Ted's 17T Cryomagnet

The outside dimensions are shown in the diagrams. The solenoid is within the “box” part which is 480 mm long by 360 mm wide. The base of the box is a 480 mm square plate which will be used to mount the magnet on a beamline. It has four holes on a 420 mm square to take mounting bolts. The base-plate has a circular centring recess which will take a 100 mm diameter 5 mm high boss, if required.

Field & neutron access

The maximum field at the sample position is to be 17 T horizontal, parallel to the beam, with 0.1% homogeneity over a 10 mm diameter spherical volume. The input neutron access is a cone extending ±10° around the horizontal field axis illuminating a 10 mm diameter sample. The output scattering angles can extend to a cone ±11° around the horizontal axis. The magnet is be tilt-able by ±10° at field and to operate with horizontal forces up to 100 kg.

With a central field of 17 T, the field at the windows is calculated to be 1.8 T. At 1 m from the centre of the coil, along the axis, it is 25 mT. At 1 m perpendicular to the axis, the field is -12 mT. At larger distances, these fields fall off as 1/r3, as expected for a dipole.

Sample

The sample temperature is to be controllable from 1.6 to 300 K (down to 60 mK with the dilution fridge). Sample exchange is to be “side loading”, with the sample holder loaded into the cryostat vacuum (while the magnet is cold) using a demountable dedicated manipulator. For sample change, the cryostat will be rotated so that its axis is 90° to the beam, to allow the manipulator to insert the sample holder along the magnet axis.

There is to be an option to plug in up to 8 low-current leads on the sample holder, to allow additional thermometry, in-situ measurements, or the application of electric fields. For both the low T stages, an optional unit can be used to rotate the sample about a vertical axis by up to 180°. This unit is not very suitable for scanning sample angle, only for adjusting it, and it provides and holds a limited torque, so not suitable for very magnetically anisotropic samples. All of these should be used in collaboration with B'ham/Lund, since changing from VTI to room-temperature bore or dilution fridge is pretty technical and sample change ditto, except for the room-temperature bore which is easy.

Material in the beam

The cryostat will have a very low background, since the only material in the beam (apart from the sample) will be 2 x 5 mm thick sapphire windows (these could be made half this thickness, or of Si, but with increased risk of breakage by any impact), plus < 0.5 mm total thickness of pure Al for LN2T windows. It is envisaged to make an optional re-entrant window which would give access to the magnet centre at room temperature & atmospheric pressure.

By purchase of Be windows, the entire cryostat could be used for X-ray measurements.

Facilities required

Single phase electrical supply. Crane to lift < 400 kg weight of cryostat. Goniometer for tilting, with motors not too close to magnet. Reasonably non-magnetic surroundings to reduce magnetic forces (Motors and forces will have to be tested offline before use). Liquid nitrogen & liquid helium filling, which should give an autonomy for at least 3 days. 16 m3 /hr or greater helium rotary pump for VTI cooling. Turbo pump to evacuate sample change manipulator. Beamtime!