Design of a Pair of Superconducting Solenoids for a Neutron Spin-Echo Spectrometer at the SNS

A neutron spin-echo spectrometer (NSE) of the next generation will be build for the spallation neutron source (SNS) in Oak Ridge, USA. A NSE spectrometer measures tiny velocity changes of the neutrons encoded by the neutrons spin clock at a sample while the neutron spin precesses in large magnetic fields following Bloch´s equation. This instrument will be the best of its class both with respect to resolution and dynamic range. In order to reach this ambitious goal, a large magnetic precision field integral before and after the sample is required which directly scales linearly with the resolution of the instrument. Therefore superconducting technology will be used to allow for a higher magnetic field integral. Here, we present the first basic design of the solenoids which has been developed in a joint project study by Julich research center (FZJ) and Babcock Noell GmbH (BNG). The solenoids will generate an integrated magnetic flux density of 1.8 Tm. To reduce the fringe field, active shielding is foreseen. To minimize vibrations modern pulse-tube cryocoolers cool the NbTi windings below Tc. A special feature of the magnet system is the position measurement with an accuracy in the order of micrometers for the windings in the cryostat during operation. This information is required for the adjustment of so called Fresnel coils outside the cryostat. Together with the setup of these correction elements, a field integral homogeneity better than 10^-6 for different neutron paths through one of the solenoids will be achieved. In addition special care has been taken in the selection procedure for the SC strands. This is necessary for the minimization of its hysteresis values and subsequently for full determination and control of the magnetic field with an accuracy of about 10^-5.