[en] For ICF hohlraums driven by long pulses, such as will be needed for ignition on the NIF, the high-Z wall must be held back to avoid excessive laser spot motion and time-dependent symmetry swings. One means of accomplishing this is to fill the hohlraum with a low density, low-Z gas. We report on gas-wall interface characterization by axial x-ray backlighting and self-emission, on gas filled hohlraums fielded at the Omega facility. Up to 30 drive beams are fired, forming a single ring of illumination on the hohlraum wall to emulate the near 2D cylindrically symmetric NIF hohlraum drive conditions. We compare the observed motion with predictions. In addition, the gas-gold interface is Rayleigh-Taylor (R-T) unstable during deceleration. This R-T instability could be further exacerbated in NIF ignition hohlraums designed with intentionally roughened walls to provide smoothing of infrared heating used to prepare smooth DT ice layers in the capsule. We have therefore intentionally prepared initial perturbations on one half of the gold wall to quantify the amount of increased penetration, due to mix of the gold into the gas, at stagnation