  In order to update the beam power from 100 kW to 250 kW in China spallation
neutron source (CSNS) Phase II, one of the important measures is to replace the
80 meters long beam transport line between the present 80 MeV linac injector
and the RCS to another kind of acceleration structure. In this paper, we
proposed a scheme based on 324 MHz double-spoke superconducting cavities.
Unlike the superconducting elliptical cavity and normal conducting CCL
structure, the double-spoke cavity belongs to TE mode structure and has smaller
transvers dimension compared with that of TH mode one. It can work at base
frequency as the DTL section, so that the cost and complexity of the RF system
will be much decreased, and the behaviors of the beam dynamics are also
improved significantly because of the low charge density and larger
longitudinal acceptance. Furthermore, because of the relatively longer
interactive length between charged particle and the electromagnetic field per
cell, it needs relatively less cell numbers and it has larger velocity
acceptance compared with the double frequency TH structures. The
superconducting section consists of 14 cryomodules, each of which includes 3
superconducting cavities and a doublet. The general considerations on cavity
and beam dynamics design are discussed and the main results are presented.
