Fermi chopper developed at KEK

We developed a Fermi chopper by modification of a turbo molecular pump with a magnetic bearing system, and the actual device, rotating at f=100–600 Hz using a rotation system similar to the turbo molecular pump, was successfully produced for inelastic neutron scattering experiments at the Japan Proton Accelerator Research Complex (J-PARC). We assumed a phase control accuracy of Δ τ ≤ 0.3 μ s , i.e., a frequency resolution of Δ f ≤ 0.01 Hz at a rotational frequency of f=600 Hz, as the specification for the development. The phase control system was developed using a direct digital synthesizer (DDS) system with a frequency control unit of 0.00058 Hz at f=600 Hz. The phase control accuracy in the produced Fermi chopper was observed to be Δ τ = 0.08 μ s at f=600 Hz, well within the specification parameters. After mounting the produced Fermi chopper on the High Resolution Chopper Spectrometer (HRC) at J-PARC, the energy resolution for an incident neutron energy Ei=206 meV was observed to be identical to the designed standard resolution of Δ E / E i = 2.5 % . We were able to produce a state-of-the-art Fermi chopper.