[en] A multi-detector chopper spectrometer enables measurements of the scattering function S(Q,E) to be made over a wide range of momentum and energy transfer (Q,E). The application of pulsed-source chopper spectrometers for inelastic measurements at thermal and epithermal energies (50 meV < E < 1000 meV) is well known. Recently at IPNS, we have extended the energy-transfer region down to about 0.5 meV with a resolution of the order of 150 μeV. It is made possible by utilizing the cold-neutron incident spectrum of the 100 K methane moderator in conjunction with a dual beryllium-body rotor system. Neutron incident energies can be changed efficiently over the 4 to 1000 meV region while maintaining an undisturbed sample environment. We describe the operation of the IPNS chopper spectrometers (HRMECS and LRMECS), the instrumental resolution and the background-suppression performance. The capability of measuring inelastic features from 0.5 to 100 meV with an energy resolution of ΔE/E₀ = 2.5% is demonstrated by experimental results of crystal-field excitation spectra of a high-Tc superconductor ErBa₂Cu₃O₇. Preliminary data of quasielastic scattering from a room-temperature molten salt AlCl₃-EMIC are presented

[en] Several of the instruments at the Intense Pulsed Neutron Source (IPNS) at Argonne use rotating Fermi choppers. The techniques used to control the speed and phase of these rotating devices are discussed

[en] In the rapid cycling synchrotron (RCS), a single proton bunch (h=1) is accelerated from 50 MeV to 450 MeV in approximately 14.2 ms. The bunch experiences an instability shortly after injection (<1 ms). During the first millisecond, the beam is bunched but little acceleration takes place; therefore this period is similar to a storage ring mode of operation. Natural vertical oscillations (assumed to be tune lines) show the vertical tune to be rising toward the bare tune value, suggesting neutralization of space charge and a reduction of its detuning effects. Neutralization time near injection ranges from 250 mus - 500 mus, depending on the background gas pressure. Oscillations move from the LSB to the USB before disappearing. Tune measurements made with a recently installed ferrite- magnet pinger system show the horizontal chromaticity to be positive early but approaching zero later in the cycle; on the other hand, the vertical chromaticity is negative throughout the cycle. During pinger studies near injection, two vertical lines are observed. Neutralization of the beam space charge implies the generation of plasma in the beam volume early in the cycle which may then dissipate as the time-varying electric fields of the beam become stronger.