Rocket nozzle & diffuser interaction

[frankylu]

My rocket engine is tested in a vacuum chamber. The high vacuum is achieved by steam ejectors. However, to further reduce chamber pressure during engine firing, the supersonic nozzle of the engine is inserted into a diffuser (basically a straight tube). The exit area of nozzle is slightly smaller than inner diameter of diffuser. So when the engine is fired, the exhaust produces additional ejector effect to draw air out of vacuum chamber, reducing chamber pressure. My question is, does the size of flow channel between nozzle exit and diffuser wall affect my thrust measurement? Is there any wave interaction with diffuser wall that affect my thrust measurement? Thanks.