Journal of large-scale research facilities, 3, A115 (2017) http://dx.doi.org/10.17815/jlsrf-3-111 Published: 07.06.2017 HFM/EXED: The High Magnetic Field Facility for Neutron Scattering at BER II Helmholtz-Zentrum Berlin für Materialien und Energie * Instrument Scientists: - Dr. Oleksandr Prokhnenko, Helmholtz-Zentrum Berlin für Materialien und Energie, phone: +49 30 8062-43068, email: prokhnenko@helmholtz-berlin.de - Dr. Peter Smeibidl, Helmholtz-Zentrum Berlin für Materialien und Energie, phone: +49 30 8062-43080, email: peter.smeibidl@helmholtz-berlin.de - Dr. Wolf-Dieter Stein, Helmholtz-Zentrum Berlin für Materialien und Energie, phone: +49 30 8062-43079, email: wolf-dieter.stein@helmholtz-berlin.de - Dr. Maciej Bartkowiak, Helmholtz-Zentrum Berlin für Materialien und Energie, phone: +49 30 8062-42318, email: maciej.bartkowiak@helmholtz-berlin.de - Dr. Norbert Stüsser, Helmholtz-Zentrum Berlin für Materialien und Energie phone: +49 30 806243171, email: stuesser@helmholtz-berlin.de Abstract: An overview of the high magnetic �eld facility for neutron scattering at Helmholtz-Zentrum Berlin (HZB) is given. The facility enables elastic and inelastic neutron scattering experiments in con- tinuous magnetic �elds up to 26.3 T combined with temperatures down to 0.6 K. 1 Introduction HFM/EXED – the high magnetic �eld facility for neutron scattering Figure 1 - consists of two main com- ponents: the High Field Magnet System (HFM) and the Extreme Environment Di�ractometer (EXED) (Lieutenant et al., 2006; Peters et al., 2006; Prokhnenko et al., 2015; Smeibidl et al., 2010). The former is a continuous �eld hybrid magnet, built by the HZB in collaboration with the National High Magnetic Field Laboratory, Tallahassee, FL, USA (NHMFL) (Smeibidl et al., 2016). The latter is a time-of-�ight instrument optimized for neutron scattering in the restricted angular geometry of the magnet. The facility has been installed in the second guide hall of the BERII research reactor and commissioned in the �rst half of 2015. *Cite article as: Helmholtz-Zentrum Berlin für Materialien und Energie . (2017). HFM/EXED: The High Magnetic Field Facility for Neutron Scattering at BER II. Journal of large-scale research facilities, 3, A115. http://dx.doi.org/10.17815/jlsrf-3-111 1 http://jlsrf.org/ http://dx.doi.org/10.17815/jlsrf-3-111 http://dx.doi.org/10.17815/jlsrf-3-111 https://creativecommons.org/licenses/by/4.0/ Journal of large-scale research facilities, 3, A115 (2017) http://dx.doi.org/10.17815/jlsrf-3-111 The High Field Hybrid Magnet System The HFM is a "�rst of its kind" hybrid system which is capable of reaching magnetic �elds up to 26.3 T, making it by far the strongest continuous �eld available for neutron scattering experiments worldwide (Smeibidl et al., 2016). It is a series-connected magnet system with an outer superconducting coil and two inner resistive coils (Bird et al., 2009; Smeibidl et al., 2010). The total �eld of 26.3 T is achieved with a 4 MW insert coil set, which has the potential to be upgraded to 8 MW and a total �eld of 31 T. The magnet is designed taking into account the special geometric constraints of performing neutron- scattering experiments. As a result, the inner resistive coil provides a conical bore at each end to allow neutron-scattering to detectors up to ±15° o� the �eld axis. The superconducting coil is a 13-Tesla, 500-mm cold bore coil consisting of Nb3Sn cable-in-conduit conductor (CICC) and weights 5 tons (6 tons full cold mass including �anges, joints and piping) (Bonito Oliva et al., 2008; Dixon et al., 2009, 2010). The magnet central bore is horizontal so that it can align with the neutron beam axis. In addition, the magnet system sits on an instrument table so it can rotate ±15° for increased neutron scattering- angle. All cryogenic and electrical utilities port through an upper “turret” for interface with the supply systems (Dixon et al., 2015). The main technical parameters are listed in Table 1 and a vertical section of the magnet system is shown in Figure 2a. Central Field 26.3 T (31) T Bore 50 mm horizontal Opening Angle 30° Power Resistive Insert 4 MW (8 MW) Field Homogeneity < 0.5% (15 mm x 15 mm) Operating