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In order to study the physics responsible for a hmitation in angular momentum in heavy-ion fusion reactions, fusion cross sections for the reactions 2sSi+isSi, '60+~Ca, 3tS+s Si, and ' C+' Cr have been measured at several incident energies covering lower and higher energy regions by detecting the yields of the fusion-evaporation residues. The limiting angular momenta for fusion are extracted from the measured fusion cross sections and compared with calculations based upon several models. Through the systematic study of different entrance channels forming the same compound nucleus, a strong entrance channel effect has been observed for very asymmetric systems. Our conclusion is that the entrance channel is significant in limiting the fusion cross section in the higher energy region.

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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