High Magnetic Field
8 horizontal and 9 vertical field cryomagnets are available. Most of them are available from the pool on request. Sufficient advance notice to sane(at)ill.fr is required for their reservation and preparation. A very few cryomagnets are assigned to specific instruments. The scientific and technical staff of the instruments concerned are in charge of their upkeep. However major repairs and modifications are carried out in the workshop of the Services for Advanced Neutron Environment team.
The use of the cryomagnets is under the responsibility of the instrument scientists and local-contacts. However, we provide all the necessary equipment and technical support for measuring the magnetic forces and establish the conditions in which each cryomagnet can be used. A set of angular limits and minimum distances is defined from these measurements and added to our Safety section.
The coil of this magnet was designed and built by our colleagues from LNCMI-Toulouse. It is equipped with liquid nitrogen cooling channels to increase the duty-cycle. The conical shape provides ±15° incident and ±30° scattered beam accesses in the scattering plane. The vertical access is ±7°. Neutron windows are made from single crystalline silicon to minimize background issues.
The coil is installed inside a cryostat developed by the ILL and constructed by AS Scientific (UK). To allow easy replacement and efficient cooling of the coil, the liquid nitrogen bath is central, large and surrounded by a liquid helium jacket. The initial cool-down time is of 2.5 hours.
The coil produces a maximum horizontal pulsed-field of 40 Tesla with a rise time of 23 ms every 8-9 min using a 1.15 MJ transportable power supply also developed at the LNCMI-Toulouse. At fields lower than 20 Tesla, the duty cycle is limited by the time necessary to charge the capacitors, i.e. about a minute. The maximum liquid nitrogen consumption is ≈1 000 litres per day at 40 Tesla.
A standard ILL cold-valve regulates the He flow brought to the sample through a Torlon/Sapphire heat exchanger developed at ILL to avoid eddy currents and reduce neutron background. The sample temperature is regulated between 2 and 300 K by controlling the He flow and the heater power. The maximum sample diameter that fits inside the coil is Ø8 mm and different sample holders are supplied to allow 45° rotations around the field axis without realignment and reglueing.
Used on three-axis spectrometers and some of the diffractometers, this recent cryomagnet has a low liquid-helium boil-off and does not require frequent refills. Indeed, there is a standard liquid helium reservoir (about 40l) cooled down by a 1.5W@4.2K cryocooler. The centre of the cryostat was equipped by ILL with a large variable temperature insert (VTI) allowing the sample to be maintained at any temperature between 1.5 and 300 K. Temperatures down to 40 mK are accessible with an ILL dilution insert.
The access in the horizontal plane is defined by a two-pillar design proposed by ILL and calculated by Oxford Instruments. This corresponds to a ±130° horizontal opening angle to Ø38 mm sample + 1 mm for screwing cadmium plates on all sides. The two half-coils suspended in the vacuum are kept apart using only two 10 mm thick Al5154 aluminium rings capable of withstanding the magnetic attraction of the coils. With this design, the neutron window thickness is 20 mm radial compared to the 33 mm required in the commonly adopted continuous ring design. In the vertical plane, the neutron access is ±5° to a Ø38 x 30 mm3 sample (minimum gap of 40 mm).
We have recently commissioned an asymmetric vertical-field cryomagnet dedicated to polarised neutron reflectometry (i.e. for ADAM and D17). The access in the horizontal plane is ±20° to a 20x20 mm2 sample + 1 mm for screwing cadmium plates on all sides. As regards the vertical plane, the neutron access is ±5°. For reducing diffuse scattering, the neutron windows are made from single-crystalline sapphire.