For S2 and S3 the frames around the end plugs are in a region shadowed by shielding. This is not the case for the upper end of the straws in array S1. The end plugs in this region need to contain minimal materials. We plan to use diffusion from one end of the S1 tubes, so that the plug on the upper end need provide only gas sealing and wire support. This technique of gas handling has been used successfully [69], for straw tubes of similar diameter and length.
According to Ref. [70], straw tubes of similar diameter and wall thickness have been tested with an over-pressure of up to 40 psi. We expect the operation of S1 in vacuum to be a soluble problem, with the straw tube material withstanding the pressure differential in the active region of the array.
Prototype testing is planned to check the long-term mechanical stability of unsupported tubes with a 15 psi pressure differential. As a backup, a minimal frame structure is being considered for the upper end of the S1 straws.