  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 minimising
the footprint and maximising the efficiency of the system. However such set-ups
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 minimised 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.
