A quadruple-bend achromatic (QBA) cell, defined as a supercell made of two double-bend cells with different outer and inner dipole bend angles, is found to provide a factor of 2 in lowering the beam emittance relative to the more conventional double-bend achromat. The ratio of bending angles of the inner dipoles to that of the outer dipoles is numerically found to be about 1.5-1.6 for an optimal low beam emittance in the isomagnetic condition. The QBA lattice provides an advantage over the double-bend achromat or the double-bend nonachromat in performance by providing a small natural beam emittance and some zero-dispersion straight sections. A lattice with 12 QBA cells and a preliminary dynamic aperture study serves as an example.

The correlation between the horizontal beam size versus the cavity temperature, observed at the Taiwan Light Source, is explained by the combined effects of (i) the cavity resonance frequency shift resulting from temperature change, (ii) the cavity voltage and synchronous phase angle change resulting from uncompensated beam loading in the low-level rf-feedback system, (iii) the synchrotron frequency shift due to the cavity voltage, and (iv) the rf cavity voltage modulation for alleviating the coupled-bunch instability. We find that the effect of higher-order parasitic-mode losses on the change of the horizontal beam size is small. This experimental method can be used to evaluate the intrinsic resolution of the low-level rf-feedback system.

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