In order for sources of coherent high brightness and intensity THz and x-ray radiation to be accepted
by university or industrial R&D laboratories, truly compact, high current and efficient particle
accelerators are required. The demand for compactness and efficiency can be satisfied by superconducting
rf energy recovery linear accelerators (SRF ERL) allowing effectively minimizing the
footprint and maximizing the efficiency of the system. However such setups are affected by regenerative
beam break-up (BBU) instabilities which limit the beam current and may terminate the beam transport
as well as energy recuperation. In this paper we suggest and discuss a SRF ERL with asymmetric
configuration of resonantly coupled accelerating and decelerating cavities. In this type of SRF ERL an
electron bunch is passing through accelerating and decelerating cavities once and, as we show in this
case, the regenerative BBU instability can be minimized allowing high currents to be achieved. We
study the BBU start current in such an asymmetric ERL via analytical and numerical models and
discuss the properties of such a system