Optimization of beamtube pumping to mitigate acquired water vapor

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Abstract:: Water vapor ingested from 2008 to 2012 appears to have increased the LLO Y tube's outgassing, slightly exceeding specified limits. A finite element diffusion model was used to assess resulting gas index phase noise. The model highlighted a historical oversight leading to systematic noise underestimation in situations with substantial conduction gradients, such as end-pumping of water vapor (the nominal default). The aLIGO configuration at LLO is especially vulnerable. A limited deployment of ion pumps is found to restore adequate noise margin.

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LIGO Document T1500202-v1