Method for permanent magnet undulator parameter stabilizing against temperature variation

摘要

Stability of undulator parameter (K_(und)) is very important for operation of synchrotron radiation sources. Because K_unddefines radiation harmonics energies, its variation causes drift of harmonic energies and undesirable photon flux change during data collection. Very often, the origin of K_(und) drift is temperature change. To stabilize K_und, the temperature of the undulator and ambient should be kept constant with ～ ± 0.1°C or better precision. Although keeping the temperature constant in a large space with such precision is a challenge, it can be done with adequate resources. However, the time needed to reach operating temperature with required precision after interruption of standard operation can be quite long. Observation made at Cornell High Energy Synchrotron Source (CHESS) indicated that this time can be ～12 h or longer. During this time period, K_(und) of undulator magnets is drifting, causing variation of photon flux. This compromises beam line operation. Reducing the sensitivity of K_(und) to temperature variation would be beneficial. In this paper, we developed the concept of stabilizing of K_(und) against temperature change utilizing the effect of differential thermo-expansion. According to the concept, girders should include specific parts made of materials with different coefficients of thermo-expansion (CTE). When temperature changes, it will create the desired deformation which can compensate for the change in magnetic field caused by temperature sensitivity of permanent magnet residual induction B_r. Engineering FEM modeling and analysis of the magnetic field of the specific girder design reveal feasibility and practicality of the concept. In the paper we present results of the engineering modeling, magnetic field analysis, and supporting experimental data.