Undulator a magnetic properties and spectral performance.

Abstract

The Undulator A at the Advanced Photon Source (APS) is a planar permanent magnet hybrid device optimized for generating x-rays from 3 keV to 45 keV by using the first, third, and fifth radiation harmonics. It also produces x-rays above this energy. For high-energy experiments, users are relying on using higher harmonics, which has become possible because of improved undulator technology over the past decade. The Undulator A has been designed to provide continuous energy coverage with no significant drop in brilliance when switching between the harmonics, i.e., the tuning curve from one harmonic to the next intersect. The undulator has a period length of 3.30 cm and has 72 magnetic periods (144 poles) for a total length of 2.4 m. The undulator was initially described in Technical Bulletin ANL/APS/TB-3 (1993) [1] and subsequently in ANL/APS/TB-17 (1994) [2]. Both documents were published before the first undulator had been delivered to the APS so that the information given was based on design specifications. A three-dimensional (3D) magnetic modeling code was used to estimate the magnetic field vs. gap, and computer simulations were used to predict the on-axis brilliance, flux, and power for the APS design lattice using an ideal undulator magnetic field, i.e., pure sinusoidal variation of the magnetic field along the undulator. The magnetic field strength given in earlier publications was what was required by the undulator purchase contract. Since then, 23 Undulator A devices have been measured, tuned, and installed in the storage ring. It should be noted that undulators are removed periodically from the storage ring for retuning, and the values listed in this document are therefore subject to change. This document focuses on the measured magnetic properties and the spectral performance of these devices. We will show the calculated on-axis brilliance and flux for the present APS lattice (''low-emittance'' lattice with 1.0% coupling) and compare those with the APS design lattice. The radiated power and on-axis power density will also be given. We will also look into the future for prospects to obtain even higher brilliances and fluxes by further improvements to the lattice and by using longer undulators with shorter period lengths. (We provide four appendices with additional information on (A) the APS storage ring, (B) Undulator A measured magnetic properties, and (C) one shorter period device. In Appendix D, we also compare the Undulator A on-axis brilliance vs. other sources at the APS for the present low-emittance operation.) All spectral and power calculations in this document were performed with computer codes from the XOP suite of x-ray optics programs that assume an ideal undulator magnetic field [3]. We have also been able to accurately predict the reduction in brilliance of higher harmonics due to magnetic field errors by using the code UR that uses a measured magnetic field as input [4], and specific references are given in the text where such experience was used. Although a variety of properties of general interest have been addressed here, specific needs by individual users will be met by contacting the authors for more information.