  RF power coupler is one of the key components in superconducting (SC) linac.
It provides RF power to the SC cavity and interconnects different temperature
layers (1.8K, 4.2K, 70K and 300K). TTF-III coupler is one of the most promising
candidates for the High Energy (HE) linac of Project X, but it cannot meet the
average power requirements because of the relatively high temperature rise on
the warm inner conductor, some design modifications will be required. In this
paper, we describe our simulation studies on the copper coating thickness on
the warm inner conductor with RRR value of 10 and 100. Our purpose is to
rebalance the dynamic and static loads, and finally lower the temperature rise
along the warm inner conductor. In addition, to get stronger coupling, better
power handling and less multipacting probability, one new cold part design was
proposed using 60mm coaxial line; the corresponding multipacting simulation
studies have also been investigated.
