Adjustment of emittance ratio by coupling control in electron-positron storage rings

In electron-positron rings of a given magnetic structure, the transverse emittances are governed by synchroton radiation and by the coupling of the transverse motions. Some conditions on the emittances are always prescribed to optimize the luminosity. The emittance ratio should be adjusted either to equalize the tune shifts in the two planes or to put the vertical tune shift at the limit, when the number of particles is restricted. Theories on linear coupling in a three-dimensional field and on amplitude variation in the presence of radiation have been combined in order to establish analytical expressions for the emittances. These allow, in particular, a determination of the smallest possible emittance ratio. Application to LEP has shown that this ratio could be varied over a large range of values by means of a limited number of skew quadrupoles, which have, in any case, to be present for the compensation of experimental solenoids. (Auth.)