  The X-ray Timing and Polarization (XTP) is a mission concept for a future
space borne X-ray observatory and is currently selected for early phase study.
We present a new design of X-ray polarimeter based on the time projection gas
chamber. The polarimeter, placed above the focal plane, has an additional rear
window that allows hard X-rays to penetrate (a transmission of nearly 80% at 6
keV) through it and reach the detector on the focal plane. Such a design is to
compensate the low detection efficiency of gas detectors, at a low cost of
sensitivity, and can maximize the science return of multilayer hard X-ray
telescopes without the risk of moving focal plane instruments. The sensitivity
in terms of minimum detectable polarization, based on current instrument
configuration, is expected to be 3% for a 1mCrab source given an observing time
of 10^5 s. We present preliminary test results, including photoelectron tracks
and modulation curves, using a test chamber and polarized X-ray sources in the
lab.
