Parameters optimization of <i>e</i> ⁺ <i>e</i> ^- crab waist colliders at low energy

The projects of Super Tau-Charm factory, operating in the beam energy range 1500–3500 MeV, aim to achieve luminosity of 10[Formula: see text] cm[Formula: see text]s[Formula: see text] utilizing the crab waist collision scheme. The main limitation at such energies is a small beam lifetime due to Touschek effect, aggravated by high bunch intensity, small emittance and small dynamic aperture. The paper describes optimization principles that led to a new set of parameters with higher Touschek beam lifetime, larger emittance, unchanged luminosity and dynamic aperture. The new optimized parameters are tested in two modifications of the present design of Super Tau-Charm factory in Novosibirsk (Russia). Both modifications improve beam lifetime by factor [Formula: see text] with horizontal emittance increased by factor 2. One modification achieves higher emittance by reduction of the arc unit cell phase advance, the other by introduction of the dispersion in the damping wiggler. The merits of the both modifications are discussed.