Future directions in the storage ring development for light sources

Major performance objectives for storage-ring based light sources are small emittance and energy spread, high beam stability, and full exploitation of the photon energy from a few eV up to hard X-rays. Most of the recent-generation light sources are built up by highly optimized lattices of the Double Bend- or Triple-Bend Achromat type. There is little room left to further optimize these lattices to even smaller emittances. Only a local adjustment of the optics at the source points might still bring some benefit. With decreasing emittance orbit stability is becoming more and more important. Cures for vibrations, thermal effects and ground motion have to start at the basic design of the building, the storage ring infrastructure and the magnet support system. Beam instabilities are especially harmful as they increase the energy spread and as a consequence deteriorate the higher harmonics of the undulator radiation. Well optimized beams suffer from lifetime problems due to Touschek scattering. Particular problems arise when short bunches are requested, for instance for storage ring based lasers. Medium-energy machines are able to produce high brilliance in the hard X-ray range by means of mini-undulators with small gaps. This, again, has an adverse effect on the beam lifetime due to beam gas scattering. The current trend in storage ring development for future Light Sources is discussed. What are the crucial issues and where is improvement desirable and possible? Is there still room for improving the basic design of a light source? Are there alternatives to the `classical´ lattices which could be used for the next generation? Or is improvement possible only on the operational field?