Current 20 kA Magnetic Field of Resistive Insert 13 T – 18 T (4 MW / 8 MW) Magnetic Field of Superconducting Coil 13 T Height ~5 m Total Weight ~25 t Cold Mass ~6 t Table 1: Hybrid magnet system operating parameters. Operation of the magnet system requires a dedicated technical infrastructure located in the separate technical building for the HFM beside the neutron guide-hall Figure 1a. The He-refrigerator system for the CICC coil and the 8 MW power supply as well as the high pressure water circulation required to cool the resistive insert magnet were constructed using standardised industrial components. A specially designed horizontal continuous �ow 3He-sample-cryostat allows combining high �elds with temperatures as low as ~0.6 K. The vacuum container of the cryostat has the shape of the magnet cone (Figure 2b). The sample size cross section inside the cryostat is limited to about 13 x 13 mm2. 2 http://dx.doi.org/10.17815/jlsrf-3-111 https://creativecommons.org/licenses/by/4.0/ http://dx.doi.org/10.17815/jlsrf-3-111 Journal of large-scale research facilities, 3, A115 (2017) Figure 1: a) BERII reactor, neutron instrument halls and HFM technical infrastructure building at the Helmholtz- Zentrum Berlin with the HFM/EXED facility. b) Photograph of the HFM/EXED facility showing the magnet and the EXED instrument components around (neutron detectors and guide lifting device). The Neutron Instrument EXED The EXED shown in Figure 3 is a dedicated neutron instrument optimized to work with the restrictions imposed by the magnet geometry (Lieutenant et al., 2006; Peters et al., 2006; Prokhnenko et al., 2015) [2-4]. To achieve that it utilizes polychromatic (time-of-�ight) technique. Equipped with a bispectral extraction system, EXED has an access to broad wavelength range. The supermirror guide (m = 1-3) with a cross section 100x60 mm2 (H x W) transfers the neutrons from both thermal and cold moder- ators to the sample position located about 75 m away from the source (Figure 3). Before reaching the sample the neutron beam is compressed spatially in both directions by a factor of two by means of an elliptically converging focusing guide section. For applications requiring low beam divergence, the focusing section can be replaced by a pin-hole collimation section with variable apertures. Flexibility of the primary instrument is ensured by three alternative systems that are available to cre- ate neutron pulses: a curved Fermi chopper for very high resolution (neutron pulse width, ∆t ~6 µ s at 600 Hz), a straight Fermi chopper for high resolution (∆t ~15 µ s at 600 Hz) and a counter- or parallel- rotating double-disc chopper for medium to low resolution (∆t ~125 µ s at 200 Hz). A number of single disc choppers (5-120 Hz) located downstream prevents the frame overlap and de�nes the bandwidth of interest. The chopper system allows operating the instrument with di�erent wavelength bands, from narrow (~0.6 Å) to wide (~14.4 Å), centred at the region of interest, and easily trade resolution for in- tensity. The secondary instrument is equipped with 12.7 mm diameter position-sensitive 3He detector tubes. The e�ective length of the tubes is 0.9 m and position resolution is 1%. The tubes are combined in 4 detector banks that are positioned in forward- and backward scattering to re�ect the geometry of the magnet. The typical sample-detector distance is about 2.5 m. A large Ar- or He-�lled detector chamber allows positioning of two detector panels at 6 m away from the sample avoiding air scattering. Technical instrument characteristics are summarized in Table 2. 