We present the first long-duration and high duty cycle 40-T pulsed-field cryomagnet addressed to single crystal neutron diffraction experiments at temperatures down to 2 K. The magnet produces a horizontal field in a bi-conical geometry, ±15° and ±30° upstream and downstream of the sample, respectively. Using a 1.15 MJ mobile generator, magnetic field pulses of 100 ms length are generated in the magnet, with a rise time of 23 ms and a repetition rate of 6-7 pulses per hour at 40 T. The setup was validated for neutron diffraction on the CEA-CRG three-axis spectrometer IN22 at the Institut Laue Langevin.
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Unfortunately the definitive shutdown of the 10 MW research-reactor BER II at HZB is planned at the end of 2019, leaving the future of the HFM-EXED facility uncertain in spite of its heavy costs and infrastructure investments.
Defining tuseful as the useful time of a field pulse and tcool as the time to cool down the magnet between two pulses, the duty-cycle of a pulsed field coil is defined by the ratio tuseful/tcool. In the neutron experiments described here, data are recorded over the whole pulse. So, tuseful corresponds to the total pulse duration.
From the capacitor bank to the top of the nitrogen cryostat, the current flows in a 25 mm2 Omerin flexible coaxial cable. From the top of the cryostat and the coil, the current flows in a rigid coaxial connector made with a 6 mm diameter copper rod insulated with Kapton and glued with Stycast 2850FT Blue in a 18 mm diameter and 1 mm thickness copper tube. Links between each coaxial part and with the coil wire are ensured by brass clamps.
The field factor B/I is the ratio between B measured by a pick-up coil inserted inside the 40 T magnet and generated by a small current I measured itself by a calibrated Keithley ammeter. The pick-up coil area is determined, thanks to a calibration solenoid whose geometrical properties are given in the Ref. 39. The main error source of the field factor comes from the evaluation of the pick-up coil area.
The rotation of the sample is only possible at room temperature. To do this handling, the liquid nitrogen is removed and the cryomagnet is warmed up to room temperature before breaking the vacuum. The cryomagnet is then opened to have access to the cold finger and sample holder.
