Minimum electron beam size of a triple bend lattice

The positive effect of the dispersive straight section to improve the brightness in the third-generation light source with a triple bend lattice is discussed. In order to evaluate the brightness performance of a non-achromat case, used is σ x 2 / β x used is at the insertion straight center that reduces to the usual beam emittance in the achromat case. The derived minimum of σ x 2 / β x in the non-achromat case is smaller than the minimum emittance of the achromat case, just as it happens in the double bend lattice.