3 http://dx.doi.org/10.17815/jlsrf-3-111 https://creativecommons.org/licenses/by/4.0/ Journal of large-scale research facilities, 3, A115 (2017) http://dx.doi.org/10.17815/jlsrf-3-111 Figure 2: a) Cross-section through HFM showing the superconducting CICC coil and resistive insert coils. The cryogenic and electrical utilities enter through the upper supply turret. b) Dedicated 3He sample-cryostat that can be installed into one of the magnet cones to combine high �elds with low temperatures. Modes of Operation In order to enable a broad range of scienti�c applications using unique combination of neutron scat- tering and high magnetic �elds, EXED has several modes of operation which are described below. Elastic Neutron Scattering is represented by di�raction and low-Q modes. The former is the main mode at the moment and is used to study single crystal and powder samples in high �elds. The mode is char- acterized by high resolution in backscattering (∆d/d ≥ 2·10−3) and large dynamic range (0.5 – 100 Å). The low-Q mode o�ers small angle scattering capabilities. It extends the low Q-range beyond 10−2 Å−1 using a 6 m-long pin-hole collimation combined with sample-detector distance of 6 m. The latter enables studies of matter on mesoscales in high magnetic �elds such as e.g. vortex state in type-two superconductors. Inelastic Neutron Scattering: A major development took place to complement the instrument portfolio by inelastic capabilities turning EXED into a direct TOF spectrometer (Bartkowiak et al., 2015). The upgrade includes an evacuated detector chamber for forward scattering with a built-in 3He detector array covering 30° in- and out- of plane and positioned 4.5 m away from the sample, a new focusing guide section that accommodates a monochromating chopper assembly and an inelastic doppler system which is at 2.5 m distance from the sample, the upgraded EXED will enable energy-resolved measure- ments over a limited Q-range < 3.25/λ (Å−1) and energy range < 25 meV in addition to the existing elastic capabilities. 2 Instrument application Typical applications are: • Quantum magnets and quantum phase transitions • Superconductivity • Multiferroic and magnetoelectric materials • Correlated electrons in 3d, 4f and 5f metal compounds 4 http://dx.doi.org/10.17815/jlsrf-3-111 https://creativecommons.org/licenses/by/4.0/ http://dx.doi.org/10.17815/jlsrf-3-111 Journal of large-scale research facilities, 3, A115 (2017) • Spin, charge and lattice degrees of freedom in transition metal oxides • Frustrated magnets • Novel states of matter 3 Instrument layout Figure 3: Schematic view of HFM/EXED. 5 http://dx.doi.org/10.17815/jlsrf-3-111 https://creativecommons.org/licenses/by/4.0/ Journal of large-scale research facilities, 3, A115 (2017) http://dx.doi.org/10.17815/jlsrf-3-111 4 Technical Data Beam tube NL 4A, 75 m long ballistic multispectral guide: a straight section (60 x 100 mm2) with a kink and a 7.5 m long focusing section at the end (elliptically tapered down to 30 x 50 mm2) Collimation i) None (standard con�guration with the focusing guide) ii) 6 m long pin-hole collimation (low-Q con�gura- tion without the focusing section) Wavelength 0.7 < λ < 15 Å Flux ~109 n/cm2/s - continuous �ux Range of scattering angles Elastic 0 – 30°, 150° – 170° Inelastic 0 – 30° Range of lattice spacing Forward scattering: 1.5 < d < 1000 Å Backward scattering: 0.5 < d < 7 Å d-resolution Forward scattering: ∆d/d > 2·10−2 Backward scattering: ∆d/d≥2·10−3 Sample size <13 x 13 mm2 Detector 192 3He linear position sensitive detectors combined in 4 sections, each containing 48 detector tubes of 900 mm e�ective length and 12.7 mm diameter SInstrument options Elastic: Di�raction; Low-Q Inelastic: direct TOF spectrometer (under construction) Sample environment B = 26.3 T (B||ki ± 15°) T = 0.6 K – RT Software Egraph (event recording data reduction) Mantid (TOF data reduction) Chopper speed range 5 – 600 Hz (Fermi chopper) 5 – 215 Hz (double disc choppers) 5 – 120 Hz (single disc choppers) Sample-detector distance 2.5 - 6 m Table 2: Technical parameters of HFM/EXED. 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Journal of Low Temperature Physics, 159(1), 402–405. http://dx.doi.org/10.1007/s10909-009-0062-1 7 http://dx.doi.org/10.17815/jlsrf-3-111 http://dx.doi.org/10.1109/TASC.2008.921228 http://dx.doi.org/10.1109/TASC.2014.2361098 http://dx.doi.org/10.1109/TASC.2009.2018810 http://dx.doi.org/10.1109/TASC.2009.2039123 http://dx.doi.org/10.1080/10238160600766294 http://dx.doi.org/10.1524/9783486992526-033 http://dx.doi.org/10.1063/1.4913656 http://dx.doi.org/10.1109/TASC.2016.2525773 http://dx.doi.org/10.1007/s10909-009-0062-1 https://creativecommons.org/licenses/by/4.0/ Introduction Instrument application Instrument layout Technical Data