Dream of isochronous ring, again

Experimental and theoretical studies of the isochronous synchrotron radiation ring, so far, suggest nonlinear effect resulted from higher order dispersion and chromaticity declines the "complete" isochronous system [1]. In addition, we have recently found that path-length deviation originated from the betatron motion is a much seriouse obstacle to preserve the bunch length [2]. However, It has been found out that a careful lattice design for the path-length cancellation seems to be very promissing, at least for a wavelength region of THz. Tolerance of the path length along a turn of the ring may be within our reach. A concept to preserve bunch form-factor (or very short bunch) by using quasi-dispersion-free arcs having proper betatron phase advances in order to produce coherent THz radiation from every bending section. Furthermore, its extension toward prebunched-FEL and SASE-FEL will be also discussed.