Top-up operation allows SPring-8 to provide highly stable X-ray beams with arbitrary filling patterns. The implementation of top-up operation is described, with a focus on the simultaneous achievement of stability of stored current, beam orbit, purity of an isolated single bunch, and beam injection efficiency. Stored-current fluctuations have been routinely reduced to a level of 10 À3 . Stored-beam oscillation on frequent beam injection, which was originally regarded as the most serious problem, has been successfully suppressed to a sufficiently low level that it never perturbs imaging experiments. Current impurities in nominally empty buckets have been reduced to a level of 10 À9 over more than one week of operation, making possible the measurement of timeresolved spectra using high-current bunches. Finally, excellent injection efficiency, higher than 80%, is routinely obtained, even for small undulator gaps, which is critical for preventing radiation damage to insertion-device magnets and to reduce leakage radiation. The process of achieving highly stabilized top-up operation at SPring-8 and its utility for user experiments are described.

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Possibilities of manipulating the Rabi frequency and luminescence rate from degenerate-level systems as well as the velocity of self-induced transparency of multi-level media are studied using a unitary transformation. The Rabi frequency and luminescence rate of an electronic system whose ground level is degenerate and coupled to a resonant mode are found to depend on the level of the degeneracy. The velocity of multi-mode optical solitons in a multi-level medium is found to be influenced by the number of propagating resonant pulses. Physical realizations of relevant systems are proposed.

We fabricated improved DC septum magnets that have smaller leakage fields for continuous beam injection. We changed the shape of the coil and placed field clamps at both ends of the magnets. We increased the thickness of the septum-plane shields and extended them to the field clamps. Leakage fields were measured and the effect they had on an electron beam during injection was studied. After reducing the leakage fields, the orbit changes during injection were less than the beam sizes, which satisfied the requirements for continuous beam injection.Index Terms-Leakage field, septum magnet, synchrotron radiation source, top up operation.