  A time projection chamber (TPC) can be used to measure the polarization of
gamma rays with excellent angular precision and sensitivity in the MeV-GeV
energy range through the conversion of photons to e+e- pairs. The Hermetic
ARgon POlarimeter (HARPO) prototype was built to demonstrate this concept. It
was recently tested in the polarized photon beam at the NewSUBARU facility in
Japan. We present this data-taking run, which demonstrated the excellent
performance of the HARPO TPC.
