Designing an elliptical supermirror guide for the high-pressure material science beamline of J-PARC

The design of an elliptical neutron guide for the time-of-flight (TOF) powder diffractometer of the high-pressure material science beamline at J-PARC was optimized using the McStas Monte Carlo simulation program to increase neutron flux for a small sample size. We investigated the analytical design in accordance with some requirements such as the gain factor at the small sample size and sufficient resolution at shorter wavelength of incident neutrons. The intensity and divergence with respect to energy dependence for elliptical guide designs were compared with those for the linearly tapered and straight guides. Optimizing the design of the elliptical guide results in the largest gain below 0.70 Å wavelength and gives constant instrumental resolution greater than 1 Å. The guide can support a flux gain of about 2.2 (high-intensity mode, Δd/d of ca. 0.6%) and 1.2 (high-resolution mode; Δd/d of ca. 0.5%) greater than 1 Å compared to the beamline of the natural flight path with resolution similar to that provided by the optimized elliptical guide.