  To merge the beam from either of the two injectors to the main linac, a
dog-leg system will be employed in the second Medium Energy Beam Transport
(MEBT2) line of the China ADS driving accelerator. The achromatic condition has
to be guaranteed to avoid beam center excursion against energy jitter. RF
cavities were found indispensable to control the bunch length growth in the
dog-leg system of MEBT2. The full uncoupling between transverse and
longitudinal plane is desired to minimize the growth of projected rms
emittances. The uncoupled achromatic condition of this dogleg system with the
presence of RF bunching cavities will be deduced using the method of transfer
matrixes. It is found that to fulfil the uncoupling condition, the distance
between the bunching cavities is uniquely determined by the maximum energy gain
of the RF cavities. The theoretical analysis is verified by the simulation code
TraceWin. The space charge effect on the uncoupled achromatic condition and the
beam emittance growth will also be discussed.
