High field resistive magnets may be built by stacking hundreds of so-called Bitter disks to form a coil. Nowadays, these disks have strongly elongated cooling holes arranged in a staggered pattern. Near the outer diameter of the disks one finds larger holes, round or flattened, to accommodate a tie-rod. These tie-rods assist in the assembly of the coils and are usually pretensioned to ensure a net compressive clamping force on the coil. However, at high fields, the axial magnetic forces toward the center of the coil dominate over any pretension in the system, reducing the net resulting clamping force at the end plates to a very low value. This is of particular importance in hybrid magnets where the resistive insert coils experience the background magnetic field of a large superconducting magnet. Several different strategies have been reported to mitigate the effect. We describe a novel clamping method that employs a water-filled, pressurized bellow that exerts a compressive force to the different subcoils of the resistive coil assembly. In contrast to the tie-rods, the force exerted by the bellow will remain almost constant when the magnet is energized and contracts. This new clamping method will be a key element for the mechanical design of the resistive insert coils of the HFML 45 T hybrid magnet.
The main science aim of the BlackGEM array is to detect optical counterparts to gravitational wave mergers. Additionally, the array will perform a set of synoptic surveys to detect Local Universe transients and short time-scale variability in stars and binaries, as well as a six-filter all-sky survey down to ∼22 nd mag. The BlackGEM Phase-I array consists of three optical wide-field unit telescopes. Each unit uses an f/5.5 modified Dall-Kirkham (Harmer-Wynne) design with a triplet corrector lens, and a 65 cm primary mirror, coupled with a 110Mpix CCD detector, that provides an instantaneous field-of-view of 2.7 square degrees, sampled at 0.564 ′′ /pixel. The total field-of-view for the array is 8.2 square degrees. Each telescope is equipped with a six-slot filter wheel containing an optimised Sloan set (BG-u, BG-g, BG-r, BG-i, BG-z) and a wider-band 440-720 nm (BG-q) filter. Each unit telescope is independent from the others. Cloud-based data processing is done in real time, and includes a transient-detection routine as well as a full-source optimal-photometry module. BlackGEM has been installed at the ESO La Silla observatory as of October 2019. After a prolonged COVID-19 hiatus, science operations started on April 1, 2023 and will run for five years. Aside from its core scientific program, BlackGEM will give rise to a multitude of additional science cases in multi-colour time-domain astronomy, to the benefit of a variety of topics in astrophysics, such as infant supernovae, luminous red novae, asteroseismology of post-main-sequence objects, (ultracompact) binary stars, and the relation between gravitational wave counterparts and other classes of transients.
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