Often a window is found to be necessary to separate the Front End vacuum from the User beamline. This is the case in the early phases of beamline operation to accommodate shielding verification. The window is positioned as needed between different vacuum transports and/or experimental stations. The Front End window is located at the end of the Front End transport and separates the Storage Ring vacuum (front-end vacuum) from the experimental beamline vacuum. For ease of maintenance and access, the window is located outside the Storage Ring tunnel and within the FOE. The window is separated from the FE via a UHV valve known as the Front End Valve (FEV).

The APS window is a dual diaphragm assembly with a gas flow in between. Dual window diaphragms lend added assurance for two reasons. The second window on the downstream side does not absorb much heat because the first window intercepts most of the absorbable soft x-ray photons. As such the second window remains structurally strong to absorb atmospheric shock. Also, in the event of one window failure, the second serves to preserve the isolation. The traditional window material is beryllium and/or diamond. The purpose of a gas barrier is to keep the wafer surfaces free from oxidation/carbonization.