4203504 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 22, NO. 3, JUNE 2012 A Commercial HTS Dipole Magnet for X-Ray Magnetic Circular Dichroism (XMCD) Experiments T. Huang, X. Gao, D. Pooke, V. Chamritski, N. Briggs, M. Christian, S. Gibson, J. Mitchell, M. Miles, and J. de Feijter Abstract—A commercial HTS magnet was designed, built and tested by HTS-110 Ltd for X-ray Magnetic Circular Dichroism (XMCD) experiments. The magnet was integrated with a XMCD UHV (Ultra High Vacuum) chamber and installed at an existing soft X-ray beamline at Singapore Synchrotron Light Source (SSLS). The dipole magnet has a bore of 40 mm in diameter and a gap of 90 mm to accommodate the XMCD chamber with nine access ports for sample manipulation and XMCD analysis. To achieve a UHV of inside the chamber, an indepen- dent bakeout system was developed for this magnet to allow the XMCD UHV chamber to be baked out at a temperature over 100 degrees Celsius without affecting the HTS magnet. A small GM type cryocooler is employed to cool the magnet coils down to 17 K by thermally intercepting the heat load from the 2nd stage to the 1st stage. The magnet can provide variable magnetic fields up to 2.16 T and with the field homogeneity better than 0.1% over a 1 cm diameter sphere. Limited by the peak voltage of the bipolar power supply used for the magnet, the minimum time of full field energizing is around 20 seconds and the minimum time of full field reversal is around 40 seconds. The technical issues related to UHV compatibility and ramping rate in developing similar HTS applications are discussed in this paper. Index Terms—Fast ramping, HTS magnet, UHV, XMCD. I. INTRODUCTION X -RAY magnetic circular dichroism (XMCD) measures the difference between two X-ray absorption spectra (XAS) taken by switching either the helicity of the circularly polarized x rays or the magnetization for a magnetized sample [1], [2]. By analysing the difference spectrum, magnetic properties of the relevant element can be obtained such as its spin and or- bital magnetic moment. With improved synchrotron radiation sources and end stations, XMCD technique has been become a powerful tool for material scientists, chemists and biologists. One of the key ingredients for an XMCD measurement is a magnet for magnetizing a sample. To take full advantage of XMCD experiments, the combined requirements for the magnet must be considered: magnetic field, sufficient sample Manuscript received September 08, 2011; accepted October 13, 2011. Date of publication November 02, 2011; date of current version May 24, 2012. T. Huang, D. Pooke, V. Chamritski, N. Briggs, M. Christian, S. Gibson, J. Mitchell, M. Miles, and J. de Feijter are with HTS-110 Ltd, Lower Hutt 5040, New Zealand (e-mail: t.huang@hts-110.com). X. Gao was with National University of Singapore, Singapore 117542, Sin- gapore. He is now with Shanghai Institute of Applied Physics, Pudong New District, Shanghai 201203, China (e-mail: gaoxingyu@sinap.ac.cn). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TASC.2011.2174599 and detector access areas, UHV compatibility and high re- versing rate. Permanent magnets, electromagnets and Low Temperature Superconducting (LTS) magnets were used for XMCD experiments in the past [4], [5]. However, permanent magnets and electromagnets can only provide a small field. Although a LTS magnet can achieve a high magnetic field, there are some limitations in use such as volume, reversing rate and maintenance. Recently, with development of HTS conductors and cry- ocoolers, commercial cryogen-free HTS magnets are available for many applications [6]–[8]. In contrast to a LTS magnet, a cryogen-free HTS magnet has several advantages: compact- ness, fast ramping rate and low maintenance and operating costs. These advantages make it a preferable solution for XMCD experiments. A commercial HTS magnet was designed, built and tested by HTS-110 Ltd for XMCD experiments. The magnet was inte- grated with a XMCD UHV (Ultra High Vacuum) chamber and installed at an existing soft X-ray beamline at Singapore Syn- chrotron Light Source (SSLS) [3]. The magnet can provide vari- able magnetic fields up to 2.16 T and field homogeneity better than 0.1% over a 1 cm diameter sphere. Limited by the bipolar power supply used for the magnet, the minimum time of full field energizing is around 20 seconds and the minimum time of full field reversal is around 40 seconds. II. SYSTEM REQUIREMENTS The magnet specifications are summarized in Table I. The HTS magnet is required to generate a maximum magnetic field of approximately 2.2 Tesla with field homogeneity of 1% in a 10 mm Diameter Sphere Volume (DSV). The dimensions of the magnet are restricted to be compatible with the existing geom- etry between the end station and the refocusing mirror on the SINS beamline at SSLS. A bore of 40 mm diameter and a gap of 90 mm are required to accommodate the XMCD chamber with nine access ports for sample manipulation and XMCD analysis. To achieve a UHV of inside the chamber, an in- dependent bake-out system is required for this magnet to allow the XMCD UHV chamber to be baked out to a temperature over 100 degrees Celsius without affecting the HTS magnet. In addi- tion, a fast field ramping rate is desirable for obtaining spectra of high precision by reversing the applied magnetic field for each photon energy. III. DESIGN AND CONSTRUCTION The complete assembly of the HTS magnet shown in Fig. 1 consists of the following main components: HTS coil packs, iron poles, integrated cryostat, cryocooler and the XMCD 1051-8223/$26.00 © 2011 IEEE