LIGO Document T1300553-v1

Effect of leak-test port separation; measurement of He transient at z = 250m

Document #:
LIGO-T1300553-v1
Document type:
T - Technical notes
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Abstract:
In T1200375 (Weiss and MZ, July 2012) we calculated P(t,z) for impulse and finite-step helium injections into the beam tube. A practical application is to determine degradation of He mass-spectrometer leak detector (MSLD) test sensitivity as a function of separation z between test site and sampling port.

Here I plot expected MSLD response to a nominal 30-second injection of helium at various distances z. The expected peak deflection is found to be degraded by about a factor of five at z = 250m (corresponding to the separation between beamtube pump ports), and by a factor of ten at z = 500m. The time delays from injection to peak response are about 1 and 5 minutes at these locations, respectively.

I then compare the prediction to a test injection at z = 250m, performed 15 May 2013 on the LLO Y tube. Agreement looks adequate for the present purpose, but the detailed transient is not completely understood. More investigation of the leak detector s impulse response is needed.

Because the limiting MSLD test sensitivity at any position is not comfortably below our threshold for concern, I conclude that wherever feasible it makes sense to locate the MSLD on one of the test ports nearest each zone under investigation.

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