KK DA ΦNE TECHNICAL NOTE INFN - LNF, Accelerator Division Frascati, July 28, 2004 Note: G-61 Terahertz Coherent Synchrotron Radiation at DAΦNE C. Biscari 1 , J. M. Byrd 2 , M. Castellano 1 , M. Cestelli Guidi 1 , A. Gallo 1 , A. Ghigo 1 A. Marcelli 1 , F. Marcellini 1 , M.C. Martin 2 , G. Mazzitelli 1 , C. Milardi 1 , P. Morini 3 , M. Piccinini 1 , M. Preger 1 , P. Raimondi 1 , D. Sali 3 , F. Sannibale 1,2 , M. Serio 1 , M. Zobov 1 1 INFN Laboratori Nazionali di Frascati - C.P. 13 00044 Frascati – Italy 2 Lawrence Berkeley National Laboratory – One Cyclotron Rd, Berkeley 49720, CA USA 3 Bruker Optics, Via G. Pascoli, 70/3, 20133 Milano, Italy Abstract We investigate the phenomena related to the emission of coherent synchrotron radiation (CSR) in the Terahertz frequency range at the DAΦNE collider. In particular, we calculate the threshold for the synchrotron radiation (SR) induced single bunch instability, usually referred as microbunching instability (MBI) [1-4]. The MBI can potentially have some effect at very high current in DAΦNE as well as in the foreseen strong RF focusing mode presently under investigation [5]. We also study the possibility of using DAΦNE as a high stability broadband CSR source in the Terahertz frequency. The cases for the existing normal-conductive RF system and for the super-conductive one proposed in [6] are evaluated following the optimization scheme of references [7, 8]. Such special mode of operation requires a quasi- isochronous lattice. Two possible solutions, with wigglers ON and OFF respectively, are also presented. Introduction An important issue for the high current operation of DAΦNE and for the proposed strong RF focusing experiment is the evaluation and understanding of the effect of the machine impedance on the bunch length and energy spread. One effect which has not yet been considered is a microbunching instability (MBI) driven by the radiation impedance. We would like to determine the importance of the MBI for these modes of operation. In fact, when the current per bunch is above the MBI threshold, the longitudinal dynamics of the bunch starts to be perturbed by the instability. For most storage rings and just above the MBI threshold, the perturbation is weak and shows measurable effects only as bursts of CSR in the far infrared (FIR). For currents much higher than the MBI threshold the effects could be relevant inducing average bunch lengthening and strong transient modulation of the longitudinal distribution.