunch length or a small bunch current. An appropriate strategy to utilize the undulator is also important, for example, operating a collinear double in-vacuum undulator or a canted in-vacuum undulator with a small canting angle may result in avalanche meltdown of the magnet foil. In this paper, we present methods to calculate the linear power density of SR and the resistive wall heating on a conductive magnet foil. The beam-induced heat load of a double undulator in Taiwan Photon Source (TPS) is presented as an example.